Lepton number violation in top quark and neutral B meson decays

Lepton number violation (LNV) can be induced by Majorana neutrinos in four-body decays of the neutral B meson and the top quark. We study the effects of Majorana neutrinos in these |\Delta L|=2 decays in an scenario where a single heavy neutrino can enhance the amplitude via the resonant mechanism. Using current bounds on heavy neutrino mixings, the most optimistic branching ratios turn out to be at the level of 10^{-6} for \bar{B} -> D^+e^-e^-\pi^+ and t -> bl^+l^+W^- decays. Searches for these LNV decays at future facilities can provide complementary constraints on masses and mixings of Majorana neutrinos.Comment: 15 pages, 6 figures, "Added comments on sensitivities. Version accepted for publication

Locating the Gribov horizon

We explore whether a tree-level expression for the gluon two-point function, supposed to express effects of an horizon term introduced to eliminate the Gribov ambiguity, is consistent with the propagator obtained in simulations of lattice-regularised quantum chromodynamics (QCD). In doing so, we insist that the gluon two-point function obey constraints that ensure a minimal level of consistency with parton-like behaviour at ultraviolet momenta. In consequence, we are led to a position which supports a conjecture that the gluon mass and horizon scale are equivalent emergent mass-scales, each with a value of roughly $0.5\,$GeV; and wherefrom it appears plausible that the dynamical generation of a running gluon mass may alone be sufficient to remove the Gribov ambiguity.Comment: 7 pages, 2 figures, 2 table

Process-independent effective coupling. From QCD Green's functions to phenomenology

This article reports on a very recent proposal for a new type of process-independent QCD effective charge [Phys.Rev.D96(2017)054026] defined, as an anologue of the Gell-Mann-Low effective charge in QCD, on the ground of nothing but the knowledge of the gauge-field two-point Green's function, albeit modified within a particular computational framework; namely, the combination of pinch technique and background field method which makes possible a systematic rearranging of classes of diagrams in order to redefine the Green's function and have them obey linear QED-like Slavnov-Taylor identities. We have here calculated that effective charge, shown how strikingly well it compares to a process-dependent effective charge based on the Bjorken sum rule; and, finally, employed it in an exploratory calculation of the proton electromagnetic form factor in the hard scattering regime.Comment: 13 pages, 3 figues; contribution to NStar 2017 (Columbia, USA

Scale-setting, flavour dependence and chiral symmetry restoration

We determine the flavour dependence of the renormalisation-group-invariant running interaction through judicious use of both unquenched Dyson-Schwinger equation and lattice results for QCD's gauge-sector two-point functions. An important step is the introduction of a physical scale setting procedure that enables a realistic expression of the effect of different numbers of active quark flavours on the interaction. Using this running interaction in concert with a well constrained class of dressed--gluon-quark vertices, we estimate the critical number of active lighter-quarks above which dynamical chiral symmetry breaking becomes impossible: $n_f^{\rm cr}\approx 9$; and hence in whose neighbourhood QCD is plausibly a conformal theory.Comment: 8 pages, 4 figures, 2 table

Gains from the upgrade of the cold neutron triple-axis spectrometer FLEXX at the BER-II reactor

The upgrade of the cold neutron triple-axis spectrometer FLEXX is described. We discuss the characterisation of the gains from the new primary spectrometer, including a larger guide and double focussing monochromator, and present measurements of the energy and momentum resolution and of the neutron flux of the instrument. We found an order of magnitude gain in intensity (at the cost of coarser momentum resolution), and that the incoherent elastic energy widths are measurably narrower than before the upgrade. The much improved count rate should allow the use of smaller single crystals samples and thus enable the upgraded FLEXX spectrometer to continue making leading edge measurements.Comment: 8 pages, 7 figures, 5 table