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Register a new userMeasurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV
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The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents ($p_T>0.2$ GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at mid-rapidity, for charged-jet transverse momentum $p_T^mathrm{jet,ch}<30$ GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at $sqrt{s_{NN}}$ = 2.76 TeV, to calculations for p+p collisions at $sqrt{s}$ = 200 GeV based on the PYTHIA Monte Carlo generator and on a Next-to-Leading Order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of $2.8pm0.2mathrm{(stat)}pm1.5mathrm{(sys)}$ GeV/c, for $10<p_T^mathrm{jet,ch}<20$ GeV/c and R = 0.5. No medium-induced change in jet shape is observed for $R<0.5$. The azimuthal distribution of low-$p_T^mathrm{jet,ch}$ recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasi-particles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of $50pm30mathrm{(sys)%}$ of the large-angle yield in p+pcollisions predicted by PYTHIA.
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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.
Azimuthal angular correlations of charged hadrons with respect to the axis of a reconstructed (trigger) jet in Au+Au and p+p collisions at $sqrt{s_{text{NN}}} = 200 text{GeV}$ in STAR are presented. The trigger jet population in Au+Au collisions is biased towards jets that have not interacted with the medium, allowing easier matching of jet energies between Au+Au and p+p collisions while enhancing medium effects on the recoil jet. The associated hadron yield of the recoil jet is significantly suppressed at high transverse momentum ($p_{text{T}}^{text{assoc}}$) and enhanced at low $p_{text{T}}^{text{assoc}}$ in 0-20% central Au+Au collisions compared to p+p collisions, which is indicative of medium-induced parton energy loss in ultrarelativistic heavy-ion collisions.
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 presents new measurements of inclusive charged hadron distributions for $p_{T} <$ 12 GeV/c from Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV. Charged hadron suppression at high $p_{T}$ is similar in shape and magnitude at all centralities to that observed previously at $sqrt{s_{NN}}$ = 130 GeV for $p_{T} <$ 6 GeV/c. The ratio of spectra from central and peripheral Au+Au collisions shows that hadron suppression is approximately constant within 6 $< p_{T} <$ 12 GeV/c. The ratios of charged hadron spectra at the two beam energies show a 15-20% increase in yield at low $p_{T}$. At high $p_{T}$, the ratios show a larger increase that agrees well with pQCD calculations of the $sqrt{s_{NN}}$ dependence of particle production in Au+Au collisions.
We present measurements of $e^+e^-$ production at midrapidity in Au$+$Au collisions at $sqrt{s_{_{NN}}}$ = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of mass ($m_{ee} <$ 5 GeV/$c^2$) and pair transverse momentum ($p_T$ $<$ 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 ($m_{ee}=0.30$--0.76 GeV/$c^2$) 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 $2.3pm0.4({rm stat})pm0.4({rm syst})pm0.2^{rm model}$ or to $1.7pm0.3({rm stat})pm0.3({rm syst})pm0.2^{rm model}$ for minimum bias collisions when the open-heavy-flavor contribution is calculated with {sc pythia} or {sc mc@nlo}, respectively. The inclusive mass and $p_T$ distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the $rho$ meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region ($m_{ee}$ = 1.2--2.8 GeV/$c^2$), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
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