Jet charge is an observable formed from the electric charges of tracks inside a reconstructed jet, typically weighted by transverse momentum to emphasize hard fragments. Jet charge provides a probabilistic handle on the net electric charge of the initiating parton and therefore on its flavor, a capability that has been demonstrated experimentally by the CMS Collaboration at CERN and by the ATLAS Collaboration at CERN. These measurements supply the empirical grounding needed to incorporate jet charge into modern flavor-tagging strategies.
How jet charge distinguishes flavors
A common estimator computes the weighted sum of track charges divided by the weighted sum of track transverse momenta, with a tunable exponent that controls sensitivity to soft radiation. Because up quarks and down quarks fragment into different charged-hadron spectra, the mean jet charge differs between those flavors. The effect is statistical rather than absolute; single-jet assignments remain probabilistic, but ensemble separation improves. For heavy flavors such as bottom quarks the situation is more complex because B meson decays and neutral B oscillations dilute the original parton charge, making fragmentation and decay modeling critical to interpretation. Precision measurements by CMS and ATLAS thus serve both to calibrate tagging algorithms and to test hadronization models in event generators.
Experimental and practical consequences
In practice, including jet charge in multivariate flavor-tagging increases discrimination power for processes where the quark charge matters, such as determining the charge of a hadronically decaying W boson, measuring top quark charge asymmetries, or separating signal from backgrounds in electroweak and Higgs analyses. The main limiting factors are detector resolution, tracking efficiency, pileup contamination, and modeling of the underlying event. These are addressed through detailed calibration studies and Monte Carlo tuning informed by the published results of the CMS Collaboration at CERN and the ATLAS Collaboration at CERN. Improvements in inner-tracker technologies and global computing resources distributed across the international collaborations also affect achievable performance, linking technical advances to broader scientific and societal investments in skills and infrastructure. Environmentally, the large-scale computing and detector operations that enable these measurements have nontrivial energy footprints, motivating efforts to optimize algorithms and infrastructures.
Validated jet-charge based discriminants therefore strengthen flavor-tagging, reduce systematic uncertainties in flavor-sensitive measurements, and enhance sensitivity to new physics signatures that manifest through quark-flavor or charge-dependent effects. Their utility depends critically on ongoing experimental calibration and on transparent modeling of nonperturbative fragmentation.