arXiv subjects

A. M. Teixeira

Publications and source records attributed to A. M. Teixeira.

At least 37 records · Page 2Linked to original sources

arXiv2019

Interference effects in semileptonic decays from heavy Majorana neutrinos

Several beyond the Standard Model scenarios introduce new heavy neutrinos, whose Dirac or Majorana nature could be tested by comparing the rates of lepton number violating and lepton number conserving processes: a Dirac fermion induces only the latter, while a Majorana one predicts the same rate for both of them. Nevertheless, in the presence of more than one Majorana fermion, this picture may change drastically due to interference effects. We focus on lepton number violating and lepton flavour violating semileptonic meson decays induced by two heavy Majorana fermions, exploring the necessary conditions to have sizeable interference effects and discussing their implications for current experimental constraints and possible future observations. In particular, we show how the $CP$ violating phases may lead to an enhancement of the lepton number violating modes and suppression of the lepton number conserving ones, and vice-versa.

hep-ph↗

arXiv2019

Interference effects in LNV and LFV semileptonic decays: the Majorana hypothesis

In the case where the Standard Model is extended by one heavy Majorana fermion, the branching fractions of semileptonic meson decays into same-sign and opposite-sign dileptons are expected to be of the same order. As we discuss here, this need not be the case in extensions by at least two sterile fermions, due to the possible destructive and constructive interferences that might arise. Depending on the $CP$ violating phases, one can have an enhancement of the lepton number violating modes and suppression of the lepton number conserving ones (and vice-versa). We explore for the first time the interference effects in semileptonic decays, and illustrate them for a future observation of kaon decays at NA62. We also argue that a non-observation of a given mode need not be interpreted in terms of reduced active-sterile mixings, but that it could instead be understood in terms of interference effects due to the presence of several sterile states; in particular, for different-flavour final state charged leptons, observing a lepton number conserving process and not a lepton number violating one does not rule out that the mediators are Majorana fermions.

hep-ph↗

arXiv2018

COMET - A submission to the 2020 update of the European Strategy for Particle Physics on behalf of the COMET collaboration

The search for charged lepton flavour violation (CLFV) has enormous discovery potential in probing new physics Beyond the Standard Model (BSM). Among the muonic CLFV processes, $μ\to e$ conversion is one of the most important processes, having several advantages compared to other such processes. We describe the COMET experiment, which is searching for $μ\to e$ conversion in a muonic atom at the J-PARC proton accelerator laboratory in Japan. The COMET experiment has taken a staged approach; the first stage, COMET Phase-I, is currently under construction at J-PARC, and is aiming at a factor 100 improvement over the current limit. The second stage, COMET Phase-II is seeking another 100 improvement (a total of 10,000), allowing a single event sensitivity (SES) of $2.6 \times 10^{-17}$ with $2\times 10^{7}$ seconds of data-taking. Further improvements by one order of magnitude, which arise from refinements to the experimental design and operation, are being considered whilst staying within the originally-assumed beam power and beam time. Such a sensitivity could be translated into probing many new physics constructions up to $\mathcal{O}(10^{4})$ TeV energy scales, which would go far beyond the level that can be reached directly by collider experiments. The search for CLFV $μ\to e$ conversion is thus highly complementary to BSM searches at the LHC.

hep-ex↗

arXiv2017

In-flight cLFV conversion: $e-μ$, $e-τ$ and $μ-τ$ in minimal extensions of the Standard Model with sterile fermions

We revisit charged lepton flavour in-flight conversions, in which a beam of electrons or muons is directed onto a fixed target, $e + N \to μ+N$, $e + N \to τ+N$ and $μ+ N \to τ+N$, focusing on elastic interactions with a nucleus $N$. After a general discussion of this observable, we carry a full phenomenological analysis in the framework of minimal Standard Model extensions via sterile neutrinos, with a strong emphasis on the rôle of the increasingly more stringent constraints arising from other (low-energy) charged lepton flavour violation observables. Despite the potential interest of this observable, in particular in the light of certain upcoming facilities with the capability of very intense lepton beams, our study suggests that due to current bounds on three-body decays ($\ell_i \to 3 \ell_j$) and $μ-e$ conversion in Nuclei, the expected number of conversions in such a minimal framework is dramatically reduced. An experimental observation of such a conversion would thus signal the presence of another source of flavour violation, possibly at tree-level.

