The PHENIX collaboration presents a systematic study of $\pi^0$ production from $p$ $+$ $p$, $p$ $+$Al, $p$ $+$Au, $d$ $+$Au, and $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Measurements were performed with different centrality selections as well as the total inelastic, 0%--100%, selection for all collision systems. For 0%--100% collisions, the nuclear modification factors, $R_{xA}$, are consistent with unity for $p_T$ above 8 GeV/$c$, but exhibit an enhancement in peripheral collisions and a suppression in central collisions. The enhancement and suppression characteristics are similar for all systems for the same centrality class. It is shown that for high-$p_T$-$\pi^0$ production, the nucleons in the $d$ and $^3$He interact mostly independently with the Au nucleus and that the counter intuitive centrality dependence is likely due to a physical correlation between multiplicity and the presence of a hard scattering process. These observations disfavor models where parton energy loss has a significant contribution to nuclear modifications in small systems. Nuclear modifications at lower $p_T$ resemble the Cronin effect -- an increase followed by a peak in central or inelastic collisions and a plateau in peripheral collisions. The peak height has a characteristic ordering by system size as $p$ $+$Au $>$ $d$ $+$Au $>$ $^{3}$He$+$Au $>$ $p$ $+$Al. For collisions with Au ions, current calculations based on initial state cold nuclear matter effects result in the opposite order, suggesting the presence of other contributions to nuclear modifications, in particular at lower $p_T$.
Differential cross section of $\pi^0$ in p+p collisions at $\sqrt{s}$ = 200 GeV
Invariant yield of $\pi^0$ from (a) p+Al, (b) p+Au, (c) d+Au, and (d) $^{3}$HeAu in different centrality selections at $\sqrt{s}$ = 200 GeV
Nuclear modification factors from inelastic (a) p+Al, (b) p+Au, (c) d+Au, and (d) $^{3}$HeAu collisions at $\sqrt{s}$ = 200 GeV. The right boxes are the $N_{coll}$ uncertainties from the Glauber model, while the left box represents the overall normalization uncertainty from p+p collisions
We present direct photon-hadron correlations in 200 GeV/A Au+Au, d+Au, and p+p collisions, for direct photon pT from 5–12 GeV/c, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in d+Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au+Au compared to p+p and d+Au. As the momentum fraction decreases, the yield of hadrons in Au+Au increases to an excess over the yield in p+p collisions. The excess is at large angles and at low hadron pT and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.
Per-trigger yield of hadrons associated to direct photons in Au+Au collisions for direct photon $p_T$ 5-9 GeV/$c$, compared with p+p baseline, in various $\xi$ bins.
Per-trigger yield of hadrons associated to direct photons in d+Au collisions for direct photon $p_T$ 7-9 GeV/$c$, compared with p+p baseline, in various $\xi$ bins.
Integrated away-side $\gamma_{dir}$-h per-trigger yields of Au+Au, d+Au, and p+p, as a function of $\xi$.
The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $dN_{\rm ch}/d\eta$ reveals that the low-momentum ($>$1\,GeV/$c$) direct-photon yield $dN_{\gamma}^{\rm dir}/d\eta$ is a smooth function of $dN_{\rm ch}/d\eta$ and can be well described as proportional to $(dN_{\rm ch}/d\eta)^\alpha$ with $\alpha{\sim}$1.25. This new scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$\,GeV/$c$) but when results from different collision energies are compared, an additional $\sqrt{s_{_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
Direct photon spectra(Physical Review C87, 054907 (2013)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 200 GeV.
Direct photon spectra(Physics Letters B94, 106 (1980)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 62.4 GeV.
Direct photon spectra(Nucl. Part. Phys. 23, A1 (1997) and Sov. J. Nucl. Phys. 51, 836 (1990)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 63 GeV.
We have measured direct photons for $p_T<5~$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $e^{+}e^{-}$ contribution from quasi-real direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}e^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p$$+$$p$ fit is observed for $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ spectra are consistent with the Au$+$Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p$$+$$p$ baseline are 285$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$c$ for minimum bias and 0\%--40\% most central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $dN_{\rm ch}/d\eta$ observed in Au$+$Au at the same collision energy.
Direct photon fraction measured with the virtual photon method for different systems in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu collisions.
The direct photon spectra for Minimum Bias and 0-40% centrality in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu collisions.
We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions after subtracting an underlying event using a model that includes higher-order azimuthal anisotropy $v_2$, $v_3$, and $v_4$. After subtraction, the away-side ($\Delta\phi\sim\pi)$ of the highest transverse-momentum trigger ($p_T>4$ GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$, the away-side shape and yield are modified. These observations are consistent with the scenario of radiative-jet energy loss. For the lowest-$p_T$ trigger correlations, an away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_n$. The angular correlations show an asymmetry when selecting the sign of the trigger-particle azimuthal angle with respect to the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.
Higher-order flow harmonics for charged hadrons at midrapidity in Au$+$Au collisions at $\sqrt{s_{NN}}$ and their systematics: $v_2$, $v_3$, $v_4$, and $v_4\{\Psi_2\}$. The source of systematic uncertainties are difference among RXN event-planes, matching cut width for CNT hadron tracks, and difference between $v_n$ measured with RXN and BBC event planes .
Per-trigger yields $Y(\Delta\phi)$ of dihadrons pairs measured in Au$+$Au collisions at$\sqrt{s_{NN}}$ after subtracting the underlying event model with several $p_T$ selections and centralities. Systematic uncertainties are due to track matching and the $v_n$ and due to ZYAM.
Per-trigger yields $Y(\Delta\phi)$ of dihadron pairs measured in Au$+$Au collisions after subtracting the underlying event-model with several $p_T$ selections of the trigger and associated particles ($p_T^{t,a}$ and several centralities. Systematic uncertainties are due to track matching and the $v_n$ and due to ZYAM.
We present measurements of two-particle angular correlations between high-transverse-momentum ($2<p_T<11$ GeV/$c$) $\pi^0$ observed at midrapidity ($|\eta|<0.35$) and particles produced either at forward ($3.1<\eta<3.9$) or backward ($-3.7<\eta<-3.1$) rapidity in $d$$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The azimuthal angle correlations for particle pairs with this large rapidity gap in the Au-going direction exhibit a ridge-like structure that persists up to $p_T{\approx}6$ GeV/$c$ and which strongly depends on collision centrality, which is a similar characteristic to the hydrodynamical particle flow in A+A collisions. The ridge-like structure is absent in the $d$-going direction as well as in $p$$+$$p$ collisions, in the transverse-momentum range studied. The results indicate that the ridge-like structure is shifted in the Au-going direction toward more central collisions, similar to the charged-particle pseudorapidity distributions.
Fourier fit coefficients for CNT-MPCS (Au-going) correlations, as a function of collision system and $\pi^0$ $p_T$: (a) the negative of the dipole coefficient, $-c_1$; (b) the quadrupole coefficient $c_2$; (c) the ratio ${-c_2}/{c_1}$.
Fourier fit coefficients for CNT-MPCS (Au-going) correlations, as a function of collision system and $\pi^0$ $p_T$: Fractional systematic uncertainty on the quadrupole coefficient $c_2$ for $d$+Au.
Fourier fit coefficients for CNT-MPCS (Au-going) correlations, as a function of collision system and $\pi^0$ $p_T$: Fractional systematic uncertainty on the quadrupole coefficient $c_2$ for $p$+$p$.
We report a measurement of $e^+e^-$ pairs from semileptonic heavy-flavor decays in $p$+$p$ collisions at $\sqrt{s_{NN}}=200$~GeV. The $e^+e^-$ pair yield from $b\bar{b}$ and $c\bar{c}$ is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and $p_T$. We used three different event generators, {\sc pythia}, {\sc mc@nlo}, and {\sc powheg}, to simulate the $e^+e^-$ spectra from $c\bar{c}$ and $b\bar{b}$ production. The data can be well described by all three generators within the detector acceptance. However, when using the generators to extrapolate to $4\pi$, significant differences are observed for the total cross section. These difference are less pronounced for $b\bar{b}$ than for $c\bar{c}$. The same model dependence was observed in already published $d$+$A$ data. The $p$+$p$ data are also directly compared with $d$+$A$ data in mass and $p_T$, and within the statistical accuracy no nuclear modification is seen.
