Measurement of quark- and gluon-like jet fractions using jet charge in PbPb and pp collisions at 5.02 TeV

Abstract

The momentum-weighted sum of the electric charges of particles inside a jet, known as jet charge, is sensitive to the electric charge of the particle initiating the parton shower. This paper presents jet charge distributions in √ s NN = 5.02 TeV lead-lead (PbPb) and proton-proton (pp) collisions recorded with the CMS detector at the LHC. These data correspond to integrated luminosities of 404 µb −1 and 27.4 pb −1 for PbPb and pp collisions, respectively. Leveraging the sensitivity of the jet charge to fundamental differences in the electric charges of quarks and gluons, the jet charge distributions from simulated events are used as templates to extract the quark-and gluon-like jet fractions from data. The modification of these jet fractions is examined by comparing pp and PbPb data as a function of the overlap of the colliding Pb nuclei (centrality). This measurement tests the color charge dependence of jet energy loss due to interactions with the quark-gluon plasma. No significant modification between different centrality classes and with respect to pp results is observed in the extracted quark-and gluon-like jet fractions.

Studying the QGP with Jets at the LHC and RHIC

“…Interestingly, like in the fragmentation function measurements from ATLAS above, a harder and narrower fragmentation is consistent with a more quark-like fragmentation and the results agree well with Pythia quark distributions as shown in [132]. It is however worth noting that the measurement of the jet charge [134] does not indicate a Does jet quenching depend on quark flavour and mass? At leading order in vacuum QCD, differences between quark and gluon fragmentation are dictated by color factors: the splitting rate is enhanced by the color factor and is 2.25 times higher for gluons than for quarks, leading to broader and softer parton showers.…”

“…At low-x, gluons dominate and toward the valence region there is a greater fraction of quarks. An attempt to measure the quark and gluon fractions in jets in Pb+Pb collisions has not found any significant difference from that measured in pp collisions [135], but such measurements have substantial systematic uncertainties and model dependence. Other techniques have been used to attempt to enhance the quark-jet fraction and to look at the effect on the jet quenching.…”

Heavy quarks and jets as probes of the QGP

“…has been measured recently by CMS in P bP b collisions [203]. This observable is, by construction, sensitive to the charge of the initiating jet particle, being initially used to measure the electric charge of the parton initiated-jet in DIS experiments [204,205].…”

Jets as a probe of the quark-gluon plasma

“…There has been recent work aiming to use Bayesian analysis to extract R AA separately for quark and gluon jets from experimental data [40] which found large differences in the quenching of quark-and gluon-initiated jets. A recent method for extracting quark and gluon fractions from data using P templates, on the hand, has suggested minimal differences between proton-proton and heavy-ion collisions [41]. A method for extracting quark and gluon fractions without using templates has recently been proposed [42] and applied to heavy-ion collisions [43] and may enable future data-driven extraction of quark and gluon jet modification.…”