Fluctuations of various observables in heavy-ion collisions at ultra-relativistic energies have been extensively studied in literature as they provide important signals regarding the formation of the Quark Gluon Plasma. ‘Multiplicity’ denotes the number of produced particles after the collision. Multiplicity distribution and their fluctuations shed light into mechanism of particle production and provide constraints to many models aiming at describing these collisions. Multiplicity fluctuations are related to fundamental properties of the system such as isothermal compressibility, quark number susceptibility, etc. So, a detailed study of event-by-event multiplicity fluctuations, which has been possible due to large number of particle production in each event at ultra-relativistic energies, has been proposed as one of the basic signatures of a phase transition.
Moreover, multiplicity fluctuation is the basic fluctuation which affects fluctuations in many other important observables such as energy density, temperature, etc. By studying the charged particle multiplicity fluctuations in heavy-ion collisions as a function of varying number of participating nucleons or centrality it may be possible to obtain information on the nature of the QGP matter.
The observed fluctuations will have contributions from statistical fluctuations and those which have dynamical origin. The contribution from dynamical origin comprises of fluctuations, which do not change event-by-event, e.g., those from Bose-Einstein (BE) correlations, resonance decays and the fluctuations, which have new physics origin and may vary from event-to-event. We are interested mainly in extracting the dynamical fluctuation, minimizing the statistical fluctuation as much as possible.
Multiplicity fluctuation is a very important study especially in higher energies as at higher energies (especially LHC-energies) Bjorken-x is very low (~10-4)so we can have a connection to initial state collisions too.
Recently, the source of the multiplicity fluctuation has been extensively studied theoretically in the microscopic level. Within framework of the relativistic fluctuating hydrodynamics, entropy and in turn, event-by-event multiplicity fluctuation has been studied as the outcome of noises during hydrodynamic evolution of the quark-gluon fluid created in high-energy nuclear collisions, even if the initial state is the same in a macroscopic sense.
In my work, I have observed the centrality-dependence and beam-energy dependence (from RHIC to LHC energies,i.e, from 7.7 GeV/A to 2.76 TeV/A) of multiplicity fluctuation (measured by obtaining scaled variance, defined by variance scaled over mean, from the multiplicity distributions) using event generators HIJING, AMPT-Default and AMPT-String Melting to observe the trend and its change with respect to energy as well as centrality. I have used a different set of centrality selection by taking 1% ,2% and 5% cross section bins, such as 0-1%, 1-2%, 2-3% and, 0-2%, 2-4%, 4-6%, 6-8% and, 0-5%, 5-10%, 10-15% etc. Results for 2% and 5% centrality bins have been binwidth corrected to get rid of the impact parameter variations arising due to finite centrality bins. The parameters from multiplicity distributions are obtained by fitting the distributions and these parameters are connected to multiplicity fluctuation.
