Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Preparatory work toward measurement of the azimuthal anisotropy√of heavy quark electrons in Au+Au collisions at sN N = 200 GeV
Y. Ueda∗1,∗2 for the PHENIX Collaboration

∗1
∗2103

mismatch BG
random hit match
random track match

102

RP resolution

studies that this BG contains two subcomponents:
mis-reconstruction of fake tracks and mis-connection ofreal tracks. The simulated DCA distribution is shownin Fig. 1 (left), with the mis-reconstruction due to therandom association of hits in red with a broad tail, andthe mis-connection in blue with a large central peak.
The blue subcomponent shows the possibility that aVTX track accidentally matches with another real DC
track after the rotation due to the high-multiplicity
environment.
The RP resolution is studied as a function of collision centrality. The RP is mainly measured with two
forward detectors in PHENIX, the beam beam counters (BBC), and the forward silicon vertex trackers(FVTX), on both sides of the nominal collision point
along the beam axis. The observed v2 needs to be corrected for the RP resolution.2) The RP resolution has
been evaluated for the two detectors using the three
sub-event method, from the diﬀerence of RPs measured with three diﬀerent detectors,2) i.e. the northand south side of BBC (or FVTX) and the central
arm. The results are shown in Fig. 1 (right), where
the larger the parameter is, the better the RP resolution is. The black and red points show the RP resolutions with BBC and FVTX, respectively. The RP
resolutions with the two detectors have a similar trend,
while that with FVTX is about twice better than thatwith BBC. The study will be continued with FVTX,
which is better suited for the v2 measurement.
CountThe azimuthal anisotropy (v2 ) of heavy (charm andbottom) quarks is a good probe for the study ofthe quark gluon plasma (QGP). Perturbative QCD(pQCD) calculations can be applied in the production of heavy quark pairs and fragmentation into heavy
quark hadrons. It allows a precise comparison of themeasured v2 of heavy quark hadrons, that of their decay electrons, and theoretical models.
In 2007, v2 of single electrons from c and b hadron
decay was measured, without their separation, inAu+Au collisions,1) leading to an unexpected discovery of substantial v2 . The detailed study of v2 of heavyquarks, i.e. measurement of v2 with c and b hadron
separation, has been recognized to be important; however, it is diﬃcult due to their small yields.
A silicon vertex tracker (VTX) consisting of four layers of silicon detectors was installed in the PHENIX experiment in 2011, in order to separate electrons fromc and b hadrons by using the distance of closest approach (DCA) method. The tracks of charged particles in the PHENIX central arm are projected backward from the drift chamber (DC) and associated tohits in the VTX. The DCA is calculated for each associated VTX track, to statistically separate electrons
from the semileptonic decays of c and b hadrons, basedon the correlation between the DCA and the lifetimeof the parent hadron. PHENIX collected about 15 bil√lion minimum bias Au+Au events at sN N = 200 GeV
in 2014. The new dataset should be large enough tostudy c and b hadrons with statistical separation.
For the precise v2 measurement utilizing the DCA
method, it is necessary to understand the background(BG) in the DCA distribution, and to precisely determine the reaction plane (RP), which is deﬁned as theplane containing the beam axis and the impact parameter vector between the two colliding nuclei.
The BG is expected to include the mismatched component (due to mismatching of tracks in DC andVTX, especially in high-multiplicity events), hadrons
misidentiﬁed as electrons, decay electrons from kaonsand quarkonia, and photonic electrons. The study ofthe mismatched component is described in this review.
It is estimated with a method called small angle rotation (SAR) using VTX and DC, where each VTX trackis rotated by a small angle. The rotated track is nolonger connected to the true DC track, and can only
be misconnected randomly to resemble the mismatchedcomponent. It has been found through simulation

10.9

BBC resolution

0.8

FVTX resolution

0.7
0.6
0.5
0.4

10

0.3
0.2
0.1

1

-0.6

-0.4

-0.2

00.2

0.4

0.6
DCA [cm]00102030

40506070 80
centralityFig. 1. (Left) Simulated DCA distribution of the mismatched BG component with SAR method. Red histogram shows mis-reconstruction with random hits;
blue shows mis-connection of diﬀerent tracks; blackshows the total. (Right) RP resolutions with BBC(black) and FVTX (red).

References
1) A. Adare et al., Phys. Rev. Lett. 98, 172301 (2007).
2) A. M. Poskanzer, S. A. Voloshin, Phys. Rev. C 58, 1671
(1998).

RIKEN Nishina Center
Department of Physics, Hiroshima University- 109 -