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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). Test of CP Invariance in Higgs Boson Vector-Boson-Fusion Production Using the H → γγ Channel with the ATLAS Detector. Phys. Rev. Lett., 131(6), 061802–23pp.
Abstract: A test of CP invariance in Higgs boson production via vector-boson fusion has been performed in the H -> gamma gamma channel using 139 fb(-1) of proton-proton collision data at root s = 13 TeV collected by the ATLAS detector at the LHC. The optimal observable method is used to probe the CP structure of interactions between the Higgs boson and electroweak gauge bosons, as described by an effective field theory. No sign of CP violation is observed in the data. Constraints are set on the parameters describing the strength of the CP-odd component in the coupling between the Higgs boson and the electroweak gauge bosons in two effective field theory bases: (d) over tilde in the HISZ basis and c(H (W) over tilde) in the Warsaw basis. The results presented are the most stringent constraints on CP violation in the coupling between Higgs and weak bosons. The 95% C.L. constraint on (d) over tilde is derived for the first time and the 95% C.L. constraint on c(H (W) over tilde) has been improved by a factor of 5 compared to the previous measurement.
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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. (2023). Combined Measurement of the Higgs Boson Mass from the H → γγ and H → ZZ* → 4l Decay Channels with the ATLAS Detector Using √s=7, 8, and 13 TeV pp Collision Data. Phys. Rev. Lett., 131(25), 251802–21pp.
Abstract: A measurement of the mass of the Higgs boson combining the H -> ZZ* -> 4l and H -> gamma gamma decay channels is presented. The result is based on 140 fb(-1) of proton-proton collision data collected by the ATLAS detector during LHC run 2 at a center-of-mass energy of 13 TeV combined with the run 1 ATLAS mass measurement, performed at center-of-mass energies of 7 and 8 TeV, yielding a Higgs boson mass of 125.11 +/- 0.09(stat) +/- 0.06(syst) = 125.11 +/- 0.11 GeV. This corresponds to a 0.09% precision achieved on this fundamental parameter of the Standard Model of particle physics.
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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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Kogler, R., Nachman, B., Schmidt, A., Asquith, L., Winkels, E., Campanelli, M., et al. (2019). Jet substructure at the Large Hadron Collider. Rev. Mod. Phys., 91(4), 045003–44pp.
Abstract: Jet substructure has emerged to play a central role at the Large Hadron Collider, where it has provided numerous innovative ways to search for new physics and to probe the standard model, particularly in extreme regions of phase space. This review focuses on the development and use of state-of-the-art jet substructure techniques by the ATLAS and CMS experiments.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2012). A Particle Consistent with the Higgs Boson Observed with the ATLAS Detector at the Large Hadron Collider. Science, 338(6114), 1576–1582.
Abstract: Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga-electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself.
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Andricek, L. et al, Lacasta, C., Marinas, C., & Vos, M. (2011). Intrinsic resolutions of DEPFET detector prototypes measured at beam tests. Nucl. Instrum. Methods Phys. Res. A, 638(1), 24–32.
Abstract: The paper is based on the data of the 2009 DEPFET beam test at CERN SPS. The beam test used beams of pions and electrons with energies between 40 and 120 GeV, and the sensors tested were prototypes with thickness of 450 μm and pixel pitch between 20 and 32 μm. Intrinsic resolutions of the detectors are calculated by disentangling the contributions of measurement errors and multiple scattering in tracking residuals. Properties of the intrinsic resolution estimates and factors that influence them are discussed. For the DEPFET detectors in the beam test, the calculation yields intrinsic resolutions of approximate to 1 μm, with a typical accuracy of 0.1 μm. Bias scan, angle scan, and energy scan are used as example studies to show that the intrinsic resolutions are a useful tool in studies of detector properties. With sufficiently precise telescopes, detailed resolution maps can be constructed and used to study and optimize detector performance.
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