STAR has recently published its first paper to Phys. Rev. Letters on measurements enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system. The results are the first from a detector based on Monolithic Active Pixel Sensor (MAPS) technology in a collider environment and are the first measurements of D⁰ elliptic flow, v₂, in Au+Au collisions at √s_NN = 200 GeV.

Heavy flavor quarks, due to their large masses, are considered to offer unique information about QGP dynamics in heavy-ion collisions. A measurement of heavy flavor hadron v₂, particularly in the low-to-intermediate p_T region, will provide us a better understanding of medium thermalization, and can help quantitatively measure the heavy quark diffusion coefficient – one of the intrinsic transport parameters of the QGP.

The HFT consists of three subsystems: one layer of Silicon Strip Detector (SSD), one layer of Intermediate Silicon Tracker (IST) and two layers of silicon PiXeL (PXL) detectors. The HFT-PXL detector is the first application of the MAPS technology in a collider experiment. Its unique features include fine pixel size and thin material budget which provides superior track pointing resolution for charged particles over a broad momentum range. The HFT was designed for precision measurements of charmed hadron production via topological reconstruction of displaced vertices in heavy-ion collisions. The HFT was installed and taken physics data during RHIC Runs 2014-2016. The dataset used in the PRL was about 1.1B minimum-bias-triggered Au+Au 200 GeV events taken in 2014. Fig. 1 (right) shows the track pointing resolution in the transverse plane as a function of particle momentum (or transverse momentum) at mid-rapidity from experiments at RHIC and the LHC.

Fig. 2 (left) shows the v₂ normalized by the number-of-constituent-quark (n_q) vs. the transverse kinetic energy (also normalized by n_q) for D⁰ mesons from this measurement and other light hadron results. With unprecedented precision, the result shows that D⁰ v₂ follows the same trend as light hadrons with this scaling. In the low p_T region, this indicates a clear mass ordering for light hadrons and D⁰ mesons. In the intermediate p_T region, the magnitude of the D-meson v₂ is the same as light mesons. This result suggests that charm quarks have gained a similar amount of collectivity in these collisions as light mesons.

Fig. 2 (right) shows the D-meson v₂ compared to various theoretical model calculations. One interesting observation is that the measured D-meson v₂ can be well-described by a 3D viscous hydrodynamic model calculation, which indicates that charm quarks may have reached local thermal equilibrium. The precision of the current data allows us to distinguish different models but there are non-trivial differences between different models that need to be settled. One important physics goal, for the future, is to constrain the temperature dependence of the heavy quark diffusion coefficient parameter via joint investigations between theorists and experimentalists.

The D⁰ v₂ results, together with other heavy flavor results from STAR (e.g. the enhancement observed in the D_s and Λ_C production in mid-central Au+Au collisions as well as the charm/bottom-separated single-electron R_AA measurements) have been reported and highlighted in the recent Quark Matter 2017 conference in Chicago in February [see Guannan's contribution to STAR Newsletter February 2017 edition]. These measurements strongly suggest that charm quarks may have reached thermalization in the Au+Au collisions at RHIC energy.

The paper was made possible with significant contributions from colleagues at BNL, CCNU, KSU, LBNL, MIT, Purdue, SINAP, UIC, USTC, and UT Austin, with critical support from the STAR operation and computing teams as well as the paper's GPC.

Posted Jun. 30, 2017

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