The Sixth Order Cumulant Of Net-proton Ultiplicity Distributions At Star

The theory of strong interactions, Quantum Chromodynamics (QCD), predicts a phase transition from hadronic matter to the Quark-Gluon Plasma (QGP) at sufficiently high temperature and/or density. Such a phase transition can be achieved in ultra-relativistic heavy ion collisions. The prime goal of the heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is to search for the possible formation of QGP and understand the QCD phase structure.Because several observations already indicated the formation of strong coupled quark gluon matter at RHIC top energy, the beam energy scan program was proposed to un-derstand the QCD phase diagram. The Lattice QCD calculations find that the transition from QGP to hadronic phase is not a real phase transition, but an analytic crossover at μB=0. Several QCD based calculations also predict first order phase transitions at large μB.Experimentally, the QCD phase diagram can be accessed by varying the beam energy, which can provide multiple trajectories in the T-μB plane.The ratios of cumulants of conserved quantities, such as net-baryon, net-charge and net-strangeness, are applied to study the QCD phase transition as these ratios are sen-sitive to the correlation length. It is found that in the crossover region of the QCD phase diagram, the ratios of cumulants of conserved quantities will deviate from unity as the Poisson expectation. The ratio of higher order cumulant C6/C2is expected to show larger deviation from unity than that of lower order C4/C2. In particular, change in sign of C6/C2may be observed if the freeze-out curve occurs close to phase transition line. C6/C2becomes an important ingredient in STAR’S Beam Energy Scan program regarding the QCD phase structure.Theoretical studies predict the net-baryon number fluctuation can be approximated by the fluctuation of net-proton multiplicity distributions. Based on the thermal Monte Carlo event generator, Therminator, at (1/2)SNN=200GeV, we study the centrality de-pendence of C6/C2in the following four cases:net-baryon multiplicity distributions with and without decay, net-proton multiplicity distributions with and without decay. We find that CC6/C2is consistent with unity at mid-rapitidy (|y|<0.5) within0.4<pT<0.8GeV/c. We also study the rapidity dependence of C6/C2from|y|<0.3to|y|<1.5, the results are still consistent with unity. So in thermal model, the effects from limited acceptance and resonance decay are small in CqJCi analysis.We present the first results on the ratio of the sixth to second order cumulants of the net-proton multiplicity distributions for Au+Au collisions for (1/2)sNN=7.7,11.6,19.6,27,39,62.4and200GeV. Both protons and anti-protons are cleanly identified within|y|<0.5and0.4<pT<0.8GeV/c by the STAR Time Projection Chamber.We present a detail study of the statistical and truncation effects on the ratio C6/C2of net-proton multiplicity distributions. We find that the effect of event statistics is reflected in our estimate of the statistical errors using Delta theorem and there is no effect of truncation in Au+Au collisions. We also study the effects of the efficiency and contamination on C6/C2of net-proton multiplicity distributions. The effects of the contamination are small and these effects have been incorporated to the systematic uncertainties. The effects of efficiency are studied by investigating the results in Hijing and let Hijing pass through GEANT simulation. It is also found that the efficiency effects is small within current uncertainties, based on simulations.We study the centrality and rapidity dependence of C6/C2of net-proton multiplicity distributions at (1/2)SNN=200and39GeV. Deviations from Poisson expectation are clearly observed for those results in rapidity windows of|y|<0.5,0.4and0.3, while the narrowing of the rapidity window takes the C6/C2values closer to unity. And the results with|y|<0.4and|y|<0.5are very close. We also present the energy dependence of the net-proton ratio C6/C2within|y|<0.5and0.4<PT<0.8GeV/c. C6/C2is measured to be below unity (Poisson expectation) for beam energies above39GeV. It is close to unity for (1/2)sNN<39GeV within the statistical uncertainties. So higher statistics are necessary.To compare with data, we present first results of C6/C2in UrQMD transport model. We find the values of C6/C2decrease as beam energy increases for peripheral collisions (40-80%) at mid-rapidity within0.4<pT<0.8GeV/c. This trend can be explained by global conservation law of baryon number in heavy ion collisions. The results are consistent with unity and experimental data due to large statistical uncertainties in central collisons. We also compare the results to PQM model (thermal model with phase transition). We find the results of PQM model are consistent with STAR data with current statistics.Finally, we do not observe negative values of C6/C2as expected in Lattice QCD calculations within current statistics in experimental data. We will look forward to the future Beam Energy Scan program at RHIC STAR.