Measurements of open charm hadron production in Au+Au Collisions at s NN − − − √ = 200 GeV at STAR
MMeasurements of open charm hadron production inAu+Au Collisions at √ s NN = 200 GeV at STAR Guannan Xie ∗ (for the STAR Collaboration) University of Illinois at Chicago, Chicago, IL 60607, USAE-mail: [email protected]
We report on the measurements of production of various charmed hadrons in Au+Au collisionsat √ s NN = 200 GeV (including D ( D ) and Λ ± c ) obtained via topological reconstruction, utiliz-ing the Heavy Flavor Tracker at STAR. Precise results on the D yields from the 2014 data arereported for a wide transverse momentum range down to 0 in various centrality bins. With thehigh-statistics data collected in 2014 and 2016, and the usage of a supervised machine learningalgorithm for signal-to-background separation, the first measurement of the centrality and trans-verse momentum dependences of Λ ± c production is shown. Finally, the total charm quark crosssection extracted from these measurements in Au+Au collisions at √ s NN = 200 GeV is presented. International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions30 September - 5 October 2018Aix-Les-Bains, Savoie, France ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] D ec pen charm hadron production at STAR Guannan Xie
1. Introduction
Because of their large mass, heavy quarks (charm and bottom) are predominately createdthrough initial hard scatterings in heavy-ion collisions at RHIC and the LHC. They experiencethe whole evolution of the system and thus are suggested to be an important tool for studying theproperties of the Quark Gluon Plasma (QGP) produced in heavy-ion collisions [1, 2]. The mod-ification of their production in transverse momentum ( p T ) due to energy loss and in azimuth dueto anisotropic flows is sensitive to heavy-quark dynamics in the partonic QGP phase. Recently,measurements at RHIC and the LHC have indicated strong energy loss and large elliptic flow foropen charm hadrons, similar in magnitude to those of light-flavor hadrons [3, 4, 5]. The observedenhancements of Λ ± c and D ± s production yields in Au+Au collisions suggest that the coalescencemechanism also plays an important role for charm quark hadronization. Study of the charm quarkhadronization mechanism in the QGP is also crucial for understanding the charm meson suppres-sion in heavy-ion collisions and charm quark energy loss in QGP [6].In these proceedings, we report on the production of various charmed hadrons in Au+Aucollisions at √ s NN = 200 GeV obtained via topological reconstruction, utilizing the Heavy FlavorTracker (HFT) at STAR. Precise measurements of the D yields are reported for a wide p T rangedown to 0. The D R AA and D R CP are also reported in various centrality bins and compared tothose of light-flavor hadrons as well as model calculations. The first measurement of the centralityand p T dependences of the Λ ± c / D ratio in heavy-ion collisions is shown. Finally, the total charmquark cross section extracted from these measurements in Au+Au collisions is presented.
2. Experimental and Analysis
The STAR experiment at RHIC is a large-acceptance detector covering full azimuth and pseu-dorapidity of | η | <
1. Data taken by the STAR experiment with the HFT installed were used for thisanalysis. The HFT consists of four sub-detectors, two layers of Pixel detectors (PXL) close to thebeam pipe, the Intermediate Silicon Tracker (IST) and the Silicon Strip Detector (SSD) at the outer-most layer. The HFT is a high resolution silicon detector which provides a track pointing resolutionof less than 50 µ m for kaons with p T = 750 MeV/ c . The excellent pointing resolution significantlyimproves the signal-to-background ratio by reducing the combinatorial background when topolog-ically reconstructing charm hadrons decaying close to the collision vertex, especially in heavy-ioncollisions. The particle identification ( π , K , p ) is performed by measuring the ionization energyloss (dE/dx) in the Time Projection Chamber (TPC) and velocity using the Time-Of-Flight (TOF)detector. In total, about 900 million minimum-bias (MB) Au+Au events from the year 2014 and 1billion events from the year 2016 were used for this analysis. These events were required to haveprimary vertices (PV) within 6 cm from the center of the STAR detector along the beam directionto ensure uniform HFT acceptance.The D mesons were reconstructed via the hadronic decay channel: D → K − π + ( B . R . ∼ Λ + c baryons through Λ + c → p + K − π + ( B . R . ∼ pen charm hadron production at STAR Guannan Xie
Analysis (TMVA) package integrated in the ROOT framework in order to obtain the highest signalsignificance [7]. The Rectangular Cut optimization method from the TMVA package is chosen for D , and a supervised learning algorithm, Boosted Decision Trees (BDT) from the Toolkit is used for Λ ± c signal and background separation. The TPC acceptance and tracking efficiency are obtainedusing the standard STAR TPC embedding technique. The particle identification efficiency andthe HFT acceptance and tracking plus topological cut efficiency are obtained using a data-drivensimulation method in order to fully capture the real-time detector performance.
