Hadron multiplicity fluctuations in perturbative QCD

TL;DR

Dokshitzer and Webber develop and test a perturbative QCD framework for hadron multiplicity fluctuations in hard processes by extending MDLA to arbitrary jet ensembles at a common hardness scale $Q$ and employing P-KNO scaling governed by the multiplicity anomalous dimension $\gamma(\alpha_s)$. They derive tail predictions for the P-KNO distribution $\Psi(\nu)$ parameterized by $\gamma$ and a source strength $\rho$, and validate them against $e^+e^-$ data and ATLAS high-$p_T$ jets, finding reasonable agreement with no free parameters after fixing $\rho_q=2/3$. The analysis identifies and discusses two high-multiplicity puzzles, notably tail flattening in ATLAS jets and CMS long-range correlations, and proposes the rattlesnake effect (RSE) as a common perturbative origin linking jet substructure to the observed phenomena. The work emphasizes that local parton-hadron duality (LPHD) remains compatible with observations, offers a coherent QCD explanation for rare high-multiplicity fluctuations, and suggests concrete experimental probes with jet radius and substructure to test the RSE scenario. Overall, the paper strengthens the perturbative description of multiplicity fluctuations in jets and highlights intriguing connections between jet substructure and collective-like signatures in high-energy hadronic collisions.

Abstract

We examine hadron multiplicity fluctuations in hard processes and confront analytic QCD predictions with the pattern of multiplicity fluctuations observed in $e^+e^-$ annihilation and high-$p_t$ jets produced in $pp$ collisions at the LHC. Special emphasis is placed on high-multiplicity fluctuations in jets. Selecting events with hadronic multiplicity exceeding the average value by a factor of 3 or more in various processes has been a source of conundrums for many years. We discuss two recent high-multiplicity puzzles and attempt to reveal their common origin.

Figures (9)

• Figure 5: Multiplicity fluctuations in ${e^+e^-}$ annihilation: checking $\rho_q$
• Figure 6: Hint at scaling violation in the P-KNO tail
• Figure 7: Comparison of multiplicity fluctuations in one hemisphere with full ${e^+e^-}$ event DELPHI.
• Figure 8: P-KNO distributions of separated quark and gluon jets at $1000<p_t<1200$ GeV.
• Figure 9: P-KNO distributions of unseparated jets at $100<p_t<800$ GeV.