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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Search for the Lepton-Flavor-Violating Decays B-s(0) -> pi(+/-mu -/+) and B-0 -> tau(+/-mu -/+). Phys. Rev. Lett., 123(21), 211801–11pp.
Abstract: Results are reported from a search for the rare decays B-s(0) -> tau(+/-)mu(+/-) 1T and B -> tau(+/-)mu(+/-), where the r lepton is reconstructed in the channel r- irrg vr. These processes are effectively forbidden in the standard model, hut they can potentially occur at detectable rates in models of new physics that can induce lepton flavor-violating decays. The search is based on a data sample corresponding to 3 fb-1 of proton -proton collisions recorded by the LHCb experiment in 2011 and 2012. The event yields observed in the signal regions for both processes are consistent with the expected standard model backgrounds. Because of the limited mass resolution arising from the undetected r neutrino, the 1.3 and B signal regions are highly overlapping. Assuming no contribution from B r1, r, the upper limit 8(B r 71) < 4.2 x 10-5 is obtained at 95% confidence level. If no contribution from tau(+/-)mu(+/-) is assumed, a limit of B(B r prn < 1.4 x 10-5 is obtained at 95% confidence level. These results represent the first limit on B(B? r5+) and the most stringent limit on B(B tau(+/-)mu(+/-).)
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Search for the rare decay B+ -> mu(+) mu(-) mu(+)nu(mu). Eur. Phys. J. C, 79(8), 675–12pp.
Abstract: A search for the rare leptonic decay B +. μ+ μ- μ+.mu is performed using proton- proton collision data corresponding to an integrated luminosity of 4.7 fb – 1 collected by the LHCb experiment. The search is carried out in the region where the lowest of the two μ+ μ- mass combinations is below 980 MeV/ c2. The data are consistent with the background- only hypothesis and an upper limit of 1.6x10 – 8 at 95% confidence level is set on the branching fraction in the stated kinematic region.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., et al. (2019). Measurement of the CP-violating phase phi(s) from B-s(0) -> J/psi pi(+)pi(-) decays in 13 TeV pp collisions. Phys. Lett. B, 797, 134789–12pp.
Abstract: Decays of B-s(0) and (B) over bar (0)(s) mesons into J/psi pi(+)pi(-) final states are studied in a data sample corresponding to 1.9 fb(-1) of integrated luminosity collected with the LHCb detector in 13 TeV pp collisions. A time-dependent amplitude analysis is used to determine the final-state resonance contributions, the CP-violating phase phi(s) = -0.057 +/- 0.060 +/- 0.011 rad, the decay-width difference between the heavier mass B-s(0) eigenstate and the B-0 meson of -0.050 +/- 0.004 +/- 0.004 ps(-1), and the CP-violating parameter vertical bar lambda vertical bar = 1.01(-0.06)(+0.08) +/- 0.03, where the first uncertainty is statistical and the second systematic. These results are combined with previous LHCb measurements in the same decay channel using 7 TeV and 8 TeV pp collisions obtaining phi(s) = 0.002 +/- 0.044 +/- 0.012 rad, and vertical bar lambda vertical bar = 0.949 +/- 0.036 +/- 0.019.
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Begone, G., Deisenroth, M. P., Kim, J. S., Liem, S., Ruiz de Austri, R., & Welling, M. (2019). Accelerating the BSM interpretation of LHC data with machine learning. Phys. Dark Universe, 24, 100293–5pp.
Abstract: The interpretation of Large Hadron Collider (LHC) data in the framework of Beyond the Standard Model (BSM) theories is hampered by the need to run computationally expensive event generators and detector simulators. Performing statistically convergent scans of high-dimensional BSM theories is consequently challenging, and in practice unfeasible for very high-dimensional BSM theories. We present here a new machine learning method that accelerates the interpretation of LHC data, by learning the relationship between BSM theory parameters and data. As a proof-of-concept, we demonstrate that this technique accurately predicts natural SUSY signal events in two signal regions at the High Luminosity LHC, up to four orders of magnitude faster than standard techniques. The new approach makes it possible to rapidly and accurately reconstruct the theory parameters of complex BSM theories, should an excess in the data be discovered at the LHC.
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Beltran Jimenez, J., & Delhom, A. (2019). Ghosts in metric-affine higher order curvature gravity. Eur. Phys. J. C, 79(8), 656–7pp.
Abstract: We disprove the widespread belief that higher order curvature theories of gravity in the metric-affine formalism are generally ghost-free. This is clarified by considering a sub-class of theories constructed only with the Ricci tensor and showing that the non-projectively invariant sector propagates ghost-like degrees of freedom. We also explain how these pathologies can be avoided either by imposing a projective symmetry or additional constraints in the gravity sector. Our results put forward that higher order curvature gravity theories generally remain pathological in the metric-affine (and hybrid) formalisms and highlight the key importance of the projective symmetry and/or additional constraints for their physical viability and, by extension, of general metric-affine theories.
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