596 research outputs found

    Low-Energy Positronium Scattering from O2

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    The total cross-section of positronium scattering from molecular oxygen has been measured in the velocity range 0.27 ‚ąí 1.50 a.u. (energy range 2 ‚ąí 61 eV) and found to be close to the corresponding equivelocity electron cross-section above 0.87 a.u. (20 eV), as previously found by Brawley et al. Science 330 789 (2010). However, below this value the cross-section for positronium is observed to exceed that for electrons by up to a factor of 4 at the lowest energy. Measurements are compared to the predictions of low-energy resonant peaks in the elastic scattering cross-section calculated within a free-electron-gas model refined by applying corrections to the correlation energy for the interaction between Ps and the electron gas. Additionally, cross-sections for O‚ąí 2 formation and positronium break-up have been calculated using a classical trajectory Monte Carlo approach. Comparisons are made with earlier calculations and discussed in terms of both experimental and theoretical uncertainties

    Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at root s=13 TeV

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    A search is presented for new particles produced at the LHC in proton-proton collisions at root s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb(-1), collected in 2017-2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb(-1), collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.Peer reviewe

    Combined searches for the production of supersymmetric top quark partners in proton-proton collisions at root s=13 TeV