  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.