hep-ph↗

arXiv2016

Impact of sterile neutrinos on nuclear-assisted cLFV processes

We discuss charged lepton flavour violating processes occurring in the presence of muonic atoms, such as muon-electron conversion in nuclei $\text{CR}(μ-e, \text{ N})$, the (Coulomb enhanced) decay of muonic atoms into a pair of electrons BR($μ^- e^- \to e^- e^-$, N), as well as Muonium conversion and decay, $\text{Mu}-\bar{\text{Mu}}$ and $\text{Mu}\to e^+ e^-$. Any experimental signal of these observables calls for scenarios of physics beyond the Standard Model. In this work, we consider minimal extensions of the Standard Model via the addition of sterile fermions, providing the corresponding complete analytical expressions for all the considered observables. We first consider an "ad hoc" extension with a single sterile fermion state, and investigate its impact on the above observables. Two well motivated mechanisms of neutrino mass generation are then considered: the Inverse Seesaw embedded into the Standard Model, and the $ν$MSM. Our study reveals that, depending on their mass range and on the active-sterile mixing angles, sterile neutrinos can give significant contributions to the above mentioned observables, some of them even lying within present and future sensitivity of dedicated cLFV experiments. We complete the analysis by confronting our results to other (direct and indirect) searches for sterile fermions.

hep-ph↗

arXiv2014

Indirect searches for sterile neutrinos at a high-luminosity Z-factory

A future high-luminosity $Z$-factory will offer the possibility to study rare $Z$ decays, as those leading to lepton flavour violating final states. Processes such as $Z \to \ell_1^\mp \ell_2^\pm$ are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one sterile fermion state, and two well-motivated frameworks of neutrino mass generation, the Inverse Seesaw embedded into the Standard Model, and the $ν$MSM. Our study shows that sterile neutrinos can give rise to contributions to BR($Z \to \ell_1^\mp \ell_2^\pm$) within reach of the FCC-ee. We also discuss the complementarity between a high-luminosity $Z$-factory and low-energy charged lepton flavour violation facilities.

hep-ph↗

arXiv2014

Effect of steriles states on lepton magnetic moments and neutrinoless double beta decay

We address the impact of sterile fermion states on the anomalous magnetic moment of charged leptons, as well as their contribution to neutrinoless double beta decays. We illustrate our results in a minimal, effective extension of the Standard Model by one sterile fermion state, and in a well-motivated framework of neutrino mass generation, embedding the Inverse Seesaw into the Standard Model. The simple "3+1" effective case succeeds in alleviating the tension related to the muon anomalous magnetic moment, albeit only at the 3$σ$ level, and for light sterile states (corresponding to a }cosmologically disfavoured regime). Interestingly, our analysis shows that a future $0 ν2 β$ observation does not necessarily imply an inverted hierarchy for the active neutrinos in this simple extension. Although the Inverse Seesaw realisation here addressed could indeed ease the tension in $(g-2)_μ$, bounds from lepton universality in kaon decays mostly preclude this from happening. However, these scenarios can also have a strong impact on the interpretation of a future $0 ν2 β$ signal regarding the hierarchy of the active neutrino mass spectrum.

hep-ph↗

arXiv2014

Sterile neutrinos in leptonic and semileptonic decays

We address the impact of a modified $W \ell ν$ coupling on a wide range of observables, such as $τ$ leptonic and mesonic decays, leptonic decays of pseudoscalar mesons, as well as semileptonic meson decays. In particular, we concentrate on deviations from lepton flavour universality, focusing on the ratios $R_{P} = Γ(P \to \ell ν) / Γ(P \to \ell' ν)$, with $P=K, π, D, D_s$, $R(D)={Γ(B^+ \to D τ^+ ν)}/{Γ(B^+ \to D\ell^+ ν)}$, $R τ={Γ(τ\to μνν)}/{Γ(τ\to eνν)}$, $R^{\ell τ}_P=Γ(τ\to Pν)/Γ(P\to \ell ν)$, and $\text{BR}(B \to τν)$. We further consider leptonic gauge boson decays, such as $W\to \ell ν$ and $Z \to νν$. For all the above observables, we provide the corresponding complete analytical expressions, derived for the case of massive neutrinos. Working in the framework of the Standard Model extended by additional sterile fermions, which mix with the active (left-handed) neutrinos, we numerically study the impact of active-sterile mixings on the above mentioned observables.

hep-ph↗

arXiv2014

Lepton flavour violation at high energies: the LHC and a Linear Collider

We discuss several manifestations of charged lepton flavour violation at high energies. Focusing on a supersymmetric type I seesaw, considering constrained and semi-constrained supersymmetry breaking scenarios, we analyse different observables, both at the LHC and at a future Linear Collider. We further discuss how the synergy between low- and high-energy observables can shed some light on the underlying mechanism of lepton flavour violation.

