Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Long-range correlation in p-Pb collisions atthe ALICE detector√
sN N =5.02 TeV withY. Sekiguchi,∗1,∗2 H. Hamagaki,∗2 and T. Gunji∗2
Measuring of the correlations in particle productionin the azimuthal angular space and rapidity space isvery useful for investigating the underlying mechanismand dynamics of particle production in high-energy
nucleus-nucleus collisions. The long-range correlationsin the rapidity space in near-side angular pairs in dihadron correlations was ﬁrst observed in Au-Au col√lisions at sN N = 200 GeV at RHIC.1,2) This longrange correlation was derived from the collective expansion of the initial geometry ﬂuctuations. Unexpectedly, a similar structure has also√ been observedin high-multiplicity p-p collisions at s=7 TeV with
the LHC-CMS experiment.3) The high-density gluon
ﬁelds in a small x of nucleus and the collision of two
high-density gluon sheets can explain the long-rangecorrelation.4) It is very interesting to study the correlation in p-Pb collisions since the initial gluon density
and magnitude of the collective expansion are very different from those in other collision systems (pp andPb-Pb).
A Large Ion Collider Experiment (ALICE) is dedicated to understand the state of matter as it existed
shortly after the Big Bang, called Quark Gluon Plasma(QGP). The main subsystems in the ALICE for thestudy are the inner tracking system (ITS), time projection chamber (TPC), and time of ﬂight (TOF). The
ITS consists of 6 layers of silicon detectors for vertex
ﬁnding and tracking. The TPC is the main tracking
detector and is used for particle identiﬁcation by measuring the speciﬁc energy loss dE/dx. The TOF is used
to identify particles by measuring the time of ﬂight.
They have a common acceptance |η| <0.9.
The correlation function as a function of ∆η and ∆ϕ
between two charged particles is deﬁned as:
d2 NassoS(∆η, ∆ϕ)=
Ntrig d∆ηd∆ϕB(∆η, ∆ϕ)
1in the same event multiplicity class. The α factor ischosen so that it is unity for pairs at (∆η, ∆ϕ) = (0, 0).
The correlation function in peripheral collisions (lowmultiplicity event class) is subtracted from that in central collisions (high-multiplicity event class) to remove
the auto-correlations from jets. Figure 1 shows theassociated yield per trigger particle in 0-20% eventclass subtracted by 60-100% event class with 1<pT,trig ,pT,asso <2 GeV. The projection onto ∆ϕ averaged over0.8< |∆η| <1.6 on the near side and |η| <1.6 on theaway side is shown in Fig. 2. The double ridge structure is observed.
Measurements of the correlation functions by tagging identiﬁed hadrons and strange baryons are ongoing to evaluate the collectivity in p-Pb collisions andthe initial state eﬀects.

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20∆φFig. 1. Associated yield per trigger partile in 0-20% eventclass subtracted by 60-100% event class with 1<pT,trig ,pT,asso <2 GeV

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where this correlation function is studied for diﬀerent pT intervals and diﬀerent event classes according to the event multiplicity, and Ntrig is the total
number of triggered particles in the event class andpT,trig interval. The signal distribution S(∆η, ∆ϕ) =1/Ntrig d2 Nsame /d∆ηd∆ϕ is the associated yield pertrigger particle for particle pairs from the sameevent. The background distribution B(∆η, ∆ϕ) =α1/Ntrig d2 Nmixed /d∆ηd∆ϕ corrects for pair acceptance, pair eﬃciency, and uncorrelated pairs. It isconstructed by correlating the trigger particle in one
event with the associated particles from other events∗1∗2RIKEN Nishina CenterCenter for Nuclear Study, the University of Tokyo

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-1-10011223344Δ∆φφFig. 2. Projection onto ∆η averaged over 0.8< |η| <1.6 on
the near side and |η| < 1.6 on the away sideReferences
1) STAR Collaboration, Phys. Rev. C80 (2009) 064912
2) PHOBOS Collaboration, Phys. Rev. Lett. 104 (2010)
062301
3) CMS Collaboration, JHEP 09 (2010) 091
4) K. Duslingand and R. Venugopalan, Phys, Rev, D 87
(2013) 054014- 119 -

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