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Conference: Bucharest University Faculty of Physics 2017 Meeting

Section: Nuclear and Elementary Particles Physics

Title:

Two-particle correlations from pp collisions at 7 TeV measured with ALICE at LHC

Authors:

Madalina TARZILA(1, 2)

^*
Affiliation:

1) Hadron Physics Department, Horia Hulubei National Institute of Physics and Nuclear Engineering, Magurele

2) Faculty of Physics, University of Bucharest

E-mail

mtarzila@niham.nipne.ro

Keywords:

two-particle correlations, double ridge, event shape observables

Abstract:

Two-particle correlation analysis is frequently used in a broad range of studies in hadron collisions from proton-proton to heavy ions. One of the most intriguing feature of these kind of correlations is the so-called long-range ridge-like structures, quantified by the Fourier parameter v2, one on the jet side - near side (∆φ≈0) and one on the away side ∆φ≈π. The ridge at small ∆φ and large ∆η was evidenced for the first time in Au–Au collisions at √sNN=200GeV at RHIC by the STAR experiment[1, 2] and confirmed later by PHOBOS[3]. The standard explanation of such ridges in heavy-ion collisions is associated with collective flow phenomena, characteristic for bulk macroscopic systems [4, 5]. By selecting high multiplicity events in pp collisions at √s=7TeV, the CMS Collaboration evidenced similar “long range rapidity correlation” [6]. All three major experiments at LHC, ATLS, ALICE and CMS [7–9] observed the same correlation in p–Pb collisions at √sNN=5.02TeV. By subtracting the correlations in low-multiplicity events from the high-multiplicity correlations, a symmetric ridge at ∆φ≈π is revealed, which is reminiscent of the ridges attributed to flow in heavy ion collisions but their origin is still under discussion. While up to the LHC energies the pp collision was considered important as baseline for heavy ion collisions studies, recent collective type phenomena and deconfinment signatures evidenced in such collisions and their understanding seems to be the only way to understand in detail the processes taking place in heavy ion collisions. When studying collective effects in pp collisions one has to be aware that jets can mimic the flow when the information is extracted from azimuthal distribution based on Fourier decomposition method. Events containing jets can be removed in some extent with the help of event shape observables such as directivity and sphericity. Results on two-particle correlation studies as a function of charged particle multiplicity and event shape selection based on directivity and sphericity for pp collision at 7 TeV data taken by ALICE experiment at LHC will be presented.

References:

[1] J. Adams et al., (STAR Collaboration),Phys.Rev.Lett., vol. 95, p. 152301, 2005.

[2] F. Wang, (STAR Collaboration),Int.J.Mod.Phys., vol. E16, pp. 3168–3175, 2007.

[3] B. Alver et al., (PHOBOS Collaboration),Phys.Rev.Lett., vol. 104, p. 062301, 2010.

[4] E. Shuryak, Phys.Rev., vol. C76, p. 047901, 2007.

[5] S. A. Voloshin, Phys.Lett., vol. B632, pp. 490–494, 2006.

[6] V. Khachatryan et al., (CMS Collaboration), JHEP, vol. 1009, p. 091, 2010.

[7] S. Chatrchyan et al., (CMS Collaboration), Phys.Lett., vol. B718, pp. 795–814, 2013.

[8] B. Abelev et al., (ALICE Collaboration), Phys.Lett., vol. B719, pp. 29–41,2013.

[9] G. Aad et al., (ATLAS Collaboration), Phys.Rev.Lett., vol. 110, p. 182302, 2013.

Acknowledgement:

This work was carried out within the 44/05.10.2011 and 2/03.01.2012 projects sponsored by Ministry of Research and Inovation via CNCSI and IFA coordinating agencies. The data analysis was done within AliAna group of HPD, special thanks go to Prof. Dr. Mihai Petrovici, Dr. Amalia Pop, Dr. Alexandru Bercuci, Dr. Andrei Cristian and Dr. Andrei Herghelegiu.