Measurement of inclusive and leading subjet fragmentation in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

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.

 

Submitted to: JHEP
e-Print: arXiv:2204.10270 | PDF | inSPIRE
Figure group

Figure 1

Cartoon of a subjet of radius $r$ inside a jet of radius $R$. We consider charged-particle subjets clustered using the anti-\kT{} algorithm from the constituents of inclusive charged-particle jets.

Figure 2

ALICE measurements of inclusive subjet \zr{} distribution in \pp{} collisions for two different subjet radii, compared to PYTHIA8~.

Figure 3

ALICE measurements of leading subjet \zr{} distribution in \pp{} collisions for two different subjet radii, compared to PYTHIA8~.

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.

Figure 5

Measurements of subjet \zr{} distributions for subjet radius $r=0.1$ in \pp{} and 0--10\% central \PbPb{} collisions The bottom panel displays the ratio of the distributions in \PbPb{} to \pp{} collisions, along with comparison to theoretical predictions~.

Figure 6

Measurements of subjet \zr{} distributions for subjet radius $r=0.2$ in \pp{} and 0--10\% central \PbPb{} collisions. The bottom panel displays the ratio of the distributions in \PbPb{} to \pp{} collisions, along with comparison to theoretical predictions~.

Figure 7

Measurements of subjet \zr{} distributions for subjet radii $r=0.1$ and $r=0.2$ in 0--10\% central \PbPb{} collisions The bottom panel displays the ratio of the distributions for $r=0.1$ to $r=0.2$, along with comparison to JEWEL and JETSCAPE model predictions~.