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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). Search for nonresonant pair production of Higgs bosons in the b b-bar b b-bar final state in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(5), 052003–38pp.
Abstract: A search for nonresonant Higgs boson pair production in the b (b) over barb (b) over bar final state is presented. The analysis uses 126 fb(-1) of pp collision data at root s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider, and targets both the gluon-gluon fusion and vector-boson fusion production modes. No evidence of the signal is found and the observed (expected) upper limit on the cross section for nonresonant Higgs boson pair production is determined to be 5.4 (8.1) times the Standard Model predicted cross section at 95% confidence level. Constraints are placed on modifiers to the HHH and HHVV couplings. The observed (expected) 2 sigma constraints on the HHH coupling modifier, kappa(lambda), are determined to be [-3.5, 11.3] ([-5.4, 11.4]), while the corresponding constraints for the HHVV coupling modifier, kappa(2V), are [-0.0, 2.1] ([-0.1, 2.1]). In addition, constraints on relevant coefficients are derived in the context of the Standard Model effective field theory and Higgs effective field theory, and upper limits on the HH production cross section are placed in seven Higgs effective field theory benchmark scenarios.
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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). Measurement of the total cross section and ρ-parameter from elastic scattering in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(5), 441–49pp.
Abstract: In a special run of the LHC with beta star=2.5 km, proton-proton elastic-scattering events were recorded at root s=13 TeV with an integrated luminosity of 340 μb(-1) using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam t variable in the range from -t=2.5 center dot 10(-4) GeV2 to -t=0.46 GeV2 using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section sigma(tot), parameters of the nuclear slope, and the rho-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit t -> 0. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the t-dependence. The results for sigma(tot) and rho are sigma(tot) (PP -> X ) =104.7 +/- 1.1 mob , rho=0.098 +/- 0.011. The uncertainty in sigma(tot) is dominated by the luminosity measurement, and in rho by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.
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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). Measurement of substructure-dependent jet suppression in Pb plus Pb collisions at 5.02 TeV with the ATLAS detector. Phys. Rev. C, 107(5), 054909–32pp.
Abstract: The ATLAS detector at the Large Hadron Collider has been used to measure jet substructure modification and suppression in Pb+Pb collisions at a nucleon-nucleon center-of-mass energy root sNN = 5.02 TeV in comparison with proton-proton (pp) collisions at root s = 5.02 TeV. The Pb+Pb data, collected in 2018, have an integrated luminosity of 1.72 nb(-1), while the pp data, collected in 2017, have an integrated luminosity of 260 pb(-1). Jets used in this analysis are clustered using the anti-k(t) algorithm with a radius parameter R = 0.4. The jet constituents, defined by both tracking and calorimeter information, are used to determine the angular scale rg of the first hard splitting inside the jet by reclustering them using the Cambridge-Aachen algorithm and employing the soft-drop grooming technique. The nuclear modification factor, RAA, used to characterize jet suppression in Pb+Pb collisions, is presented differentially in rg, jet transverse momentum, and in intervals of collision centrality. The RAA value is observed to depend significantly on jet r(g). Jets produced with the largest measured r(g) are found to be twice as suppressed as those with the smallest rg in central Pb+Pb collisions. The RAA values do not exhibit a strong variation with jet p(T) in any of the rg intervals. The r(g) and p(T) dependence of jet RAA is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Escobar, C., et al. (2011). Search for quark contact interactions in dijet angular distributions in pp collisions at sqrt(s)=7 TeV measured with the ATLAS detector. Phys. Lett. B, 694(4-5), 327–345.
Abstract: Dijet angular distributions from the first LHC pp collisions at center-of-mass energy root s = 7 TeV have been measured with the ATLAS detector. The dataset used for this analysis represents an integrated luminosity of 3.1 pb(-1). Dijet chi distributions and centrality ratios have been measured up to dijet masses of 2.8 TeV, and found to be in good agreement with Standard Model predictions. Analysis of the chi distributions excludes quark contact interactions with a compositeness scale Lambda below 3.4 TeV, at 95% confidence level, significantly exceeding previous limits.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Escobar, C., et al. (2012). Measurement of the W -> tau nu(tau) cross section in pp collisions at root s=7 TeV with the ATLAS experiment. Phys. Lett. B, 706(4-5), 276–294.
Abstract: The cross section for the production of W bosons with subsequent decay W -> tau nu(tau) is measured with the ATLAS detector at the LHC. The analysis is based on a data sample that was recorded in 2010 at a proton-proton center-of-mass energy of root s = 7 TeV and corresponds to an integrated luminosity of 34 pb(-1). The cross section is measured in a region of high detector acceptance and then extrapolated to the full phase space. The product of the total W production cross section and the W -> tau nu(tau) branching ratio is measured to be sigma(tot)(W -> tau nu tau) = 11.1 +/- 0.3 (stat) +/- 1.7 (syst) +/- 0.4 (lumi) nb. (C) 2011 CERN.
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