Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 49 (2016)√Long-range correlation of V0 particles in p-Pb collisions at
5.02 TeV with the ALICE detector

sN N =Y. Sekiguchi,∗1,∗2 H. Hamagaki,∗2 and T. Gunji∗2∗1∗2RIKEN Nishina Center
Center for Nuclear Study, the University of Tokyo- 110 -tion of the subtracted correlation functions onto ∆φ.
To quantify azimuthal anisotropy (v2 ), the Fourier coefficients are extracted by fitting with the functiona0 + 2a1 cos(∆φ) + 2a2 cos(2∆φ). The vn coefficientK0 ,ΛK0 ,Λ
can be obtained as vn s= Vn s / Vnh−h , whereVni = ain /(ai0 + bi ), in which i is the index of h-h orh-V0 pairs (h denotes undefined hadrons) and b is thebaseline determined by averaging over 1.2< |∆η| <1.6
on the near side of the 60-100% event class. Figure
2 shows the extracted v2 coefficient for K0s and Λ(Λ̄)
compared to p and K as a function of pT . Mass ordering between the v2 of K0s and Λ(Λ̄) as well as the kaonand proton is observed.
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12
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1234−1∆φ01234Fig. 1. Projection of the subtracted correlation functionsof the associated K0s (top) and Λ (bottom) yield per
trigger particle with 1.5<pT,trig , pT,asso <2.5 GeV.

v2 (PC sub)Measuring correlations in particle production as afunction of the azimuthal angular space and rapidity space is very useful for investigating particle production in high-energy nucleus-nucleus collisions. Thelong-range correlations in the rapidity space in nearside angular pairs of dihadron correlations were first√
observed in Au-Au collisions at sN N = 200 GeV atRHIC1,2) . This long-range correlations are derived
from the collective expansion of the initial geometry
fluctuations. Unexpectedly, a similar structure has
also been observed in high-multiplicity pp collisions√at sN N = 7 TeV by the CMS experiment3) . Itis very interesting to study the correlation in p-Pb
collisions because the initial gluon density and magnitude of the collective expansion are very different
from those in other collision systems. The azimuthal
anisotropy parameter v2 of K, π, and p shows massordering at low transverse momentum (pT ) and thetrend is similar to Pb-Pb collisions4) . The mass ordering is a characteristic feature of collective expansion. This analysis aims to further explore the partonic collectivity by extracting v2 of K0s and Λ in p√
Pb collisions at sNN = 5.02 TeV with the ALICE
detector. The correlations between the unidentified
charged hadrons as trigger particle and K0s and Λ(Λ̄)
as associated particles at |η| <0.8 are measured asa function of the azimuthal angle difference ∆φ andpseudo-rapidity difference ∆η. K0s and Λ decay into
π + + π − and p+ + π − with a characteristic decay pattern, called V0 . Topological cuts are required to reducethe combinatorial background. The correlation function as a function of ∆η and ∆φ between two chargedS(∆η,∆φ)
d2 Nasso
1
particles is defined as Ntrigd∆ηd∆φ = B(∆η,∆φ) , whereNtrig is the total number of triggered particles in theevent class and pT interval, the signal distribution
1 d2 NsameS (∆η, ∆φ) = Ntrigd∆ηd∆φ is the associated yield pertrigger particle from the same event, and the back2
Nmixedaccounts
ground distribution B(∆η, ∆φ) = α dd∆ηd∆φfor pair acceptance and pair efficiency. B is constructedby taking the correlations between the trigger particles in one event and the associated particles in otherevents in the same event class. The α factor is chosen
so that it is unity at ∆η ∼ 0 because the acceptanceis flat along ∆φ. This correlation function is studiedfor different pT intervals and different event classes.
The correlation function in peripheral collisions is subtracted from that in central collisions to remove theauto-correlations from jets. Figure 1 shows the projec-

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work in progress

0.25-K+(K )
0Ksp+(p )
Λ

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TFig. 2. v2 of K0s , Λ(Λ̄) compared with one of kaon andproton. Error bars and shaded bands show statisticaluncertainties and systematic, respectively.

References1) STAR Collaboration, Phys. Rev. C80 064912 (2009)2) PHOBOS Collaboration, Phys. Rev. Lett. 104062301(2010)3) CMS Collaboration, JHEP 09 091(2010)4) ALICE Collaboration, Phys. Lett. B, 726 164177(2013)∆φ