Step by step extrapolation from the number of $e^+e^-$ pairs for $m_{e^+e^-}$ > 1.16 GeV/$c^2$ from $c\bar{c}$ in the PHENIX acceptance to the number of $c\bar{c}$ pairs in 4$\pi$ for PYTHIA, MC@NLO, and POWHEG. Numbers are in units of pairs per event using the $c\bar{c}$ cross sections determined in this paper.
Step by step extrapolation from the number of $e^+e^-$ pairs for $m_{e^+e^-}$ > 1.16 GeV/$c^2$ from $b\bar{b}$ in the PHENIX acceptance to the number of $b\bar{b}$ pairs in 4$\pi$ for PYTHIA, MC@NLO, and POWHEG. Numbers are in units of pairs per event using the $b\bar{b}$ cross sections determined in this paper.
Summary of $c\bar{c}$ and $b\bar{b}$ cross sections measured in $p$+$p$ collisions using three different generators, PYTHIA, MC@NLO, and POWHEG.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured 2nd and 3rd order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of $0.4<p_{T}<4.0$ GeV/$c$. At low $p_T$ the second-order coefficients, $v_2$, are similar to the ones observed in hadrons. Third order coefficients, $v_3$, are nonzero and almost independent of centrality. These new results on $v_2$ and $v_3$, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.
Direct photon $v_2$ and $v_3$ at midrapidity ($|\eta|$ < 0.35), for different centralities, measured with the conversion method. The event plane was determined with the reaction plane detector (1 < $|\eta|$ < 2.8).
Direct photon $v_2$ and $v_3$ at midrapidity ($|\eta|$ < 0.35), for different centralities, measured with the calorimeter method. The event plane was determined with the reaction plane detector (1 < $|\eta|$ < 2.8).
Measurements of midrapidity charged particle multiplicity distributions, $dN_{\rm ch}/d\eta$, and midrapidity transverse-energy distributions, $dE_T/d\eta$, are presented for a variety of collision systems and energies. Included are distributions for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ and 62.4 GeV, Cu$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, U$+$U collisions at $\sqrt{s_{_{NN}}}=193$ GeV, $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, $N_{\rm part}$, and the number of constituent quark participants, $N_{q{\rm p}}$. For all $A$$+$$A$ collisions down to $\sqrt{s_{_{NN}}}=7.7$ GeV, it is observed that the midrapidity data are better described by scaling with $N_{q{\rm p}}$ than scaling with $N_{\rm part}$. Also presented are estimates of the Bjorken energy density, $\varepsilon_{\rm BJ}$, and the ratio of $dE_T/d\eta$ to $dN_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Multiplicity in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 130 GeV
Measurements of the fractional momentum loss ($S_{\rm loss}\equiv{\delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using $\pi^0$ in Au$+$Au and Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{\rm loss}$ as a function of a number of variables: the number of participants, $N_{\rm part}$, the number of quark participants, $N_{\rm qp}$, the charged-particle density, $dN_{\rm ch}/d\eta$, and the Bjorken energy density times the equilibration time, $\varepsilon_{\rm Bj}\tau_{0}$. We find that the $p_T$ where $S_{\rm loss}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{\rm loss}$ tends to follow a power-law function with all four scaling variables. The data at $\sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of $S_{\rm loss}$ with $dN_{\rm ch}/d\eta$ and $\varepsilon_{\rm Bj}\tau_{0}$, lending insight on the physics of parton energy loss.
Global variables for Au+Au collisions at RHIC from PHENIX.
Global variables for Au+Au collisions at RHIC from PHENIX.
Global variables for Cu+Cu collisions at RHIC from PHENIX.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured $\phi$ meson production and its nuclear modification in asymmetric Cu$+$Au heavy-ion collisions at $\sqrt{s_{NN}}=200$ GeV at both forward Cu-going direction ($1.2<y<2.2$) and backward Au-going direction ($-2.2<y<-1.2$), rapidities. The measurements are performed via the dimuon decay channel and reported as a function of the number of participating nucleons, rapidity, and transverse momentum. In the most central events, 0\%--20\% centrality, the $\phi$ meson yield integrated over $1<p_T<5$ GeV/$c$ prefers a smaller value, which means a larger nuclear modification, in the Cu-going direction compared to the Au-going direction. Additionally, the nuclear-modification factor in Cu$+$Au collisions averaged over all centrality is measured to be similar to the previous PHENIX result in $d$$+$Au collisions for these rapidities.
Invariant yield as a function of the number of participating nucleons for 1.2 < $|y|$ < 2.2 and 1 < $p_T$ < 5 GeV/$c$. Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
Invariant yield as a function of transverse momentum for 1.2 < $|y|$ < 2.2 and 0%–93% centrality. Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
Invariant yield as a function of rapidity for 1 < $p_T$ < 5 GeV/$c$ and 0%–93% centrality. Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
We present the first measurement of elliptic ($v_2$) and triangular ($v_3$) flow in high-multiplicity $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in $^{3}$He$+$Au and in $p$$+$$p$ collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the $^{3}$He$+$Au system. The collective behavior is quantified in terms of elliptic $v_2$ and triangular $v_3$ anisotropy coefficients measured with respect to their corresponding event planes. The $v_2$ values are comparable to those previously measured in $d$$+$Au collisions at the same nucleon-nucleon center-of-mass energy. Comparison with various theoretical predictions are made, including to models where the hot spots created by the impact of the three $^{3}$He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.
Results for $v_2$ and $v_3$ as a function of $p_T$ for inclusive charged hadrons at midrapidity in 0-5% central $^3$He+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
The PHENIX experiment has measured $\phi$ meson production in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV using the dimuon and dielectron decay channels. The $\phi$ meson is measured in the forward (backward) $d$-going (Au-going) direction, $1.2<y<2.2$ ($-2.2<y<-1.2$) in the transverse-momentum ($p_T$) range from 1--7 GeV/$c$, and at midrapidity $|y|<0.35$ in the $p_T$ range below 7 GeV/$c$. The $\phi$ meson invariant yields and nuclear-modification factors as a function of $p_T$, rapidity, and centrality are reported. An enhancement of $\phi$ meson production is observed in the Au-going direction, while suppression is seen in the $d$-going direction, and no modification is observed at midrapidity relative to the yield in $p$$+$$p$ collisions scaled by the number of binary collisions. Similar behavior was previously observed for inclusive charged hadrons and open heavy flavor indicating similar cold-nuclear-matter effects.
Invariant yields of $\phi$ meson production as a function of $p_T$ at different $d$+Au centrality classes. Type B represents uncertainties that are correlated from point to point.
Invariant yields of $\phi$ meson production as a function of $p_T$ at different $d$+Au centrality classes. Type B represents uncertainties that are correlated from point to point.
Invariant yields of $\phi$ meson production as a function of $p_T$ at different $d$+Au centrality classes. Type B represents uncertainties that are correlated from point to point.
We report the measurement of cumulants ($C_n, n=1\ldots4$) of the net-charge distributions measured within pseudorapidity ($|\eta|<0.35$) in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=7.7-200$ GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g. $C_1/C_2$, $C_3/C_1$) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. The measured values of $C_1/C_2 = \mu/\sigma^2$ and $C_3/C_1 = S\sigma^3/\mu$ can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
We present a systematic study of charged pion and kaon interferometry in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. The kaon mean source radii are found to be larger than pion radii in the outward and longitudinal directions for the same transverse mass; this difference increases for more central collisions. The azimuthal-angle dependence of the radii was measured with respect to the second-order event plane and similar oscillations of the source radii were found for pions and kaons. Hydrodynamic models qualitatively describe the similar oscillations of the mean source radii for pions and kaons, but they do not fully describe the transverse-mass dependence of the oscillations.
