AION/Magis

Abstract:

The Lorentz-contracted electromagnetic fields of the ions in ultra-relativistic heavy-ion collisions generate intense quasi-real photon fluxes. These lead to photon-induced interactions that are observed in ultra-peripheral collisions (UPCs), such as vector meson and lepton-pair production. The high photon flux also enables the occurrence of multiple photon-induced processes in a single collision. Presented is the first measurement of the coincident production of γ γ → μ + μ − and γ + A → ρ 0 + A in UPC Pb+Pb collisions at centre-of-mass energies of 5.02 TeV and 5.36 TeV with the ATLAS detector at the Large Hadron Collider. The coincident ρ 0 is identified via its decay into a π + π − pair, with the selected π + π − sample also containing the non-resonant continuum contribution. The rate of the coincident process relative to the exclusive γ γ → μ + μ − process is measured differentially in intervals of forward event activity, quantified by the Zero Degree Calorimeters. The relative rate of coincident ρ 0 production, including non-resonant π + π − contributions, is measured to be ( 9.3 ± 0.4 ( stat . ) ± 0.2 ( syst . ) ) × 10 − 3 , summed over forward event activity. Correlations between the dimuon kinematic properties, such as its mass, and the coincident ρ 0 meson production rate, are also presented. These measurements confirm the presence of multi photon-induced processes in UPC collisions, and can provide new insight into the impact parameter dependence of photon-induced vector meson production.

Abstract:

A search for quantum black holes in electron+jet or muon+jet final states with high invariant mass is performed. The analysis uses data from s = 13.6 TeV pp collisions recorded by the ATLAS detector between 2022 and 2024 during Run 3 of the Large Hadron Collider, corresponding to an integrated luminosity of 164 fb − 1 . This search is strongly motivated by a dramatic increase of the production cross-section by up to an order of magnitude for the highest masses considered, thanks to the small increase of 0.6 TeV in centre-of-mass energy between Run 2 and Run 3. No significant excess above the Standard Model background is observed, and 95% CL upper limits are set on the production cross-section times branching ratio in several benchmark models, reaching a mass scale of 9.4 TeV. These represent the strongest exclusion limits to date on quantum black hole production.

Abstract:

This Letter presents the first evidence of the simultaneous production of two $Z$ bosons and one photon with the ATLAS detector at the Large Hadron Collider. The measurement is performed using the full Run-2 dataset, recorded from 2015 to 2018, of proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ . The fully leptonic final state with four leptons and one photon is analyzed, $pp\to ZZ\gamma \to {\ell }^{+}{\ell }^{-}{\ell }^{\prime +}{\ell }^{\prime -}\gamma$ with $\ell ,{\ell }^{\prime }=e$ or $\mu$ . This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of $p_{\mathrm{T}}^{\gamma }>20\mathrm{GeV}$ . Eight events are selected, with a background estimate of $0.92\pm 0.15$ . This results in an observed (expected) significance of the $ZZ\gamma$ final state of $4.4\sigma$ ( $4.0\sigma$ ). The measured cross section for $pp\to ZZ\gamma \to {\ell }^{+}{\ell }^{-}{\ell }^{\prime +}{\ell }^{\prime -}\gamma$ in the fiducial region is ${\sigma }_{ZZ\gamma }=0.144_{-0.051}^{+0.065}{(\mathrm{stat})}_{-0.004}^{+0.008}(\mathrm{syst})\mathrm{fb}$ , in agreement with the standard model prediction of ${\sigma }_{\mathrm{fid}}^{\mathrm{SM}}=0.130\pm 0.006\mathrm{fb}$ .