This article presents new measurements of the fragmentation properties of jets in both proton-proton (pp) and heavy-ion collisions with the ALICE experiment at the Large Hadron Collider (LHC). We report distributions of the fraction $z_r$ of transverse momentum carried by subjets of radius $r$ within jets of radius $R$. Charged-particle jets are reconstructed at midrapidity using the anti-$k_{\rm{T}}$ algorithm with jet radius $R=0.4$, and subjets are reconstructed by reclustering the jet constituents using the anti-$k_{\rm{T}}$ algorithm with radii $r=0.1$ and $r=0.2$. In proton-proton collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the $z_r$ distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet fragmentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The $z_r$ distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark-gluon plasma (QGP). We find indications that there is a turnover in the ratio between the distributions in Pb-Pb and pp collisions as $z_r \rightarrow 1$, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.
Accepted by: JHEP
HEP Data
e-Print: arXiv:2204.10270 | PDF | inSPIRE
Figure group
Figure 4
ALICE measurements of inclusive (top) and leading (bottom) subjet zr distributions in pp collisions at √s = 5.02 TeV, compared to NLL′ predictions carried out with SCET [46, 47] and corrected for missing neutral-particle energy and multi-parton interaction effects using PYTHIA8 [62] or HERWIG7 [78]. The shaded bands denote systematic uncertainty on the NLL′ calculations. The distributions are normalized such that the integral of the region defined by 0.7 < zr < zNP r is unity, where zNP r is denoted by the dashed vertical blue lines. The non- perturbative scale in Eq. 4 is taken to be Λ = 1 GeV/c. In determining the normalization, bins that overlap with the dashed blue line are considered to be in the non-perturbative (right) region. |   |