HBT parameters of positive pion pairs, shown as value $\pm$ statistical uncertainty [absolute value] $\pm$ systematic uncertainty [%] for the centrality bins shown in Fig. 3.
HBT parameters of negative pion pairs, shown as value $\pm$ statistical uncertainty [absolute value] $\pm$ systematic uncertainty [%] for the centrality bins shown in Fig. 3.
HBT parameters of charge-combined kaon pairs, shown as value $\pm$ statistical uncertainty [absolute value] $\pm$ systematic uncertainty [%] for the centrality bins shown in Fig. 3.
New PHENIX measurements of the anisotropic flow coefficients $v_2\{\Psi_2\}$, $v_3\{\Psi_3\}$, $v_4\{\Psi_4\}$ and $v_4\{\Psi_2\}$ for identified particles ($\pi^{\pm}$, $K^{\pm}$, and $p+\bar{p}$) obtained relative to the event planes $\Psi_n$ in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV are presented as functions of collision centrality and particle transverse momenta $p_T$. The $v_n$ coefficients show characteristic patterns consistent with hydrodynamical expansion of the matter produced in the collisions. For each harmonic $n$, a modified valence quark number $n_q$ scaling plotting $v_n/(n_q)^{n/2}$ versus ${\rm KE}_T/n_q$ is observed to yield a single curve for all the measured particle species for a broad range of transverse kinetic energies ${\rm KE}_T$. A simultaneous blast wave model fit to the observed particle spectra and $v_n(p_T)$ coefficients identifies spatial eccentricities $s_n$ at freeze-out, which are much smaller than the initial-state geometric values.
Azimuthal anisotropy $v_n$ via the event-plane method for charge-combined $\pi^{\pm}$ in 0%–50% central Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV.
Azimuthal anisotropy $v_2$ and $v_3$ via the two-particle correlation method for charge-combined $\pi^{\pm}$ in 0%–50% central Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV.
Azimuthal anisotropy $v_4$ via the two-particle correlation method for charge-combined $\pi^{\pm}$ in 0%–50% central Au+Au collisions at $\sqrt{s_{NN}} =$ 200 GeV.
We have studied the dependence of azimuthal anisotropy $v_2$ for inclusive and identified charged hadrons in Au$+$Au and Cu$+$Cu collisions on collision energy, species, and centrality. The values of $v_2$ as a function of transverse momentum $p_T$ and centrality in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV and 62.4 GeV are the same within uncertainties. However, in Cu$+$Cu collisions we observe a decrease in $v_2$ values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both Au$+$Au and Cu$+$Cu collisions we find that $v_2$ depends both on eccentricity and the number of participants, $N_{\rm part}$. We observe that $v_2$ divided by eccentricity ($\varepsilon$) monotonically increases with $N_{\rm part}$ and scales as ${N_{\rm part}^{1/3}}$. The Cu$+$Cu data at 62.4 GeV falls below the other scaled $v_{2}$ data. For identified hadrons, $v_2$ divided by the number of constituent quarks $n_q$ is independent of hadron species as a function of transverse kinetic energy $KE_T=m_T-m$ between $0.1<KE_T/n_q<1$ GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the Cu$+$Cu data at 62.4 GeV, of $v_2/(n_q\cdot\varepsilon\cdot N^{1/3}_{\rm part})$ vs $KE_T/n_q$ for all measured particles.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
We present the midrapidity charged pion invariant cross sections and the ratio of $\pi^-$-to-$\pi^+$ production ($5<p_T<13$ GeV/$c$), together with the double-helicity asymmetries ($5<p_T<12$ GeV/$c$) in polarized $p$$+$$p$ collisions at $\sqrt{s} = 200$ GeV. The cross section measurements are consistent with perturbative calculations in quantum chromodynamics within large uncertainties in the calculation due to the choice of factorization, renormalization, and fragmentation scales. However, the theoretical calculation of the ratio of $\pi^-$-to-$\pi^+$ production when considering these scale uncertainties overestimates the measured value, suggesting further investigation of the uncertainties on the charge-separated pion fragmentation functions is needed. Due to cancellations of uncertainties in the charge ratio, direct inclusion of these ratio data in future parameterizations should improve constraints on the flavor dependence of quark fragmentation functions to pions. By measuring charge-separated pion asymmetries, one can gain sensitivity to the sign of $\Delta G$ through the opposite sign of the up and down quark helicity distributions in conjunction with preferential fragmentation of positive pions from up quarks and negative pions from down quarks. The double-helicity asymmetries presented are sensitive to the gluon helicity distribution over an $x$ range of $\sim$0.03--0.16.
Invariant cross section for $\pi^+$ and $\pi^-$ hadrons, as well as the statistical and systematic uncertainties. In addition, there is an absolute scale uncertainty of 9.6$\%$.
Double-helicity asymmetries and statistical uncertainties for $\pi^+$ and $\pi^-$ hadrons. The primary systematic uncertainties, which are fully correlated between points, are $1.4\times10^{-3}$ from relative luminosity and a $^{+7.0\%}_{-7.7\%}$ scaling uncertainty from beam polarization.
Ratio of charged pion cross section, as shown in Fig.6.
The standard model (SM) of particle physics is spectacularly successful, yet the measured value of the muon anomalous magnetic moment $(g-2)_\mu$ deviates from SM calculations by 3.6$\sigma$. Several theoretical models attribute this to the existence of a "dark photon," an additional U(1) gauge boson, which is weakly coupled to ordinary photons. The PHENIX experiment at the Relativistic Heavy Ion Collider has searched for a dark photon, $U$, in $\pi^0,\eta \rightarrow \gamma e^+e^-$ decays and obtained upper limits of $\mathcal{O}(2\times10^{-6})$ on $U$-$\gamma$ mixing at 90% CL for the mass range $30<m_U<90$ MeV/$c^2$. Combined with other experimental limits, the remaining region in the $U$-$\gamma$ mixing parameter space that can explain the $(g-2)_\mu$ deviation from its SM value is nearly completely excluded at the 90% confidence level, with only a small region of $29<m_U<32$ MeV/$c^2$ remaining.
The experimental sensitivity and observed limit on the number of dark photon candidates as a function of the assumed dark photon mass.
The experimental sensitivity and observed limit on the number of dark photon candidates as a function of the assumed dark photon mass.
The experimental sensitivity and observed limit on the number of dark photon candidates as a function of the assumed dark photon mass.
We present a measurement of the cross section and transverse single-spin asymmetry ($A_N$) for $\eta$ mesons at large pseudorapidity from $\sqrt{s}=200$~GeV $p^{\uparrow}+p$ collisions. The measured cross section for $0.5<p_T<5.0$~GeV/$c$ and $3.0<|\eta|<3.8$ is well described by a next-to-leading-order perturbative-quantum-chromodynamics calculation. The asymmetries $A_N$ have been measured as a function of Feynman-$x$ ($x_F$) from $0.2<|x_{F}|<0.7$, as well as transverse momentum ($p_T$) from $1.0<p_T<4.5$~GeV/$c$. The asymmetry averaged over positive $x_F$ is $\langle{A_{N}}\rangle=0.061{\pm}0.014$. The results are consistent with prior transverse single-spin measurements of forward $\eta$ and $\pi^{0}$ mesons at various energies in overlapping $x_F$ ranges. Comparison of different particle species can help to determine the origin of the large observed asymmetries in $p^{\uparrow}+p$ collisions.
The measured ETA meson cross section, E*D3(SIG)/DP**3, versus PT at forward rapidity. The statistical and systematic uncertainties are type-A and type-B uncertainties respectively.
