Multiplicity and transverse momentum evolution of charge-dependent correlations in pp, p–Pb, and Pb–Pb collisions at the LHC

Abstract

We report on two-particle charge-dependent correlations in pp, p–Pb, and Pb–Pb collisions as a function of the pseudorapidity and azimuthal angle difference, and respectively. These correlations are studied using the balance function that probes the charge creation time and the development of collectivity in the produced system. The dependence of the balance function on the event multiplicity as well as on the trigger and associated particle transverse momentum () in pp, p–Pb, and Pb–Pb collisions at 7, 5.02, and 2.76 TeV, respectively, are presented. In the low transverse momentum region, for GeV/c, the balance function becomes narrower in both and directions in all three systems for events with higher multiplicity. The experimental findings favor models that either incorporate some collective behavior (e.g. AMPT) or different mechanisms that lead to effects that resemble collective behavior (e.g. PYTHIA8 with color reconnection). For higher values of transverse momenta the balance function becomes even narrower but exhibits no multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low is a feature of bulk particle production.

Two-particle differential transverse momentum and number density correlations in p−Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV at the CERN Large Hadron Collider

“…The narrowing of R (CD) 2 observed with increasing produced-particle multiplicity in Figs. 8 and 9 is qualitatively similar to the narrowing of the balance function (BF) reported by the ALICE collaboration [62,63]. A quantitative comparison of the widths obtained from R (CD) 2 correlations and those already reported for the BF is presented in Sec.…”

“…The STAR experiment has measured balance functions in Au-Au, d-Au, and pp collisions at √ s NN = 130 and 200 GeV [58][59][60][61]. More recently, the ALICE collaboration reported observations of a narrowing of the balance function with increasing produced charged-particle multiplicity ( [62,63]. Measurements in Au-Au and Pb-Pb are in qualitative agreement with the scenario, proposed by Pratt et al [36,37,57], of two-stage quark production in high-energy central heavy-ion collisions but observations of a narrowing of the balance function with increasing N ch in p-Pb and pp put this simple interpretation into question.…”

Rapidity correlation structure in nuclear collisions

“…Rapidity correlations provide the space-time information that allows us to probe the onset of hydrodynamic behavior in collisions. Two particle correlation measurements show a near-side peak that sits atop a flat ridge in relative rapidity; see, e.g., [8][9][10][11][12][13][14][15][16]. This result affirms the long-standing principle that longitudinal expansion roughly follows a one-dimensional Hubble-like behavior [17][18][19].…”

Experimental overview on small collision systems at the LHC

“…Alternatively, however, microscopic effects such as the Color Reconnection (CR) mechanism of strings implemented in PYTHIA can also qualitatively explain the data [157]. PYTHIA with CR describes the increase of the average p T with multiplicity [158] and the multiplicity dependence of charge-dependent correlations (balance function) [159] quite well. Measured particle ratios increase with increasing multiplicity in the same way as in pPb collisions [48].…”