The jet cross section and jet-substructure observables in p +p collisions at root 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 |eta| <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). Also measured was the distribution of xi =-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.

The PHENIX experiment measured the centrality dependence of two-pion Bose-Einstein correlation functions in root sNN = 200 GeV Au + Au collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The data are well represented by Levy-stable source distributions. The extracted source parameters are the correlation-strength parameter lambda, the Levy index of stability a, and the Levy-scale parameter R as a function of transverse mass m(T) and centrality. The lambda(m(T)) parameter is constant at larger values of m(T), but decreases as m(T) decreases. The Levy-scale parameter R(m(T)) decreases with mT and exhibits proportionality to the length scale of the nuclear overlap region. The Levy exponent alpha(m(T)) is independent of m(T) within uncertainties in each investigated centrality bin, but shows a clear centrality dependence. At all centralities, the Levy exponent a is significantly different from that of Gaussian ( alpha= 2) or Cauchy ( alpha = 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 eta meson is included. In each centrality class, the best value of the in-medium eta mass is compared to the mass of the. meson, as well as to several theoretical predictions that consider restoration of U-A(1) symmetry in hot hadronic matter.

The PHENIX experiment has performed a systematic study of identified charged-hadron $({\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},$ ${K}^{\ifmmode\pm\else\textpm\fi{}},$ $p,$ $\overline{p})$ production at midrapidity in $p+\mathrm{Al}$, $^{3}\mathrm{He}+\mathrm{Au}$, and $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{{s}_{{}_{NN}}}=200\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ and $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{{s}_{{}_{NN}}}=193\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$. 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 $K/\ensuremath{\pi}$ and $p/\ensuremath{\pi}$ have been measured in different centrality ranges of large ($\mathrm{Cu}+\mathrm{Au}$ and $\mathrm{U}+\mathrm{U}$) and small ($p+\mathrm{Al}$ and $^{3}\mathrm{He}+\mathrm{Au}$) collision systems. The values of $K/\ensuremath{\pi}$ ratios measured in all considered collision systems were found to be consistent with those measured in $p+p$ collisions. However, the values of $p/\ensuremath{\pi}$ ratios measured in large collision systems reach the values of $\ensuremath{\approx}0.6$, which is a factor of $\ensuremath{\approx}2$ 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 (${R}_{AB}$) are also presented. Enhancement of proton ${R}_{AB}$ values over meson ${R}_{AB}$ values was observed in central $^{3}\mathrm{He}+\mathrm{Au}$, $\mathrm{Cu}+\mathrm{Au}$, and $\mathrm{U}+\mathrm{U}$ collisions. The proton ${R}_{AB}$ values measured in the $p+\mathrm{Al}$ collision system were found to be consistent with ${R}_{AB}$ values of $\ensuremath{\phi}$, ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$, ${K}^{\ifmmode\pm\else\textpm\fi{}}$, and ${\ensuremath{\pi}}^{0}$ mesons, which may indicate that the size of the system produced in $p+\mathrm{Al}$ collisions is too small for recombination to cause a noticeable increase in proton production.

The PHENIX experiment has performed a systematic study of identified charged-hadron ( p +/-, K +/-, p, p) production at midrapidity in p + Al, He-3+Au, and Cu + Au collisions at root s(NN) = 200 GeV and U + U collisions at root 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/ p and p/ p have been measured in different centrality ranges of large (Cu + Au and U + U) and small ( p + Al and He-3+Au) collision systems. The values of K/pi ratios measured in all considered collision systems were found to be consistent with those measured in p + p collisions. However, the values of p/pi ratios measured in large collision systems reach the values of approximate to 0.6, which is a factor of approximate to 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-AB) are also presented. Enhancement of proton R-AB values over meson RAB values was observed in central He-3+Au, Cu + Au, and U + U collisions. The proton R-AB values measured in the p + Al collision system were found to be consistent with R-AB phi values of phi, pi(+/-), K-+/-, and pi(0) 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.