ASYM(PEAK) and ASYM(BG) for ETA mesons measured as a function of XF in the range 0.3 < ABS(XF) < 0.7 from the 4X4B triggered dataset. The values represented are the weighted mean of the South and North MPC (Muon Piston Calorimeter). The uncertainties listed are statistical only.
ASYM for ETA mesons measured as a function of XF in the range 0.2 < ABS(XF) < 0.7. Uncertainties listed are those due to the statistics, the XF uncorrelated uncertainties due to extracting the yields, and the correlated relative luminosity uncertainty.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured low mass vector meson, $\omega$, $\rho$, and $\phi$, production through the dimuon decay channel at forward rapidity ($1.2<|y|<2.2$) in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV. The differential cross sections for these mesons are measured as a function of both $p_T$ and rapidity. We also report the integrated differential cross sections over $1<p_T<7$ GeV/$c$ and $1.2<|y|<2.2$: $d\sigma/dy(\omega+\rho\rightarrow\mu\mu) = 80 \pm 6 \mbox{(stat)} \pm 12 \mbox{(syst)}$ nb and $d\sigma/dy(\phi\rightarrow\mu\mu) = 27 \pm 3 \mbox{(stat)} \pm 4 \mbox{(syst)}$ nb. These results are compared with midrapidity measurements and calculations.
Differential cross sections of (OMEGA + RHO) and PHI as functions of PT. The statistical uncertainty includes the type-A systematic uncertainty. The systematic uncertainty is the type-B systematic uncertainty.
Differential cross sections of (OMEGA + RHO) and PHI as functions of rapidity. The statistical uncertainty includes the type-A systematic uncertainty. The systematic uncertainty is the type-B systematic uncertainty.
N(PHI) / ( N(OMEGA) + N(RHO) ) as a function of PT. The statistical uncertainty includes the type-A systematic uncertainty. The systematic uncertainty is the type-B systematic uncertainty.
We report a measurement of $e^+e^-$ pairs from semileptonic heavy-flavor decays in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Exploring the mass and transverse-momentum dependence of the yield, the bottom decay contribution can be isolated from charm, and quantified by comparison to {\sc pythia} and {\sc mc@nlo} simulations. The resulting $b\bar{b}$-production cross section is $\sigma^{d{\rm Au}}_{b\bar{b}}=1.37{\pm}0.28({\rm stat}){\pm}0.46({\rm syst})$~mb, which is equivalent to a nucleon-nucleon cross section of $\sigma^{NN}_{bb}=3.4\pm0.8({\rm stat}){\pm}1.1({\rm syst})\ \mu$b.
$b\bar{b}$ cross section
The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV down to $p_T=0.4$ GeV/$c$. Photons are detected via photon conversions to $e^+e^-$ pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low $p_T$. We find an excess of direct photons above the $N_{\rm coll}$-scaled yield measured in $p$$+$$p$ collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240 MeV/$c$ in the $p_T$ range from 0.6--2.0 GeV/$c$. While the shape of the $p_T$ distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with $N_{\rm part}^\alpha$, where $\alpha=1.48{\pm}0.08({\rm stat}){\pm}0.04({\rm syst})$.
Ratio $R_{\gamma}$ as function of photon $p_T$ from the 2007 and 2010 data sets in minimum-bias Au+Au collisions, and the $R_{\gamma}$ in the combined 2007+2010 measurement.
Ratio $R_{\gamma}$ as function of photon $p_T$ for the combined 2007 and 2010 data sets in different centrality bins.
Direct photon $p_T$ spectra in different centrality bins.
We present azimuthal angular correlations between charged hadrons and energy deposited in calorimeter towers in central $d$$+$Au and minimum bias $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The charged hadron is measured at midrapidity $|\eta|<0.35$, and the energy is measured at large rapidity ($-3.7<\eta<-3.1$, Au-going direction). An enhanced near-side angular correlation across $|\Delta\eta| >$ 2.75 is observed in $d$$+$Au collisions. Using the event plane method applied to the Au-going energy distribution, we extract the anisotropy strength $v_2$ for inclusive charged hadrons at midrapidity up to $p_T=4.5$ GeV/$c$. We also present the measurement of $v_2$ for identified $\pi^{\pm}$ and (anti)protons in central $d$$+$Au collisions, and observe a mass-ordering pattern similar to that seen in heavy ion collisions. These results are compared with viscous hydrodynamic calculations and measurements from $p$$+$Pb at $\sqrt{s_{_{NN}}}=5.02$ TeV. The magnitude of the mass-ordering in $d$$+$Au is found to be smaller than that in $p$$+$Pb collisions, which may indicate smaller radial flow in lower energy $d$$+$Au collisions.
$c_2$ ($p_T$) for track lower-tower pairs from 0-5% $d$+Au collisions and $c_2$ ($p_T$) for pairs in minimum bias $p$+$p$ collisions times the dilution factor.
Measured $v_2$ for midrapidity charged tracks in 0-5% central $d$+Au at $\sqrt{s_{NN}}$ = 200 GeV using the event plane method.
Measured $v_2$ ($p_T$) for identified pions, each charged combined, 0-5% central $d$+Au collisions at RHIC.
Measurements of bottomonium production in heavy ion and $p$$+$$p$ collisions at the Relativistic Heavy Ion Collider (RHIC) are presented. The inclusive yield of the three $\Upsilon$ states, $\Upsilon(1S+2S+3S)$, was measured in the PHENIX experiment via electron-positron decay pairs at midrapidity for Au$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $\Upsilon(1S+2S+3S)\rightarrow e^+e^-$ differential cross section at midrapidity was found to be $B_{\rm ee} d\sigma/dy =$ 108 $\pm$ 38 (stat) $\pm$ 15(syst) $\pm$ 11 (luminosity) pb in $p$$+$$p$ collisions. The nuclear modification factor in the 30\% most central Au$+$Au collisions indicates a suppression of the total $\Upsilon$ state yield relative to the extrapolation from $p$$+$$p$ collision data. The suppression is consistent with measurements made by STAR at RHIC and at higher energies by the CMS experiment at the Large Hadron Collider.
Summary of the measured $\Upsilon$ invariant multiplicities, $BdN/dy$, for one $p + p$ three Au + Au data sets.
Summary of the measured $\Upsilon$ nuclear modification factors, $R_{AA}$, for Au + Au data sets.
Summary of the measured $\Upsilon$ nuclear modification factors, $R_{AA}$, for Au + Au data sets.
Results are presented from data recorded in 2009 by the PHENIX experiment at the Relativistic Heavy Ion Collider for the double-longitudinal spin asymmetry, $A_{LL}$, for $\pi^0$ and $\eta$ production in $\sqrt{s} = 200$ GeV polarized $p$$+$$p$ collisions. Comparison of the $\pi^0$ results with different theory expectations based on fits of other published data showed a preference for small positive values of gluon polarization, $\Delta G$, in the proton in the probed Bjorken $x$ range. The effect of adding the new 2009 \pz data to a recent global analysis of polarized scattering data is also shown, resulting in a best fit value $\Delta G^{[0.05,0.2]}_{\mbox{DSSV}} = 0.06^{+0.11}_{-0.15}$ in the range $0.05<x<0.2$, with the uncertainty at $\Delta \chi^2 = 9$ when considering only statistical experimental uncertainties. Shifting the PHENIX data points by their systematic uncertainty leads to a variation of the best-fit value of $\Delta G^{[0.05,0.2]}_{\mbox{DSSV}}$ between $0.02$ and $0.12$, demonstrating the need for full treatment of the experimental systematic uncertainties in future global analyses.
PI0 ASYM(LL) measurements from 2005.
PI0 ASYM(LL) measurements from 2006.
PI0 ASYM(LL) measurements from 2009.
