Measurements of double-helicity asymmetries for inclusive hadron production in polarized p+p collisions are sensitive to helicity--dependent parton distribution functions, in particular to the gluon helicity distribution, Delta(g). This study focuses on the extraction of the double-helicity asymmetry in eta production: polarized p+p --> eta + X, the eta cross section, and the eta/pi^0 cross section ratio. The cross section and ratio measurements provide essential input for the extraction of fragmentation functions that are needed to access the helicity-dependent parton distribution functions.
Cross section for midrapidity inclusive of $\eta$ productions at $\sqrt{s}$ = 200 GeV as a function of $p_T$ and its comparison to NLO pQCD calculations at three different scales $\mu$. The error bars shown are the statistical and systematic uncertainties added in quadrature. Note that the fragmentation functions used in the calculations are partially constrained by this data.
Cross section ratio for the midrapidity production of inclusive $\eta$ to $\pi^0$ mesons at $\sqrt{s}$ = 200 GeV as a function of $p_T$. The error bars show the statistical and systematic uncertainties added in quadrature. The solid curve shows the ratio of the NLO pQCD calculations shown in Fig. 3 and the corresponding one for the $\pi^0$. The dashed curve shows the result of a PYTHIA Monte-Carlo simulation.
Double helicity asymmetry for midrapidity inclusive $\eta$ production from the combined 2005 and 2006 data at $\sqrt{s}$ = 200 GeV as a function of $p_T$. An additional 4.8% systematic uncertainty in the beam polarizations is not shown.
We report the first measurement of transverse single-spin asymmetries in $J/\psi$ production from transversely polarized $p+p$ collisions at $\sqrt{s} = 200$ GeV with data taken by the PHENIX experiment in 2006 and 2008. The measurement was performed over the rapidity ranges $1.2 < |y| < 2.2$ and $ |y| < 0.35$ for transverse momenta up to 6 GeV/$c$. $J/\psi$ production at RHIC is dominated by processes involving initial-state gluons, and transverse single-spin asymmetries of the $J/\psi$ can provide access to gluon dynamics within the nucleon. Such asymmetries may also shed light on the long-standing question in QCD of the $J/\psi$ production mechanism. Asymmetries were obtained as a function of $J/\psi$ transverse momentum and Feynman-$x$, with a value of $-0.086 \pm 0.026^{\rm stat} \pm 0.003^{\rm syst}$ in the forward region. This result suggests possible nonzero trigluon correlation functions in transversely polarized protons and, if well defined in this reaction, a nonzero gluon Sivers distribution function.
Transverse single-spin asymmetry in $J/\psi$ production as a function of $x_F$ for 2006 and 2008 data sets. Systematic uncertainties $\delta$$A^f_N$ Type B and $\delta$$A^P_N$ Type B are due to the geometric scale factor and the polarization, respectively.
Transverse single-spin asymmetry in $J/\psi$ production as a function of $x_F$ for 2006 and 2008 data sets. Systematic uncertainties $\delta$$A^f_N$ Type B and $\delta$$A^P_N$ Type B are due to the geometric scale factor and the polarization, respectively.
Transverse single-spin asymmetry in $J/\psi$ production as a function of $x_F$ for 2006 and 2008 data sets. Systematic uncertainties $\delta$$A^f_N$ Type B and $\delta$$A^P_N$ Type B are due to the geometric scale factor and the polarization, respectively.
Correlations of charged hadrons of 1 < pT < 10 GeV/c with high pT direct photons and pi^ 0 mesons in the range 5
Away-side charged hadron yield per π 0 trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1 & Away-side isolated direct photon trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1.
Away-side charged hadron yield per π 0 trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1 & Away-side isolated direct photon trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1.
Away-side charged hadron yield per π 0 trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1 & Away-side isolated direct photon trigger as a function of xE, which is equivalent to zT in the collinear limit cos(∆φ) = 1.
