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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., et al. (2018). Observation of the decay Lambda(0)(b) -> Lambda(+)(c)p(p)over-bar pi(-). Phys. Lett. B, 784, 101–111.
Abstract: The decay Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-) is observed using pp collision data collected with the LHCb detector at centre-of-mass energies of root s = 7 and 8 Tev, corresponding to an integrated luminosity of 3 fb(-1). The ratio of branching fractions between Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-) and Lambda(0)(b) -> Lambda(+)(c)pi(-) decays is measured to be B(Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-))/B(Lambda(0)(b) -> Lambda(+)(c)pi(-) = 0.0540 +/- 0.0023 +/- 0.0032. Two resonant structures are observed in the Lambda(+)(c)pi(-) mass spectrum of the Lambda(0)(b) -> Lambda(+)(c)pp pi(-) decays, corresponding to the Xc(2455) and X (2520) states. The ratios of branching fractions with respect to the decay Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-) are B(Lambda(0)(b) -> Sigma(0)(c)p (p) over bar x B(Sigma(0)(b) -> Lambda(+)(c)pi(-))/B(Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-)) = 0.089 +/- 0.015 +/- 0.006, B(Lambda(0)(b) -> Sigma(c)*(0)p (p) over bar x B(Sigma(c)*(0) -> Lambda(+)(c)pi(-))/B(Lambda(0)(b) -> Lambda(+)(c)p (p) over bar pi(-)) = 0.119 +/- 0.020 +/- 0.014. In all of the above results, the first uncertainty is statistical and the second is systematic. The phase space is also examined for the presence of dibaryon resonances. No evidence for such resonances is found.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Search for new phenomena in high-mass final states with a photon and a jet from pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 78(2), 102–25pp.
Abstract: A search is performed for new phenomena in events having a photon with high transverse momentum and a jet collected in 36.7 fb(-1) of proton-proton collisions at a centre-of-mass energy of root s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The invariant mass distribution of the leading photon and jet is examined to look for the resonant production of new particles or the presence of new high-mass states beyond the Standard Model. No significant deviation from the background-only hypothesis is observed and cross-section limits for generic Gaussian-shaped resonances are extracted. Excited quarks hypothesized in quark compositeness models and high-mass states predicted in quantum black hole models with extra dimensions are also examined in the analysis. The observed data exclude, at 95% confidence level, the mass range below 5.3 TeV for excited quarks and 7.1 TeV (4.4 TeV) for quantum black holes in the Arkani-Hamed-Dimopoulos-Dvali (Randall-Sundrum) model with six (one) extra dimensions.
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Alcaide, J., Chala, M., & Santamaria, A. (2018). LHC signals of radiatively-induced neutrino masses and implications for the Zee-Babu model. Phys. Lett. B, 779, 107–116.
Abstract: Contrary to the see-saw models, extended Higgs sectors leading to radiatively-induced neutrino masses do require the extra particles to be at the TeV scale. However, these new states have often exotic decays, to which experimental LHC searches performed so far, focused on scalars decaying into pairs of same-sign leptons, are not sensitive. In this paper we show that their experimental signatures can start to be tested with current LHC data if dedicated multi-region analyses correlating different observables are used. We also provide high-accuracy estimations of the complicated Standard Model backgrounds involved. For the case of the Zee-Babu model, we show that regions not yet constrained by neutrino data and low-energy experiments can be already probed, while most of the parameter space could be excluded at the 95% C.L. in a high-luminosity phase of the LHC.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Search for supersymmetry in final states with missing transverse momentum and multiple b-jets in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 107–58pp.
Abstract: A search for supersymmetry involving the pair production of gluinos decaying via third-generation squarks into the lightest neutralino ((chi) over tilde (0)(1)) is reported. It uses LHC proton-proton collision data at a centre-of-mass energy root s = 13TeV with an integrated luminosity of 36.1 fb(-1) collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing large missing transverse momentum and several energetic jets, at least three of which must be identified as originating from b-quarks. To increase the sensitivity, the sample is divided into subsamples based on the presence or absence of electrons or muons. No excess is found above the predicted background. For (chi) over tilde (0)(1) masses below approximately 300 GeV, gluino masses of less than 1.97 (1.92) TeV are excluded at 95% confidence level in simplified models involving the pair production of gluinos that decay via top (bottom) squarks. An interpretation of the limits in terms of the branching ratios of the gluinos into third-generation squarks is also provided. These results improve upon the exclusion limits obtained with the 3.2 fb(-1) of data collected in 2015.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Search for top-squark pair production in final states with one lepton, jets, and missing transverse momentum using 36 fb(-1) of root s=13 TeV pp collision data with the ATLAS detector. J. High Energy Phys., 06(6), 108–96pp.
Abstract: The results of a search for the direct pair production of top squarks, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, several energetic jets, and missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where the mediator decays into a pair of dark-matter particles and is produced in association with a pair of top quarks. The search uses data from proton-proton collisions delivered by the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of root s = 13TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb(-1). A wide range of signal scenarios with different mass-splittings between the top squark, the lightest neutralino and possible intermediate supersymmetric particles are considered, including cases where the W bosons or the top quarks produced in the decay chain are off-shell. No significant excess over the Standard Model prediction is observed. The null results are used to set exclusion limits at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator models, upper limits are set on the visible cross-section.
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