We present a measurement of inclusive \(J/\psi\) production at mid-rapidity (\(|y|<1\)) in \(p+p\) collisions at a center-of-mass energy of \(\sqrt{s}\) = 200 GeV with the STAR experiment at the ...Relativistic Heavy Ion Collider (RHIC). The differential production cross section for \(J/\psi\) as a function of transverse momentum (\(p_T\)) for \(0<p_T<14\) GeV/\(c\) and the total cross section are reported and compared to calculations from the color evaporation model and the non-relativistic Quantum Chromodynamics model. The dependence of \(J/\psi\) relative yields in three \(p_T\) intervals on charged-particle multiplicity at mid-rapidity is measured for the first time in \(p+p\) collisions at \(\sqrt{s}\) = 200 GeV and compared with that measured at \(\sqrt{s}\) = 7 TeV, PYTHIA8 and EPOS3 Monte Carlo generators, and the Percolation model prediction.
PHENIX has measured the centrality dependence of midrapidity pion, kaon, and proton transverse momentum distributions in d+Au and p+p collisions at {radical}(s{sub NN})=200 GeV. The p+p data provide ...a reference for nuclear effects in d+Au and previously measured Au+Au collisions. Hadron production is enhanced in d+Au, relative to independent nucleon-nucleon scattering, as was observed in lower energy collisions. The nuclear modification factor for (anti)protons is larger than that for pions. The difference increases with centrality but is not sufficient to account for the abundance of baryon production observed in central Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The centrality dependence in d+Au shows that the nuclear modification factor increases gradually with the number of collisions encountered by each participant nucleon. We also present comparisons with lower energy data as well as with parton recombination and other theoretical models of nuclear effects on particle production.
We report first measurements of \(e^{+}e^{-}\) pair production in the mass region 0.4 \(<M_{ee}<\) 2.6 GeV/\(c^{2}\) at low transverse momentum (\(p_T<\) 0.15 GeV/\(c\)) in non-central Au\(+\)Au ...collisions at \(\sqrt{s_{NN}}\) = 200 GeV and U\(+\)U collisions at \(\sqrt{s_{NN}}\) = 193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40-80% centrality of these collisions. The excess yields peak distinctly at low-\(p_T\) with a width (\(\sqrt{\langle p^2_T\rangle}\)) between 40 to 60 MeV/\(c\). The absolute cross section of the excess depends weakly on centrality while those from a theoretical model calculation incorporating an in-medium broadened \(\rho\) spectral function and radiation from a Quark Gluon Plasma or hadronic cocktail contributions increase dramatically with increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the \(p^{2}_{T}\) distributions.
We report the timing and spatial resolution from the Muon Telescope Detector (MTD) installed in the STAR experiment at RHIC. Cosmic ray muons traversing the STAR detector have an average transverse ...momentum of 6 GeV/c. Due to their very small multiple scattering, these cosmic muons provide an ideal tool to calibrate the detectors and measure their timing and spatial resolution. The values obtained were ~100 ps and ~1-2 cm, respectively. These values are comparable to those obtained from cosmic-ray bench tests and test beams.
Phys. Rev. C 91, 034905 (2015) Event-by-event fluctuations of the ratio of inclusive charged to photon
multiplicities at forward rapidity in Au+Au collision at $\sqrt{s_{NN}}$=200
GeV have been ...studied. Dominant contribution to such fluctuations is expected
to come from correlated production of charged and neutral pions. We search for
evidences of dynamical fluctuations of different physical origins. Observables
constructed out of moments of multiplicities are used as measures of
fluctuations. Mixed events and model calculations are used as baselines.
Results are compared to the dynamical net-charge fluctuations measured in the
same acceptance. A non-zero statistically significant signal of dynamical
fluctuations is observed in excess to the model prediction when charged
particles and photons are measured in the same acceptance. We find that, unlike
dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated
by correlation due to particle decay. Results are compared to the expectations
based on the generic production mechanism of pions due to isospin symmetry, for
which no significant (<1%) deviation is observed.
We report measurements of single- and double- spin asymmetries for \(W^{\pm}\) and \(Z/\gamma^*\) boson production in longitudinally polarized \(p+p\) collisions at \(\sqrt{s} = 510\) GeV by the STAR ...experiment at RHIC. The asymmetries for \(W^{\pm}\) were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton's polarized quark distributions at the scale of the \(W\) mass. The results are compared to theoretical predictions, constrained by recent polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range \(0.05<x<0.2\).
Global polarization of \(\Lambda\) hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at \(\sqrt{s_{_{NN}}}\) = 200 GeV, with no significant difference ...between \(\Lambda\) and \(\bar{\Lambda}\). These new results reveal the collision energy dependence of the global polarization together with the results previously observed at \(\sqrt{s_{_{NN}}}\) = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity \(\eta^H\) within \(|\eta^H|<1\). An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the \(2\sigma\) level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.
The azimuthal anisotropic flow of identified and unidentified charged particles has been systematically studied in Cu+Au collisions at \(\sqrt{s_{_{NN}}}\) = 200 GeV for harmonics \(n=\) 1-4 in the ...pseudorapidity range \(|\eta|<1\). The directed flow in Cu+Au collisions is compared with the rapidity-odd and, for the first time, the rapidity-even components of charged particle directed flow in Au+Au collisions at \(\sqrt{s_{_{NN}}}\) = 200~GeV. The slope of the directed flow pseudorapidity dependence in Cu+Au collisions is found to be similar to that in Au+Au collisions, with the intercept shifted toward positive \(\eta\) values, i.e., the Cu-going direction. The mean transverse momentum projected onto the spectator plane, \(\langle p_x\rangle\), in Cu+Au collision also exhibits approximately linear dependence on \(\eta\) with the intercept at about \(\eta\approx-0.4\), closer to the rapidity of the Cu+Au system center-of-mass. The observed dependencies find natural explanation in a picture of the directed flow originating partly due the "tilted source" and partly due to the rapidity dependent asymmetry in the initial density distribution. Charge-dependence of the \(\langle p_x\rangle\) was also observed in Cu+Au collisions, indicating an effect of the initial electric field created by charge difference of the spectator protons in two colliding nuclei. The rapidity-even component of directed flow in Au+Au collisions is close to that in Pb+Pb collisions at \(\sqrt{s_{_{NN}}}\) = 2.76 TeV, indicating a similar magnitude of dipole-like fluctuations in the initial-state density distribution. Higher harmonic flow in Cu+Au collisions exhibits similar trends to those observed in Au+Au and Pb+Pb collisions and is qualitatively reproduced by a viscous hydrodynamic model and a multi-phase transport model. For all harmonics with \(n\ge2\) we observe an approximate scaling of \(v_n\) with the number of constituent quarks.
Two particle correlations between identified meson and baryon trigger particles with 2.5<p{sub T}<4.0 GeV/c and lower p{sub T} charged hadrons have been measured at midrapidity by the PHENIX ...experiment at RHIC in p+p,d+Au, and Au+Au collisions at {radical}(s{sub NN})=200 GeV. In noncentral Au+Au collisions, the probability of finding a hadron near in azimuthal angle to the trigger particles is almost identical for mesons and baryons and significantly higher than in p+p collisions. The associated yields for trigger baryons decrease in the most central collisions, consistent with some baryon production by thermal recombination in addition to hard scattering.
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