Beam Energy Dependence of Jet-Quenching Effects in Au+Au Collisions at sNN=7.7

Abstract

We report measurements of the nuclear modification factor R_{CP} for charged hadrons as well as identified π^{+(-)}, K^{+(-)}, and p(p[over ¯]) for Au+Au collision energies of sqrt[s_{NN}]=7.7, 11.5, 14.5, 19.6, 27, 39, and 62.4 GeV. We observe a clear high-p_{T} net suppression in central collisions at 62.4 GeV for charged hadrons which evolves smoothly to a large net enhancement at lower energies. This trend is driven by the evolution of the pion spectra but is also very similar for the kaon spectra. While the magnitude of the proton R_{CP} at high p_{T} does depend on the collision energy, neither the proton nor the antiproton R_{CP} at high p_{T} exhibit net suppression at any energy. A study of how the binary collision-scaled high-p_{T} yield evolves with centrality reveals a nonmonotonic shape that is consistent with the idea that jet quenching is increasing faster than the combined phenomena that lead to enhancement.

Constraining the particle production mechanism in Au+Au collisions at sNN=7.7 , 27, and 200 GeV using a multiphase transport mode

“…The measurements from the STAR experiment cover the μ B interval from 20 to 450 MeV. This is also believed to be the region in which the transition from hadronic matter to QGP takes place [19][20][21][22][23][24][25].…”

Absence of jet quenching in peripheral nucleus–nucleus collisions

“…For peripheral classes of 85% and beyond, another bias becomes increasingly important, namely the multiplicity selection in AA collisions effectively cuts into the pp cross section, suppressing the rate of hard processes relative to minimum bias pp collisions. While at center-of-mass energies of below 40 GeV [18,19], the effect from multiple interactions can be neglected, the latter, so called "jet-veto" [9], bias is strongly enhanced due to auto-correlations, since centrality estimators at mid-rapidity were used. Hence, the ratio of yields in central to peripheral collisions, R CP , which is intended to quantify nuclear modification in absence of pp reference data, would be larger than unity in absence of nuclear effects.…”

“…In principle, all hard probes should be similarly affected; examples are the most peripheral R AA of inclusive J/ψ [20] or jets [21] at LHC energies. Furthermore, the discussion of the onset of parton energy loss studied with the RHIC beam energy scan [18,19] should take the biases into account when considering the ratio of yields in central to peripheral collisions, which may lead to an artificial enhancement of R CP in absence of nuclear effects. Calculations that attempt to address parton energy loss in peripheral collisions have to account for the effect, or else they will overestimate the suppression caused by parton energy loss.…”

Search for QCD critical point by transverse velocity dependence of anti-deuteron to deuteron ratio *

“…The βT dependence of 0-5% centralp/p (left) and 0-10% centrald/d (right) are derived from the pT spectra in Au+Au collisions measured by the STAR experiment at RHIC-BES energies[34][35][36]. The dashed lines are linear fit.…”