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Register a new userMeasurement of the sixth-order cumulant of net-proton multiplicity distributions in Au+Au collisions at $sqrt{s_{rm NN}}=$ 27, 54.4, and 200 GeV at RHIC
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According to first principle Lattice QCD calculations, the transition from quark-gluon plasma to hadronic matter is a smooth crossover in the region $mu_{rm B}leq T_{c}$. As a result, higher-order cumulants and their ratios are predicted to be negative, $C_{6}/C_{2}<0$, for example. In this paper, we report the first measurement of the midrapidity net-proton $C_{6}/C_{2}$ from 27, 54.4 and 200 GeV Au+Au collisions at RHIC. The dependence on collision centrality and kinematic acceptance in ($p_{T}$, $y$) are analyzed. While for 27 and 54.4 GeV collisions the $C_{6}/C_{2}$ values are close to zero within uncertainties, it is observed that for 200 GeV collisions, the $C_{6}/C_{2}$ ratio becomes progressively negative from peripheral to central collisions. Transport model calculations without critical dynamics predict values around zero. These observations seem to favor a smooth crossover in the high energy nuclear collisions at RHIC.
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We report on the first measurement of the charmed baryon $Lambda_c^{pm}$ production at midrapidity ($|y|$ $<$ 1) in Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV collected by the STAR experiment at the Relativistic Heavy Ion Collider. The $Lambda_c$/$D^0$ (denoting ($Lambda_c^++Lambda_c^-$)/($D^0+bar{D^0}$)) yield ratio is measured to be 1.08 $pm$ 0.16 (stat.) $pm$ 0.26 (sys.) in the 0--20% most central Au+Au collisions for the transverse momentum ($p_T$) range 3 $<$ $p_T$ $<$ 6 GeV/$c$. This is significantly larger than the PYTHIA model calculations for $p+p$ collisions. The measured $Lambda_c$/$D^0$ ratio, as a function of $p_T$ and collision centrality, is comparable to the baryon-to-meson ratios for light and strange hadrons in Au+Au collisions. Model calculations including coalescence hadronization for charmed baryon and meson formation reproduce the features of our measured $Lambda_c$/$D^0$ ratio.
We report the measurement of cumulants ($C_n, n=1ldots4$) of the net-charge distributions measured within pseudorapidity ($|eta|<0.35$) in Au$+$Au collisions at $sqrt{s_{_{NN}}}=7.7-200$ GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g. $C_1/C_2$, $C_3/C_1$) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. The measured values of $C_1/C_2 = mu/sigma^2$ and $C_3/C_1 = Ssigma^3/mu$ can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy.
Higher-order cumulants of the net-baryon multiplicity distributions are predicted to be sensitive to the properties of the nuclear matter created in high-energy nuclear collisions. In this talk, we present the collision centrality and acceptance (rapidity and transverse momentum) dependence of the ratio of the 6$^{{rm th}}$- to the 2$^{rm nd}$-order cumulant ratio ($C_{6}/C_{2}$) of net-proton in Au+Au collisions at $sqrt{s_{rm NN}}=54.4$ and 200 GeV measured by the STAR detector at RHIC. The new results are compared to hadron transport model and lattice QCD calculations.
We report on the measurements of production of various charmed hadrons in Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 200 GeV (including $D^{0}(overline{D^{0}})$ and $Lambda_{c}^{pm}$) obtained via topological reconstruction, utilizing the Heavy Flavor Tracker at STAR. Precise results on the $D^{0}$ yields from the 2014 data are reported for a wide transverse momentum range down to 0 in various centrality bins. With the high-statistics data collected in 2014 and 2016, and the usage of a supervised machine learning algorithm for signal-to-background separation, the first measurement of the centrality and transverse momentum dependences of $Lambda_{c}^{pm}$ production is shown. Finally, the total charm quark cross section extracted from these measurements in Au+Au collisions at $sqrt{s_{rm{NN}}}$ = 200 GeV is presented.
The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at $sqrt{s_{NN}}$=200 GeV. Jets are reconstructed with the anti-k$_{T}$ algorithm using charged tracks with pseudorapidity $|eta|<1.0$ and transverse momentum $0.2<p_{T,jet}^{ch}<30$ GeV/$c$, with jet resolution parameter $R$=0.2, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-$p_T$) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the $p_T$ region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for $5<p_{T,jet}^{ch}<25$ GeV/$c$ and $5<p_{T,jet}^{ch}<30$ GeV/$c$, respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the $pp$ yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high $p_T$, and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of $R$ exhibits no significant evidence for medium-induced broadening of the transverse jet profile for $R<0.4$ in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.
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