We have measured the azimuthal anisotropy of pi0's for 1 < pT < 18 GeV/c for Au+Au collisions at sqrt s_NN = 200 GeV. The observed anisotropy shows a gradual decrease in 3 < pT < 7 - 10 GeV/c, but remains positive beyond 10 GeV/c. The magnitude of this anisotropy is under-predicted, up to at least 10 GeV/c, by current perturbative QCD (pQCD) energy-loss model calculations. An estimate of the increase in anisotropy expected from initial-geometry modification due to gluon saturation effects and initial-geometry fluctuations is insufficient to account for this discrepancy. Calculations which implement a path length dependence steeper than what is implied by current pQCD energy-loss models, show reasonable agreement with the data.
$\pi^0$ $v_2$ using the reaction plane determined with MPC and RXN$_{in}$ combined as a function of $p_T$ for different centralities.
Ratios of $v_2$ measured separately using MPC and RXN$_{in}$ to combine results.
$v_2$ vs $N_{part}$ in two $p_T$ ranges and $R_{AA}$ vs $N_{part}$ in the same $p_T$ ranges.
New measurements by the PHENIX experiment at RHIC for eta production at midrapidity as a function of transverse momentum (p_T) and collision centrality in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions are presented. They indicate nuclear modification factors (R_AA) that are similar both in magnitude and trend to those found in earlier pi^0 measurements. Linear fits to R_AA in the 5--20 GeV/c p_T region show that the slope is consistent with zero within two standard deviations at all centralities although a slow rise cannot be excluded. Having different statistical and systematic uncertainties the pi^0 and eta measurements are complementary at high p_T/ thus, along with the extended p_T range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.
$E\frac{dN^3}{dp^3}$ vs. $p_T$, 0% to 5% centrality $Au+Au$. 90% Limit on 18-20 and 20-22 GeV/c bins.
$E\frac{dN^3}{dp^3}$ vs. $p_T$, 0% to 10% centrality $Au+Au$. 90% Limit on 18-20 and 20-22 GeV/c bins.
$E\frac{dN^3}{dp^3}$ vs. $p_T$, 10% to 20% centrality $Au+Au$.
The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the invariant differential cross section for production of K^0_S , \omega, \eta prime, and \phi mesons in p + p collisions at = 200 GeV. Measurements \omega and \phi production in different decay channels give consistent results. New results for the \phi are in agreement with previously published data and extend the measured pT coverage. The spectral shapes of all hadron transverse momentum distributions measured by PHENIX are well described by a Tsallis distribution functional form with only two parameters, n and T, determining the high-pT and characterizing the low-pT regions of the spectra, respectively. The values of these parameters are very similar for all analyzed meson spectra, but with a lower parameter T extracted for protons. The integrated invariant cross sections calculated from the fitted distributions are found to be consistent with existing measurements and with statistical model predictions.
Parameters of the Tsallis fit with Eq. 8 in the paper with all parameters free to vary. Cross sections are in $\mu$b for $J/\psi$ and $\psi^{\prime}$ and in mb for all other particles.
Parameters of the power law fit with Eq. 3 in the paper. Units of $A$ are mb(GeV/$c$)$^{\upsilon + 2}$.
