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A. Vicente

Publications and source records attributed to A. Vicente.

At least 19 recordsLinked to original sources
arXiv2022

Intrinsic polynomial squeezing for Balakrishnan-Taylor beam models

We explore the energy decay properties related to a model in extensible beams with the so-called energy damping. We investigate the influence of the nonloncal damping coefficient in the stability of the model. We prove, for the first time, that the corresponding energy functional is squeezed by polynomial-like functions involving the power of the damping coefficient, which arises intrinsically from the Balakrishnan-Taylor beam models. As a consequence, it is shown that such models with nonlocal energy damping are never exponentially stable in its essence.

math.AP↗
arXiv2017

Remarks on the Standard Model predictions for $R(D)$ and $R(D^*)$

Semileptonic $b \to c$ transitions, and in particular the ratios $R(D^{(*)}) = \frac{Γ(B \rightarrow D^{(*)} τν)}{Γ(B \rightarrow D^{(*)} \ell ν)}$, can be used to test the universality of the weak interactions. In light of the recent discrepancies between the experimental measurements of these observables by BaBar, Belle and LHCb and the Standard Model predicted values, we study the robustness of the latter. Our analysis reveals that $R(D)$ might be enhanced by lepton mass effects associated to the mostly unknown scalar form factor. In constrast, the Standard Model prediction for $R(D^*)$ is found to be more robust, since possible pollutions from $B^*$ contributions turn out to be negligibly small, which indicates that $R(D^*)$ is a promising observable for searches of new physics.

hep-ph↗
arXiv2015

Diboson anomaly: heavy Higgs resonance and QCD vector-like exotics

The ATLAS collaboration (and also CMS) has recently reported an excess over Standard Model expectations for gauge boson pair production in the invariant mass region $1.8-2.2$ TeV. In the light of these results, we argue that such signal might be the first manifestation of the production and further decay of a heavy CP-even Higgs resulting from a type-I Two Higgs Doublet Model. We demonstrate that in the presence of colored vector-like fermions, its gluon fusion production cross-section is strongly enhanced, with the enhancement depending on the color representation of the new fermion states. Our findings show that barring the color triplet case, any QCD "exotic" representation can fit the ATLAS result in fairly large portions of the parameter space. We have found that if the diboson excess is confirmed and this mechanism is indeed responsible for it, then the LHC Run-2 should find: (i) a CP-odd scalar with mass below $\sim 2.3$ TeV, (ii) new colored states with masses below $\sim 2$ TeV, (iii) no statistically significant diboson events in the $W^\pm Z$ channel, (iv) events in the triboson channels $W^\pm W^\mp\,Z$ and $ZZZ$ with invariant mass amounting to the mass of the CP-odd scalar.

hep-ph↗
arXiv2014

Explaining the CMS Higgs flavor violating decay excess

Direct searches for lepton flavor violating Higgs boson decays in the $τμ$ channel have been recently reported by the CMS collaboration. The results display a slight excess of signal events with a significance of 2.5$σ$, which translates into a branching ratio of about $1$%. By interpreting these findings as a hint for beyond the standard model physics, we show that the Type-III 2HDM is capable of reproducing such signal while at the same time satisfying vacuum stability, perturbativity, electroweak precision data, measured Higgs standard decay modes and low-energy lepton flavor violating constraints. We have found that the allowed signal strength ranges for the $b\bar b$, $WW^*$ and $ZZ^*$ standard channels shrink as soon as BR$(h\toτμ)\sim 1$% is enforced. Thus, we point out that if the excess persists, improved measurements of these channels may be used to test our Type-III 2HDM scenario.

hep-ph↗
arXiv2014

Lepton flavor violation in low-scale seesaw models: SUSY and non-SUSY contributions

Taking the supersymmetric inverse seesaw mechanism as the explanation for neutrino oscillation data, we investigate charged lepton flavor violation in radiative and 3-body lepton decays as well as in neutrinoless $μ-e$ conversion in muonic atoms. In contrast to former studies, we take into account all possible contributions: supersymmetric as well as non-supersymmetric. We take CMSSM-like boundary conditions for the soft supersymmetry breaking parameters. We find several regions where cancellations between various contributions exist, reducing the lepton flavor violating rates by an order of magnitude compared to the case where only the dominant contribution is taken into account. This is in particular important for the correct interpretation of existing data as well as for estimating the reach of near future experiments where the sensitivity will be improved by one to two orders of magnitude. Moreover, we demonstrate that ratios like BR($τ\to 3 μ$)/BR($τ\to μe^+ e^-$) can be used to determine whether the supersymmetric contributions dominate over the $W^\pm$ and $H^\pm$ contributions or vice versa.