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Received 2 October 2023DOI:https://doi.org/10.1103/PhysRevC.108.049905©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle correlations & fluctuationsQuark-gluon plasmaRelativistic heavy-ion collisionsPhysical SystemsBose-Einstein condensatesPionsNuclear Physics

The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$ for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.

The PHENIX experiment reports systematic measurements at the Relativistic Heavy Ion Collider of phi-meson production in asymmetric Cu + Au collisions at root sNN = 200 GeV and in U + U collisions at root sNN = 193 GeV. Measurements were performed via the phi -> K+K- decay channel at midrapidity |n| < 0.35. Features of phi -meson production measured in Cu + Cu, Cu + Au, Au + Au, and U + U collisions were found to not depend on the collision geometry, which was expected because the yields are averaged over the azimuthal angle and follow the expected scaling with nuclear-overlap size. The elliptic flow of the 0 meson in Cu + Au, Au + Au, and U + U collisions scales with second-order-participant eccentricity and the length scale of the nuclear -overlap region (estimated with the number of participating nucleons). At moderate pT, phi-meson production measured in Cu + Au and U + U collisions is consistent with coalescence-model predictions, whereas at high p(T) the production is in agreement with expectations for in-medium energy loss of parent partons prior to their fragmentation. The elliptic flow for 0 mesons measured in Cu + Au and U + U collisions is well described by a (2+1)-dimensional viscous-hydrodynamic model with specific-shear viscosity eta/s = 1/4 pi.

The measurement of direct photons from Au+Au collisions at √sNN=39 and 62.4 GeV in the transverse-momentum range 0.4

The PHENIX experiment reports systematic measurements at the Relativistic Heavy Ion Collider of ϕ-meson production in asymmetric Cu+Au collisions at sNN=200GeV and in U+U collisions at sNN=193GeV. Measurements were performed via the ϕ→K+K− decay channel at midrapidity |η|<0.35. Features of ϕ-meson production measured in Cu+Cu, Cu+Au, Au+Au, and U+U collisions were found to not depend on the collision geometry, which was expected because the yields are averaged over the azimuthal angle and follow the expected scaling with nuclear-overlap size. The elliptic flow of the ϕ meson in Cu+Au, Au+Au, and U+U collisions scales with second-order-participant eccentricity and the length scale of the nuclear-overlap region (estimated with the number of participating nucleons). At moderate pT, ϕ-meson production measured in Cu+Au and U+U collisions is consistent with coalescence-model predictions, whereas at high pT the production is in agreement with expectations for in-medium energy loss of parent partons prior to their fragmentation. The elliptic flow for ϕ mesons measured in Cu+Au and U+U collisions is well described by a (2+1)-dimensional viscous-hydrodynamic model with specific-shear viscosity η/s=1/4π.5 MoreReceived 25 July 2022Accepted 28 September 2022DOI:https://doi.org/10.1103/PhysRevC.107.014907©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLeptonic, semileptonic & radiative decaysParticle productionRelativistic heavy-ion collisionsPhysical SystemsStrange quarkVector mesonsTechniquesHadron collidersNuclear Physics

The PHENIX experiment reports systematic measurements at the Relativistic Heavy Ion Collider of ϕ-meson production in asymmetric Cu+Au collisions at √(s__NN)=200 GeV and in U+U collisions at √(s__NN)=193 GeV. Measurements were performed via the ϕ→ K^+K^- decay channel at midrapidity |η|<0.35. Features of ϕ-meson production measured in Cu+Cu, Cu+Au, Au+Au, and U+U collisions were found to not depend on the collision geometry, which was expected because the yields are averaged over the azimuthal angle and follow the expected scaling with nuclear-overlap size. The elliptic flow of the ϕ meson in Cu+Au, Au+Au, and U+U collisions scales with second-order-participant eccentricity and the length scale of the nuclear-overlap region (estimated with the number of participating nucleons). At moderate p_T, ϕ-meson production measured in Cu+Au and U+U collisions is consistent with coalescence-model predictions, whereas at high p_T the production is in agreement with expectations for in-medium energy loss of parent partons prior to their fragmentation. The elliptic flow for ϕ mesons measured in Cu+Au and U+U collisions is well described by a (2+1)D viscous-hydrodynamic model with specific-shear viscosity η/s=1/4π.

