arXiv subjects

B. A. Couto e Silva

Publications and source records attributed to B. A. Couto e Silva.

6 recordsLinked to original sources

arXiv2026

Energy-independent neutrino dephasing from an ultralight axion-like background

We study neutrino oscillations in an ultralight axion-like particle (ALP) dark-matter background using a bottom-up derivative interaction for active neutrinos. For couplings diagonal in the neutrino mass basis, the leading relative phase is fixed by the difference of the ALP field between production and detection. More generally, an unresolved background multiplies each interference term by the characteristic function of this endpoint field difference. For a single fixed-amplitude coherent mode with uniformly sampled phase, the factor is $J_0(A_{ij})$. In the slow-flight and small-dephasing regimes, this result and a Gaussian virialized-halo treatment agree at quadratic order and produce a leading $L^2E^0$ suppression. We identify the finite-exposure and coherence conditions under which the two statistical descriptions apply. As an application, we analytically map the energy-independent damping constraint derived from the first JUNO data onto the flavor benchmark $g_2=g_3$. For a local ALP density of $0.4\mathrm{GeV}/\mathrm{cm}^3$, the low-mass plateau gives $|Δg_{21}|=|Δg_{31}|\lesssim0.90\mathrm{GeV}^{-1}$. This number is a baseline-matched EFT recast of the reference damping analysis. No specific ultraviolet completion is assumed.

hep-ph↗

arXiv2026

Mixed Freeze-In and Freeze-Out Histories and Dark-Sector Decays in a $\mathbb{Z}_4$ Two-Scalar Model

We present a systematic non-equilibrium analysis of a renormalisable $\mathbb{Z}_4$ Higgs-portal dark sector with a complex scalar $S_A$ and a real scalar $S_B$, including conversion, semi-annihilation, and, when kinematically allowed, the dark-sector decay $S_B\to S_A S_A$. We impose theoretical consistency, Higgs invisible-decay limits, relic-fraction-rescaled LZ bounds, and an a posteriori thermalisation check of the WIMP/FIMP assignments. Allowing one or both dark states to remain out of equilibrium qualitatively enlarges the set of viable cosmological histories. In the stable regime, FIMP--FIMP and mixed WIMP--FIMP configurations can share the observed relic density between the two components, while the WIMP-like state remains subject to the usual Higgs-portal correlation between freeze-out and spin-independent scattering and the FIMP-like state can provide a substantial abundance with a negligible direct-detection rate. For $M_{S_B}>2M_{S_A}$, the decay of the unstable $S_B$ opens three distinct abundance-transfer mechanisms: SuperWIMP production, injection-assisted freeze-out, and sequential freeze-in. Because the decay proceeds entirely within the dark sector, the usual electromagnetic and hadronic BBN energy-injection bounds do not apply; we instead track the parent energy fraction and use an equivalent thermal-WDM mass to diagnose the warmness of the decay-produced daughter population. The minimal $\mathbb{Z}_4$ Higgs portal thus supports thermal, mixed, and fully non-thermal relic histories within the same renormalisable framework, with dark-sector decay linking relic-density production to small-scale structure.

hep-ph↗

arXiv2026

Photon Propagation through Axion Clouds around Magnetized Compact Objects: Time Delays and Polarimetric Signatures

