The experimental study of the collisions of heavy nuclei at relativistic energies has established the properties of the quark-gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks and gluons are not bound into hadrons. In this state, matter behaves as a nearly inviscid fluid that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the produced particles, producing a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton-proton ($p$$+$$p$) and proton-nucleus ($p$$+$$A$) collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here, we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton-gold ($p$$+$Au), deuteron-gold ($d$$+$Au), and helium-gold ($^3$He$+$Au) collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_{NN}}}$~=~200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide a simultaneous description of these measurements.
$v_2$for 0-5% central p+Au collisions
$v_2$for 0-5% central d+Au collisions
$v_2$for 0-5% central $^3$He+Au collisions
The PHENIX collaboration presents first measurements of low-momentum ($0.4
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.
Production of $\pi^0$ and $\eta$ mesons has been measured at midrapidity in Cu$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. Measurements were performed in $\pi^0(\eta)\rightarrow\gamma\gamma$ decay channel in the 1(2)-20 GeV/$c$ transverse momentum range. A strong suppression is observed for $\pi^0$ and $\eta$ meson production at high transverse momentum in central Cu$+$Au collisions relative to the $p$$+$$p$ results scaled by the number of nucleon-nucleon collisions. In central collisions the suppression is similar to Au$+$Au with comparable nuclear overlap. The $\eta/\pi^0$ ratio measured as a function of transverse momentum is consistent with $m_T$-scaling parameterization down to $p_T=$2 GeV/$c$, its asymptotic value is constant and consistent with Au$+$Au and $p$$+$$p$ and does not show any significant dependence on collision centrality. Similar results were obtained in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions as well as in $e^+e^-$ collisions in a range of collision energies $\sqrt{s_{_{NN}}}=$3--1800 GeV. This suggests that the quark-gluon-plasma medium produced in Cu$+$Cu collisions either does not affect the jet fragmentation into light mesons or it affects the $\pi^0$ and $\eta$ the same way.
$\pi^0$ spectra from figure 3a from minimum bias Cu+Au collisions. Type A uncertainties are uncorrelated point-to-point. Type B uncertainties are correlated point-to-point. Type C uncertainties affect the scale of the data.
$\pi^0$ spectra from figure 3a from 0-10% central Cu+Au collisions. Type A uncertainties are uncorrelated point-to-point. Type B uncertainties are correlated point-to-point. Type C uncertainties affect the scale of the data.
$\pi^0$ spectra from figure 3a from 10-20% central Cu+Au collisions. Type A uncertainties are uncorrelated point-to-point. Type B uncertainties are correlated point-to-point. Type C uncertainties affect the scale of the data.
Asymmetric nuclear collisions of $p$$+$Al, $p$$+$Au, $d$$+$Au, and $^{3}$He$+$Au at $\sqrt{s_{_{NN}}}=200$ GeV provide an excellent laboratory for understanding particle production, as well as exploring interactions among these particles after their initial creation in the collision. We present measurements of charged hadron production $dN_{\rm ch}/d\eta$ in all such collision systems over a broad pseudorapidity range and as a function of collision multiplicity. A simple wounded quark model is remarkably successful at describing the full data set. We also measure the elliptic flow $v_{2}$ over a similarly broad pseudorapidity range. These measurements provide key constraints on models of particle emission and their translation into flow.
Charged hadron $dN_{ch}/d\eta$ as a function of pseudorapidity in high-multiplicity 0%-5% central $^3$He+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
Charged hadron $dN_{ch}/d\eta$ as a function of pseudorapidity in high-multiplicity 0%-5% central $d$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
Charged hadron $dN_{ch}/d\eta$ as a function of pseudorapidity in high-multiplicity 0%-5% central $p$+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV.
The PHENIX collaboration has measured high-$p_T$ dihadron correlations in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The correlations arise from inter- and intra-jet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of $p_{\rm out}$, the transverse momentum component of the associated hadron perpendicular to the trigger hadron, are sensitive to initial and final state transverse momenta. These distributions are measured multi-differentially as a function of $x_E$, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side $p_{\rm out}$ widths, sensitive to fragmentation transverse momentum, show no significant broadening between $p$$+$Au, $p$$+$Al, and $p$$+$$p$. The away-side nonperturbative $p_{\rm out}$ widths are found to be broadened in $p$$+$Au when compared to $p$$+$$p$; however, there is no significant broadening in $p$$+$Al compared to $p$$+$$p$ collisions. The data also suggest that the away-side $p_{\rm out}$ broadening is a function of $N_{\rm coll}$, the number of binary nucleon-nucleon collisions, in the interaction. The potential implications of these results with regard to initial and final state transverse momentum broadening and energy loss of partons in a nucleus, among other nuclear effects, are discussed.
The Gaussian width differences between $p$+$A$ and $p$+$p$ are shown in two $x_E$ bins as a function of $N_{coll}$.