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The PHENIX experiment at the Relativistic Heavy Ion Collider measured pi(0) and eta mesons at midrapidity in U + U collisions at root S-NN = 192 GeV in a wide transverse momentum range. Measurements were performed in the pi(0)(eta) -> gamma gamma decay modes. A strong suppression of pi(0) and eta meson production at high transverse momentum was observed in central U + U collisions relative to binary scaled p + p results. Yields of pi(0) and eta mesons measured in U + U collisions show similar suppression pattern to those measured in Au + Au collisions at root S-NN = 200 GeV for similar numbers of participant nucleons. The eta/pi(0) ratios do not show dependence on centrality or transverse momentum and are consistent with previously measured values in hadron-hadron, hadron-nucleus, nucleus-nucleus, and e(+)e(-) collisions.

The PHENIX experiment at the Relativistic Heavy Ion Collider measured π^0 and η mesons at midrapidity in U+U collisions at √(s__NN)=192 GeV in a wide transverse momentum range. Measurements were performed in the π^0(η)→γγ decay modes. A strong suppression of π^0 and η meson production at high transverse momentum was observed in central U+U collisions relative to binary scaled p+p results. Yields of π^0 and η mesons measured in U+U collisions show similar suppression pattern to the ones measured in Au+Au collisions at √(s__NN)=200 GeV for similar numbers of participant nucleons. The η/π^0 ratios do not show dependence on centrality or transverse momentum, and are consistent with previously measured values in hadron-hadron, hadron-nucleus, nucleus-nucleus, and e^+e^- collisions.

We present direct photon-hadron correlations in 200 GeV/A Au + Au, d + Au, and p + p collisions, for direct photon p(T) 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 p(T) 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.

This corrects the article DOI: 10.1103/PhysRevLett.109.152301.

As a colorless probe, direct photons balance the p_T of the away-side jet at leading order. Direct photon-hadron correlations are thus an excellent probe for nuclear structure and QCD effects, including parton energy loss in the Quark-Gluon Plasma. PHENIX has measured π^0 and direct photon-triggered two-particle azimuthal correlations in a variety of collision systems ranging from p+p to Au+Au at 200 GeV. In p+Au and d+Au collisions, no modification of the per-trigger jet yield or away-side correlation width compared to p+p collisions is observed for direct photon triggered correlations while an increase in the away-side width for π^0 triggered correlations in p+Au has been measured. In Au+Au collisions, direct photons have been identified statistically as well as using an isolation cut. Combining data sets from different collision systems allows us to quantify the transition from suppression at high z_T (p_T,h/p_T,γ) to the enhancement of low z_T particles relative to p+p, and to study this transition as a function of trigger p_T.

Experimental results at RHIC and at the LHC show a similar strong suppression for light and heavy quark probes at high pT, and a possible quark mass dependence of suppression at low pT. More high precision measurements of separated charm and bottom are needed to quantify the dependence of medium effects on the quark mass. The PHENIX experiment measured separated electrons from bottom and charm decays using displaced vertex distributions at mid-rapidity |y|<0.35. Azimuthal anisotropy of electrons from separated charm and bottom decays are obtained from high statistics Au+Au data taken in 2014. PHENIX also measures muons from heavy quark decays in d+Au collisions at forward rapidity to study collective effects on heavy flavor production in small systems. These proceedings report the azimuthal anisotropy of charm and bottom at mid-rapidity in minimum bias Au+Au collisions and the anisotropy of single muons from heavy quark decays in d+Au collisions.