Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Improvement in the track reconstruction eﬃciency with VTX in thePHENIX experiment
Y. Ueda∗1∗2 for the PHENIX Collaboration
Heavy quarks (charm and bottom quarks) with largemasses (Mc ≈ 1 GeV/c2 and Mb ≈ 4 GeV/c2 ) are agood probe to study the quark gluon plasma (QGP).
They are mainly produced by hard scattering in heavyion collisions and interact with soft partons in theQGP. The momentum and angular distributions ofheavy quarks reveal the properties of the QGP.
For the measurement of heavy quarks, the SiliconVertex Tracker (VTX) consisting of four layers of silicon detectors was installed in the PHENIX experiment. It reconstructs charged particle trajectories andtheir distances of closest approach (DCAs) to the primary collision vertex precisely. Since the DCA is related to the lifetime and mass of the particle, we canstatistically separate electrons from semileptonic decays of the charm and bottom quarks using the DCA.
In 2014, PHENIX collected about 15 billion minimum bias Au+Au events. We measure the centrality
dependence of the nuclear modiﬁcation factors1) andthe azimuthal anisotropies of the charm and bottom
quarks over a wide range of values of pT .
The track reconstruction is important for this measurement. The method of track reconstruction with
VTX is as follows. First, a line from the primary vertex to a hit point on the 1st VTX layer is deﬁned as abasic track. Then, it is propagated to the 2nd layer. Ifa hit point on the 2nd layer is found within certain windows in both the φ and z directions, called the DPHIand DZ windows, from the propagated track, the track
is reﬁtted, including the new hit. The same steps arerepeated for 3rd and/or 4th layer.
The window sizes of the 2nd, 3rd and 4th layers are
named DPHI0/DZ0, DPHI1/DZ1, and DPHI2/DZ2,respectively. They are changed with the multiplicity
because the track density is diﬀerent.
Precise determination of the primary vertex is necessary to measure the DCA precisely. However, the measurement is diﬃcult in low multiplicity events. Furthermore, the original window sizes used in the peripheral Au+Au collisions were too tight, and a signiﬁcant
fraction of the track was lost. I have optimized theDPHI/DZ window sizes for low multiplicity events toimprove the eﬃciency of track reconstruction, keeping
the number of fake tracks as small as possible. Here,the fake track rate is estimated by a simulation.
In low multiplicity events (50–100% centrality), DZ0
is no longer used for the new track reconstruction. DZ1
and 2 are 1.5 times larger than the original window
sizes for low multiplicity events, while the sizes for

DPHI are decreased by a factor of 10. This is because
an increase in only the window sizes of DZ, especially
DZ0, is more eﬀective for improving the eﬃciency without increasing the number of fake tracks.
The number of tracks in low multiplicity events is increased by a factor of 2.5 using the new window sizes
over the pT range and around the DCA peak (Fig. 1).
In addition, the DCA tail part around |0.1| cm is increased only by a factor of four. The fake track rateincreases only by a small amount. Therefore, we will
apply the new window sizes for the reconstruction of
the VTX data.

∗1Reference∗2

RIKEN Nishina Center
Department of Physics, Hiroshima University

Fig. 1. (a) pT and (b) DCA distributions in low multiplicity
events with the original and new window sizes. Blueand red lines are with the original and new window
sizes, respectively.

1) K. Nagashima et al.: In this report.

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