hep-ph↗
arXiv2014

FlavorKit: a brief overview

We give a brief overview of FlavorKit, a kit for the study of flavor observables beyond the standard model. In contrast to previous flavor codes, FlavorKit is not restricted to a single model, but can be used to obtain predictions for flavor observables in a wide range of models. FlavorKit can be used in two different ways. The basic usage of FlavorKit allows for the computation of a large number of lepton and quark flavor observables, using generic analytical expressions for the Wilson coefficients of the relevant operators. The setup is based on the public codes SARAH and SPheno, and thus allows for the analytical and numerical computation of the observables in the model defined by the user. If necessary, the user can also go beyond the basic usage and define his own operators and/or observables. For this purpose, a Mathematica package called PreSARAH has been developed. This tool uses FeynArts and FormCalc to compute generic expressions for the required Wilson coefficients at the tree- and 1-loop levels. Similarly, the user can easily implement new observables. With all these tools properly combined, the user can obtain analytical and numerical results for the observables of his interest in the model of his choice.

hep-ph↗
arXiv2014

Leptogenesis with a dynamical seesaw scale

In the simplest type-I seesaw leptogenesis scenario right-handed neutrino annihilation processes are absent. However, in the presence of new interactions these processes are possible and can affect the resulting $B-L$ asymmetry in an important way. A prominent example is provided by models with spontaneous lepton number violation, where the existence of new dynamical degrees of freedom can play a crucial role. In this context, we provide a model-independent discussion of the effects of right-handed neutrino annihilations. We show that in the weak washout regime, as long as the scattering processes remain slow compared with the Hubble expansion rate throughout the relevant temperature range, the efficiency can be largely enhanced, reaching in some cases maximal values. Moreover, the $B-L$ asymmetry yield turns out to be independent upon initial conditions, in contrast to the "standard" case. On the other hand, when the annihilation processes are fast, the right-handed neutrino distribution tends to a thermal one down to low temperatures, implying a drastic suppression of the efficiency which in some cases can render the $B-L$ generation mechanism inoperative.

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↗
arXiv2013

Charged lepton flavor violation beyond minimal supersymmetry

We discuss charged lepton flavor violation in supersymmetric models with extended leptonic sectors at low energies. Contrary to the usual high-scale realizations of the seesaw mechanism, these non-minimal supersymmetric models have new superfields and/or interactions at the supersymmetric scale. As a consequence of this, the resulting lepton flavor violating phenomenology may be very different from that of minimal models.

hep-ph↗
arXiv2013

Flavor origin of R-parity

Proton stability is guaranteed in the MSSM by assuming a discrete symmetry, R-parity. However, there are additional R-parity conserving higher dimensional operators which violate lepton and baryon numbers and induce fast proton decay. Here we study the possibility that all renormalizable, as well as the most dangerous non-renormalizable, R-parity violating operators are forbidden by a flavor symmetry, providing a common origin for fermion mixing and proton and dark matter stability. We propose a specific model based on the Delta(27) discrete symmetry.

hep-ph↗
arXiv2013

Sterile neutrinos and $R_K$

We study the violation of lepton flavour universality in light meson decays due to the presence of non-zero mixings between the active neutrinos with new sterile states. The modified $W \ell ν$ vertices, arising from a non-unitarity 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 $P=K, π$. These enhancements are illustrated in the case of the inverse seesaw, 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↗
arXiv2013

Bilinear R-parity violation with flavor symmetry

Bilinear R-parity violation (BRPV) provides the simplest intrinsically supersymmetric neutrino mass generation scheme. While neutrino mixing parameters can be probed in high energy accelerators, they are unfortunately not predicted by the theory. Here we propose a model based on the discrete flavor symmetry $A_4$ with a single R-parity violating parameter, leading to (i) correct Cabbibo mixing given by the Gatto-Sartori-Tonin formula, and a successful unification-like b-tau mass relation, and (ii) a correlation between the lepton mixing angles $θ_{13}$ and $θ_{23}$ in agreement with recent neutrino oscillation data, as well as a (nearly) massless neutrino, leading to absence of neutrinoless double beta decay.

