Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Distance of closest approach analysis with VTX at RHIC-PHENIX
H. Asano,∗1,∗2 A. Adare,∗3 Y. Akiba,∗2 S. Bathe,∗4,∗2 J. Bryslawskyj,∗4 T. Hachiya,∗2 T. Koblesky,∗3M. Kurosawa,∗2 D. McGlinchey,∗3 T. Moon,∗7,∗2 H. Nakagomi,∗10,∗2 R. Nouicer,∗11,∗2 T. Rinn∗8 Z. Rowan,∗4T. Sumita,∗2 A. Taketani,∗2 and the PHENIX VTX group
Heavy quark production is being studied via aPHENIX experiment by measuring of electrons fromsemi-leptonic decays of hadrons containing charm andbottom quarks. A large suppression and strong elliptic
flow of single electron heavy
flavor has been observed√in Au+Au collisions at s = 200 GeV1) . In past measurement, PHENIX was unable to distinguish electrons
from charm and bottom quarks.
In order to understand the medium effects in moredetail, the Silicon Vertex Tracker (VTX) was developed and installed in year 2011 in the RHIC-PHENIXexperiment. The VTX can measure charms and bottoms separately using the distance of closest approach(DCA) to the primary vertex. After the preliminary result of the fraction of b → e/(b → e+c → e) in Au+Au
collision was reported2) , we improved the DCA analysis to obtain the final result.
(i) In the high multiplicity environment, hits on theVTX can be associated with uncorrelated tracks.
They are one of the main sources of backgroundof DCA distribution. We studied the chi-square
distribution of track fitting using real data anda Monte Carlo simulation. We determined thethreshold of the chi-square value of tracks to reject random association candidates as much as
possible. The real tracks are expected to be removed around 5% from the simulation study.
(ii) Electrons from photon conversion and Dalitz decay of neutral mesons are also the backgroundsource in the electron DCA measurement. We
identify e+ , e− pairs by VTX and removed thesepairs from the DCA distribution. The tagging
efficiency of the conversion and the Dalitz decay are estimated thorough the simulation. The
yield of the remaining electrons and the shapeof the DCA distribution from conversions andDalitz decays are also estimated using the pastmeasurement data1) and the simulation.
(iii) Electrons from J/ψ and Kaon also cause small
background noise. The yield of these electrons∗1∗2∗3∗4∗5∗6∗7∗8Department of Physics, Kyoto University
RIKEN Nishina Center
Department of Physics, University of Colorado Boulder
Department of Physics, City University of New York
Department of Physics, Yonsei University
Graduate School of Pure and Applied Sciences, Universityof Tsukuba
Brookhaven National Laboratory
Department of Physics and Astronomy, Iowa State University

and the shape of DCA distribution are estimatedusing the past measurement data and the simulation.
After completing the study, we decomposed electron
DCA distribution. In Fig. 1, the shape of each component is obtained through the simulation and each component is overlaid on the DCA distribution of real data.
Currently, the ratio of charm and bottom quarks are
based on theoretical prediction. The fraction of each
DCA background (conversion, Dalitz, Kaon, J/ψ...) isobtained from (ii) and (iii). The sum of those DCA
distribution values describes the shape of the electron
DCA distribution in real data very well.
To obtain the precise ratio of b → e/(b → e + c → e)
as a function of transverse momentum and momentum
distribution of parent mesons (D and B mesons) fromDCA measurement, an invariant yield of heavy flavor
electrons and DCA distribution are being analyzed simultaneously. The results will be published soon.
2.00 < p < 2.50
T

Data
CharmBottom
RandomHadron Contamination
DalitzConversion
Ke3J/ ψSum

310102

10

110-1
-0.15

-0.1

-0.05

00.05

0.1

0.15

c*DCA2d [cm]Fig. 1. Electron DCA distribution at 2.0-2.5 GeV/c.
Charm (blue) and bottom (yellow) are signal. The
other components are backgrounds. The sum of thesignals and backgrounds (light blue) obtained throughthe simulation is consistent with data (black).

References
1) A. Adare et al. Phys.Rev., C84:044905, (2011).
2) R. Nouicer et al. Nucl.Phys., A904-905 (2013) 647c-652c- 113 -

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