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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for high-mass dilepton resonances using 139 fb(-1) of pp collision data collected at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 796, 68–87.
Abstract: A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-ofmass energy of root s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb(-1). A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E-6-motivated Z(psi)' boson. Also presented are limits on Heavy Vector Triplet model couplings.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for low-mass resonances decaying into two jets and produced in association with a photon using pp collisions root s=13 TeV with the ATLAS detector. Phys. Lett. B, 795, 56–75.
Abstract: A search is performed for localised excesses in dijet mass distributions of low-dijet-mass events produced in association with a high transverse energy photon. The search uses up to 79.8 fb(-1) of LHC proton-proton collisions collected by the ATLAS experiment at a centre-of-mass energy of 13 TeV during 2015-2017. Two variants are presented: one which makes no jet flavour requirements and one which requires both jets to be tagged as b-jets. The observed mass distributions are consistent with multi-jet processes in the Standard Model. The data are used to set upper limits on the production cross-section for a benchmark Z' model and, separately, on generic Gaussian-shape contributions to the mass distributions, extending the current ATLAS constraints on dijet resonances to the mass range between 225 and 1100 GeV.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Search for New Phenomena in Two-Body Invariant Mass Distributions Using Unsupervised Machine Learning for Anomaly Detection at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 132(8), 081801–23pp.
Abstract: Searches for new resonances are performed using an unsupervised anomaly-detection technique. Events with at least one electron or muon are selected from 140 fb-1 of pp collisions at p ffi s ffi= 13 TeV recorded by ATLAS at the Large Hadron Collider. The approach involves training an autoencoder on data, and subsequently defining anomalous regions based on the reconstruction loss of the decoder. Studies focus on nine invariant mass spectra that contain pairs of objects consisting of one light jet or b jet and either one lepton (e; mu), photon, or second light jet or b jet in the anomalous regions. No significant deviations from the background hypotheses are observed. Limits on contributions from generic Gaussian signals with various widths of the resonance mass are obtained for nine invariant masses in the anomalous regions.
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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. (2024). Observation of WZγ Production in pp Collisions at √s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 132(2), 021802–21pp.
Abstract: This Letter reports the observation of WZ gamma production and a measurement of its cross section using 140.1 +/- 1.2 fb(-1) of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the bosons decaying leptonically, pp -> WZ gamma -> l'(perpendicular to)nu l(+) l(-)gamma (l(')(+) = e, mu), is measured in a fiducial phasespace region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross section is found to be 2.01 +/- 0.30(stat) +/- 0.16(syst) fb. The corresponding standard model predicted cross section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is 1.50 +/- 0.06 fb. The observed significance of the WZ gamma signal is 6.3 sigma, compared with an expected significance of 5.0 sigma.
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ATLAS and CMS Collaborations(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Evidence for the Higgs Boson Decay to a Z Boson and a Photon at the LHC. Phys. Rev. Lett., 132(2), 021803–32pp.
Abstract: The first evidence for the Higgs boson decay to a Z boson and a photon is presented, with a statistical significance of 3.4 standard deviations. The result is derived from a combined analysis of the searches performed by the ATLAS and CMS Collaborations with proton -proton collision datasets collected at the CERN Large Hadron Collider (LHC) from 2015 to 2018. These correspond to integrated luminosities of around 140 fb-1 for each experiment, at a center -of -mass energy of 13 TeV. The measured signal yield is 2.2 +/- 0.7 times the standard model prediction, and agrees with the theoretical expectation within 1.9 standard deviations.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Measurement of the Centrality Dependence of the Dijet Yield in p plus Pb Collisions at √sNN=8.16 TeV with the ATLAS Detector. Phys. Rev. Lett., 132(10), 102301–22pp.
Abstract: ATLAS measured the centrality dependence of the dijet yield using 165 nb-1 of p + Pb data collected at root sNN = 8.16 TeV in 2016. The event centrality, which reflects the p + Pb impact parameter, is characterized by the total transverse energy registered in the Pb-going side of the forward calorimeter. The central-to-peripheral ratio of the scaled dijet yields, RCP, is evaluated, and the results are presented as a function of variables that reflect the kinematics of the initial hard parton scattering process. The RCP shows a scaling with the Bjorken x of the parton originating from the proton, xp, while no such trend is observed as a function of xPb. This analysis provides unique input to understanding the role of small proton spatial configurations in p + Pb collisions by covering parton momentum fractions from the valence region down to xp similar to 10-3 and xPb similar to 4 x 10-4.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Study of High-Transverse-Momentum Higgs Boson Production in Association with a Vector Boson in the qqbb Final State with the ATLAS Detector. Phys. Rev. Lett., 132(13), 131802–23pp.
Abstract: This Letter presents the first study of Higgs boson production in association with a vector boson (V = W or Z) in the fully hadronic qqbb final state using data recorded by the ATLAS detector at the LHC in ffiffiproton-proton collisions at root root s= 13 TeV and corresponding to an integrated luminosity of 137fb(-1). The vector bosons and Higgs bosons are each reconstructed as large-radius jets and tagged using jet substructure techniques. Dedicated tagging algorithms exploiting b-tagging properties are used to identify jets consistent with Higgs bosons decaying into b (b) over bar. Dominant backgrounds from multijet production are determined directly from the data, and a likelihood fit to the jet mass distribution of Higgs boson candidates is used to extract the number of signal events. The VH production cross section is measured inclusively and differentially in several ranges of Higgs boson transverse momentum: 250-450, 450-650, and greater than 650 GeV. The inclusive signal yield relative to the standard model expectation is observed to be μ= 1.4(-0.9)(+1.0) and the corresponding cross section is 3.1 +/- 1.3(stat)(-1.4)(+1.8) (syst) pb.
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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). Strong Constraints on Jet Quenching in Centrality-Dependent p plus Pb Collisions at 5.02 TeV from ATLAS. Phys. Rev. Lett., 131(7), 072301–21pp.
Abstract: Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including pp and p+ Pb collisions. In contrast, while jet quenching is observed in Pb + Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb-1 of p+ Pb and 3.6 pb-1 of pp collisions at 5.02 TeV. The yields of charged hadrons with p(T)(ch) > 0.5 GeV near and opposite in azimuth to jets with p(t)(je) T > 30 or 60 GeV, and the ratios of these yields between p+ Pb and pp collisions, IpPb, are reported. The collision centrality of p+ Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The IpPb values are consistent with unity within a few percent for hadrons with p(T )(ch)> 4 GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p+ Pb collisions.
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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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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Observation of Single-Top-Quark Production in Association with a Photon Using the ATLAS Detector. Phys. Rev. Lett., 131(18), 181901–22pp.
Abstract: This Letter reports the observation of single top quarks produced together with a photon, which directly probes the electroweak coupling of the top quark. The analysis uses 139 fb(-1) of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be 688 +/- 23(stat)(-71)(+75) (syst) fb, to be compared with the standard model prediction of 515(-42)(+36) fb at next-to-leading order in QCD.
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