Charged-pion-interferometry measurements were made with respect to the 2$^{\rm nd}$- and 3$^{\rm rd}$-order event plane for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. A strong azimuthal-angle dependence of the extracted Gaussian-source radii was observed with respect to both the 2$^{\rm nd}$- and 3$^{\rm rd}$-order event planes. The results for the 2$^{\rm nd}$-order dependence indicate that the initial eccentricity is reduced during the medium evolution, but not reversed in the final state, which is consistent with previous results. In contrast, the results for the 3$^{\rm rd}$-order dependence indicate that the initial triangular shape is significantly reduced and potentially reversed by the end of the medium evolution, and that the 3$^{\rm rd}$-order oscillations are largely dominated by the dynamical effects from triangular flow.
The azimuthal dependence of $R^2_s$, $R^2_o$, $R^2_l$, and $R^2_{os}$ for charged pions in 0.2 < $k_T$ < 2.0 GeV/$c$ with respect to second-(a)-(d) and third-order (e)-(h) event plane in Au + Au collisions at $\sqrt{S_{NN}}$ 200 GeV.
The azimuthal dependence of $R^2_s$, $R^2_o$, $R^2_l$, and $R^2_{os}$ for charged pions in 0.2 < $k_T$ < 2.0 GeV/$c$ with respect to second-(a)-(d) and third-order (e)-(h) event plane in Au + Au collisions at $\sqrt{S_{NN}}$ 200 GeV.
The solid points are the oscillation amplitudes relative to the average of HBT radii for four different combinations (a) $2R^{2}_{s,n}/R^{2}_{s,0}$, (b) $2R^{2}_{os,n}/R^{2}_{s,0}$, (c) $2R^{2}_{o,n}/R^{2}_{o,0}$, and (d) $2R^{2}_{o,n}/R^{2}_{s,0}$ as a function of initial spatial anisotropy ($\varepsilon_{n}$), which are calculated using the Glauber model.
Measurements of the midrapidity transverse energy distribution, $d\Et/d\eta$, are presented for $p$$+$$p$, $d$$+$Au, and Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV and additionally for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=62.4$ and 130 GeV. The $d\Et/d\eta$ distributions are first compared with the number of nucleon participants $N_{\rm part}$, number of binary collisions $N_{\rm coll}$, and number of constituent-quark participants $N_{qp}$ calculated from a Glauber model based on the nuclear geometry. For Au$+$Au, $\mean{d\Et/d\eta}/N_{\rm part}$ increases with $N_{\rm part}$, while $\mean{d\Et/d\eta}/N_{qp}$ is approximately constant for all three energies. This indicates that the two component ansatz, $dE_{T}/d\eta \propto (1-x) N_{\rm part}/2 + x N_{\rm coll}$, which has been used to represent $E_T$ distributions, is simply a proxy for $N_{qp}$, and that the $N_{\rm coll}$ term does not represent a hard-scattering component in $E_T$ distributions. The $dE_{T}/d\eta$ distributions of Au$+$Au and $d$$+$Au are then calculated from the measured $p$$+$$p$ $E_T$ distribution using two models that both reproduce the Au$+$Au data. However, while the number-of-constituent-quark-participant model agrees well with the $d$$+$Au data, the additive-quark model does not.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Measurements of transverse-single-spin asymmetries ($A_{N}$) in $p$$+$$p$ collisions at $\sqrt{s}=$62.4 and 200 GeV with the PHENIX detector at RHIC are presented. At midrapidity, $A_{N}$ is measured for neutral pion and eta mesons reconstructed from diphoton decay, and at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of $A_{N}$ at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in $\sqrt{s}$ and $p_T$, and it is found that the asymmetries depend only weakly on $\sqrt{s}$. The origin of the forward $A_{N}$ is presently not understood quantitatively. The extended reach to higher $p_T$ probes the transition between transverse momentum dependent effects at low $p_T$ and multi-parton dynamics at high $p_T$.
Neutral pion $A_N$ at $\sqrt{s} = 62.4$ GeV as a function of $x_F$ in pseudorapidity $3.1 < |\eta| < 3.5$, with statistical and systematic uncertainties.
Neutral pion $A_N$ at $\sqrt{s} = 62.4$ GeV as a function of $x_F$ in pseudorapidity $3.5 < |\eta| < 3.8$, with statistical and systematic uncertainties.
Neutral pion $A_N$ at $\sqrt{s}$ = 62.4 GeV as function of transverse momentum $p_T$.
We report $e^\pm-\mu^\mp$ pair yield from charm decay measured between midrapidity electrons ($|\eta|<0.35$ and $p_T>0.5$ GeV/$c$) and forward rapidity muons ($1.4<\eta<2.1$ and $p_T>1.0$ GeV/$c$) as a function of $\Delta\phi$ in both $p$$+$$p$ and in $d$+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Comparing the $p$$+$$p$ results with several different models, we find the results are consistent with a total charm cross section $\sigma_{c\bar{c}} =$ 538 $\pm$ 46 (stat) $\pm$ 197 (data syst) $\pm$ 174 (model syst) $\mu$b. These generators also indicate that the back-to-back peak at $\Delta\phi = \pi$ is dominantly from the leading order contributions (gluon fusion), while higher order processes (flavor excitation and gluon splitting) contribute to the yield at all $\Delta\phi$. We observe a suppression in the pair yield per collision in $d$+Au. We find the pair yield suppression factor for $2.7<\Delta\phi<3.2$ rad is $J_{dA}$ = 0.433 $\pm$ 0.087 (stat) $\pm$ 0.135 (syst), indicating cold nuclear matter modification of $c\bar{c}$ pairs.
The fully-corrected like-sign-subtracted heavy flavor $e$-$\mu$ pair yield in $p$+$p$.
The fully corrected like-sign-subtracted heavy flavor $e$-$\mu$ pair yield in $d$+Au.
$J_{dA}$ plotted as a function of $\Delta\phi$.
The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}$=200 GeV through the measurement of electrons at midrapidity that originate from semileptonic decays of charm and bottom hadrons. In peripheral Cu$+$Cu collisions an enhanced production of electrons is observed relative to $p$$+$$p$ collisions scaled by the number of binary collisions. In the transverse momentum range from 1 to 5 GeV/$c$ the nuclear modification factor is $R_{AA}$$\sim$1.4. As the system size increases to more central Cu$+$Cu collisions, the enhancement gradually disappears and turns into a suppression. For $p_T>3$ GeV/$c$, the suppression reaches $R_{AA}$$\sim$0.8 in the most central collisions. The $p_T$ and centrality dependence of $R_{AA}$ in Cu$+$Cu collisions agree quantitatively with $R_{AA}$ in $d+$Au and Au$+$Au collisions, if compared at similar number of participating nucleons $\langle N_{\rm part} \rangle$.
The $p_T$ spectra of electrons from the decays of open heavy flavor hadrons produced in Cu+Cu collisions, separated by centrality.
The $p_T$ spectra of electrons from the decays of open heavy flavor hadrons produced in Cu+Cu collisions, separated by centrality.
The $p_T$ spectra of electrons from the decays of open heavy flavor hadrons produced in Cu+Cu collisions, separated by centrality.
High-energy proton- and deuteron-nucleus collisions provide an excellent tool for studying a wide array of physics effects, including modifications of parton distribution functions in nuclei, gluon saturation, and color neutralization and hadronization in a nuclear environment, among others. All of these effects are expected to have a significant dependence on the size of the nuclear target and the impact parameter of the collision, also known as the collision centrality. In this article, we detail a method for determining centrality classes in p(d)+A collisions via cuts on the multiplicity at backward rapidity (i.e., the nucleus-going direction) and for determining systematic uncertainties in this procedure. For d+Au collisions at sqrt(s_NN) = 200 GeV we find that the connection to geometry is confirmed by measuring the fraction of events in which a neutron from the deuteron does not interact with the nucleus. As an application, we consider the nuclear modification factors R_{p(d)+A}, for which there is a potential bias in the measured centrality dependent yields due to auto-correlations between the process of interest and the backward rapidity multiplicity. We determine the bias correction factor within this framework. This method is further tested using the HIJING Monte Carlo generator. We find that for d+Au collisions at sqrt(s_NN)=200 GeV, these bias corrections are small and vary by less than 5% (10%) up to p_T = 10 (20) GeV. In contrast, for p+Pb collisions at sqrt(s_NN) = 5.02 TeV we find these bias factors are an order of magnitude larger and strongly p_T dependent, likely due to the larger effect of multi-parton interactions.
