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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb-1 of ATLAS √s=13 TeV proton-proton collision data. Eur. Phys. J. C, 83(9), 781–33pp.
Abstract: Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the mu(+)mu(-) gamma final state with a proton-proton collision data sample corresponding to an integrated luminosity of 139 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/psi gamma, psi(2S)gamma, and Upsilon(1S, 2S, 3S)gamma are found to be 2.0 x 10(-4), 10.5x10(-4), and (2.5, 4.2, 3.4) x10(-4), respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 x 10(-6), 2.4 x 10(-6), and (1.1, 1.3, 2.4) x10(-6). An observed 95% CL interval of (-133, 175) is obtained for the kappa(c)/kappa(gamma) ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for kappa(b)/kappa(gamma).
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Exclusive dielectron production in ultraperipheral Pb+Pb collisions at √s_NN=5.02 TeV with ATLAS. J. High Energy Phys., 06(6), 182–42pp.
Abstract: Exclusive production of dielectron pairs, gamma gamma -> e(+) e(-), is studied using L-int = 1.72 nb(-1) of data from ultraperipheral collisions of lead nuclei at root s(NN) = 5.02TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon-photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum p(T)(e) > 2.5 GeV, absolute electron pseudorapidity |eta(e)| < 2.5, dielectron invariant mass m(ee) > 5 GeV, and dielectron transverse momentum p(T)(ee) < 2 GeV. Differential cross-sections are measured as a function of mee, average peT, absolute dielectron rapidity |y(ee)|, and scattering angle in the dielectron rest frame, | cos theta* |, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215 +/- 1(stat.) (+23)(-20)(syst.) +/- 4(lumi.) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high |y(ee)| and | cos theta* | values.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2023). Measurement of the production of a W boson in association with a charmed hadron in pp collisions at root s=13 TeV with the ATLAS detector . Phys. Rev. D, 108(3), 032012–54pp.
Abstract: The production of a W boson in association with a single charm quark is studied using 140 fb(-1) of vS = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. ffiffis The charm quark is tagged by the presence of a charmed hadron reconstructed with a secondary-vertex fit. The W boson is reconstructed from the decay to either an electron or a muon and the missing transverse momentum present in the event. The charmed mesons reconstructed are D+ ?K-p+p+ and D*+ ? D0p+ ? (K-p+)p+ and the charge conjugate decays in the fiducial regions where pT(e; mu) > 30 GeV, l?(e; mu)l < 2.5, pT(D(*)) > 8 GeV, and l?(D(*))l < 2.2. The integrated and normalized differential cross sections as a function of the pseudorapidity of the lepton from the W boson decay, and of the transverse momentum of the charmed hadron, are extracted from the data using a profile likelihood fit. The measured total fiducial cross sections are sfidOS-SS(W- + D+) = 50.2 + 0.2(stat)+2.4 -2.3(syst) pb, s(OS-SS) (fid)(W- + D+) = 48.5 + 0.2(stat)+2.3-2.2(syst) pb, sfidOS-SS(W- + D*+) = 51.1 + 0.4(stat)+1.9 -1.8 (syst) pb, and s(OS-SS) (fid)(W+ + D*-) = 50.0 + 0.4(stat)+1.9 -1.8 (syst) pb. Results are compared with the predictions of next-to-leading-order quantum chromodynamics calculations performed using state-of-the-art parton distribution functions. Additionally, the ratio of charm to anticharm production cross sections is studied to probe the s -s- quark asymmetry. The ratio is found to be R+ c = 0.971 + 0.006(stat) + 0.011(syst). The ratio and cross-section measurements are consistent with the predictions obtained with parton distribution function sets that have a symmetric s -s- sea, indicating that any s -s- asymmetry in the Bjorken-x region relevant for this measurement is small.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of Z γ γ production in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(6), 539–30pp.
Abstract: Cross-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) 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(-> ll)gamma gamma 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 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.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in root s=13 TeV pp Collisions with the ATLAS Detector. Phys. Rev. Lett., 131(6), 061803–23pp.
Abstract: A search for a long-lived, heavy neutral lepton (N) in 139 fb(-1) of vs = 13 TeV pp collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W ?N μor W -Ne and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV < m(N) < 15 GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.
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