Constant and linear fits to the power law and Tsallis fit parameters. The last column (Prob.) gives the probability estimated by the $\chi^2$/$n.d.f.$ of the fit.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has performed systematic measurements of phi meson production in the K+K- decay channel at midrapidity in p+p, d+Au, Cu+Cu and Au+Au collisions at sqrt(S_NN)=200 GeV. Results are presented on the phi invariant yield and the nuclear modification factor R_AA for Au+Au and Cu+Cu, and R_dA for d+Au collisions, studied as a function of transverse momentum (1<p_T<7 GeV/c) and centrality. In central and mid-central Au+Au collisions, the R_AA of phi exhibits a suppression relative to expectations from binary scaled p+p results. The amount of suppression is smaller than that of the neutral pion and the eta meson in the intermediate p_T range (2--5 GeV/c); whereas at higher p_T the phi, pi^0, and eta show similar suppression. The baryon (protons and anti-protons) excess observed in central Au+Au collisions at intermediate p_T is not observed for the phi meson despite the similar mass of the proton and the phi. This suggests that the excess is linked to the number of constituent quarks rather than the hadron mass. The difference gradually disappears with decreasing centrality and for peripheral collisions the R_AA values for both particles are consistent with binary scaling. Cu+Cu collisions show the same yield and suppression as Au+Au collisions for the same number of N_part. The R_dA of phi shows no evidence for cold nuclear effects within uncertainties.
Invariant $p_T$ spectra of the $\phi$ meson for different centrality bins in Au+Au, Cu+Cu, $d$+Au, and $p$+$p$ collisions at $\sqrt{s_{NN}}$ = 200 GeV.
$R_{AA}$ vs. $p_T$ for $\phi$ in central Au+Au collisions, $R_{AA}$ vs. $p_T$ for $\phi$ and $\pi^0$ in 10-20% mid-central Au+Au collisions, and $R_{AA}$ vs. $p_T$ for $\phi$ and $p$+$\bar{p}$ in 60-92% and for $\pi^0$ in 80-92% peripheral Au+Au collisions. The global uncertainty of ~ 10% related to the $p$+$p$ reference normalization is not shown.
$R_{AA}$ vs. $p_T$ for $\phi$ for 30-40% centrality Au+Au and 0-10% centrality Cu+Cu collisions, and $R_{AA}$ vs. $p_T$ for $\phi$ and $\pi^0$ for 40-50% centrality Au+Au and 10-20% centrality Cu+Cu collisions. The global uncertainty of ~ 10% related to the $p$+$p$ reference normalization is not shown.
The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte Carlo models predict.
Average value of charged particle multiplicity per event and unit of pseudorapidity in the pseudorapidity range from -0.2 to 0.2.
Charged particle multiplicity as a function of pseudorapidity.
Charged particle multiplicity as a function of transverse momentum.
Differential measurements of the elliptic (v_2) and hexadecapole (v_4) Fourier flow coefficients are reported for charged hadrons as a function of transverse momentum (p_T) and collision centrality or the number of participant nucleons (N_part) for Au+Au collisions at sqrt(s_NN)=200 GeV. The v_{2,4} measurements at pseudorapidity |\eta|<=0.35 obtained with four separate reaction plane detectors positioned in the range 1.0<|\eta|<3.9 show good agreement, indicating the absence of significant \eta-dependent nonflow perturbations. Sizable values for v_4(p_T) are observed with a ratio v_4(p_T,N_part)/v_2^2(p_T,N_part)~0.8 for 50
Glauber quantities ($N_{part}$, $N_{coll}$, $b$) for Au+Au collisions at 200 GeV (PHENIX Run 2007)
Event-plane resolution factors vs. $N_{part}$ for $v_2$ and $v_4$ measurements for the indicated event planes.
Comparison of $v_2$ vs. $N_{part}$ and $v_4$ vs. $N_{part}$ for charged hadrons obtained with several reaction plane detectors for the $p_T$ selections indicated.
We present a measurement of direct photon pair production cross sections using 4.2 fb-1 of data collected with the D0 detector at the Fermilab Tevatron proton-antiproton Collider. We measure single differential cross sections as a function of the diphoton mass, the transverse momentum of the diphoton system, the azimuthal angle between the photons, and the polar scattering angle of the photons, as well as the double differential cross sections considering the last three kinematic variables in three diphoton mass bins. The results are compared with different perturbative QCD predictions and event generators.
Single differential cross section DSIG/DM.
Single differential cross section DSIG/DPT.
Single differential cross section DSIG/DPHI.
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