Search

The PHENIX Collaboration presents a systematic study of inclusive π0 production from p+p, p+Al, p+Au, d+Au, and 3He+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, RxA, are consistent with unity for pT 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-pT − π0 production, the nucleons in the d and 3He interact mostly independently with the Au nucleus and that the counterintuitive 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 pT 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 > 3He+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 pT.

In 2015, the PHENIX Collaboration has measured single-spin asymmetries for charged pions in transversely polarized p+p collisions at the center-of-mass energy of s=200 GeV. The pions were detected at central rapidities of |η

In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded p+p, p+Al, and p+Au collision data at center of mass energies of sNN=200 GeV with the proton beam(s) transversely polarized. At very forward rapidities η>6.8 relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction xF. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultraperipheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on xF. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for p+p collisions, moderate for p+Al collisions, and large for p+Au collisions.

We present a measurement of the transverse single- spin asymmetry for π0 and η mesons in p+p collisions in the pseudorapidity range |η

Studying spin-momentum correlations in hadronic collisions offers a glimpse into a three- dimensional picture of proton structure. The transverse single-spin asymmetry for midrapidity isolated direct photons in p↑+p collisions at √s=200 GeV is measured with the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). Because direct photons in particular are produced from the hard scattering and do not interact via the strong force, this measurement is a clean probe of initial-state spin-momentum correlations inside the proton and is in particular sensitive to gluon interference effects within the proton. This is the first time direct photons have been used as a probe of spin-momentum correlations at RHIC. The uncertainties on the results are a 50- fold improvement with respect to those of the one prior measurement for the same observable, from the Fermilab E704 experiment. These results constrain gluon spin-momentum correlations in transversely polarized protons.

In 2015, the PHENIX collaboration has measured very forward (η > 6.8) single spin asymmetries of inclusive neutrons in transversely polarized proton-proton and proton-nucleus collisions at a center of mass energy of 200 GeV. A previous publication from this dataset concentrated on the nuclear dependence of such asymmetries. In this measurement the explicit transverse momentum dependence of inclusive neutron single spin asymmetries for proton-proton collisions is extracted using a bootstrapping unfolding technique on the transverse momenta. This explicit transverse momentum dependence will help improve the understanding of the mechanisms that create these asymmetries.

The PHENIX collaboration has measured high-pT dihadron correlations in p+p, p+Al, and p+Au collisions at √sNN=200 GeV. The correlations arise from inter- and intrajet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of pout, 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 multidifferentially as a function of xE, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side pout widths, sensitive to fragmentation transverse momentum, show no significant broadening between p+Au, p+Al, and p+p. The away- side nonperturbative pout 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 pout broadening is a function of Ncoll, 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.

Measurements of the differential production of electrons from open-heavy-flavor hadrons with charm- and bottom-quark content in p+p collisions at √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.

We present a detailed measurement of charged two- pion correlation functions in 0–30% centrality √sNN=200 GeV Au+Au collisions by the PHENIX experiment at the Relativistic Heavy Ion Collider. The data are well described by Bose- Einstein correlation functions stemming from Lévy-stable source distributions. Using a fine transverse momentum binning, we extract the correlation strength parameter λ, the Lévy index of stability α, and the Lévy length scale parameter R as a function of average transverse mass of the pair mT. We find that the positively and the negatively charged pion pairs yield consistent results, and their correlation functions are represented, within uncertainties, by the same Lévy-stable source functions. The λ(mT) measurements indicate a decrease of the strength of the correlations at low mT. The Lévy length scale parameter R(mT) decreases with increasing mT, following a hydrodynamically predicted type of scaling behavior. The values of the Lévy index of stability α are found to be significantly lower than the Gaussian case of α=2, but also significantly larger than the conjectured value that may characterize the critical point of a second-order quark-hadron phase transition.

We present measurements of the transverse- momentum dependence of elliptic flow v2 for identified pions and (anti)protons at midrapidity (|η

We present a detailed measurement of charged two- pion correlation functions in 0–30% centrality sNN=200 GeV Au+Au collisions by the PHENIX experiment at the Relativistic Heavy Ion Collider. The data are well described by Bose- Einstein correlation functions stemming from Lévy-stable source distributions. Using a fine transverse momentum binning, we extract the correlation strength parameter λ, the Lévy index of stability α, and the Lévy length scale parameter R as a function of average transverse mass of the pair mT. We find that the positively and the negatively charged pion pairs yield consistent results, and their correlation functions are represented, within uncertainties, by the same Lévy-stable source functions. The λ(mT) measurements indicate a decrease of the strength of the correlations at low mT. The Lévy length scale parameter R(mT) decreases with increasing mT, following a hydrodynamically predicted type of scaling behavior. The values of the Lévy index of stability α are found to be significantly lower than the Gaussian case of α=2, but also significantly larger than the conjectured value that may characterize the critical point of a second-order quark-hadron phase transition.