hep-ph↗
arXiv2012

Neutrino masses from R-parity violation with a $Z_3$ symmetry

We consider a supersymmetric model where the neutrino mass matrix arises from bilinear and trilinear R-parity violation, both restricted by a $Z_3$ flavor symmetry. Assuming flavor blind soft supersymmetry (SUSY) breaking conditions, corrected at low energies due to running effects, we obtain a neutrino mass matrix in agreement with oscillation data. In particular, a large $θ_{13}$ angle can be easily accommodated. We also find a correlation between the reactor and atmospheric mixing angles. This leads in some scenarios to a clear deviation from $θ_{23} = π/4$. The Lightest Supersymmetric Particle (LSP) decay, dominated by the trilinear couplings, provides a direct way to test the model at colliders.

hep-ph↗
arXiv2012

General NMSSM signatures at the LHC

We study the possible LHC collider signatures in the next-to-minimal supersymmetric Standard Model (NMSSM). The general NMSSM consists of 29 supersymmetric (SUSY) particles which can be mass ordered in $29! \simeq 9 \cdot 10^{30}$ ways. To reduce the number of hierarchies to a more manageable amount we assume a degeneracy of the sfermions of the first two generations with the same quantum numbers. Further assumptions about the neutralino and chargino masses leave 15 unrelated parameters. We check all $15! \approx 10^{12}$ relevant mass orderings for the dominant decay chains and the corresponding collider signatures at the LHC. As preferred signatures, we consider charged leptons, missing transverse momentum, jets, and W, Z or Higgs bosons. We present the results for three different choices of the singlet to Higgs coupling $λ$: (a) small: $O(λ)< O(Y_τ)$, (b) large: $O(λ)\simeq O(Y_{top},Y_b, Y_τ)$ and (c) dominant: $O(λ)>O(Y_{top})$. We compare these three scenarios with the MSSM expectations as well as among each other. We also mention a possible mass hierarchy leading to 7 jets plus 1 lepton signatures at the LHC and comment briefly on the consequence of possible R-parity violation.

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

Constrained SUSY seesaws with a 125 GeV Higgs

Motivated by the ATLAS and CMS discovery of a Higgs-like boson with a mass around 125 GeV, and by the need of explaining neutrino masses, we analyse the three canonical SUSY versions of the seesaw mechanism (type I, II and III) with CMSSM boundary conditions. In type II and III cases, SUSY particles are lighter than in the CMSSM (or the constrained type I seesaw), for the same set of input parameters at the universality scale. Thus, to explain $m_{h^0} \simeq 125 GeV$ at low energies, one is forced into regions of parameter space with very large values of $m_0$, $M_{1/2}$ or $A_0$. We compare the squark and gluino masses allowed by the ATLAS and CMS ranges for $m_{h^0}$ (extracted from the 2011-2012 data), and discuss the possibility of distinguishing seesaw models in view of future results on SUSY searches. In particular, we briefly comment on the discovery potential of LHC upgrades, for squark/gluino mass ranges required by present Higgs mass constraints. A discrimination between different seesaw models cannot rely on the Higgs mass data alone, therefore we also take into account the MEG upper limit on BR$(μ\to e γ)$ and show that, in some cases, this may help to restrict the SUSY parameter space, as well as to set complementary limits on the seesaw scale.

hep-ph↗
arXiv2012

General MSSM signatures at the LHC with and without R-parity

We present the possible signatures appearing in general realizations of the MSSM based on 14 unrelated mass parameters at the SUSY scale. The parameters of the general MSSM are reduced by assuming a degeneracy of the sfermions of the first two generations with the same quantum numbers. We also assume no mass-splitting between neutral and charged Higgsinos. We do allow for separate soft breaking terms for the third generation sfermons. We consider all possible resulting $14! \approx 9 \cdot10^{10}$ relevant mass orderings and check for the dominant decay cascades and the corresponding collider signatures. In determining the dominant decay modes we assume that mixing between sparticles is sub-dominant. As preferred signatures, we consider charged leptons, missing transverse momentum, jets, and $W, Z$ or Higgs bosons. We include also the cases of bi- and trilinear R-parity violation and show that specific signatures can be used to distinguish the different scenarios.

hep-ph↗