Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Study of medium properties with two particle correlations in d+Au√
collisions at sN N = 200 GeV at PHENIX
C.-H. Chen ∗1
Two-particle correlation is a powerful method tostudy jet-medium interaction and the collective motionof particles. Interesting new results are revealed byLHC data when p + p collisions with two-particle correlations are studied. Upon observing low-multiplicity
p + p collisions at 7 TeV, the ∆η-∆φ correlation function is, as expected, found to have a single nearside
peak at ∆η ≈ 0 and ∆η ≈ 0 and an away-side peakat ∆φ ≈ π along ∆η. For high-multiplicity p + p collisions at the same energy, an enhancement along ∆η at∆φ ≈ 0, or a “ridge” structure, is observed 1) . Finally,
p+Pb collisions at 5.02 TeV with similar multiplicity
selection, exhibit ridge structure as well 2) .
This long-range correlation along the ∆η directionat ∆φ ≈ 0 has been observed at RHIC previously.
In two-particle ∆η-∆φ correlations in central Au+Au
collisions, an enhancement along ∆η at ∆φ ≈ 0 has
been observed 3) . It has also been found that thislong-range correlation extends to as far as ∆η ≈ 44). Similar phenomena has been confirmed in Pb+Pb
collisions at LHC 5) .
This long-range correlation along ∆η, or “ridge”,
was originally believed to exist only in central Au+Au
collisions, but now has also been observed in p + p andp+Pb collisions in LHC. The fact that the ridge appears in both system leads to the question of whether
the ridge observed in p + p and p+Pb in LHC is thesame as that seen in heavy-ion collisions at RHIC.
Triggered by the new results from LHC, it is important to investigate whether a similar effect exists
in d+Au collisions at RHIC. Studying d+Au collisions
will certainly provide new insights into the p+Pb dataat LHC. First, d+Au is collided at 200 GeV, which isconsiderably smaller than p+Pb at 5.02 TeV at LHC.
Further, in d+Au collisions, the two nucleons in thedeuteron may make the initial colliding geometry more
complicated than in p+Pb collisions.
At PHENIX, it is possible to measure the two particle correlations with a large η gap by correlatinga charged hadron in the central arm spectrometer(|η| < 0.35) and the energy cluster in the Muon Piston
Calorimeter (MPC, 3.1 < |η| < 3.9). A large ∆η separation can strongly suppress the non-flow contribution,and thus the remaining correlation should reflect theproperties of the produced medium.
Since d+Au is an asymmetric system, in central d+Au collision, the multiplicity distribution, ordN/dη, is asymmetric along the direction of η 6) , where∗1Fig. 1. The unidentified charged hadron in the central arm
correlated with energy clusters in MPC in the Au-goingdirection (−3.9 < η < −3.1) in d+Au and p + p collisions.

the multiplicity is larger in the Au-going direction thanin the d-going direction. Therefore a comparison of thecorrelation in d+Au to p + p, might reveal some newproperties in d+Au collisions.
Figure 1 depicts the correlation function of thecharged hadron in mid-rapidity correlated with the energy cluster in MPC in the Au-going direction in themost central d+Au collisions (0-5%) for various hadronpT . This is compared with the same correlation function measured in p+ p collisions. In p+ p collisions, thecorrelation function has a local minimum at ∆φ ≈ 0.
In the case of d+Au correlation functions, the nearsideshape is significantly different from the shape in p + p.
Instead of showing a local minimum, it is either peakedat ∆φ ≈ 0, or there is a strong correlation at ∆φ ≈ 0.
We further measure the Fourier coefficients of thecorrelation functions. In p+p, the correlation functions
are well described by c1 , which could be understood as
conservation of momentum with very little contribution from other harmonics. In central d+Au collisions,
we observe a significant contribution not only from c1 ,but also c2 . This indicates that in central d+Au collisions, something similar to elliptic flow in heavy ion
collisions has been seen.
References
1) V. Khachatryan et al., J. High Energy Phys. 09 (2010)091
2) S. Chatrchyan et al., Phys. Lett. B 718 (2013) 795
3) B. Abelev et al., Phys. Rev. C 80 (2009) 064912
4) B. Alver et al., Phys. Rev. Lett. 104 (2010) 062301
5) S. Chatrchyan et al., J. High Energy Phys. 07 (2011)766) B. Beck et al., Phys. Rev. C 72 (2005) 031901

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