We present measurements of the transverse- momentum dependence of elliptic flow v2 for identified pions and (anti)protons at midrapidity (|η

Recently, multiparticle-correlation measurements of relativistic p / d / 3 He + Au , p + Pb , and even p + p collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of two-, four-, and six-particle angular correlations in d + Au collisions at √sNN = 200, 62.4, 39, and 19.6 GeV. We also present measurements of two- and four-particle angular correlations in p + Au collisions at √sNN = 200 GeV . We find the four-particle cumulant to be real valued for d + Au collisions at all four energies. We also find that the four-particle cumulant in p + Au has the opposite sign as that in d + Au . Further, we find that the six-particle cumulant agrees with the four-particle cumulant in d + Au collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity.

During 2015, the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized p+p collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in p+p collisions predicts only a moderate atomic-mass-number (A) dependence. In contrast, the asymmetries observed at RHIC in p+A collisions showed a surprisingly strong A dependence in inclusive forward neutron production. The observed asymmetry in p+Al collisions is much smaller, while the asymmetry in p+Au collisions is a factor of 3 larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed A dependence.

We report a measurement of e(+)e(-) pairs from semileptonic heavy-flavor decays in p + p collisions at root S-NN = 200 GeV. The e(+)e(-) pair yield from b (b) over bar and c (c) over bar is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and p(T). We used three different event generators, PYTHIA, MC@NLO, and POWHEG, to simulate the e(+)e(-) spectra from c (c) over bar and b (b) over bar 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 (b) over bar than for c (c) over bar. 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.

The fraction of J/ψ mesons which come from B-meson decay, FB→J/ψ, is measured for J/ψ rapidity 1.20 in p+p and Cu+Au collisions at √sNN = 200 GeV with the PHENIX detector. The extracted fraction is FB→J/ ψ=0.025±0.006 (stat) ± 0.010(syst) for p+p collisions. For Cu+Au collisions, FB→J/ψ is 0.094 ± 0.028(stat) ± 0.037(syst) in the Au-going direction (−2.2

The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in minimum bias Au+Au collisions at √sNN=200 GeV via the yields of electrons from semileptonic decays of charm and bottom hadrons. Previous heavy flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks owing to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au+Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in p+p collisions at √sNN=200 GeV and find the fractions to be similar within the large uncertainties on both measurements for pT>4GeV/c. We use the bottom electron fractions in Au+Au and p+p along with the previously measured heavy flavor electron RAA to calculate the RAA for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region 3

The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in minimum bias Au+Au collisions at sqrt(s_NN)=200 GeV via the yields of electrons from semileptonic decays of charm and bottom hadrons. Previous heavy flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks owing to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au+Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in p+p collisions at sqrt(s_NN)=200 GeV and find the fractions to be similar within the large uncertainties on both measurements for pT>4 GeV/c. We use the bottom electron fractions in Au+Au and p+p along with the previously measured heavy flavor electron RAA to calculate the RAA for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region 3

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured φ meson production and its nuclear modification in asymmetric Cu + Au heavy-ion collisions at √sNN = 200 GeV at both forward Cu-going direction (1.2

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured ϕ 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

Jet production rates are measured in p+p and d+Au collisions at √sNN=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the R=0.3 anti-k_t algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multiwire proportional chambers, and the jet transverse momentum (p_T) spectra are corrected for the detector response. Spectra are reported for jets with 12

We present the first measurement of elliptic (v2) and triangular (v3) flow in high-multiplicity 3He+Au collisions at √sNN=200 GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in 3He+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the 3He+Au system. The collective behavior is quantified in terms of elliptic v2 and triangular v3 anisotropy coefficients measured with respect to their corresponding event planes. The v2 values are comparable to those previously measured in d+Au collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three 3He 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.

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured low-mass vector-meson, ω, ρ, and ϕ, production through the dimuon decay channel at forward rapidity (1.2

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured low-mass vector- meson, ω, ρ, and ϕ, production through the dimuon decay channel at forward rapidity (1.2

The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at sNN=200 GeV. The pT of the photon is an excellent approximation to the initial pT of the jet and the ratio zT=pTh/pTγ 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. IAA, the ratio of hadron yield opposite the photon 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 zT. The associated hadron yield at low zT is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.

The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √sNN=200 GeV. The pT of the photon is an excellent approximation to the initial pT of the jet and the ratio zT=phT/pγT 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. IAA, the ratio of hadron yield opposite the photon 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 zT. The associated hadron yield at low zT is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.