RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-4. Hadron Physics

Measurements of J/ψ mesons from B meson decays with thePHENIX-VTX detector
T. Hachiya ∗1 for the PHENIX collaboration
Heavy quarks, bottoms and charms, are a usefulprobe to study the properties of quark gluon plasma(QGP) created in high-energy heavy-ion collisions.
Due to their large masses (Mb ≈ 4.2 GeV/c2 andMc ≈ 1.3 GeV/c2 ), heavy quarks are dominantly produced via initial hard scattering between partons in theincoming nuclei. Since none of the heavy quarks canbe created during space-time evolution of the QGP,
the number of heavy quarks is ﬁxed at the beginningof the collisions. However, the ﬁnal state interactionsof heavy quarks in the QGP such as energy loss andcollective ﬂow can modify the momentum and angular
distribution of the heavy quarks.
The PHENIX experiment recently measured single
electron yields from bottom and charm quark decays√
separately in Au+Au collisions at sN N = 200 GeV1)
using the silicon vertex detector (VTX). VTX consistsof four layers of silicon sensors covering |η| < 1.2 andalmost 2π in azimuth. VTX provides a precise displaced tracking to measure the decay length of heavy
ﬂavor hadrons. We observed that bottom quarks wereless suppressed than charm quarks at 3 < pT < 4GeV/c and were similarly suppressed at high pT .
J/ψ produced from B meson decay is a direct channel to measure the bottom production, and it is especially good for low pT . All J/ψ are promptly produced at the collision vertex except for J/ψ from B
decay, called non-prompt J/ψ, because of the decay
length (cτB 0 ≈ 455μm). Therefore, we can separate
non-prompt and prompt J/ψ by measuring their decay lengths.
The analysis is performed using data from p + p and√Au+Au collisions at sN N = 200 GeV recorded in2015 and 2014. J/ψ is reconstructed by an unlikesign di-electron pair. Electrons are identiﬁed using the ring-imaging Čerenkov detector and energymomentum matching, where the energies and momenta are measured by the electromegnetic calorimeter and the drift chamber at the PHENIX central arm
spectrometer, respectively. Electron tracks are associated with hits in VTX to remove the background tracks
caused by photon conversions and Kaon decays. Figure 1 shows the invariant mass distributions of electron
pairs in p + p collisions. The red and blue histograms
correspond to the unlike- and like-sign pairs, respectively. The like-sign distribution represents the combinatorial background qualitatively. In the distribution,
the J/ψ signal is clearly seen and there is almost no
background around the J/ψ mass region.
We then reconstruct the secondary vertex position∗1of electron pair from the J/ψ decay and the collisionvertex point using VTX. To identify non-prompt J/ψ,
we calculate the pseudo proper time, X, which is deﬁned as follows:
Lxy · MJ/ψ.
(1)
X=pT J/ψ
Here, Lxy is the length between the secondary vertex
and the collision vertex projected in the direction of
the momentum vector of J/ψ, and MJ/ψ and pT J/ψ
are the measured mass and pT of the electron pairs,
respectively. The kinematic relation of these variables
is described in Fig. 2 (left). The pseudo proper timeof J/ψ is an approximation of the decay length, cτ ,of B meson since the mass of J/ψ is close to thatof the B meson. Figure 2 (right) shows the pseudo
proper time distributions of simulated non-prompt andprompt J/ψ samples using PYTHIA and PHENIX detector simulation. These distributions are normalized
to match their peaks in order to demonstrate the difference in the shape between non-prompt and prompt
J/ψ. The non-prompt J/ψ has a longer tail on the positive side, which represents B meson decays far fromthe collision vertex, while the prompt J/ψ shows thesymmetric shape caused by the detector resolution.
We are measuring the pseudo proper time of J/ψ using data from both p + p and Au+Au collisions. Theseresults will be published soon.

10310210100.5

1

1.5

2

2.5

3

3.5

4

4.5
5Mee (GeV/c2)

Fig. 1. Invariant mass of unlike sign(red) and likesign(blue) electron pairs.
Non−prompt J/ψ → ee-+e-

3

B → J/ψ-/BL

Collision
VertexDCA

prompt J/ ψ10102e+
SecondaryVertex

Lxy101-0.3-0.2-0.1

00.1
0.2
0.3
Pseudo proper time (cm)Fig. 2. (Left) Decay kinematics of non-prompt J/ψ.
(Right) Pseudo proper time distribution of simulated
non-prompt (blue) and prompt(red) J/ψ.

Reference1) A. Adare et al., Phys. Rev. C 93, 034904 (2016).

RIKEN Nishina Center- 114 -