Cosmic-ray fermion decay through tau-antitau emission with Lorentz violation

We study CPT and Lorentz violation in the tau-lepton sector of the Standard Model in the context of the Standard-Model Extension, parametrized by a coefficient which is thus far unbounded by experiment. We show that any nonzero value of this coefficient implies that, for sufficiently large energies, Standard Model fermions become unstable against decay due to the emission of a pair of tau-antitau leptons. We calculate the induced fermion energy-loss rate and we deduce the first limit on the Lorentz- and CPT-violation coefficient.Fundacao para a Ciencia e a Tecnologia of Portugal (FCT) [UID/FIS/00099/2013, SFRH/BPD/101403/2014]; CONACyT [234745]info:eu-repo/semantics/publishedVersio

Cosmic-ray fermion decay by emission of on-shell W bosons with CPT violation

We study CPT and Lorentz violation in the electroweak gauge sector of the Standard Model in the context of the Standard-Model Extension. In particular, we consider the Lorentz-violating and CPT-odd Chern-Simons like parameter for the W boson, which is thus far unbounded by experiment. We demonstrate that any nonzero value of this parameter implies that, for sufficiently large energies, one of the polarization modes of the W boson propagates with spacelike four-momentum. In this scenario, an emission of W bosons by ultra-high-energy cosmic rays is possible. We calculate the induced fermion energy-loss rate, and we deduce the first limit on the pertinent Lorentz-and CPT-violating parameter that couples to the W boson. Consistency between the quantum description in various reference frames is preserved by using a recently formulated covariant quantization procedure for massive photons and applying it to the W bosons.Fundacao para a Ciencia e a Tecnologia of Portugal (FCT) (SFRH/BPD/101403/2014, POPH/FSE

Cherenkov-like emission of Z bosons

We study CPT and Lorentz violation in the electroweak gauge sector of the Standard Model in the context of the Standard-Model Extension (SME). In particular, we show that any non-zero value of a certain relevant Lorentz violation parameter that is thus far unbounded by experiment would imply that for sufficiently large energies one of the helicity modes of the Z boson should propagate with spacelike four-momentum and become stable against decay in vacullin. In this scenario, Cherenkov-hike radiation of Z bosons by ultra-highenergy cosmic-ray protons becomes possible. We deduce a bound on the Lorentz violation parameter from the observational data on ultra-high energy cosmic rays.Portuguese Fundacao para a Ciencia e a Tecnologia (FCT) [SFRH/BPD/101403/2014]; program POPH/FS

CPT and Lorentz violation in the electroweak sector

Long ago, Carroll, Field and Jackiw introduced CPT-violation in the photon sector by adding a dimension-3 gauge-invariant term parametrized by a constant four-vector parameter k(AF) to the usual (Maxwell) Lagrangian, deriving an ultra-tight bound from astrophysical data. Here, we will discuss recent work studying the extension of this term to the full electroweak gauge sector of the Standard Model. In the context of the Standard Model Extension, CPT and Lorentz violation arises from two gauge-invariant terms parametrized by the four vectors k(1) and k(2). First we will show how upon spontaneous breaking of the electroweak gauge symmetry these two terms yield Lorentz-violating terms for the photon and the W and Z bosons. As it turns out, the resulting modified dispersion relations for the W bosons yield spacelike momentum for one of its propagating modes at sufficiently large energy. This in turn allows for the possibility of Cherenkov-like W-boson emission by high-energy fermions such as protons, provoking their decay. Analysis of ultra-high-energy cosmic ray data allows for bounding the previously unbound parameter k(2), and, by combination with the ultra-tight bound on k(AF), the parameter k(1).Portuguese Fundacao para a Ciencia e a Tecnologia (FCT) - SFRH/BPD/101403/2014program POPH/FSE New College of Floridainfo:eu-repo/semantics/publishedVersio

Quantum Gravity Phenomenology without Lorentz Invariance Violation: a detailed proposal

We describe a scheme for the exploration of quantum gravity phenomenology focussing on effects that could be thought as arising from a fundamental granularity of space-time. In contrast with the simplest assumptions, such granularity is assumed to respect Lorentz Invariance but is otherwise left unspecified. The proposal is fully observer covariant, it involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. We present the effective Hamiltonian which could be used to analyze concrete experimental situations, some of which are briefly described, and we shortly discuss the degree to which the present proposal is in line with the fundamental ideas behind the equivalence principle.Comment: LaTeX, 24 pages. To be published in Classical and Quantum Gravit

Transmittivity of a Bose-Einstein condensate on a lattice: interference from period doubling and the effect of disorder

We evaluate the particle current flowing in steady state through a Bose-Einstein condensate subject to a constant force in a quasi-onedimensional lattice and to attractive interactions from fermionic atoms that are localized in various configurations inside the lattice wells. The system is treated within a Bose-Hubbard tight binding model by an out-of-equilibrium Green's function approach. A new band gap opens up when the lattice period is doubled by locating the fermions in alternate wells and yields an interference pattern in the transmittivity on varying the intensity of the driving force. The positions of the transmittivity minima are determined by matching the period of Bloch oscillations and the time for tunnelling across the band gap. Massive disorder in the distribution of the fermions will wash out the interference pattern, but the same period doubling of the lattice can be experimentally realized in a four-beam set-up. We report illustrative numerical results for a mixture of 87Rb and 40K atoms in an optical lattice created by laser beams with a wavelength of 763 nm.Comment: 13 pages, 5 figure

Sphaleron Transition Rate in Presence of Dynamical Fermions

We investigate the effect of dynamical fermions on the sphaleron transition rate at finite temperature for the Abelian Higgs model in one spatial dimension. The fermion degrees of freedom are included through bosonization. Using a numerical simulation, we find that massless fermions do not change the rate within the measurement accuracy. Surprisingly, the exponential dependence of the sphaleron energy on the Yukawa coupling is not borne out by the transition rate, which shows a very weak dependence on the fermion mass.Comment: 20 pages, 7 figures, LaTeX, psfi

Introduction to Quantum-Gravity Phenomenology

After a brief review of the first phase of development of Quantum-Gravity Phenomenology, I argue that this research line is now ready to enter a more advanced phase: while at first it was legitimate to resort to heuristic order-of-magnitude estimates, which were sufficient to establish that sensitivity to Planck-scale effects can be achieved, we should now rely on detailed analyses of some reference test theories. I illustrate this point in the specific example of studies of Planck-scale modifications of the energy/momentum dispersion relation, for which I consider two test theories. Both the photon-stability analyses and the Crab-nebula synchrotron-radiation analyses, which had raised high hopes of ``beyond-Plankian'' experimental bounds, turn out to be rather ineffective in constraining the two test theories. Examples of analyses which can provide constraints of rather wide applicability are the so-called ``time-of-flight analyses'', in the context of observations of gamma-ray bursts, and the analyses of the cosmic-ray spectrum near the GZK scale.Comment: 46 pages, LaTex. Based on lectures given at the 40th Karpacz Winter School in Theoretical Physic