Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)√Measurement of dielectron production in sN N = 5.02 TeV p-Pb
collisions at LHC-ALICE
S. Hayashi,∗1 H. Hamagaki,∗1 and T. Gunji∗1

1/Nevt dN/dMee [(GeV/c2)-1]

A clear enhancement of electron samples by more than
20 times can be observed above 3 GeV/c for the TRD
triggered data.

contaminants such as of kaons, protons and deuterons
up to 2 GeV/c. The hadron contaminations can be
reduced less than 1% up to 6 GeV/c by the TOF information. Figure 1 shows the single-electron spectrum.
∗1Center for Nuclear Study, Graduate School of Science, theUniversity of Tokyo

Minimum bias
TRD trigger-3

1010-4-5

10-6

10

00.5

11.5

2102

2.5

33.5
Mee [GeV/c2]

Minimum bias
TRD trigger

101-1

10-2

10

00.5

1

1.5

2

2.5

33.5
2
Mee [GeV/c ]Fig. 2. Upper panel: The dielectron spectrum after background subtraction. Lower panel: the signal to background ratio.

For the estimation of the dielectron background,like-sign pairs in the same-events technique are used.
Since the acceptance depends on the sign of charge,
acceptance diﬀerence between unlike-sign and like-signpairs is evaluated using mixed unlike-sign and mixed
like-sign pairs. Equation 1 is calculated, and the Rfactor is corrected to like-sign pairs to estimate thebackground.
R=Fig. 1. Single-electron pT distributions for the minimumbias trigger and the TRD single-electron trigger.

10-2

S/B

In high-energy heavy-ion collisions, heavy quarks areprimary produced during the initial hard scatteringand experience the entire evolution of system. Therefore, they are sensitive to the transport properties of
the hot and dense matter created in the collisons. In
paticular, the correlation of heavy quark pairs provides
key insight into the mechanisms of the energy loss andthermalization1) . This can be studied through themeasurement of dielectron production because correlated electron-positron pairs from semi-leptonic decaysof heavy quarks are the dominant source of dielectrons
above 1 GeV/c2 . The dielectron measurement aroundthe intermediate-mass region in p-Pb collisions reveals
cold-nuclear-matter eﬀects such as gluon shadowing
and gluon saturation on the heavy-quark production.
In the ALICE experiment, the Transition Radiation Detector (TRD) has a capability of the online
electron identiﬁcation and provides an electron trigger to enrich the data samples for the study of heavyﬂavor electron production. In 2013, ALICE successfully collected data in p-Pb collisions with the TRD
trigger (Lint = 1.4 nb−1 ) and the minimum bias trigger (Lint = 0.067 nb−1 ).
In the central barrel of the ALICE detector, charged
tracks are reconstructed with the Inner Tracking System (ITS) and the Time Projection Chamber (TPC)2) .
Electrons are identiﬁed using dE/dx obtained in theTPC and time-of-ﬂight measurement with the TOF detector (120ps resolution). TOF is essential to remove

N+− |mix + N−+ |mix
N++ |mix + N−− |mix

(1)

where N++ |mix , N−− |mix and N+− |mix , N−+ |mix arelike-sign pairs and unlike-sign pairs in mixed events,respectively.
√ The estimated background is deﬁned as
NCB = 2R N++ N−− , where N++ , N−− are like-signpairs in the same event.
Figure 2 shows the inclusive dielectron spectrum
after background subtraction, and the signal-tobackground ratio. The vector mesons and J/ψ peaks
can be seen clearly. The signal-to-background ratio
agrees with the result of the pp collisions3) . in diﬀerent pT bins.
References
1) X. Zhu et al., Phys. Rev. Lett 100, 152301 (2008)
2) F. Carminati et al., J. Phys. G: Nucl. Part. Phys. 30(2004) 1517 1763
3) Markus K. Koehler, arXiv:1302.2049 (2013)- 101 -