• Energy Research

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.

AbstractCross-sections for the production of a Z boson in association with two photons are measured in proton–proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb$$^{-1}$$ - 1 recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated $$Z(\rightarrow \ell \ell )\gamma \gamma $$ Z ( → ℓ ℓ ) γ γ cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the $$Z\gamma \gamma $$ Z γ γ system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.

This paper presents a measurement of jet-track correlations in photon-jet events, using 1.72 nb$^{-1}$ of Pb+Pb data at $\sqrt{s_{\text{NN}}}$ = 5.02 TeV recorded with the ATLAS detector at the LHC. Events with energetic photon-jet pairs are selected, where the photon and jet are approximately back-to-back in azimuth. The angular correlation between jets and charged-particle tracks with transverse momentum ($p_\text{T}$) in the range 0.5-2.0 GeV in the hemisphere opposite to the jet, $|Δϕ\text{(jet,track)}|>π/2$, is measured as a function of their relative pseudorapidity difference, $|Δη\text{(jet,track)}|$. In central Pb+Pb collisions, these correlations are predicted to be sensitive to the diffusion wake in the quark-gluon plasma resulting from the lost energy of high-$p_\text{T}$ partons traversing the plasma, with a characteristic modification as a function of $|Δη\text{(jet,track)}|$. The correlations are examined with different selections on the jet-to-photon $p_\text{T}$ ratio to select events with different degrees of energy loss. No diffusion wake signal is observed within the current sensitivity and upper limits at 95% confidence level on the diffusion wake amplitude are reported. 36 pages in total, author list starting page 19, 8 figures, 0 tables, published in Phys. Rev. C. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2024-01/

Parton energy loss in the quark-gluon plasma (QGP) is studied with a measurement of photon-tagged jet production in 1.7 nb$^{-1}$ of Pb+Pb data and 260 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, with the ATLAS detector. The process $pp \to \gamma$+jet+$X$ and its analogue in Pb+Pb collisions is measured in events containing an isolated photon with transverse momentum ($p_\mathrm{T}$) above $50$ GeV and reported as a function of jet $p_\mathrm{T}$. This selection results in a sample of jets with a steeply falling $p_\mathrm{T}$ distribution that are mostly initiated by the showering of quarks. The $pp$ and Pb+Pb measurements are used to report the nuclear modification factor, $R_\mathrm{AA}$, and the fractional energy loss, $S_\mathrm{loss}$, for photon-tagged jets. In addition, the results are compared with the analogous ones for inclusive jets, which have a significantly smaller quark-initiated fraction. The $R_\mathrm{AA}$ and $S_\mathrm{loss}$ values are found to be significantly different between those for photon-tagged jets and inclusive jets, demonstrating that energy loss in the QGP is sensitive to the colour-charge of the initiating parton. The results are also compared with a variety of theoretical models of colour-charge-dependent energy loss. Physics letters / B 846, 138154 - (2023). doi:10.1016/j.physletb.2023.138154 Published by North-Holland Publ., Amsterdam