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Dai, L. R., Abreu, L. M., Feijoo, A., & Oset, E. (2023). The isospin and compositeness of the Tcc(3875) state. Eur. Phys. J. C, 83(10), 983–11pp.
Abstract: We perform a fit to the LHCb data on the T-cc(3875) state in order to determine its nature. We use a general framework that allows to have the (DD & lowast;+)-D-0, (D+D & lowast;0) components forming a molecular state, as well as a possible nonmolecular state or contributions from missing coupled channels. From the fits to the data we conclude that the state observed is clearly of molecular nature from the (DD & lowast;+)-D-0, (D+D & lowast;0) components and the possible contribution of a nonmolecular state or missing channels is smaller than 3%, compatible with zero. We also determine that the state has isospin I=0 with a minor isospin breaking from the different masses of the channels involved, and the probabilities of the (DD & lowast;+)-D-0, (D+D & lowast;0) channels are of the order of 69% and 29% with uncertainties of 1%. The differences between these probabilities should not be interpreted as a measure of the isospin violation. Due to the short range of the strong interaction where the isospin is manifested, the isospin nature is provided by the couplings of the state found to the (DD & lowast;+)-D-0, (D+D & lowast;0) components, and our results for these couplings indicate that we have an I=0 state with a very small isospin breaking. We also find that the potential obtained provides a repulsive interaction in I=1, preventing the formation of an I=1 state, in agreement with what is observed in the experiment.
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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). Search for top-philic heavy resonances in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 84(2), 157–32pp.
Abstract: Asearch for the associated production of a heavy resonance with a top-quark or a top-antitop-quark pair, and decaying into a t (t) over bar pair is presented. The search uses the vdata recorded by the ATLAS detector in pp collisions at root s = 13 TeV at the Large Hadron Collider during the years 2015-2018, corresponding to an integrated luminosity of 139 fb(-1). Events containing exactly one electron ormuon are selected. The two hadronically decaying top quarks from the resonance decay are reconstructed using jets clustered with a large radius parameter of R = 1. The invariant mass spectrum of the two top quark candidates is used to search for a resonance signal in the range of 1.0 TeV to 3.2 TeV. The presence of a signal is examined using an approach with minimal model dependence followed by a model-dependent interpretation. No significant excess is observed over the background expectation. Upper limits on the production cross section times branching ratio at 95% confidence level are provided for a heavy Z' boson based on a simplified model, for Z' mass between 1.0 TeV and 3.0 TeV. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters.
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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). New techniques for jet calibration with the ATLAS detector. Eur. Phys. J. C, 83(8), 761–41pp.
Abstract: A determination of the jet energy scale is presented using proton-proton collision data with a centre-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 140 fb(-1) collected using the ATLAS detector at the LHC. Jets are reconstructed using the ATLAS particle-flow method that combines charged-particle tracks and topo-clusters formed from energy deposits in the calorimeter cells. The anti-kt jet algorithm with radius parameter R = 0.4 is used to define the jet. Novel jet energy scale calibration strategies developed for the LHC Run 2 are reported that lay the foundation for the jet calibration in Run 3. Jets are calibrated with a series of simulation-based corrections, including state-of-the-art techniques in jet calibration such as machine learning methods and novel in situ calibrations to achieve better performance than the baseline calibration derived using up to 81 fb(-1) of Run 2 data. The performance of these new techniques is then examined in the in situ measurements by exploiting the transverse momentum balance between a jet and a reference object. The b-quark jet energy scale using particle flow jets is measured for the first time with around 1% precision using gamma+jet events.
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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). Observation of four-top-quark production in the multilepton final state with the ATLAS detector. Eur. Phys. J. C, 83(6), 496–35pp.
Abstract: This paper presents the observation of four-topquark (t (t) over bart (t) over bar) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb-1 at a centre-of-mass energy of 13 TeVcollected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of themeasured t (t) over bart (t) over bar signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The t (t) over bart (t) over bar production cross section is measured to be 22.5(-5.5)(+6.6), consistent with the SM prediction of 12.0 +/- 2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect t (t) over bart (t) over bar production.
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Accettura, C. et al, & Zurita, J. (2023). Towards a muon collider. Eur. Phys. J. C, 83(9), 864–110pp.
Abstract: A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.
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