ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Study of the spin and parity of the Higgs boson in diboson decays with the ATLAS detector. Eur. Phys. J. C, 75(10), 476–36pp.
Abstract: Studies of the spin, parity and tensor couplings of the Higgs boson in the H -> ZZ* -> 4l, H -> WW* -> e nu μnu and H -> gamma gamma decay processes at the LHC are presented. The investigations are based on 25 fb(-1) of pp collision data collected by the ATLAS experiment at root s = 7 TeV and root s = 8 TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers J (P) = 0(+), is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9% confidence level. Using the H -> ZZ* -> 4l and H -> WW* -> e nu μnu. decays, the tensor structure of the interaction between the spin-0 boson and the SM vector bosons is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Two-particle Bose-Einstein correlations in pp collisions at root s=0.9 and 7 TeV measured with the ATLAS detector. Eur. Phys. J. C, 75(10), 466–25pp.
Abstract: The paper presents studies of Bose-EinsteinCorrelations (BEC) for pairs of like-sign charged particles measured in the kinematic range p(T) > 100 MeV and |eta| < 2.5 in proton-proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 μb(-1), 190 μb(-1) and 12.4 nb(-1) for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size parameter is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.
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Gago, A. M., Hernandez, P., Jones-Perez, J., Losada, M., & Moreno Briceño, A. (2015). Probing the Type I Seesaw mechanism with displaced vertices at the LHC. Eur. Phys. J. C, 75(10), 470–10pp.
Abstract: The observation of Higgs decays into heavy neutrinos would be strong evidence for new physics associated to neutrino masses. In this work we propose a search for such decays within the Type I Seesaw model in the few-GeV mass range via displaced vertices. Using 300 fb(-1) of integrated luminosity, at 13 TeV, we explore the region of parameter space where such decays are measurable. We show that, after imposing pseudorapidity cuts, there still exists a region where the number of events is larger than O(10). We also find that conventional triggers can greatly limit the sensitivity of our signal, so we display several relevant kinematical distributions which might aid in the optimization of a dedicated trigger selection.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider. Eur. Phys. J. C, 75(10), 510–48pp.
Abstract: This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton-proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb(-1) of collisions at a centre-of-mass energy of root s = 8 TeV, although in some case an additional 4.7 fb(-1) of collision data at root s = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.
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Amjad, M. S., Bilokin, S., Boronat, M., Doublet, P., Frisson, T., Garcia Garcia, I., et al. (2015). A precise characterisation of the top quark electro-weak vertices at the ILC. Eur. Phys. J. C, 75(10), 512–11pp.
Abstract: Top quark production in the process e(+)e(-) -> t t at a future linear electron positron collider with polarised beams is a powerful tool to determine indirectly the scale of new physics. The presented study, based on a detailed simulation of the ILD detector concept, assumes a centre-of-mass energy of root s = 500GeV and a luminosity of L = 500 fb(-1) equally shared between the incoming beam polarisations of Pe-, Pe+ = +/- 0.8, -/+ 0.3. Events are selected in which the top pair decays semi-leptonically and the cross sections and the forward-backward asymmetries are determined. Based on these results, the vector, axial vector and tensorial CP conserving couplings are extracted separately for the photon and the Z(0) component. With the expected precision, a large number of models in which the top quark acts as a messenger to new physics can be distinguished with many standard deviations. This will dramatically improve expectations from e.g. the LHC for electro-weak couplings of the top quark.
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