分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： Measurements of the fractional momentum loss (Sloss≡δpT/pT) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using π0 in Au+Au and Cu+Cu collisions at sNN−−−√=62.4 and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb+Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of Sloss as a function of a number of variables: the number of participants, Npart, the number of quark participants, Nqp, the charged-particle density, dNch/dη, and the Bjorken energy density times the equilibration time, εBjτ0. We find that the pT where Sloss has its maximum, varies both with centrality and collision energy. Above the maximum, Sloss tends to follow a power-law function with all four scaling variables. The data at sNN−−−√=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of Sloss with dNch/dη and εBjτ0, lending insight on the physics of parton energy loss.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： Measurements of midrapidity charged particle multiplicity distributions, dNch/dη, and midrapidity transverse-energy distributions, dET/dη, are presented for a variety of collision systems and energies. Included are distributions for Au+Au collisions at sNN−−−√=200, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu+Cu collisions at sNN−−−√=200 and 62.4 GeV, Cu+Au collisions at sNN−−−√=200 GeV, U+U collisions at sNN−−−√=193 GeV, d+Au collisions at sNN−−−√=200 GeV, 3He+Au collisions at sNN−−−√=200 GeV, and p+p collisions at sNN−−−√=200 GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, Npart, and the number of constituent quark participants, Nqp. For all A+Acollisions down to sNN−−−√=7.7 GeV, it is observed that the midrapidity data are better described by scaling with Nqp than scaling with Npart. Also presented are estimates of the Bjorken energy density, εBJ, and the ratio of dET/dη to dNch/dη, the latter of which is seen to be constant as a function of centrality for all systems.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured ϕ meson production and its nuclear modification in asymmetric Cu+Au heavy-ion collisions at sNN−−−−√=200 GeV at both forward Cu-going direction (1.2
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： The invariant yields for J/ψ production at forward rapidity (1.2<|y|<2.2) in U+U collisions at sNN−−−√=193 GeV have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor RAA are presented and compared with those from Au+Au collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from U+U and Au+Au collisions within the same centrality class is presented, and used to investigate the role of cc¯ coalescence. Two different parameterizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, Ncoll, and these were found to give significantly different Ncoll values. Results using Ncoll values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the J/ψ suppression, relative to binary collision scaling, is similar in U+U and Au+Au for peripheral and midcentral collisions, but that J/ψ show less suppression for the most central U+U collisions. The results are consistent with a picture in which, for central collisions, increase in the J/ψ yield due to cc¯ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between cc¯ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine Ncoll.
分类： 物理学 >> 核物理学 提交时间： 2016-09-12
摘要： We present measurements of e+e− production at midrapidity in Au+Au collisions at sNN−−−√ = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass (mee< 5 GeV/c2) and pair transverse momentum (pT < 5 GeV/c), for minimum bias and for five centrality classes. The \ee yield is compared to the expectations from known sources. In the low-mass region (mee=0.30--0.76 GeV/c2) there is an enhancement that increases with centrality and is distributed over the entire pair \pt range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to 2.3±0.4(stat)±0.4(syst)±0.2model or to 1.7±0.3(stat)±0.3(syst)±0.2model for minimum bias collisions when the open-heavy-flavor contribution is calculated with {\sc pythia} or {\sc mc@nlo}, respectively. The inclusive mass and pT distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the ρ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region (mee = 1.2--2.8 GeV/c2), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
分类： 物理学 >> 核物理学 提交时间： 2016-09-06
摘要： We report the double helicity asymmetry, AJ/ψLL, in inclusive J/ψ production at forward rapidity as a function of transverse momentum pT and rapidity |y|. The data analyzed were taken during s√=510GeV longitudinally polarized p+p collisions at the Relativistic Heavy Ion Collider (RHIC) in the 2013 run using the PHENIX detector. At this collision energy, J/ψ particles are predominantly produced through gluon-gluon scatterings, thus AJ/ψLL is sensitive to the gluon polarization inside the proton. We measured AJ/ψLL by detecting the decay daughter muon pairs μ+μ− within the PHENIX muon spectrometers in the rapidity range 1.2<|y|<2.2. In this kinematic range, we measured the AJ/ψLL to be 0.012±0.010~(stat)~±~0.003(syst). The AJ/ψLL can be expressed to be proportional to the product of the gluon polarization distributions at two distinct ranges of Bjorken x: one at moderate range x≈0.05 where recent RHIC data of jet and π0 double helicity spin asymmetries have shown evidence for significant gluon polarization, and the other one covering the poorly known small-x region x≈2×10−3. Thus our new results could be used to further constrain the gluon polarization for x<0.05.
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