Different physical quantities characterizing $d$+Au collisions, and the bias-factor corrections, for nine PHENIX centrality bins.
Different physical quantities characterizing $d$+Au collisions, and the bias-factor corrections, for nine PHENIX centrality bins.
Different physical quantities characterizing $d$+Au collisions, and the bias-factor corrections, for four PHENIX centrality bins.
The PHENIX experiment has measured open heavy-flavor production via semileptonic decay muons over the transverse momentum range 1 < pT < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in d+Au and p+p collisions at ?sNN = 200 GeV. In central d+Au collisions an enhancement (suppression) of heavy-flavor muon production is observed at backward (forward) rapidity relative to the yield in p+p collisions scaled by the number of binary collisions. Modification of the gluon density distribution in the Au nucleus contributes in terms of anti-shadowing enhancement and shadowing suppression; however, the enhancement seen at backward rapidity exceeds expectations from this effect alone. These results, implying an important role for additional cold nuclear matter effects, serves as a key baseline for heavy-quark measurements in A+A collisions and in constraining the magnitude of charmonia breakup effects at the Relativistic Heavy Ion Collider and the Large Hadron Collider.
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
Invariant yield of negatively charged heavy-flavor muons as a function of $p_T$ in $d$+Au collisions for different centralities at (a) backward rapidity (Au-going) and (b) forward rapidity (d-going).
The azimuthal anisotropy coefficients v_2 and v_4 of pi^0 and eta mesons are measured in Au+Au collisions at sqrt(s_NN)=200 GeV, as a function of transverse momentum p_T (1-14 GeV/c) and centrality. The extracted v_2 coefficients are found to be consistent between the two meson species over the measured p_T range. The ratio of v_4/v_2^2 for pi^0 mesons is found to be independent of p_T for 1-9 GeV/c, implying a lack of sensitivity of the ratio to the change of underlying physics with p_T. Furthermore, the ratio of v_4/v_2^2 is systematically larger in central collisions, which may reflect the combined effects of fluctuations in the initial collision geometry and finite viscosity in the evolving medium.
$\eta$ meson $v_2, 0-20\%$ centrality
$\eta$ meson $v_2, 20-60\%$ centrality
$\pi^0$ meson $v_4, 0-20\%$ centrality
We present results for three charmonia states (psi^prime, chi_c and J/psi) in d+Au collisions at |y|<0.35 and sqrt(s_NN)=200 GeV. We find that the modification of the psi^prime yield relative to that of the J/psi scales approximately with charged-particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large impact-parameter collisions we observe a similar suppression for the psi^prime and J/psi, while in small impact-parameter collisions the more weakly bound psi^prime is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger psi^prime suppression in central events is not explained by an increase of the nuclear absorption due to meson formation time effects.
The $\psi^{\prime}$, $\chi_c$, and $J/\psi$ $R_{dAu}$ for minimum bias centrality integrated $d$+Au collisions as a function of the quarkonia binding energy, where $J/\psi$ $R_{dAu}$ has been corrected for the effects of $\psi^{\prime}$ and $\chi_c$ feed-down.
The $\psi^{\prime}$ nuclear modification factor, $R_{dAu}$, as a function of $N_{coll}$. Also included is the previously measured $J/\psi$ $R_{dAu}$ as a function of $N_{coll}$.
The $\psi^{\prime}$ nuclear modification factor, $R_{dAu}$, as a function of proper time in nucleus. Also included is the previously measured $J/\psi$ $R_{dAu}$ as a function of $N_{coll}$.
The transverse momentum (p_T) spectra and ratios of identified charged hadrons (\pi^+/-, K^+/-, p, p^bar) produced in sqrt(s_NN)=200 GeV Au+Au and d+Au collisions are reported in five different centrality classes for each collision species. The measurements of pions and protons are reported up to p_T=6 GeV/c (5 GeV/c), and the measurements of kaons are reported up to p_T=4 GeV/c (3.5 GeV/c) in Au+Au (d+Au) collisions. In the intermediate p_T region, between 2--5 GeV/c, a significant enhancement of baryon to meson ratios compared to those measured in p+p collisions is observed. This enhancement is present in both Au+Au and d+Au collisions, and increases as the collisions become more central. We compare a class of peripheral Au+Au collisions with a class of central d+Au collisions which have a comparable number of participating nucleons and binary nucleon-nucleon collisions. The p_T dependent particle ratios for these classes display a remarkable similarity, which is then discussed.
kaon AuAu Invariant yields versus $p_T$
kaon dAu Invariant yields versus $p_T$
pion AuAu Invariant yields versus $p_T$
The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central $p$$+$Pb collisions at $\sqrt{s_{_{NN}}}$=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in $d$$+$Au collisions compared to those seen in $p$$+$Pb collisions at the LHC. The larger extracted $v_2$ values in $d$$+$Au collisions at RHIC are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from $p$$+$Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.
The second-order pair anisotropy, c2, of the central collision excess as a function of associated particle pT.
The third-order pair anisotropy, c3, of the central collision excess as a function of associated particle pT.
Charged hadron second-order anisotropy, v2, as a function of pT.
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The p_T of the photon is an excellent approximation to the initial p_T of the jet and the ratio z_T=p_T^h/p_T^\gamma is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I_ AA, the ratio of jet fragment yield in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z_T. The fragment yield at low z_T is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
Direct photon-hadron pair per-trigger yields vs Delta-phi (Au+Au and p+p)
Integrated per-trigger yields and I_AA vs xi
Integrated per-trigger yields and I_AA vs xi
The three Upsilon states, Upsilon(1S+2S+3S), are measured in d+Au and p+p collisions at sqrt(s_NN)=200 GeV and rapidities 1.2<|y|<2.2 by the PHENIX experiment at the Relativistic Heavy-Ion Collider. Cross sections for the inclusive Upsilon(1S+2S+3S) production are obtained. The inclusive yields per binary collision for d+Au collisions relative to those in p+p collisions (R_dAu) are found to be 0.62 +/- 0.26 (stat) +/- 0.13 (syst) in the gold-going direction and 0.91 +/- 0.33 (stat) +/- 0.16 (syst) in the deuteron-going direction. The measured results are compared to a nuclear-shadowing model, EPS09 [JHEP 04, 065 (2009)], combined with a final-state breakup cross section, sigma_br, and compared to lower energy p+A results. We also compare the results to the PHENIX J/psi results [Phys. Rev. Lett. 107, 142301 (2011)]. The rapidity dependence of the observed Upsilon suppression is consistent with lower energy p+A measurements.
$\Upsilon$ invariant yields and cross sections of $p$+$p$ and $d$+Au collisions.
The energy dependence of the single-transverse-spin asymmetry, A_N, and the cross section for neutron production at very forward angles were measured in the PHENIX experiment at RHIC for polarized p+p collisions at sqrt(s)=200 GeV. The neutrons were observed in forward detectors covering an angular range of up to 2.2 mrad. We report results for neutrons with momentum fraction of x_F=0.45 to 1.0. The energy dependence of the measured cross sections were consistent with x_F scaling, compared to measurements by an ISR experiment which measured neutron production in unpolarized p+p collisions at sqrt(s)=30.6--62.7 GeV. The cross sections for large x_F neutron production for p+p collisions, as well as those in e+p collisions measured at HERA, are described by a pion exchange mechanism. The observed forward neutron asymmetries were large, reaching A_N=-0.08+/-0.02 for x_F=0.8; the measured backward asymmetries, for negative x_F, were consistent with zero. The observed asymmetry for forward neutron production is discussed within the pion exchange framework, with interference between the spin-flip amplitude due to the pion exchange and nonflip amplitudes from all Reggeon exchanges. Within the pion exchange description, the measured neutron asymmetry is sensitive to the contribution of other Reggeon exchanges even for small amplitudes.
