Light-quark and Gluon Jets: Calorimeter Response, Jet Energy Scale Systematics and Properties

The calorimeter response to jets depends on how the jet fragments. Differences in the fragmentation of jets initiated by light quarks and gluons give rise to differences in their jet energy scale. Monte Carlo simulations are used to establish a sample-dependent jet energy scale systematic uncertainty. The difference in response is largely caused by differences in observable properties of light-quark-initiated and gluon-initiated jets, including the jet transverse width and the number of charged hadrons associated to the jet. These properties can also be exploited to discriminate between light-quark and gluon jets. Jets produced in proton-proton collisions at a center-of-mass-energy of $\sqrt{s} =$ 7 TeV and measured with the ATLAS detector are used to establish the accuracy of the description of such properties. The 2011 dataset, with an integrated luminosity of 4.7 fb$^{-1}$ is used. The studies are performed on inclusive and photon-jet samples. They show agreement between data and Monte Carlo simulation for the jet width but not for the number of charged hadrons in gluon jets. These results are verified using enriched samples of gluon and light-quark jets. The differences between data and Monte Carlo simulation suggest that the charged hadron multiplicity is less powerful in discriminating between light-quark and gluon jets than expected.