The PHENIX experiment measured the centrality dependence of two-pion
Bose-Einstein correlation functions in $\sqrt{s_{_{NN}}}=200$~GeV Au$+$Au
collisions at the Relativistic Heavy Ion Collider at ...Brookhaven National
Laboratory. The data are well represented by L\'evy-stable source
distributions. The extracted source parameters are the correlation-strength
parameter $\lambda$, the L\'evy index of stability $\alpha$, and the
L\'evy-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 L\'evy scale parameter $R(m_T)$ decreases
with $m_T$ and exhibits proportionality to the length scale of the nuclear
overlap region. The L\'evy 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 L\'evy exponent $\alpha$ 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 $\eta$ meson, as well as
to several theoretical predictions that consider restoration of $U_A(1)$
symmetry in hot hadronic matter.
The PHENIX experiment has measured open heavy-flavor production via semileptonic decay over the transverse momentum range 1 < p(T) < 6 GeV/c at forward and backward rapidity (1.4 < |y| < 2.0) in ...d+Au and p + p collisions at √sNN = 200 GeV. In central d+Au collisions, relative to the yield in p + p collisions scaled by the number of binary nucleon-nucleon collisions, a suppression is observed at forward rapidity (in the d-going direction) and an enhancement at backward rapidity (in the Au-going direction). Predictions using nuclear-modified-parton-distribution functions, even with additional nuclear-p(T) broadening, cannot simultaneously reproduce the data at both rapidity ranges, which implies that these models are incomplete and suggests the possible importance of final-state interactions in the asymmetric d + Au collision system. These results can be used to probe cold-nuclear-matter effects, which may significantly affect heavy-quark production, in addition to helping constrain the magnitude of charmonia-breakup effects in nuclear matter.
The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in d+Au collisions at √(s(NN))=200 GeV. These ...measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central p+Pb collisions at √(s(NN))=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in d+Au collisions at RHIC compared to those seen in p+Pb collisions at the LHC. The larger extracted v2 values in d+Au are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from p+Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.
We present results for three charmonia states (ψ', χc, and J/ψ) in d+Au collisions at |y|<0.35 and sqrts(NN)=200 GeV. We find that the modification of the ψ' yield relative to that of the J/ψ scales ...approximately with charged particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large-impact-parameter collisions we observe a similar suppression for the ψ' and J/ψ, while in small-impact-parameter collisions the more weakly bound ψ' is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger ψ' suppression in central events is not explained by an increase of the nuclear absorption owing to meson formation time effects.
The reaction 2H(e,e'p)n has been studied with full kinematic coverage for photon virtuality 1.75<Q2<5.5 GeV2. Comparisons of experimental data with theory indicate that for very low values of neutron ...recoil momentum (p(n)<100 MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For 100<p(n)<750 MeV/c, proton-neutron rescattering dominates the cross section, while Delta production followed by the NDelta-->NN transition is the primary contribution at higher momenta.
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √s(NN)=200 GeV. The p(T) of the photon is an excellent approximation to the initial ...p(T) of the jet and the ratio z(T)=p(T)(h)/p(T)(γ) 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. I(AA), 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 z(T). The associated hadron yield at low z(T) is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
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