Multiplicity Fluctuations in the String Model in pp Collisions at LHC Energies

Abstract

The fluctuation of the number of charged particles in a given rapidity interval of observation was studied. Analytical expressions and the Monte Carlo modelling in the framework of a quark–gluon string model were used to calculate scaled and robust variance of the multiplicity in the case with string fusion and without it. The used distribution of primary strings in the transverse plane is consistent with the Regge approach. The string fusion effects were taken into account by implementing of a lattice (grid) in the impact parameter plane. The scaled and robust multiplicity variance for pp collisions is calculated at three initial energies: 0.9, 2.76 and 7 TeV, and it is shown that the results obtained with string fusion included are in better agreement with the experimental values of the ALICE collaboration at CERN than those without string fusion. It is also demonstrated that for pp collisions at LHC energies, fluctuations in the number of particles from a given string cluster, due to the presence of short-range correlations between particles, significantly exceed the Poisson ones.