Measurement of the nuclear modification factor of $b$-jets in 5.02 TeV Pb+Pb collisions with the ATLAS detector

TL;DR

This paper measures how $b$-jets are suppressed in lead-lead collisions at 5.02 TeV, revealing mass and color charge effects in parton energy loss through comparison with theoretical models.

Contribution

It provides the first detailed measurement of the nuclear modification factor of $b$-jets at 5.02 TeV, highlighting differences from inclusive jets and informing energy loss mechanisms.

Findings

$b$-jet suppression decreases from peripheral to central collisions.

$b$-jets have a larger $R_{AA}$ than inclusive jets in central collisions.

Results support mass and color charge effects in partonic energy loss.

Abstract

This paper presents a measurement of $b$-jet production in Pb+Pb and $pp$ collisions at $\sqrt{s_{_\textrm{NN}}}=5.02$ TeV with the ATLAS detector at the LHC. The measurement uses 260 $\textrm{pb}^{-1}$ of $pp$ collisions collected in 2017 and 1.4 $\textrm{nb}^{-1}$ of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-$k_{t}$ algorithm. The $b$-jets are identified from a sample of jets containing muons from the semileptonic decay of $b$-quarks using template fits of the muon momentum relative to the jet axis. In $pp$ collisions, $b$-jets are reconstructed for radius parameters $R= 0.2$ and $R= 0.4$, and only $R= 0.2$ jets are used in Pb+Pb collisions. For comparison, inclusive $R= 0.2$ jets are also measured using 1.7 $\textrm{nb}^{-1}$ of Pb+Pb collisions collected in 2018 and the same $pp$ collision data as the $b$-jet measurement. The nuclear modification factor, $R_\textrm{AA}$, is calculated for both $b$-jets and inclusive jets with $R= 0.2$ over the transverse momentum range of 80--290 GeV. The nuclear modification factor for $b$-jets decreases from peripheral to central collisions. The ratio of the $b$-jet $R_\textrm{AA}$ to inclusive jet $R_\textrm{AA}$ is also presented and suggests that the $R_\textrm{AA}$ for $b$-jets is larger than that for inclusive jets in central Pb+Pb collisions. The measurements are compared with theoretical calculations and suggest a role for mass and colour-charge effects in partonic energy loss in heavy-ion collisions.