Multiplicity dependence of two-particle correlation in $\sqrt{s}$ = 7 TeV pp collisions at LHC-ALICE experiment

Early stage of universe or inside of neutron stars are supposed to be Quark Gluon Plasma (QGP) state. The QGP is a state of matter in quantum chromodynamics (QCD), which exists at extremely high temperature and/or high density. In high energy nuclear collisions experiment, hot dense matter or QGP has been studied, the collective flow of the system has been one of key issues to understand the state of matter. Large Hadron Collider (LHC) has served pp collisions at a nucleon–nucleon center-of-mass energy of 7 TeV in 2010, where the maximum charged particle multiplicity has been measured as large as 100$\sim$200 charged particles in $|\eta|<$2.5 in $dN/d\eta$ like Cu-Cu collisions $\sqrt {s_{NN}} = 200$ GeV at mid-peripheral at RHIC. A Large Ion Collider Experiment (ALICE) detector at the LHC is optimized for studying the high-temperature and high-density system called as QGP. Angular correlations between two charged particles are measured with central and forward detectors in ALICE experiment. The correlations are measured over the combination of central- and forward pseudorapidity with the coverage of full azimuthal angle in different intervals of event multiplicity and transverse momentum. In order to study the hot dense matter created in high multiplicity pp collisions, this analysis focuses on anisotropic expansion. The extension of the elongated structure along $\Delta\eta$ axis at $\Delta\varphi\approx$0 in intermediate transverse momentum ranges, which is called ``Ridge structure'' is observated in high multiplicity. The multiplicity dependence of azimuthal angle correlation functions is quantified with the second Fourier coefficients such as the elliptic flow parameter $v_{2}$, that is about 7.4$\%$ in the transverse momentum of $1-4$ GeV/c. Jet-like correlation shapes are also enhanced with increasing multiplicity selections both experimental result and PYTHIA simulation. Moreover, the double ridges, which have similar magnitude between near side and away side enhancement in $\Delta\varphi\approx0$ and $\pi$, are observed, when the associate particle yield per trigger in low multiplicity event is subtracted from the one in high multiplicity events. The multiplicity dependence of particle yields per trigger is summarized with the integrated yield in $|\Delta\varphi|<0.8$ and the integrated yield increases with increasing multiplicity selections. The observed structure both ridge and double ridge have been found to increase continuously with the event multiplicity and cannot be explained by the existing Monte-Carlo event generator such as PYTHIA. Ridge structure had been observed in high multiplicity pp collisions, which imply the formation of hot dense matter like heavy ion collisions and p-Pb collisions. The extraction of elliptic parameter coefficients could serve as an estimate of the amount of collective flow in pp collisions. Since the observed ridge structure in high multiplicity pp collisions could come from collective phenomena like heavy ion collisions.