hep-ph↗

arXiv2014

Slepton mass splittings and cLFV in the SUSY seesaw in the light of recent experimental results

Following recent experimental developments, in this study we re-evaluate if the interplay of high- and low-energy lepton flavour violating observables remains a viable probe to test the high-scale type-I supersymmetric seesaw. Our analysis shows that fully constrained supersymmetric scenarios no longer allow to explore this interplay, since recent LHC data precludes the possibility of having sizeable slepton mass differences for a slepton spectrum sufficiently light to be produced, and in association to BR(mu -> e gamma) within experimental reach. However, relaxing the strict universality of supersymmetric soft-breaking terms, and fully exploring heavy neutrino dynamics, still allows to have slepton mass splittings O(few %), for slepton masses accessible at the LHC, with associated mu -> e gamma rates within future sensitivity. For these scenarios, we illustrate how the correlation between high- and low-energy lepton flavour violating observables allows to probe the high-scale supersymmetric seesaw.

hep-ph↗

arXiv2013

Potential of a Linear Collider for Lepton Flavour Violation studies in the SUSY seesaw

We study the potential of an e+- e- Linear Collider for charged lepton flavour violation studies in a supersymmetric framework where neutrino masses and mixings are explained by a type-I seesaw. Focusing on e-mu flavour transitions, we evaluate the background from standard model and supersymmetric charged currents to the e mu + missing E_T signal. We study the energy dependence of both signal and background, and the effect of beam polarisation in increasing the signal over background significance. Finally, we consider the mu- mu- + missing E_T final state in e- e- collisions that, despite being signal suppressed by requiring two e-mu flavour transitions, is found to be a clear signature of charged lepton flavour violation due to a very reduced standard model background.

hep-ph↗

arXiv2013

Revisiting the $Γ(K \to e ν)/Γ(K \to μν)$ ratio in supersymmetric unified models

It has been pointed out that supersymmetric extensions of the Standard Model can induce significant changes to the theoretical prediction of the ratio $Γ(K\rightarrow eν)/Γ(K\rightarrowμν)\equiv R_{K}$, through lepton flavour violating couplings. In this work we carry out a full computation of all one-loop corrections to the relevant $ν\ell H^{+}$ vertex, and discuss the new contributions to $R_{K}$ arising in the context of different constrained (minimal supergravity inspired) models which succeed in accounting for neutrino data, further considering the possibility of accommodating a near future observation of a $μ\to eγ$ transition. We also re-evaluate the prospects for $R_{K}$ in the framework of unconstrained supersymmetric models. In all cases, we address the question of whether it is possible to saturate the current experimental sensitivity on $R_{K}$ while in agreement with the recent limits on $B$-meson decay observables (in particular BR($B_{s}\toμ^+μ^-$) and R($B_{u}\toτν$)), as well as BR($τ\to e γ$) and available collider constraints. Our findings reveal that in view of the recent bounds, and even when enhanced by effective sources of flavour violation in the right-handed $\tilde{e}-\tildeτ$ sector, constrained supersymmetric (seesaw) models typically provide excessively small contributions to $R_{K}$. Larger contributions can be found in more general settings, where the charged Higgs mass can be effectively lowered, and even further enhanced in the unconstrained MSSM. However, our analysis clearly shows that even in this last case SUSY contributions to $R_{K}$ are still unable to saturate the current experimental bounds on this observable, especially due to a strong tension with the $B_{u}\toτν$ bound.

hep-ph↗

arXiv2012

Tree-level lepton universality violation in the presence of sterile neutrinos: impact for $R_K$ and $R_π$

We consider a tree-level enhancement to the violation of lepton flavour universality in light meson decays arising from modified $W \ell ν$ couplings in the standard model minimally extended by sterile neutrinos. Due to the presence of additional mixings between the active (left-handed) neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix intervening in charged currents might lead to a tree-level enhancement of $R_{P} = Γ(P \to e ν) / Γ(P \to μν)$, with $P=K, π$. We illustrate these enhancements in the case of the inverse seesaw model, showing that one can saturate the current experimental bounds on $Δr_{K}$ (and $Δr_π$), while in agreement with the different experimental and observational constraints.

