Abstract
The measurement of four-particle cumulant and anisotropic elliptic flow coefficient for the second harmonic, c2{4} and v2{4}, are presented using pp data at s=5.02 and s=13 TeV and p+Pb data at sNN=5.02 TeV. The values of c2{4} are calculated using the standard cumulant method and recently proposed two and three subevent methods. The three subevent method gives a negative c2{4}, and thus a well-defined v2{4} (which equals (−c2{4})1/4), in these collision systems. v2{4} is found to be smaller than v2 measured using the two-particle correlation method (denoted as v2{2}), as expected for a collective behavior. Following a recent model framework, the measured values of v2{4} and v2{2} are used to probe the number of sources in the initial state collision geometry.
Highlights
The study of azimuthal correlations in high energy nuclear collisions at the RHIC and the LHC has been an important avenue of understanding the multi-parton dynamics of QCD in the strongly-coupled nonperturbative region
The standard multi-particle cumulant method reduces the non-flow correlations, it is important to study how effective the suppression of non-flow is in this method compared with the new subevent method. c2{4} is calculated in each event class defined by NcShel
For most of the Nch range, the c2{4} values with three subevent method are always negative, as expected for collectivity. This is because three subevent cumulant suppresses the short-range non-flow contributions and dijet correlation, so that the residual non-flow effects are smaller than the one using the standard cumulant method
Summary
The study of azimuthal correlations in high energy nuclear collisions at the RHIC and the LHC has been an important avenue of understanding the multi-parton dynamics of QCD in the strongly-coupled nonperturbative region. The long-range ridge in large systems such as central or mid-central A+A collisions [1], is commonly interpreted as the result of collective hydrodynamic expansion of hot and dense nuclear matter created in the overlap region. Since the formation of an extended nuclear matter is not necessarily expected in small collision systems such as p+A [2] and pp [3], the origin of the ridge could be different from that in large collision systems. There remains a considerable debate in the theoretical community on whether the ridge in small systems is hydrodynamic origin similar to A+A collisions [4] or is induced by gluon saturation in the initial state of the collision [5]. An improved cumulant method based on the correlation between particles from different subevents separated in η has been proposed to further reduce to non-flow correlations, especially in small systems [7]
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