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BABAR Collaboration(Lees, J. P. et al), Martinez-Vidal, F., & Oyanguren, A. (2017). Search for Invisible Decays of a Dark Photon Produced in e(+)e(-) Collisions at BABAR. Phys. Rev. Lett., 119(13), 131804–7pp.
Abstract: We search for single-photon events in 53 fb(-1) of e(+)e(-) collision data collected with the BABAR detector at the PEP-II B-Factory. We look for events with a single high-energy photon and a large missing momentum and energy, consistent with production of a spin-1 particle A' through the process e(+)e(-) -> gamma A'; A' -> invisible. Such particles, referred to as “dark photons,” are motivated by theories applying a U(1) gauge symmetry to dark matter. We find no evidence for such processes and set 90% confidence level upper limits on the coupling strength of A' to e(+)e(-) in the mass range m(A') <= 8 GeV. In particular, our limits exclude the values of the A' coupling suggested by the dark-photon interpretation of the muon (g – 2)(mu) anomaly, as well as a broad range of parameters for the dark-sector models.
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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. (2017). Study of WW gamma and WZ gamma production in pp collisions at root s=8 TeV and search for anomalous quartic gauge couplings with the ATLAS experiment. Eur. Phys. J. C, 77(9), 646–30pp.
Abstract: This paper presents a study of WW gamma and WZ gamma triboson production using events from proton-proton collisions at a centre-of-mass energy of root s = 8 TeV recorded with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 20.2 fb(-1). The WW gamma production cross-section is determined using a final state containing an electron, a muon, a photon, and neutrinos (e upsilon μupsilon gamma). Upper limits on the production cross-section of the e upsilon μupsilon gamma final state and theWW gamma and WZ gamma final states containing an electron or a muon, two jets, a photon, and a neutrino (e upsilon j j gamma or μupsilon j j gamma) are also derived. The results are compared to the cross-sections predicted by the Standard Model at next-to-leading order in the strong-coupling constant. In addition, upper limits on the production cross-sections are derived in a fiducial region optimised for a search for newphysics beyond the Standard Model. The results are interpreted in the context of anomalous quartic gauge couplings using an effective field theory. Confidence intervals at 95% confidence level are derived for the 14 coupling coefficients to which WW gamma and WZ gamma production are sensitive.
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Binosi, D., Mezrag, C., Papavassiliou, J., Roberts, C. D., & Rodriguez-Quintero, J. (2017). Process-independent strong running coupling. Phys. Rev. D, 96(5), 054026–7pp.
Abstract: We unify two widely different approaches to understanding the infrared behavior of quantum chromodynamics (QCD), one essentially phenomenological, based on data, and the other computational, realized via quantum field equations in the continuum theory. Using the latter, we explain and calculate a process-independent running coupling for QCD, a new type of effective charge that is an analogue of the Gell-Mann-Low effective coupling in quantum electrodynamics. The result is almost identical to the process-dependent effective charge defined via the Bjorken sum rule, which provides one of the most basic constraints on our knowledge of nucleon spin structure. This reveals the Bjorken sum to be a near direct means by which to gain empirical insight into QCD's Gell-Mann-Low effective charge.
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Sakai, S., Roca, L., & Oset, E. (2017). Charm-beauty meson bound states from B (B*)D(D*) and interaction B (B*)(D)over-bar((D)over-bar*). Phys. Rev. D, 96(5), 054023–9pp.
Abstract: We evaluate the s-wave interaction of pseudoscalar and vector mesons with both charm and beauty to investigate the possible existence of molecular BD, B* D, BD*, B* D*, B (D) over bar, B* (D) over bar, B (D) over bar*, or B* (D) over bar* meson states. The scattering amplitude is obtained implementing unitarity starting from a tree level potential accounting for the dominant vector meson exchange. The diagrams are evaluated using suitable extensions to the heavy flavor sector of the hidden gauge symmetry Lagrangians involving vector and pseudoscalar mesons, respecting heavy quark spin symmetry. We obtain bound states at energies above 7 GeV for BD (J(P) = 0(+)), B* D (1(+)), BD* (1(+)), and B* D* (0(+), 1(+,) 2(+)), all in isospin 0. For B (D) over bar (0(+)), B* (D) over bar (1(+)), B (D) over bar* (1(+)), and B* (D) over bar* (0(+), 1(+), 2(+)) we also find similar bound states in I = 0, but much less bound, which would correspond to exotic meson states with _ (b) over bar and (c) over bar quarks, and for the I = 1 we find a repulsive interaction. We also evaluate the scattering lengths in all cases, which can be tested in current investigations of lattice QCD.
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Aitken, K., McKeen, D., Neder, T., & Nelson, A. E. (2017). Baryogenesis from oscillations of charmed or beautiful baryons. Phys. Rev. D, 96(7), 075009–15pp.
Abstract: We propose a model for CP-violating oscillations of neutral, heavy-flavor baryons into antibaryons at rates which are within a few orders of magnitude of their lifetimes. The flavor structure of the baryon violation suppresses neutron oscillations and baryon-number-violating nuclear decays to experimentally allowed rates. We also propose a scenario for producing such baryons in the early Universe via the out-of-equilibrium decays of a neutral particle, after hadronization but before nucleosynthesis. We find parameters where CP-violating baryon oscillations at a temperature of a few MeV could result in the observed asymmetry between baryons and antibaryons. Furthermore, part of the relevant parameter space for baryogenesis is potentially testable at Belle II via decays of heavy-flavor baryons into an exotic neutral fermion. The model introduces four new particles: three light Majorana fermions and a colored scalar. The lightest of these fermions is typically long lived on collider time scales and may be produced in decays of bottom and possibly charmed hadrons.
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