The cross section results for forward neutron production in $p$+$p$ collisions at $\sqrt{s}$ = 200 GeV are shown. Two different forms, exponential and Gaussian, were used for the $p_T$ distribution. The integrated $p_T$ region for each bin is 0 < $p_T$ < 0.11$x_F$ GeV/$c$.
The $x_F$ dependence of $A_N$ for neutron production in the ZDC trigger sample.
The $x_F$ dependence of $A_N$ for neutron production for the ZDC$\otimes$BBC trigger sample.
We report on the first measurement of double-spin asymmetry, A_LL, of electrons from the decays of hadrons containing heavy flavor in longitudinally polarized p+p collisions at sqrt(s)=200 GeV for p_T= 0.5 to 3.0 GeV/c. The asymmetry was measured at mid-rapidity (|eta|<0.35) with the PHENIX detector at the Relativistic Heavy Ion Collider. The measured asymmetries are consistent with zero within the statistical errors. We obtained a constraint for the polarized gluon distribution in the proton of |Delta g/g(log{_10}x= -1.6^+0.5_-0.4, {mu}=m_T^c)|^2 < 0.033 (1 sigma), based on a leading-order perturbative-quantum-chromodynamics model, using the measured asymmetry.
Invariant differential cross sections of electrons from heavy-flavor decays.
Double-spin asymmetry of the heavy flavor electron production.
The PHENIX experiment has measured the production of neutral pions in Au+Au collisions at sqrt(s_NN)=200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, p_T, to 20 GeV/c. The production ratio of eta/pi^0 is 0.46+/-0.01(stat)+/-0.05(syst), constant with p_T and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the p+p and d+Au values. The production of pi^0 is suppressed by a factor of 5, as in earlier findings. However, with the improved statistical precision a small but significant rise of the nuclear modification factor, R_AA, vs p_T, with a slope of 0.0106+/-^(0.0034)_(0.0029)[GeV/c]^-1, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing p_T. To study the path length dependence of suppression, the pi^0 yield was measured at different angles with respect to the event plane; a strong azimuthal dependence of the pi^0 R_AA is observed. The data are compared to theoretical models of parton energy loss as a function of the path length, L, in the medium. Models based on pQCD are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and AdS/CFT for the soft interactions is consistent with the data.
Invariant yields of neutral pions, all centralities
Invariant yields of neutral pions, all centralities
$\Eta/ \pi^0 ratios
The PHENIX experiment has measured electrons and positrons at midrapidity from the decays of hadrons containing charm and bottom quarks produced in d+Au and p+p collisions at sqrt(s_NN)=200 GeV at the Relativistic Heavy Ion Collider, in the transverse-momentum range 0.85 < pT < 8.5 GeV/c. In central d+Au collisions, the nuclear modification factor R_dA at 1.5 < pT < 5 GeV/c displays evidence of enhancement of these electrons, relative to those produced in p+p collisions, and shows that the mass-dependent Cronin enhancement observed at RHIC extends to the heavy-D-meson family. A comparison with the neutral-pion data suggests that the difference in cold-nuclear-matter effects on light- and heavy-flavor mesons could contribute to the observed differences between the pi0 and heavy-flavor-electron nuclear modification factor R_AA.
Heavy flavor electron yield, Run-8 $p$ + $p$, $d$+Au collisions. Electrons from heavy flavor decays, separated by centrality. The lines represent a fit to the previous $p+p$ result [23], scaled by $N_{coll}$. The inset shows the ratio of photonic background electrons determined by the converter and cocktail methods for Minimum Bias $d$+Au collisions, with error bars (boxes) that represent the statistical uncertainty on the converter data (systematic uncertainty on the photonic-electron cocktail).
Heavy flavor electron yield, $d$+Au $\implies$ CHARGED X. Electrons from heavy flavor decays, separated by centrality. The lines represent a fit to the previous $p+p$ result [23], scaled by $N_{coll}$. The inset shows the ratio of photonic background electrons determined by the converter and cocktail methods for Minimum Bias $d$+Au collisions, with error bars (boxes) that represent the statistical uncertainty on the converter data (systematic uncertainty on the photonic-electron cocktail).
Heavy flavor electron $R_{dA}$ 0-100% d+Au collisions. The nuclear modification factors $R_{dA}$ and $R_{AA}$ for minimum bias $d$+Au and Au+Au collisions, for the $\pi^{0}$ and $e^{\pm}_{HF}$. The two boxes on the right side of the plot represent the global uncertainties in the $d$+Au (left) and Au+Au (right) values of $N_{coll}$ . An additional common global scaling uncertainty of 9.7% on $R_{dA}$ and $R_{AA}$ from the $p+p$ reference data is omitted for clarity.
Direct photons have been measured in sqrt(s_NN)=200 GeV d+Au collisions at midrapidity. A wide p_T range is covered by measurements of nearly-real virtual photons (1<p_T<6 GeV/c) and real photons (5<p_T<16 GeV/c). The invariant yield of the direct photons in d+Au collisions over the scaled p+p cross section is consistent with unity. Theoretical calculations assuming standard cold nuclear matter effects describe the data well for the entire p_T range. This indicates that the large enhancement of direct photons observed in Au+Au collisions for 1.0<p_T<2.5 GeV/c is due to a source other than the initial-state nuclear effects.
Direct photon fraction. The direct-photon fractions from the virtual-photon analysis as a function of $p_{T}$ in $p+p$ (MB) [1] collisions. The values in the table are equal to this mean value. The statistical and systematic uncertainties are shown by the bars and bands, respectively. The curves show expectations from a NLO pQCD calculation [17, 18] with different cutoff mass scales: (solid) $\mu$ = 0.5 $p_{T}$ , (dash) $\mu$ = 1.0 $p_{T}$ , and (dash-dot) $\mu$ = 2.0 $p_{T}$.
Direct photon fraction. The direct-photon fractions from the virtual-photon analysis as a function of $p_{T}$ in $d$+Au (MB) [1] collisions. The values in the table are equal to this mean value. The statistical and systematic uncertainties are shown by the bars and bands, respectively. The curves show expectations from a NLO pQCD calculation [17, 18] with different cutoff mass scales: (solid) $\mu$ = 0.5 $p_{T}$ , (dash) $\mu$ = 1.0 $p_{T}$ , and (dash-dot) $\mu$ = 2.0 $p_{T}$.
Direct photon cross section. (a) The invariant cross sections of the direct photon in $p+p$ [3, 4] and $d$+Au collisions. The $p+p$ fit result with the empirical parameterization described in the text is shown as well as NLO pQCD calculations, and the scaled $p+p$ fit is compared with the $d$+Au data. The closed and open symbols show the results from the virtual photon and $\pi_{0}$-tagging methods, respectively. The asterisk symbols show the result from the statistical subtraction method for $d$+Au data, overlapping with the virtual photon result in 3 < $p_{T}$ < 5 GeV/c. The values in the table are equal to this mean value. The bars and bands represent the point-to-point (ptp.) and $p_{T}$-correlated (cor.) uncertainties, respectively. (b) The $p+p$ data over the fit. The uncertainties of the fit due to both point-to-point (ptp.) and pT -correlated uncertainties of the data are summed quadratically, and the sum is shown as dotted lines. The NLO pQCD calculations divided by the fit are also shown.
