Event-by-Event Fluctuation Study in UrQMD Model in Heavy Ion Interactions

Event-by-event pseudo-rapidity fluctuations of pions produced in UrQMD-simulated 16O-AgBr (60 A GeV) and 32S-AgBr (200 A GeV) interactions has been analyzed using variable ϕ, a variable defined as a measure of fluctuation and it is sensitive to the correlated pion emission in A–A collision. Non-zero ϕ values indicate the presence of correlation among the pions and hence event-by-event fluctuation for both interactions in UrQMD model. Fluctuations, measured using the ϕ parameter, show clear dependence on rapidity zone width and average multiplicity. ϕ increases with zone width, indicating long range correlations, and decreases with multiplicity, suggesting dilution from multiple particle sources. Stronger fluctuations are observed in 32S-AgBr interactions. While UrQMD captures key trends, it underestimates fluctuation magnitudes compared to experimental data highlighting limitations in modeling ultra-relativistic nucleus-nucleus collisions. Further, UrQMD events show slightly higher correlation and fluctuation than FRITIOF generated events.