369 Search Results

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 < |y| < 2.2) in 𝑝 + 𝑝 and Au + Au collisions at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200 GeV. The low-mass vector-meson yield and nuclear-modification factor were measured as a function of the average number of participating nucleons, ⟨𝑁_part⟩, and the transverse momentum 𝑝_𝑇. These results were compared with those obtained via the kaon decay channel in a similar 𝑝_𝑇 range at midrapidity. The nuclear-modification factors in both rapidity regions are consistent within the uncertainties.more » A comparison of the 𝜔 + 𝜌 and 𝐽/𝜓 mesons reveals that the light and heavy flavors are consistently suppressed across both 𝑝_𝑇 and ⟨𝑁_part⟩. Finally, in contrast, the 𝜙 meson displays a nuclear-modification factor consistent with unity, suggesting strangeness enhancement in the medium formed.« less

Recent measurements of 𝐽/𝜓 production as a function of event charged-particle multiplicity at the collision energies of both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) show enhanced 𝐽/𝜓 production yields with increasing multiplicity. One potential explanation for this type of dependence is multiparton interactions (MPI). We present the first study of potential autocorrelations at RHIC energies and forward and backward rapidity of self-normalized 𝐽/𝜓 yields and 𝜓⁡(2⁢𝑆) to 𝐽/𝜓 ratio, as a function of self-normalized multiplicity in 𝑝 + 𝑝 collisions. In addition, detailed pythia studies tuned to RHIC energies were performed to investigate themore » MPI impacts. We find that the PHENIX data at RHIC are consistent with recent LHC measurements and can only be described by pythia calculations that include MPI effects. The forward and backward 𝜓⁡(2⁢𝑆) to 𝐽/𝜓 ratio is found to be less dependent on the charged-particle multiplicity.« less

Here, we present the first forward-rapidity measurements of elliptic anisotropy of open-heavy-flavor muons at the Relativistic Heavy Ion Collider. The measurements are based on data samples of Au + Au collisions at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200 GeV collected by the PHENIX experiment in 2014 and 2016 with integrated luminosity of 14.5 nb⁻¹. The measurements are performed in the pseudorapidity range 1.2 < |𝜂| < 2 and cover transverse momenta 1< 𝑝_𝑇 < 4 GeV/𝑐. The elliptic flow of charged hadrons as a function of transverse momentum is also measured in the same kinematic range. We observe significant elliptic flow for bothmore » charged hadrons and heavy-flavor muons. The results show clear mass ordering of elliptic flow of light- and heavy-flavor particles. The magnitude of the measured 𝑣₂ is comparable to that in the midrapidity region. This indicates that there is no strong longitudinal dependence in the quark-gluon-plasma evolution between midrapidity and the rapidity range of this measurement at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200 GeV.« less

Here, we report the first measurement of the azimuthal anisotropy of 𝐽/𝜓 at forward rapidity (1.2 < |𝜂| < 2.2) in Au+Au collisions at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200 GeV at the BNL Relativistic Heavy Ion Collider. The data were collected by the PHENIX experiment in 2014 and 2016 with integrated luminosity of 14.5 nb⁻¹. The second Fourier coefficient (𝑣₂) of the azimuthal distribution of 𝐽/𝜓 is determined as a function of the transverse momentum (𝑝_𝑇) using the event-plane method. The measurements were performed for several selections of collision centrality: 0%–50%, 10%–60%, and 10%–40%. We find that in all cases the valuesmore » of 𝑣₂⁡(𝑝_𝑇), which quantify the elliptic flow of 𝐽/𝜓, are consistent with zero. Within uncertainties, the results are consistent with measurements at midrapidity, indicating no significant elliptic flow of the 𝐽/𝜓 within the quark-gluon-plasma medium at collision energies of $$\sqrt{𝑠_{𝑁⁢𝑁}}$$=200 GeV.« less

The jet cross section and jet-substructure observables in 𝑝 + 𝑝 collisions at $$\sqrt{s}$$ =200 GeV were measured by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC). Jets are reconstructed from charged-particle tracks and electromagnetic-calorimeter clusters using the anti-𝑘_𝑡 algorithm with a jet radius of 𝑅 = 0.3 for jets with transverse momentum within 8.0 < 𝑝_𝑇 < 40.0 GeV/𝑐 and pseudorapidity |𝜂| < 0.15. Measurements include the jet cross section, as well as distributions of SoftDrop-groomed momentum fraction (𝑧_𝑔), charged-particle transverse momentum with respect to jet axis (𝑗_𝑇), and radial distributions of charged particles within jets (𝑟).more » Also measured was the distribution of 𝜉 = −ln⁡(𝑧), where 𝑧 is the fraction of the jet momentum carried by the charged particle. The measurements are compared to theoretical next-to and next-to-next-to-leading-order calculations, the PYTHIA and Herwig event generators, and to other existing experimental results. Indicated from these measurements is a lower particle multiplicity in jets at RHIC energies when compared to models. Also noted are implications for future jet measurements with sPHENIX at RHIC as well as at the future Electron-Ion Collider.« less

We previously reported jet-production rates in 𝑝 + 𝑝 and 𝑑 + Au collisions at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Subsequently, we found that electronic noise in the electromagnetic calorimeter was not completely removed by the tower-timing cuts used in the analysis. We have conducted an independent re-analysis of the data that eliminated noisy towers by removing them completely from the analysis. These cuts remove any contributions from instrumental noise, but reduce the overall acceptance for electromagnetic clusters by ≈16% and hence the overall statistical power of themore » data. Finally, this re-analysis found that instrumental noise in the electromagnetic calorimeter led to an artificial increase in the jet yield for both the 𝑝 + 𝑝 and 𝑑 + Au collisions.« less