3. Physics Results (GeV/c) T p (a) 0-10% Au+Au @ 200 GeV
LBTDuke
STAR Preliminary (GeV/c) T p (b) 10-40% (GeV/c) T p0 2 4 6 8 (c) 40-80% AA R (GeV/c) T p , 0-12% p , 0-5% K, f D LBT Duke (a) 0-10%
Au+Au @ 200 GeV
STAR Preliminary (GeV/c) T p (b) 10-20% (GeV/c) T p0 2 4 6 8 (c) 20-40% ( X / - % ) c p R Figure 1: (Left) D R AA in Au+Au collisions at √ s NN = 200 GeV for 0–10% (a), 10–40% (b) and 40–80%(c) centrality bins, respectively. (Right) D R CP with the 40–60% spectrum as the reference for differentcentrality classes in Au+Au collisions compared to those of light-flavor and strange mesons ( π ± , K S and φ ). The grey bands around unity depict the systematic uncertainty due to vertex resolution correction. Thelight and dark green boxes represent the global uncertainties in the spectra for the given centrality and thereference, respectively. Figure 1 left panel shows the D R AA with the p + p measurement [8] as the reference for dif-ferent centrality bins 0–10% (a), 10–40% (b) and 40–80% (c), respectively. The new R AA measure-ments are also compared to the previously published results using only the STAR TPC after recentcorrection [3]. The p + p D reference spectrum is updated using the latest global analysis of charmfragmentation ratios from Ref. [9] and also by taking into account the p T dependence of the frag-mentation ratio between D and D ∗± from PYTHIA. The new measurement with the HFT detectorshows a nice agreement with the previous measurement without the HFT. The brackets on the datapoints depict the total systematic uncertainty dominated by the uncertainty in the p + p referencespectrum. The open circles of the first two and last two data points indicate that those are calcu-lated with an extrapolated p + p reference. From low to intermediate p T region, the D R AA shows acharacteristic structure that is qualitatively consistent with the expectation from model predictionsthat charm quarks gain sizable collective motion during the medium evolution. The large uncer-tainty in the p + p baseline needs to be further reduced before making more quantitative conclusions.2 pen charm hadron production at STAR Guannan Xie
The right panel of Fig. 1 shows the D R CP for different centralities as a function of p T with the40–60% centrality spectrum as the reference. As a comparison, R CP of charged pions, K s and φ inthe corresponding centralities are also plotted in each panel. The measured D R CP in central 0–10% collisions shows a significant suppression at p T > c . The suppression level is similar tothat of light-flavor and strange mesons and the suppression gradually decreases when moving fromcentral collisions to mid-central and peripheral collisions. The D R CP for p T < c does notshow a modification with centrality, in contrast to light-flavor hadrons. Calculations from the Dukegroup and the Linearized Boltzmann Transport (LBT) are also compared to the data [10, 11]. Bothcollisional and radiative energy losses are included in these two calculations, and the parametersused in the models are tuned to reproduce the previously published results [3]. Both model calcu-lations match our new measured data well while the improved precision of the new measurementsis expected to further constrain the theoretical model calculations. ) D + ) / ( D - c L + + c L ( part N Preliminary
STAR < 6 GeV/c T = 200 GeV, 3 < p NN sAu+Au, < 4 GeV/c T = 7 TeV, 3 < psALICE, p+p, T p Preliminary
STAR = 200 GeV NN sAu+Au, 10-80% Ko: three quark (0-5%)Ko: di-quark, (0-5%)Greco (0-20%)PYTHIA