hep-ph↗

arXiv2012

Lepton flavour violation: physics potential of a Linear Collider

We revisit the potential of a Linear Collider concerning the study of lepton flavour violation, in view of new LHC bounds and of the (very) recent developments in lepton physics. Working in the framework of a type I supersymmetric seesaw, we evaluate the prospects of observing seesaw-induced lepton flavour violating final states of the type e μ+ missing energy, arising from e+ e- and e- e- collisions. In both cases we address the potential background from standard model and supersymmetric charged currents. We also explore the possibility of electron and positron beam polarisation. The statistical significance of the signal, even in the absence of kinematical and/or detector cuts, renders the observation of such flavour violating events feasible over large regions of the parameter space. We further consider the μ-μ- + E^T_miss final state in the e- e- beam option finding that, due to a very suppressed background, this process turns out to be a truly clear probe of a supersymmetric seesaw, assuming the latter to be the unique source of lepton flavour violation.

hep-ph↗

arXiv2011

Probing the supersymmetric type III seesaw: LFV at low-energies and at the LHC

We consider a supersymmetric type III seesaw, where the additional heavy states are embedded into complete SU(5) representations to preserve gauge coupling unification. Complying with phenomenological and experimental constraints strongly tightens the viable parameter space of the model. In particular, one expects very characteristic signals of lepton flavour violation both at low-energies and at the LHC, which offer the possibility of falsifying the model.

hep-ph↗

arXiv2011

Interplay of LFV and slepton mass splittings at the LHC as a probe of the SUSY seesaw

We study the impact of a type-I SUSY seesaw concerning lepton flavour violation (LFV) both at low-energies and at the LHC. The study of the di-lepton invariant mass distribution at the LHC allows to reconstruct some of the masses of the different sparticles involved in a decay chain. In particular, the combination with other observables renders feasible the reconstruction of the masses of the intermediate sleptons involved in $ χ_2^0\to \tilde \ell \,\ell \to \ell \,\ell\,χ_1^0$ decays. Slepton mass splittings can be either interpreted as a signal of non-universality in the SUSY soft breaking-terms (signalling a deviation from constrained scenarios as the cMSSM) or as being due to the violation of lepton flavour. In the latter case, in addition to these high-energy processes, one expects further low-energy manifestations of LFV such as radiative and three-body lepton decays. Under the assumption of a type-I seesaw as the source of neutrino masses and mixings, all these LFV observables are related. Working in the framework of the cMSSM extended by three right-handed neutrino superfields, we conduct a systematic analysis addressing the simultaneous implications of the SUSY seesaw for both high- and low-energy lepton flavour violation. We discuss how the confrontation of slepton mass splittings as observed at the LHC and low-energy LFV observables may provide important information about the underlying mechanism of LFV.

hep-ph↗

arXiv2010

Phenomenology of LFV at low-energies and at the LHC: strategies to probe the SUSY seesaw

We study the impact of a type-I SUSY seesaw concerning lepton flavour violation (LFV) at low-energies and at the LHC. At the LHC, $ χ_2^0\to \tilde \ell \,\ell \to \ell \,\ell\,χ_1^0$ decays, in combination with other observables, render feasible the reconstruction of the masses of the intermediate sleptons, and hence the study of $\ell_i - \ell_j$ mass differences. If interpreted as being due to the violation of lepton flavour, high-energy observables, such as large slepton mass splittings and flavour violating neutralino and slepton decays, are expected to be accompanied by low-energy manifestations of LFV such as radiative and three-body lepton decays. We discuss how to devise strategies based in the interplay of slepton mass splittings as might be observed at the LHC and low-energy LFV observables to derive important information on the underlying mechanism of LFV.

hep-ph↗

arXiv2009

Phenomenology of the constrained NMSSM

We discuss several phenomenological aspects of the fully constrained version of the next-to-minimal supersymmetric extension of the standard model (cNMSSM). Assuming universal boundary conditions at a high energy scale for the soft supersymmetry-breaking gaugino, sfermion and Higgs masses as well as for the trilinear interactions, we find that the model can satisfy all present constraints from colliders and cosmological data on dark matter, B- and muon-physics. The phenomenologically viable region of the parameter space of the cNMSSM can be described by essentially one single parameter as the universal gaugino mass parameter M_{1/2}, and corresponds to small values for the universal scalar mass m_0. The lightest supersymmetric particle is always a singlino-like neutralino that is almost degenerate with the lightest tau slepton. We study the particle spectrum of the model and its signatures at the LHC, such as possibly long-lived tau sleptons at the end of decay chains, that would distinguish the cNMSSM from the constrained MSSM.

hep-ph↗