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Register a new userFirst observation of the directed flow of $D^{0}$ and $overline{D^0}$ in Au+Au collisions at $sqrt{s_{rm NN}}$ = 200~GeV
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We report the first measurement of rapidity-odd directed flow ($v_{1}$) for $D^{0}$ and $overline{D^{0}}$ mesons at mid-rapidity ($|y| < 0.8$) in Au+Au collisions at $sqrt{s_{rm NN}}$ = 200,GeV using the STAR detector at the Relativistic Heavy Ion Collider. In 10--80% Au+Au collisions, the slope of the $v_{1}$ rapidity dependence ($dv_{1}/dy$), averaged over $D^{0}$ and $overline{D^{0}}$ mesons, is -0.080 $pm$ 0.017 (stat.) $pm$ 0.016 (syst.) for transverse momentum $p_{rm T}$ above 1.5~GeV/$c$. The absolute value of $D^0$-meson $dv_1/dy$ is about 25 times larger than that for charged kaons, with 3.4$sigma$ significance. These data give a unique insight into the initial tilt of the produced matter, and offer constraints on the geometric and transport parameters of the hot QCD medium created in relativistic heavy-ion collisions.
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Heavy-flavor quarks are dominantly produced in initial hard scattering processes and experience the whole evolution of the system in heavy-ion collisions at RHIC energies. Thus they are suggested to be an excellent probe to the medium properties through their interaction with the medium. In this proceedings, we report our first measurement of $D^0$ production via topological reconstruction using STARs recently installed Heavy Flavor Tracker (HFT). We also report our new measurement of Nuclear Modification Factor ($R_{AA}$) of $D^0$ mesons in central Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV as a function of transverse momentum ($p_{T}$). New results confirm the strong suppression at high $p_{T}$ with a much improved precision, and show that the $R_{AA}$ at high $p_{T}$ are comparable with light hadrons ($pi$) and with D meson measurements at the LHC. Furthermore, several theoretical calculations are compared to our data, and with charm diffusion coefficient 2${pi}TD_{S}$ $sim$ 2-12 can reproduce both the $D^0$ $R_{AA}$ and $v_2$ data in Au+Au collisions at RHIC.
Due to the large masses, heavy-flavor quarks are dominantly produced in initial hard scattering processes and experience the whole evolution of the medium produced in heavy-ion collisions at RHIC energies. They are also expected to thermalize slower than light-flavor quarks. Thus the measurement of heavy quark production and azimuthal anisotropy can provide important insights into the medium properties through their interactions with the medium. In these proceedings, we report measurements of $D^0$ production and elliptic flow ($v_2$) via topological reconstruction using STARs recently installed Heavy Flavor Tracker (HFT). The new measurement of the nuclear modification factor ($R_{AA}$) of $D^0$ mesons in central Au+Au collisions at $sqrt{s_{NN}}$ = 200 GeV confirms the strong suppression at high transverse momenta ($p_{T}$) reported in the previous publication with much improved precision. We also report the measurement of elliptic flow for $D^0$ mesons in a wide transverse momentum range in 0-80% minimum-bias Au+Au collisions. The $D^0$ elliptic flow is finite for $p_{T}$ $>$ 2 GeV/c and is systematically below that of light hadrons in the same centrality interval. Furthermore, several theoretical calculations are compared to both $R_{AA}$ and $v_2$ measurements, and the charm quark diffusion coefficient is inferred to be between 2 and $sim$12.
We report the first measurement of charmed-hadron ($D^0$) production via the hadronic decay channel ($D^0rightarrow K^- + pi^+$) in Au+Au collisions at $sqrt{s_{_{mathrm{NN}}}}$ = 200,GeV with the STAR experiment. The charm production cross-section per nucleon-nucleon collision at mid-rapidity scales with the number of binary collisions, $N_{bin}$, from $p$+$p$ to central Au+Au collisions. The $D^0$ meson yields in central Au+Au collisions are strongly suppressed compared to those in $p$+$p$ scaled by $N_{bin}$, for transverse momenta $p_{T}>3$ GeV/$c$, demonstrating significant energy loss of charm quarks in the hot and dense medium. An enhancement at intermediate $p_{T}$ is also observed. Model calculations including strong charm-medium interactions and coalescence hadronization describe our measurements.
We report on the first measurement of charm-strange meson $D_s^{pm}$ production at midrapidity in Au+Au collisions at $sqrt{s_{_{rm NN}}}$ = 200 GeV from the STAR experiment. The yield ratio between strange ($D_{s}^{pm}$) and non-strange ($D^{0}$) open-charm mesons is presented and compared to model calculations. A significant enhancement, relative to a PYTHIA simulation of $p$+$p$ collisions, is observed in the $D_{s}^{pm}/D^0$ yield ratio in Au+Au collisions over a large range of collision centralities. Model calculations incorporating abundant strange-quark production in the quark-gluon plasma (QGP) and coalescence hadronization qualitatively reproduce the data. The transverse-momentum integrated yield ratio of $D_{s}^{pm}/D^0$ at midrapidity is consistent with a prediction from a statistical hadronization model with the parameters constrained by the yields of light and strange hadrons measured at the same collision energy. These results suggest that the coalescence of charm quarks with strange quarks in the QGP plays an important role in $D_{s}^{pm}$ meson production in heavy-ion collisions.
We report a new measurement of $D^0$-meson production at mid-rapidity ($|y|$,$<$,1) in Au+Au collisions at ${sqrt{s_{rm NN}} = rm{200,GeV}}$ utilizing the Heavy Flavor Tracker, a high resolution silicon detector at the STAR experiment. Invariant yields of $D^0$-mesons with transverse momentum $p_{T}$ $lesssim 9$,GeV/$c$ are reported in various centrality bins (0--10%, 10--20%, 20--40%, 40--60% and 60--80%). Blast-Wave thermal models are used to fit the $D^0$-meson $p_{T}$ spectra to study $D^0$ hadron kinetic freeze-out properties. The average radial flow velocity extracted from the fit is considerably smaller than that of light hadrons ($pi,K$ and $p$), but comparable to that of hadrons containing multiple strange quarks ($phi,Xi^-$), indicating that $D^0$ mesons kinetically decouple from the system earlier than light hadrons. The calculated $D^0$ nuclear modification factors re-affirm that charm quarks suffer large amount of energy loss in the medium, similar to those of light quarks for $p_{T}$,$>$,4,GeV/$c$ in central 0--10% Au+Au collisions. At low $p_{T}$, the nuclear modification factors show a characteristic structure qualitatively consistent with the expectation from model predictions that charm quarks gain sizable collective motion during the medium evolution. The improved measurements are expected to offer new constraints to model calculations and help gain further insights into the hot and dense medium created in these collisions.
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