Figure 2: (Left) ( Λ + c + Λ − c ) / ( D + D ) ratio as a function of N part in 3 < p T < c . (Right) ( Λ + c + Λ − c ) / ( D + D ) ratio as a function of p T for the 10–80% centrality class. Figure 2 shows in the left panel the measured ( Λ + c + Λ − c ) / ( D + D ) ratio as a function of N part in 3 < p T < c . There is a clear increasing trend towards more central collisions whilethe value in the peripheral collisions is comparable with the measurement in p + p collisions at7 TeV from ALICE [12]. The right panel shows the ( Λ + c + Λ − c ) / ( D + D ) ratio as a functionof p T for the 10–80% centrality class. The values show a significant enhancement compared tothe calculations from PYTHIA. The enhancement is also larger than the statistical hadronizationmodel (SHM) prediction [13]. In Ko’s [14] and Greco’s model calculations [15], which includecoalescence hadronization of charm quarks, the predicted ratio is comparable to our measurement,but tends to underestimate the data for high p T . However, one needs measurements at low p T tofurther differentiate between three-quark and di-quark recombination scenarios.Besides the D and Λ ± c , STAR also has performed measurements of D ± and D ± s [16, 17] inAu+Au collisions at √ s NN = 200 GeV. With these various charmed hadron measurements, the totalcharm quark cross section per binary nucleon collision was obtained and listed in Table 1. Forthe D , the measurements were performed down to p T = p T which results in sizable uncertainties for these cross section measurements.3 pen charm hadron production at STAR Guannan Xie
Table 1:
Total charm cross-section per binary nucleon collision at midrapidity in Au+Au and p + p collisionsat 200 GeV. Charm Hadron Cross Section d σ /dy( µ b)Au+Au D ± ± D + ± ± D + s ± ± Λ + c ±
13 (stat) ±
28 (sys)total cc ±
13 (stat) ±
29 (sys) p + p total cc ±
30 (stat) ±
26 (sys)The total cc cross section per binary nucleon collision in Au+Au collisions is consistent with that in p + p collisions within uncertainties. However, the charm hadrochemistry is modified in heavy-ioncollisions compared to that in p + p collisions.
4. Summary
We have presented the recent measurements of production of various charmed hadrons inAu+Au collisions at √ s NN = 200 GeV obtained by the STAR experiment at RHIC. The measured D R CP , R AA and ( Λ + c + Λ − c ) / ( D + D ) ratio are presented and compared to model calculations. References [1] J. Adams et al. (STAR Collaboration), Nucl. Phys.
A757 , 102 (2005).[2] A. Andronic et al.
The Eur. Phys. Jour. C
107 (2016)[3] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. Lett. , 142301 (2014), Erratum: Phys. Rev.Lett. , 229901 (2018).[4] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. Lett. , 212301 (2017).[5] S. Acharya et al. (ALICE Collaboration), JHEP
174 (2018).[6] M. Cacciari et al. , Phys. Rev. Lett. , 122001 (2005).[7] A. Hocker et al. PoS
ACAT , 040 (2007).[8] L. Adamczyk et al. (STAR Collaboration), Phys. Rev. D , 072013 (2012).[9] M. Lisovyi et al. The Eur. Phys. Jour. C , 397 (2016).[10] Y. Xu et al. Phys. Rev. C , 014907 (2018).[11] S. Cao et al. Phys. Rev. C , 014909 (2016).[12] S. Acharya et al. (ALICE Collaboration) JHEP
108 (2018)[13] I. Kuznetsova et al.
The European Physical Journal C et al. Phys. Rev. Lett. , 222301 (2008).[15] S. Ghosh et al.
Phys. Rev. D , 2-7 (2014).[16] J.Vanek (STAR Collaboration), Poster Contribution ID : 84. Quark Matter 2018.[17] L. Zhou (STAR Collaboration), Nuclear Physics A , 620-623 (2017), 620-623 (2017)