Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Single electron yields from semileptonic charm and bottom hadron√
decays in Au+Au collisions at sN N = 200 GeV†
T. Hachiya,∗1 A. Adare,∗2 Y. Akiba,∗1 H. Asano,∗1,∗3 S. Bathe,∗1,∗4 J. Bryslawskeyj,∗4 T. Koblesky,∗2M. Kurosawa,∗1 D. Mcglinchey,∗1 T. Moon,∗1,∗5 K. Nagashima,∗1,∗6 H. Nakagomi,∗1,∗7 R. Nouicer,∗8 T. Rinn,∗9Z. Rowan,∗4 T. Sumita,∗1 and A. Taketani∗1The PHENIX experiment measured open heavy flavor production in minimum bias Au+Au collisions at√sN N = 200 GeV using single electrons from semileptonic decays of charm and bottom hadrons. Previous measurement of electrons from inclusive heavy
flavor decays showed strong suppression at high transverse momentum, pT > 5 GeV/c, when comparedwith that in p + p collisions1) . In order to understandthe suppression, we installed the silicon vertex tracker(VTX). VTX allows separating the charm and bottom
contributions by measuring the distance of the closest
approach of electrons to the primary vertex (DCA).
Data analysis was performed in three steps. First,
we measured the DCA of inclusive electrons using
Au+Au events recorded in 2011. The electrons containa large amount of backgrounds (BG) that are Dalitz
decays of π 0 and η, photon conversions, Ke3 decays,and J/ψ → e+ e− decays. Most of them are rejectedbased on the requirement of the pair-wise hit in VTX
because the pair of hits are created by e+ e− pairs ofBG’s, such as γ → e+ e− .
Second, we determined the BG DCA distribution for1)misidentified hadrons, 2)random matching of electrons with VTX hits, 3) π 0 , η, Ke3 , and J/ψ decays
and photon conversions. 1) is evaluated by the event
swap method and 2) is evaluated by embedding single
simulated electrons into the real events. 3) is estimatedusing GEANT-based detector simulation with the input of measured spectra. All BG DCA distributions
are normalized based on the BG yield obtained in thesingle electron measurement in 20041–3) .
Third, to extract the charm and bottom components, we developed the unfolding method thatemploys the Bayesian inference technique with theMarkov chain Monte Carlo sampler. The method performed simultaneous fitting with the DCA distributions and inclusive heavy flavor electrons yield1,2) as afunction of pT .
From the unfolding result, the fraction of bottomelectrons to inclusive heavy flavors was shown in Fig.
†∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗91. The red line and the pink band represent the center
value and the systematic errors, respectively. The grayline is the FONLL prediction in p + p collisions. We
found the steeper rise in 2 < pT < 4 GeV/c with apossible peak compared with the central FONLL calculations.
The nuclear modification factors RAA of charm andbottom electrons can be calculated separately using
this result with additional constraints in p + p4) andprevious Au+Au measurement1,2) . Figure 2 indicatesthat both charm and bottom electrons are strongly
suppressed at high pT , and bottom electrons are lesssuppressed than charm electrons in 3 < pT < 4 GeV/c.
For further improvement, we are analyzing large
amounts of the Au+Au and p + p data recorded in2014 and 20155) .

Fig. 1. Bottom electron fraction in minimum bias Au+Au
collisions. The FONLL calculations are also shown(gray lines)

Fig. 2. RAA for charm and bottom electrons as a functionof pT .

Condensed from the article in arXiv:1509.04662
RIKEN Nishina Center
Department of Physics, University of Colorado, Boulder
Department of Physics, Kyoto University
Department of Natural Science, Baruch College, CUNY
Department of Physics, Yonsei University
Department of Physics, Hiroshima University
Department of Physics, University of Tsukuba
Brookhaven National Laboratory
Department of Physics, Iowa State University

References

1) A. Adare et al., Phys. Rev. Lett. 98 172301 (2007).
2) A. Adare et al., Phys. Rev. C 84 044905 (2011).
3) H. Asano at al., In this report.
4) M. M. Aggarwal et al., Phys. Rev. Lett. 105 202301(2010).
5) K. Nagashima at al., In this report.

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