Lund multiplicity in QCD jets

TL;DR

This paper calculates the average Lund multiplicity of high-energy QCD jets at the LHC with high precision, reducing theoretical uncertainties and including non-perturbative effects, to enable robust comparisons with experimental data.

Contribution

It extends previous calculations of multiplicity to LHC energies with NNDL accuracy, incorporating fixed-order, resummation, and non-perturbative effects for the first time.

Findings

NNDL accuracy reduces theoretical uncertainty by 50%.

Non-perturbative corrections are below 5% at a few GeV scales.

Lund multiplicity is suitable for precise theory-data comparisons at the LHC.

Abstract

We compute the average Lund multiplicity of high-energy QCD jets. This extends an earlier calculation, done for event-wide multiplicity in $e^+e^-$ collisions [arxiv:2205.02861], to the large energy range available at the LHC. Our calculation achieves next-to-next-to-double logarithmic (NNDL) accuracy. Our results are split into a universal collinear piece, common to the $e^+e^-$ calculation, and a non-universal large-angle contribution. The latter amounts to 10-15% of the total multiplicity. We provide accurate LHC predictions by matching our resummed calculation to fixed-order NLO results and by incorporating non-perturbative corrections via Monte Carlo simulations. Including NNDL terms leads to a 50% reduction of the theoretical uncertainty, with non-perturbative corrections remaining below 5% down to transverse momentum scales of a few GeV. This proves the suitability of Lund multiplicities for robust theory-to-data comparisons at the LHC.