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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2025). A precise measurement of the jet energy scale derived from single-particle measurements and in situ techniques in proton-proton collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 85(9), 927–32pp.
Abstract: The jet energy calibration and its uncertainties are derived from measurements of the calorimeter response to single particles in both data and Monte Carlo simulation using proton-proton collisions at root s = 13 TeV collected with the ATLAS detector during Run 2 at the Large Hadron Collider. The jet calibration uncertainty for anti-k(T) jets with a jet radius parameter of R-jet = 0.4 and in the central jet rapidity region is about 2.5% for transverse momenta (p(T)) of 20 GeV, about 0.5% for p(T) = 300 GeV and 0.7% for p(T) = 4 TeV. Excellent agreement is found with earlier determinations obtained from p(T)-balance based in situ methods (Z/gamma+jets). The combination of these two independent methods results in the most precise jet energy measurement achieved so far with the ATLAS detector with a relative uncertainty of 0.3% at p(T) = 300 GeVand 0.6% at 4 TeV. The jet energy calibration is also derived with the single-particle calorimeter response measurements separately for quark- and gluon-induced jets and furthermore for jets with R-jet varying from 0.2 to 1.0 retaining the correlations between these measurements. Differences between inclusive jets and jets from boosted top-quark decays, with and without grooming the soft jet constituents, are also studied.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2025). Azimuthal anisotropies of charged particles with high transverse momentum in Pb plus Pb collisions at √sNN=5.02 TeV with the ATLAS detector. Physical Review C, 112(2), 024910–34pp.
Abstract: A measurement is presented of elliptic (v(2)) and triangular (v(3)) azimuthal anisotropy coefficients for charged particles produced in Pb+Pb collisions at root s(NN) = 5.02 TeV using a dataset corresponding to an integrated luminosity of 0.44 nb(-1) collected with the ATLAS detector at the LHC in 2018. The values of v(2) and v(3) are measured for charged particles over a wide range of transverse momentum (p(T)), 1-400 GeV, and Pb+Pb collision centrality, 0-60%, using the scalar-product and multiparticle cumulant methods. These methods are sensitive to event-by-event fluctuations and nonflow effects in the measurements of azimuthal anisotropies. Positive values of v(2) are observed up to a p(T) of approximately 100 GeV from both methods across all centrality intervals. Positive values of v(3) are observed up to approximately 25 GeV using both methods, though the application of the three-subevent technique to the multiparticle cumulant method leads to significant changes at the highest p(T). At high p(T) (p(T) (sic) 10 GeV), charged particles are dominantly from jet fragmentation. These jets, and hence the measurements presented here, are sensitive to the path-length dependence of parton energy loss in the quark-gluon plasma produced in Pb+Pb collisions.
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., Cantero, J., et al. (2025). Search for new physics in final states with semivisible jets or anomalous signatures using the ATLAS detector. Physical Review D, 112(1), 012021–31pp.
Abstract: A search is presented for hadronic signatures of beyond the Standard Model (BSM) physics, with an emphasis on signatures of a strongly coupled hidden dark sector accessed via resonant production of a Z ' mediator. The ATLAS experiment dataset collected at the Large Hadron Collider from 2015 to 2018 is used, consisting of proton-proton collisions at root s = 13 TeV and corresponding to an integrated luminosity of 140 fb(-1). The Z ' mediator is considered to decay to two dark quarks, which each hadronize and decay to showers containing both dark and Standard Model particles, producing a topology of interacting and noninteracting particles within a jet known as “semivisible.” Machine learning methods are used to select these dark showers and reject the dominant background of mismeasured multijet events, including an anomaly detection approach to preserve broad sensitivity to a variety of BSM topologies. A resonance search is performed by fitting the transverse mass spectrum based on a functional form background estimation. No significant excess over the expected background is observed. Results are presented as limits on the production cross section of semivisible jet signals, parametrized by the fraction of invisible particles in the decay and the Z ' mass, and by quantifying the significance of any generic Gaussian-shaped mass peak in the anomaly region.
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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. (2025). The performance of missing transverse momentum reconstruction and its significance with the ATLAS detector using 140 fb-1 of √s=13 TeV pp collisions. Eur. Phys. J. C, 85(6), 606–45pp.
Abstract: This paper presents the reconstruction of missing transverse momentum (pTmiss) in proton-proton collisions, at a center-of-mass energy of 13 TeV. This is a challenging task involving many detector inputs, combining fully calibrated electrons, muons, photons, hadronically decaying tau-leptons, hadronic jets, and soft activity from remaining tracks. Possible double counting of momentum is avoided by applying a signal ambiguity resolution procedure which rejects detector inputs that have already been used. Several pTmiss 'working points' are defined with varying stringency of selections, the tightest improving the resolution at high pile-up by up to 39% compared to the loosest. The pTmiss performance is evaluated using data and Monte Carlo simulation, with an emphasis on understanding the impact of pile-up, primarily using events consistent with leptonic Z decays. The studies use 140 fb(-1) of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that pTmiss reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft pTmiss component are calculated. After various improvements the scale and resolution uncertainties are reduced by up to 76% and 51%, respectively, compared to the previous calculation at a lower luminosity.
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Bordes, J., Chan, H. M., & Tsou, S. T. (2025). A review of the FSM whereby three old problems of the SM and one beyond appear soluble or resolved. Int. J. Mod. Phys. A, .
Abstract: Constructed initially to offer [A] a solution to the generation problem, the FSM turns out to offer also [B] a solution to the strong CP problem, and, via a “hidden sector”, necessitated by the framing of color, more populous and interacting little with our own, [C] the hope of a solution to the dark matter problem as well. In doing so, it gives rise to two new predictions: [CPS] that the definition of CP should depend on scale, and [VTR1] that there could be a vacuum transition at a scale of around 17MeV, both likely to have wide repercussions. So far, [CPS] has led to (a) KM phase in the CKM (PMNS) matrix for quarks (leptons) consistent with experiment, (b) a baryon to photon ratio eta in baryogenesis of size comparable to that observed. And [VTR1], though affecting mainly the “hidden” or dark sector where little is yet explored, has implications nevertheless on several anomalies recently reported by experiment, and, when supplemented by Kaluza-Klein, suggests [D] a possible new solution to the horizon problem in cosmology.
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