Measurements of the differential production of electrons from open-heavy-flavor hadrons with charm- and bottom-quark content in $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV are presented. The measurements proceed through displaced-vertex analyses of electron tracks from the semileptonic decay of charm and bottom hadrons using the PHENIX silicon-vertex detector. The relative contribution of electrons from bottom decays to inclusive heavy-flavor-electron production is found to be consistent with fixed-order-plus-next-to-leading-log perturbative-QCD calculations within experimental and theoretical uncertainties. These new measurements in $p$$+$$p$ collisions provide a precision baseline for comparable forthcoming measurements in A$+$A collisions.
Inclusive heavy-flavor-electron invariant yield from the refolded charm and bottom yields (closed squares [red]) compared to published data (closed circles [gray]).
Inclusive heavy-flavor-electron invariant yield from the refolded charm and bottom yields (closed squares [red]) compared to published data (closed circles [gray]).
Unfolded charm and bottom hadron yields in bins of transverse momentum.
We report on the nuclear dependence of transverse single-spin asymmetries (TSSAs) in the production of positively-charged hadrons in polarized $p^{\uparrow}+p$, $p^{\uparrow}+$Al and $p^{\uparrow}+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The measurements have been performed at forward rapidity ($1.4<\eta<2.4$) over the range of $1.8
$A_N$ as a function of $A^{1/3}$ for positively-charged hadrons at 1.4 < $\eta$ < 2.4, 0.1 < $x_F$ < 0.2, and 1.8 < $p_T$ < 7.0 GeV/$c$ in $p^{\uparrow}$+$p$, $p^{\uparrow}$+Al, and $p^{\uparrow}$+Au collisions.
$A_N$ as a function of $N^{Avg.}_{coll}$ for positively-charged hadrons at 1.4 < $\eta$ < 2.4, 0.1 < $x_F$ < 0.2, and 1.8 < $p_T$ < 7.0 GeV/$c$ in $p^{\uparrow}$+$p$, $p^{\uparrow}$+Al, and $p^{\uparrow}$+Au collisions.
The PHENIX experiment has studied nuclear effects in $p$$+$Al and $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV on charged hadron production at forward rapidity ($1.4<\eta<2.4$, $p$-going direction) and backward rapidity ($-2.2<\eta<-1.2$, $A$-going direction). Such effects are quantified by measuring nuclear modification factors as a function of transverse momentum and pseudorapidity in various collision multiplicity selections. In central $p$$+$Al and $p$$+$Au collisions, a suppression (enhancement) is observed at forward (backward) rapidity compared to the binary scaled yields in $p$+$p$ collisions. The magnitude of enhancement at backward rapidity is larger in $p$$+$Au collisions than in $p$$+$Al collisions, which have a smaller number of participating nucleons. However, the results at forward rapidity show a similar suppression within uncertainties. The results in the integrated centrality are compared with calculations using nuclear parton distribution functions, which show a reasonable agreement at the forward rapidity but fail to describe the backward rapidity enhancement.
RpA of charged hadrons as a function of pT at forward and backward rapidity in p+Al 0%-100% centrality.
RpA of charged hadrons as a function of pT at forward and backward rapidity in p+Au 0%-100% centrality.
RpA of charged hadrons as a function of eta at forward and backward rapidity in p+Al and p+Au 0%-100% centrality.
The production of the $\Lambda$(1520) baryonic resonance has been measured at midrapidity in inelastic pp collisions at $\sqrt{s}$ = 7 TeV and in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV for non-single diffractive events and in multiplicity classes. The resonance is reconstructed through its hadronic decay channel $\Lambda$(1520) $\rightarrow$ pK$^{-}$ and the charge conjugate with the ALICE detector. The integrated yields and mean transverse momenta are calculated from the measured transverse momentum distributions in pp and p-Pb collisions. The mean transverse momenta follow mass ordering as previously observed for other hyperons in the same collision systems. A Blast-Wave function constrained by other light hadrons ($\pi$, K, K$_{\rm{S}}^0$, p, $\Lambda$) describes the shape of the $\Lambda$(1520) transverse momentum distribution up to 3.5 GeV/$c$ in p-Pb collisions. In the framework of this model, this observation suggests that the $\Lambda(1520)$ resonance participates in the same collective radial flow as other light hadrons. The ratio of the yield of $\Lambda(1520)$ to the yield of the ground state particle $\Lambda$ remains constant as a function of charged-particle multiplicity, suggesting that there is no net effect of the hadronic phase in p-Pb collisions on the $\Lambda$(1520) yield.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) at midrapidity in inelastic pp collisions at $\sqrt{s}$ $\mathrm{=}$ 7 TeV.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) in NSD p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ $\mathrm{=}$ 5.02 TeV. The uncertainty 'sys,$p_{\rm T}$-correlated' indicates the systematic uncertainty after removing the contributions of $p_{\rm T}$-uncorrelated uncertainty.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) in p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ $\mathrm{=}$ 5.02 TeV in multiplicity interval 0--20\%. The uncertainty 'sys,$p_{\rm T}$-correlated' indicates the systematic uncertainty after removing the contributions of $p_{\rm T}$-uncorrelated uncertainty.
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