• Energy Research

AbstractThis paper presents a measurement of b-jet production in Pb+Pb and pp collisions at $$\sqrt{s_{_\text {NN}}}=5.02$$ s NN = 5.02  TeV with the ATLAS detector at the LHC. The measurement uses 260 $$\text {pb}^{-1}$$ pb - 1 of pp collisions collected in 2017 and 1.4 $$\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018. In both collision systems, jets are reconstructed via the anti-$$k_{t}$$ 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 $$R= 0.2$$ R = 0.2 and $$R= 0.4$$ R = 0.4 , and only $$R= 0.2$$ R = 0.2 jets are used in Pb+Pb collisions. For comparison, inclusive $$R= 0.2$$ R = 0.2 jets are also measured using 1.7 $$\text {nb}^{-1}$$ nb - 1 of Pb+Pb collisions collected in 2018 and the same pp collision data as the b-jet measurement. The nuclear modification factor, $$R_\text {AA}$$ R AA , is calculated for both b-jets and inclusive jets with $$R= 0.2$$ 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 $$R_\text {AA}$$ R AA to inclusive jet $$R_\text {AA}$$ R AA is also presented and suggests that the $$R_\text {AA}$$ R 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.

This Letter presents the first study of the energy dependence of diboson polarization fractions in $WZ→ℓνℓ′ℓ′(ℓ,ℓ′=e,μ)$ production. The dataset used corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector. Two fiducial regions with an enhanced presence of events featuring two longitudinally polarized bosons are defined. A nonzero fraction of events with two longitudinally polarized bosons is measured with an observed significance of 5.3 standard deviations in the region with 100<$p_T^Z$≤200 GeV and 1.6 standard deviations in the region with $p_T^Z$>200 GeV, where $p_T^Z$ is the transverse momentum of the Z boson. This Letter also reports the first study of the radiation-amplitude-zero effect. Events with two transversely polarized bosons are analyzed for the $ΔY(ℓWZ)$ and $ΔY(WZ)$ distributions defined respectively as the rapidity difference between the lepton from the W boson decay and the Z boson and the rapidity difference between the W boson and the Z boson. Significant suppression of events near zero is observed in both distributions. Unfolded $ΔY(ℓWZ)$ and $ΔY(WZ)$ distributions are also measured and compared to theoretical predictions. Physical review letters 133, 101802 (2024). doi:10.1103/PhysRevLett.133.101802 Published by APS, College Park, Md.