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STAR focus: Evidence for Charm Quark Thermalization at RHIC

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 √sNN = 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 pT 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.



Figure 1. Left: The Heavy Flavor Tracker (HFT) system which consists of one layer of Silicon Strip Detector (SSD), one layer of Intermediate Silicon Tracker (IST), and two layers of PiXeL detector (PXL). Right: 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.

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.



Figure 2. Left: v2 normalized by the number-of-constituent-quarks (nq) as a function of transverse kinetic energy (also normalized by nq) for D0 mesons and light hadrons in 10-40% central Au+Au collisions at 200 GeV. Right: v2 as a function of pT for D0 in 0-80% Au+Au collisions at 200 GeV compared to various model calculations.


Fig. 2 (left) shows the v2 normalized by the number-of-constituent-quark (nq) vs. the transverse kinetic energy (also normalized by nq) 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 pT region, this indicates a clear mass ordering for light hadrons and D0 mesons. In the intermediate pT 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 Ds and ΛC production in mid-central Au+Au collisions as well as the charm/bottom-separated single-electron RAA 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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July 11, 2017
Congratulations to Chris Flores on successfully defended his PhD thesis at UC Davis. His thesis title is: "A Systematic, Large Phase Space Study of Pion, Kaon, and Proton Production in Au+Au Heavy-Ion Collisions from the RHIC Beam Energy Scan Program at STAR"

July 4, 2017
Congratulations to Kunsu Oh who successfully defended his PhD thesis at Pusan in May, 2017. His thesis title is: "Charm and Bottom Hadron Production via Semi-leptonic Decay Channel in Relativistic Heavy Ion Collisions".

June 30, 2017
We get to know one of our new collaborating institutions in the June 2017 edition of the STAR Newsletter, and are called to volunteer for the annual Summer Sunday at BNL. We also hear from our spokesperson about Collaboration topics, and our S&C Leader on computing efforts. The recent PAC and RHIC/AGS Users' Meetings make the news, along with recognitions for members of the STAR Collaboration.

June 23, 2017
Congratulations Prashanth Shanmuganathan (Ph.D. Supervisor Declan Keane, Kent State) and Zilong Chang (Ph.D. Supervisor Carl Gagliardi, Texas A&M University) for wining the RHIC/AGS Users Thesis awards. Their theses can be found on our web pages or via the links below:

June 18, 2017
Congratulations to Xu Wang who successfully defended his PhD thesis at SDU on May 26, 2017. His thesis title is: "Setup of a PMT Test System for LHAASO-KM2A and STAR-iTPC MWPC Prototype Design and Performance Measurement".

May 25, 2017
Congratulations to Zhengqiao Zhang who successfully defended his PhD thesis at SINAP on May 25, 2017. His thesis title is: "Measurement of interaction between antiprotons".

May 25, 2017
Congratulations to Yifei Xu who successfully defended his PhD thesis at SINAP on May 25, 2017. His thesis title is: "Experimental study of hypertriton in relativistic heavy ion collisions".

May 25, 2017
Congratulations to Long Ma who successfully defended his PhD thesis at SINAP on May 25, 2017. His thesis title is: "Measurement of D-meson triggered azimuthal correlations and study of anisotropic flow fluctuations at RHIC energies".


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