Multiplicities, fluctuations and QCD: Interplay between soft and hard physics?

TL;DR

This paper investigates multiplicity fluctuations in high-energy physics, comparing experimental data with QCD predictions and Monte Carlo models, revealing oscillatory behaviors and the limitations of certain theoretical approximations.

Contribution

It demonstrates the quasi-oscillatory behavior of multiplicity ratios and evaluates the ability of Monte Carlo models and QCD approximations to reproduce these fluctuations.

Findings

Oscillatory behavior similar to NNLLA predictions observed in data.

Monte Carlo models like JETSET reproduce fluctuations well.

First-order QCD predictions deviate significantly from experimental data.

Abstract

Multiplicity fluctuations are studied both globaly (in terms of high-order moments) and locally (in terms of small phase-space intervals). The ratio of cumulant factorial to factorial moments of the charged-particle multiplicity distribution shows a quasi-oscillatory behaviour similar to that predicted by the NNLLA of perturbative QCD. However, an analysis of the sub-jet multiplicity distribution at perturbative scales shows that these oscillations cannot be related to the NNLLA prediction. We investigate how it is possible to reproduce the oscillations within the framework of Monte-Carlo models. Furthermore, local multiplicity fluctuations in angular phase-space intervals are compared with Monte-Carlo models and with first-order QCD predictions. While JETSET reproduces the experimental data very well, the predictions of the Double Leading Log Approximations and estimates obtained in Modified Leading Log Approximations deviate significantly from the data.