We present the first measurement of pseudorapidity distribution of photons in the region 2.3 < or = eta < or = 3.7 for different centralities in Au+Au collisions at square root of (S(NN)) = 62.4 GeV. ...We find that the photon yield scales with the number of participating nucleons at all collision centralities studied. The pseudorapidity distribution of photons, dominated by pi0 decays, has been compared to those of charged pions, photons, and inclusive charged particles from heavy-ion and nucleon-nucleon collisions at various energies. The photon production has been shown to be consistent with the energy and centrality independent limiting fragmentation scenario.
Midrapidity open charm spectra from direct reconstruction of D0(D0)-->K-/+pi+/- in d+Au collisions and indirect electron-positron measurements via charm semileptonic decays in p+p and d+Au collisions ...at squarerootsNN=200 GeV are reported. The D0(D0) spectrum covers a transverse momentum (pT) range of 0.1<pT<3 GeV/c, whereas the electron spectra cover a range of 1<pT<4 GeV/c. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at midrapidity for open charm production from d+Au collisions at BNL RHIC is dsigma(NN)cc/dy=0.30+/-0.04(stat)+/-0.09(syst) mb. The results are compared to theoretical calculations. Implications for charmonium results in A+A collisions are discussed.
Results on high transverse momentum charged particle emission with respect to the reaction plane are presented for Au + Au collisions at square root s(NN)=200 GeV. Two- and four-particle correlations ...results are presented as well as a comparison of azimuthal correlations in Au + Au collisions to those in p + p at the same energy. The elliptic anisotropy v(2) is found to reach its maximum at p(t) approximately 3 GeV/c, then decrease slowly and remain significant up to p(t) approximately 7-10 GeV/c. Stronger suppression is found in the back-to-back high-p(t) particle correlations for particles emitted out of plane compared to those emitted in plane. The centrality dependence of v(2) at intermediate p(t) is compared to simple models based on jet quenching.
Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au + Au and p + p collisions at square root of s(NN) = 200 GeV. Strong short- and long-range ...correlations (LRC) are seen in central Au + Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short-range correlations are observed in peripheral Au + Au collisions. Both the dual parton model (DPM) and the color glass condensate (CGC) predict the existence of the long-range correlations. In the DPM, the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC, longitudinal color flux tubes generate the LRC. The data are in qualitative agreement with the predictions of the DPM and indicate the presence of multiple parton interactions.
STAR observes a complex picture of RHIC collisions where correlation effects of different origins–initial state geometry, semi-hard scattering, hadronization, as well as final state interactions such ...as quantum intensity interference - coexist. Presenting the measurements of flow, mini-jet deformation, modified hadronization, and the Hanbury Brown and Twiss effect, we trace the history of the system from the initial to the final state. The resulting picture is discussed in the context of identifying the relevant degrees of freedom and the likely equilibration mechanism.
Dihadron azimuthal correlations containing a high transverse momentum (\(p_T\)) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions ...occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20-60%) Au+Au collisions at \(\sqrt{s_{_{\rm NN}}}=200\) GeV as a function of the trigger particle's azimuthal angle relative to the event plane, \(\phi_s=|\phi_t-\psi_{\rm EP}|\). The azimuthal correlation is studied as a function of both the trigger and associated particle \(p_T\). The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with \(\phi_s\), with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity \(\Delta\eta\) correlation) appears to drop with increasing \(\phi_s\) while the jet-like component remains approximately constant. The correlation functions are further studied with subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the \(\phi_s\)-dependence of the ridge or the away-side double-peak structure. ...
Phys. Rev. Lett. 119, 062301 (2017) We report the first di-jet transverse momentum asymmetry measurements from
Au+Au and p+p collisions at RHIC. The two highest-energy back-to-back jets
reconstructed ...from fragments with transverse momenta above 2 GeV/c display a
significantly stronger momentum imbalance in heavy-ion collisions than in the
p+p reference. When re-examined with correlated soft particles included, we
observe that these di-jets then exhibit a unique new feature -- momentum
balance is restored to that observed in p+p for a jet resolution parameter of
R=0.4, while re-balancing is not attained with a smaller value of R=0.2.
Phys. Rev. Lett. 118, 012301 (2017) We present the first measurement of charge-dependent directed flow in Cu+Au
collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The results are presented as a
function of ...the particle transverse momentum and pseudorapidity for different
centralities. A finite difference between the directed flow of positive and
negative charged particles is observed that qualitatively agrees with the
expectations from the effects of the initial strong electric field between two
colliding ions with different nuclear charges. The measured difference in
directed flow is much smaller than that obtained from the
parton-hadron-string-dynamics (PHSD) model, which suggests that most of the
electric charges, i.e. quarks and antiquarks, have not yet been created during
the lifetime of the strong electric field, which is of the order of, or less
than, 1fm/$c$.
The inclusive $J/\psi$ transverse momentum ($p_{T}$) spectra and nuclear
modification factors are reported at midrapidity ($|y|<1.0$) in Au+Au
collisions at $\sqrt{s_{NN}}=$ 39, 62.4 and 200 GeV ...taken by the STAR
experiment. A suppression of $J/\psi$ production, with respect to
{\color{black}the production in $p+p$ scaled by the number of binary
nucleon-nucleon collisions}, is observed in central Au+Au collisions at these
three energies. No significant energy dependence of nuclear modification
factors is found within uncertainties. The measured nuclear modification
factors can be described by model calculations that take into account both
suppression of direct $J/\psi$ production due to the color screening effect and
$J/\psi$ regeneration from recombination of uncorrelated charm-anticharm quark
pairs.
Phys. Rev. C 94, 014910 2016 We present measurements of the near-side of triggered di-hadron correlations
using neutral strange baryons ($\Lambda$, $\bar{\Lambda}$) and mesons ($K^0_S$)
at ...intermediate transverse momentum (3 $<$ $p_T$ $<$ 6 GeV/$c$) to look for
possible flavor and baryon/meson dependence. This study is performed in $d$+Au,
Cu+Cu and Au+Au collisions at $\sqrt{s_{{NN}}}$ = 200 GeV measured by the STAR
experiment at RHIC. The near-side di-hadron correlation contains two
structures, a peak which is narrow in azimuth and pseudorapidity consistent
with correlations due to jet fragmentation, and a correlation in azimuth which
is broad in pseudorapidity. The particle composition of the jet-like
correlation is determined using identified associated particles. The dependence
of the conditional yield of the jet-like correlation on the trigger particle
momentum, associated particle momentum, and centrality for correlations with
unidentified trigger particles are presented. The neutral strange particle
composition in jet-like correlations with unidentified charged particle
triggers is not well described by PYTHIA. However, the yield of unidentified
particles in jet-like correlations with neutral strange particle triggers is
described reasonably well by the same model.
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