Multiplicity dependence of charged-particle intra-jet properties in pp collisions at $\sqrt{s}$ = 13 TeV

Abstract

The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton-proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at $\sqrt{s}$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region ($|\eta |<0.9$) using the sequential recombination anti-$k_{\mathrm{T}}$ algorithm with jet resolution parameters $R=0.2$, 0.3, and 0.4 for the transverse momentum ($p_{\mathrm{T}}$) interval 5-110 GeV/$c$. High-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet $p_{\mathrm{T}}$ in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables $z^{\mathrm{ch}}$ and ${\xi }^{\mathrm{ch}}$ are measured for different jet-$p_{\mathrm{T}}$ intervals. Jet-$p_{\mathrm{T}}$ independent fragmentation of leading jets is observed for wider jets except at high- and low-$z^{\mathrm{ch}}$. The observed hump-backed plateau structure in the ${\xi }^{\mathrm{ch}}$ distribution indicates suppression of low-$p_{\mathrm{T}}$ particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$z^{\mathrm{ch}}$ particles accompanied by a suppression of high-$z^{\mathrm{ch}}$ particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$p_{\mathrm{T}}$ jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet $p_{\mathrm{T}}$. These measurements provide important constraints to models of jet fragmentation.

33 pages, 9 captioned figures, 4 tables, authors from page 27, published version, figures at http://alice-publications.web.cern.ch/node/10274

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