We report the measurement of direct photons at midrapidity in Au+Au collisions at sqrt{s_NN} = 200 GeV. The direct photon signal was extracted for the transverse-momentum range of 4 GeV/c < p_T < 22 GeV/c, using a statistical method to subtract decay photons from the inclusive-photon sample. The direct-photon nuclear-modification factor R_AA was calculated as a function of p_T for different Au+Au collision centralities using the measured p+p direct-photon spectrum and compared to theoretical predictions. R_AA was found to be consistent with unity for all centralities over the entire measured p_T range. Theoretical models that account for modifications of initial-direct-photon production due to modified-parton-distribution functions in Au and the different isospin composition of the nuclei, predict a modest change of R_AA from unity and are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.
Direct photon spectra for all centrality selections in Au+Au.
Direct photon nuclear modification factor ($R_{AA}$)
The differential cross section for the production of direct photons in p+p collisions at sqrt(s)=200 GeV at midrapidity was measured in the PHENIX detector at the Relativistic Heavy Ion Collider. Inclusive-direct photons were measured in the transverse-momentum range from 5.5--25 GeV/c, extending the range beyond previous measurements. Event structure was studied with an isolation criterion. Next-to-leading-order perturbative-quantum-chromodynamics calculations give a good description of the spectrum. When the cross section is expressed versus x_T, the PHENIX data are seen to be in agreement with measurements from other experiments at different center-of-mass energies.
Cross section of midrapidity inclusive-direct photon production in $p+p$ collisions at $\sqrt{s}$ = 200 GeV as a function of transverse momentum ($p_T$). Asymmetric statistical uncertainties occur in $p_T$ bins with no tagged $π^0$ counts.
Ratio of isolated/inclusive-direct photon (Fig. 13). Upper(+) and lower bounds(-) on systematics can be different, and are listed separately.
Ratio of isolated/inclusive photon from $π^0$ (Fig. 13).
We present measured J/psi production rates in d+Au collisions at sqrt(s_NN) = 200 GeV over a broad range of transverse momentum (p_T=0-14 GeV/c) and rapidity (-2.2<y<2.2). We construct the nuclear-modification factor R_dAu for these kinematics and as a function of collision centrality (related to impact parameter for the R_dAu collision). We find that the modification is largest for collisions with small impact parameters, and observe a suppression (R_dAu<1) for p_T<4 GeV/c at positive rapidities. At negative rapidity we observe a suppression for p_T<2 GeV/c then an enhancement (R_dAu>1) for p_T>2 GeV/c. The observed enhancement at negative rapidity has implications for the observed modification in heavy-ion collisions at high p_T.
$J/\psi$ invariant yield as a function of $p_T$ for $p+p$ and 0–100% centrality integrated $d$+Au collisions. The type C systematic uncertainty for each distribution is given as a percentage in the legend. Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
$J/\psi$ invariant yield as a function of $p_T$ for $p+p$ and 0–100% centrality integrated $d$+Au collisions. The type C systematic uncertainty for each distribution is given as a percentage in the legend. Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
$J/\psi$ invariant yield as a function of $p_T$ for $p+p$ and 0–100% centrality integrated $d$+Au collisions. The type C systematic uncertainty for each distribution is given as a percentage in the legend.Type A represents uncertainties that are uncorrelated from point to point, Type B represents uncertainties that are correlated from point to point, and Type C represents uncertainties in the overall normalization.
Background: Heavy-flavor production in p+p collisions tests perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p+p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Purpose: Determine transverse-momentum, pt, spectra and the corresponding R_AA for muons from heavy-flavor mesons decay in p+p and Cu+Cu collisions at sqrt(s_NN)=200 GeV and y=1.65. Method: Results are obtained using the semi-leptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte-Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p+p collisions at sqrt{s}=200 GeV, integrated over pt and in the rapidity range 1.4<y<1.9 is found to be dsigma_ccbar/dy = 0.139 +/- 0.029 (stat) ^{+0.051}_{-0.058} (syst) mb. This result is consistent with calculations and with expectations based on the corresponding midrapidity charm-production cross section measured earlier by PHENIX. The R_AA for heavy-flavor muons in Cu+Cu collisions is measured in three centrality intervals for 1<pt<4 GeV/c. Suppression relative to binary-collision scaling (R_AA<1) increases with centrality. Conclusions: Within experimental and theoretical uncertainties, the measured heavy-flavor yield in p+p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu+Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.
Production cross section of negative muons from heavy-flavor mesons decay as a function of $p_T$ in $p$+$p$ collisions at $\sqrt{s}=200$ GeV.
Invariant production yields of negative muons from heavy-flavor-mesons decay as a function $p_T$ in Cu+Cu collisions for three different centrality intervals (40-94%, 20-40%, and 0-20%), scaled by powers of ten for clarity. The solid line associated to each set of points corresponds to a fit to the $p$+$p$ invariant yield distribution described in the text, scaled by the appropriate number of binary collisions $N_{coll}$ when comparing to the Cu+Cu measurements.
Invariant production yields of negative muons from heavy-flavor-mesons decay as a function $p_T$ in $p$+$p$ collisions at $\sqrt{s}=200$ GeV. The solid line associated to each set of points corresponds to a fit to the $p$+$p$ invariant yield distribution described in the text, scaled by the appropriate number of binary collisions $N_{coll}$ when comparing to the Cu+Cu measurements.
Measurements of the anisotropy parameter v_2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum p_T, and transverse kinetic energy KE_T at midrapidity (|\eta|<0.35) in Au+Au collisions at sqrt(s_NN) = 200 GeV are presented. Pions and protons are identified up to p_T = 6 GeV/c, and kaons up to p_T = 4 GeV/c, by combining information from time-of-flight and aerogel Cherenkov detectors in the PHENIX Experiment. The scaling of v_2 with the number of valence quarks (n_q) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KE_T/n_q in noncentral Au+Au collisions (20--60%), but this scaling remains valid in central collisions (0--10%).
Identified hadron $v_2$ in central (0–20% centrality, left panels) Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Panels (a) and (b) show $v_2$ as a function of transverse momentum $p_T$. The $v_2$ of all species for centrality 0–20% has been scaled up by a factor of 1.6 for better comparison with results of 20–60% centrality. The error bars (shaded boxes) represent the statistical (systematic) uncertainties. The systematic uncertainties shown are type A and B only.
Identified hadron $v_2$ in central (0–20% centrality, left panels) Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Panels (a) and (b) show $v_2$ as a function of transverse momentum $p_T$. The $v_2$ of all species for centrality 0–20% has been scaled up by a factor of 1.6 for better comparison with results of 20–60% centrality. The error bars (shaded boxes) represent the statistical (systematic) uncertainties. The systematic uncertainties shown are type A and B only.
Identified hadron $v_2$ in central (0–20% centrality, left panels) Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Panels (a) and (b) show $v_2$ as a function of transverse momentum $p_T$. The $v_2$ of all species for centrality 0–20% has been scaled up by a factor of 1.6 for better comparison with results of 20–60% centrality. The error bars (shaded boxes) represent the statistical (systematic) uncertainties. The systematic uncertainties shown are type A and B only.
Unpolarized cross sections and double-helicity asymmetries of single-inclusive positive and negative charged hadrons at midrapidity from p+p collisions at sqrt(s)=62.4 GeV are presented. The PHENIX measurements for 1.0 < p_T < 4.5 GeV/c are consistent with perturbative QCD calculations at next-to-leading order in the strong coupling constant, alpha_s. Resummed pQCD calculations including terms with next-to-leading-log accuracy, yielding reduced theoretical uncertainties, also agree with the data. The double-helicity asymmetry, sensitive at leading order to the gluon polarization in a momentum-fraction range of 0.05 ~< x_gluon ~< 0.2, is consistent with recent global parameterizations disfavoring large gluon polarization.
Fit-function parameters for the efficiency curves for different hadron species.
Fit-function parameters for relative fractions of different species in the hadron mix.
Systematic uncertainties of cross-section measurements from various sources.
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