Temporal offsets between Gamma-Ray Bursts (GRBs) and high-energy neutrinos probe propagation effects in extreme astrophysical environments. We investigate whether such offsets can be generated by photon propagation through dense axion clouds gravitationally bound to strongly magnetized compact objects, such as canonical pulsars. Working within the Euler--Heisenberg effective theory extended by the axion sector, we derive the photon dispersion relations in a strong magnetic background permeated by an oscillating axion field. The magnetized vacuum is birefringent already at the Euler--Heisenberg level and the axion cloud superimposes density-dependent, time-dependent, and parity-odd structure on this baseline. The resulting geometry-dependent deviations from luminal propagation yield kinematic time delays reaching $Δt_{\perp} \simeq 9.3 \times 10^{-12}$ s for $\mathbf{k} \perp \mathbf{B}$, far too small to account for macroscopic multimessenger offsets, so that propagation through such environments cannot, by itself, rule out Lorentz-invariance violation as an explanation for GRB offsets. In the polarization sector, we show that the axion-induced circular birefringence is an endpoint effect and therefore does not produce a leading net rotation for a complete vacuum-to-vacuum transit through a localized cloud. For configurations with a nonzero axion-field endpoint contrast, the same parity-odd phase defines a conditional environmental sensitivity benchmark. We find that this mechanism is optimally sensitive in the ultralight regime, yielding a benchmark reach of $g_{aγγ} \lesssim 2.5 \times 10^{-10}\,\mathrm{GeV}^{-1}$ for canonical pulsar fields and axion masses near $m_a \sim 10^{-9}$ eV, and we identify the parity-odd (chiral) birefringence induced by the oscillating cloud as the signature that distinguishes the axion contribution from the calculable QED baseline.

hep-ph↗

arXiv2025

Neutrino Theory in the Precision Era

This document summarises discussions on future directions in theoretical neutrino physics, which are the outcome of a neutrino theory workshop held at CERN in February 2025. The starting point is the realisation that neutrino physics offers unique opportunities to address some of the most fundamental questions in physics. This motivates a vigorous experimental programme which the theory community fully supports. \textbf{A strong effort in theoretical neutrino physics is paramount to optimally take advantage of upcoming neutrino experiments and to explore the synergies with other areas of particle, astroparticle, and nuclear physics, as well as cosmology.} Progress on the theory side has the potential to significantly boost the physics reach of experiments, as well as go well beyond their original scope. Strong collaboration between theory and experiment is essential in the precision era. To foster such collaboration, \textbf{we propose to establish a CERN Neutrino Physics Centre.} Taking inspiration from the highly successful LHC Physics Center at Fermilab, the CERN Neutrino Physics Centre would be the European hub of the neutrino community, covering experimental and theoretical activities.

hep-ph↗

arXiv2025

Very Special Relativity: Cherenkov Effect and an Analogy with Minkowski's Electrodynamics of Continuous Media

In this work, we explore the implications of the Cohen and Glashow Very Special Relativity (VSR) theory, a framework that introduces Lorentz invariance violation through the presence of a preferred direction. Our analysis focuses on the impact of VSR on the Cherenkov angle, revealing modifications to the dispersion relation of particles, particularly the photon and the electron, which acquire an effective inertial mass. This modification also implies a deviation in the speed of light, which can be constrained through precise experimental measurements. Using data from the RICH system of the LHCb experiment, we take advantage of its capability to reconstruct Cherenkov angles within the momentum range of the particles of 2.6-100 GeV/c. These measurements, combined with the most stringent laboratory tests of the isotropy of the speed of light ($Δc / c \sim 10^{-17}$), allow us to impose new upper bounds on the parameter $Ω$, which quantifies a deviation from the standard Special Relativity. Furthermore, we establish an analogy between VSR and Minkowski's electrodynamics in a dielectric medium for particles with very high velocity, offering a physically intuitive interpretation of the parameter $Ω$.

hep-ph↗

arXiv2022

Spin-1/2 "bosons'' with mass dimension 3/2 and fermions with mass dimension 1 cannot represent physical particle states

We delve into the first principles of quantum field theory to prove that the so-called spin-1/2 ''bosons'' and the fermions with mass dimension 1, including ELKO, cannot represent physical particle states with spin $1/2$. Specifically, we first demonstrate that both aforementioned fields are not invariant under rotational symmetry, which implies that the particles created for these fields are not eigenstates of the spin operator in the $(\frac{1}{2},0)\oplus(0,\frac{1}{2})$ representation of the Lorentz group, nor is it possible to construct a Hamiltonian density scalar under the rotational group from them. Furthermore, following Weinberg's approach to local causal fields, we prove that regardless of any discrete symmetry or adjoint structure, the relativistic fields in the $(\frac{1}{2}, 0) \oplus (0,\frac{1}{2})$ representation satisfy the Fermi-Dirac statistics in complete agreement with the well-established spin-statistics theorem and experimental results.

hep-th↗