Dr. Karolos Potamianos

Abstract:

This Letter presents a measurement of prompt charged-particle production in proton-oxygen interactions at $\sqrt{s_{\mathrm{NN}}}=9.62\mathrm{TeV}$ center-of-mass energy with the ATLAS detector, corresponding to $634\mu {\mathrm{b}}^{-1}$ of integrated luminosity. A total of $246\times {10}^6$ selected events have at least one track with transverse momentum $p_{\mathrm{T}}>500\mathrm{MeV}$ and pseudorapidity $|\eta |<2.5$ . The measured fiducial proton-oxygen cross section is ${\sigma }_{\mathrm{fid}.}^{p\mathrm{O}}=396\pm 6(\exp .)\pm 9(\mathrm{lumi}.)\mathrm{mb}$ and the extrapolated inelastic proton-air cross section is ${\sigma }_{\mathrm{inel}.}^{p+\mathrm{air}}=406\pm 6(\exp .)\pm 9(\mathrm{lumi}.)\pm 28(\mathrm{th}.)\mathrm{mb}$ . Measurements of charged-particle multiplicity, $p_{\mathrm{T}}$ , and $\eta$ distributions are an order-of-magnitude more precise than differences between hadronic-interaction models. These results enable improved modeling of cosmic-ray air showers, which is important for astroparticle physics.

Abstract:

Entanglement is a key property of quantum systems. In this Letter the first measurements of quantum entanglement between spins in pairs of $Z$ bosons are reported, using proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV, recorded with the ATLAS detector. Measurements of angular observables sensitive to $Z{Z}^{*}$ spin-density-matrix elements in the $H\to Z{Z}^{*}\to {\ell }^{+}{\ell }^{-}{\ell }^{+}{\ell }^{-}$ process yield coefficients $C_{2,1,2,-1}=-0.71\pm 0.45$ and $C_{2,2,2,-2}=0.08\pm 0.44$ , consistent with their Standard Model predictions. A complementary hypothesis test using the full angular distribution, and relying on several Standard Model assumptions in the decays, provides substantially higher sensitivity to quantum correlations and disfavors the separable-state hypothesis at a significance of 4.7 standard deviations (expected $4.9\sigma$ ) relative to the entangled Standard Model hypothesis. These results provide strong evidence of quantum entanglement between massive bosons (spin qutrits) at the electroweak scale.

Abstract:

A search is performed for the electroweak pair production of charginos and associated production of a chargino and neutralino, each of which decays through an $R$ -parity-violating coupling into a lepton and a $W$ , $Z$ , or Higgs boson. This search targets the Higgs boson decay channel of the charginos and neutralinos, using events with three or more $b$ -tagged jets and one or two electrons or muons. The analyzed data corresponds to an integrated luminosity of $140{\mathrm{fb}}^{-1}$ and $56{\mathrm{fb}}^{-1}$ of proton-proton collision data produced by the Large Hadron Collider at center-of-mass energies of $\sqrt{s}=13\mathrm{TeV}$ and $\sqrt{s}=13.6\mathrm{TeV}$ , respectively, collected by the ATLAS experiment between 2015 and 2023. The data are found to be consistent with predictions from the Standard Model. The results are interpreted as limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model. Limits are also set on the production of charginos and neutralinos for a minimal supersymmetric Standard Model with an additional $B-L$ gauge symmetry that is spontaneously broken. Charginos and neutralinos with masses between 150 GeV and 1100 GeV are excluded at 95% confidence level for a scenario in which they decay via Higgs bosons, assuming equal decay branching fractions to each lepton flavor. Additional limits on chargino and neutralino production are derived for flavor-specific decay scenarios where the charginos decay into a single lepton flavor.

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 ${\sigma }_{\mathrm{fid}}^{\mathrm{SM}}=0.130\pm 0.006\mathrm{fb}$ .