Here, PHENIX presents a simultaneous measurement of the production of direct 𝛾 and 𝜋⁰ in 𝑑 + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV over a 𝑝_𝑇 range of 7.5 to 18 GeV/𝑐 for different event samples selected by event activity, i.e., charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor, 𝑅$$^{𝜋^0}_{dAu,EXP}$$, is 0.925 ± 0.023⁢(stat) ± 0.15⁢(scale), consistent with unity for minimum-bias (MB) 𝑑+Au collisions. For event classes with low and moderate event activity, 𝑅$$^{𝜋^0}_{dAu,EXP}$$ is consistent withmore » the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-𝑝_𝑇 𝜋⁰ production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5% of events with the highest event activity, 𝑅$$^{𝜋^0}_{dAu,EXP}$$ is suppressed by 20% relative to the MB value with a significance of 4.5⁢𝜎, which may be due to final-state effects. This suppression corresponds to a 𝑝_𝑇 shift of 𝛿⁢𝑝_𝑇 = 0.213 ± 0.055 Gev/𝑐 at 9 Gev/𝑐.« less

The PHENIX experiment measured the centrality dependence of two-pion Bose-Einstein correlation functions in $$\sqrt{𝑠{𝑁⁢𝑁}}$$ = 200 GeV Au + Au collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The data are well represented by Lévy-stable source distributions. The extracted source parameters are the correlation-strength parameter 𝜆, the Lévy index of stability 𝛼, and the Lévy-scale parameter 𝑅 as a function of transverse mass 𝑚_𝑇 and centrality. The 𝜆⁡(𝑚_𝑇) parameter is constant at larger values of 𝑚_𝑇, but decreases as 𝑚_𝑇 decreases. The Lévy-scale parameter 𝑅⁡(𝑚_𝑇) decreases with 𝑚_𝑇 and exhibits proportionality to the length scale of themore » nuclear overlap region. The Lévy exponent 𝛼⁡(𝑚_𝑇) is independent of 𝑚_𝑇 within uncertainties in each investigated centrality bin, but shows a clear centrality dependence. At all centralities, the Lévy exponent 𝛼 is significantly different from that of Gaussian (𝛼 = 2) or Cauchy (𝛼 = 1) source distributions. Comparisons to the predictions of Monte-Carlo simulations of resonance-decay chains show that, in all but the most peripheral centrality class (50%–60%), the obtained results are inconsistent with the measurements, unless a significant reduction of the in-medium mass of the 𝜂′ meson is included. Finally, in each centrality class, the best value of the in-medium 𝜂′ mass is compared to the mass of the 𝜂 meson, as well as to several theoretical predictions that consider restoration of U_𝐴⁢(1) symmetry in hot hadronic matter.« less

High-momentum two-particle correlations are a useful tool for studying jet-quenching effects in the quark-gluon plasma. Angular correlations between neutral-pion triggers and charged hadrons with transverse momenta in the range 4–12 GeV/𝑐 and 0.5–7 GeV/𝑐, respectively, have been measured by the PHENIX experiment in 2014 for Au + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV. Suppression is observed in the yield of high-momentum jet fragments opposite the trigger particle, which indicates jet suppression stemming from in-medium partonic energy loss, while enhancement is observed for low-momentum particles. The ratio and differences between the yield in Au + Au collisions and 𝑝 +more » 𝑝 collisions, 𝐼_𝐴⁢𝐴 and Δ_𝐴⁢𝐴, as a function of the trigger-hadron azimuthal separation, Δ⁢𝜙, are measured for the first time at the BNL Relativistic Heavy Ion Collider. Finally, these results better quantify how the yield of low-𝑝_𝑇 associated hadrons is enhanced at wide angle, which is crucial for studying energy loss as well as medium-response effects.« less

The PHENIX experiment has performed a systematic study of identified charged-hadron (𝜋^±, 𝐾^±, 𝑝, $$\bar{𝑝}$$) production at midrapidity in 𝑝 + Al, ³He + Au, and Cu + Au collisions at $$\sqrt{s_{NN}}$$ = 200 GeV and U + U collisions at $$\sqrt{s_{NN}}$$ = 193 GeV. Identified charged-hadron invariant transverse-momentum (𝑝_𝑇) and transverse-mass (𝑚_𝑇) spectra are presented and interpreted in terms of radially expanding thermalized systems. The particle ratios of 𝐾/𝜋 and 𝑝/𝜋 have been measured in different centrality ranges of large (Cu + Au and U + U) and small (𝑝 + Al and ³He + Au) collision systems. Themore » values of 𝐾/𝜋 ratios measured in all considered collision systems were found to be consistent with those measured in 𝑝+𝑝 collisions. However, the values of 𝑝/𝜋 ratios measured in large collision systems reach the values of ≈0.6, which is a factor of ≈2 larger than in 𝑝 + 𝑝 collisions. These results can be qualitatively understood in terms of the baryon enhancement expected from hadronization by recombination. Identified charged-hadron nuclear-modification factors (𝑅_𝐴⁢𝐵) are also presented. Enhancement of proton 𝑅_𝐴⁢𝐵 values over meson 𝑅_𝐴⁢𝐵 values was observed in central ³He + Au, Cu + Au, and U + U collisions. Finally, the proton 𝑅_𝐴⁢𝐵 values measured in the 𝑝 + Al collision system were found to be consistent with 𝑅_𝐴⁢𝐵 values of 𝜙, 𝜋^±, 𝐾^±, and 𝜋⁰ mesons, which may indicate that the size of the system produced in 𝑝 + Al collisions is too small for recombination to cause a noticeable increase in proton production.« less