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-$k_{\rm T}$ algorithm with jet resolution parameters $R$ = 0.2, 0.3, and 0.4 for the transverse momentum ($p_{\rm T}$) 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 $p_{\rm T}$ in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation functions $z^{\rm ch}$ and $\xi^{\rm ch}$ are measured for different jet-$p_{\rm T}$ intervals. Jet-$p_{\rm T}$ independent fragmentation of leading jets is observed for wider jets except at high- and low-$z^{\rm ch}$. The observed "hump-backed plateau" structure in the $\xi^{\rm ch}$ distribution indicates suppression of low-$p_{\rm T}$ particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$z^{\rm ch}$ particles accompanied by a suppression of high-$z^{\rm ch}$ particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$p_{\rm T}$ 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 $p_{\rm T}$. These measurements provide important constraints to models of jet fragmentation.
Submitted to: EPJC
e-Print: arXiv:2311.13322 | PDF | inSPIRE
CERN-EP-2023-264
Figure group
Figure 4
Top: Ratios of PYTHIA 8 and EPOS LHC predictions to data for $z^{\rm ch}$ distributions in different $p_{\rm T}^{\rm jet,\,ch}$ intervals in MB events for jet $R$ = 0.2 (left), 0.3 (middle), and 0.4 (right). Bottom: Ratios of PYTHIA 8 predictions to data for $z^{\rm ch}$ distributions in different $p_{\rm T}^{\rm jet,\,ch}$ intervals in HM events for jet $R$ = 0.2 (left), 0.3 (middle), and 0.4 (right). |   |
Figure 6
Top panel: Distributions of $z^{\rm ch}$ for the jet-$\pt$ interval 10--20 GeV/$c$ for inclusive (quark- and gluon-initiated) jets with `MPI: ON, CR: ON' and `MPI: OFF, CR: OFF' configurations (left), and for gluon-initiated jets with `MPI: OFF, CR: OFF' configuration (right) using PYTHIA 8. Bottom panel: Ratio of $z^{\rm ch}$ distributions between HM and MB events. |  |
