arXiv subjects

Smaragda Lola

Publications and source records attributed to Smaragda Lola.

18 recordsLinked to original sources

arXiv2024

Muon g-2 and lepton flavor violation in supersymmetric GUTs

We present a class of supersymmetric (SUSY) GUT models that can explain the apparent discrepancy between the SM predictions and experimental values of muon g-2 while providing testable signals for lepton flavor violation in charged lepton decays. Moreover, these models predict LSP neutralino abundance that is compatible with the Planck dark matter bounds. We find that scenarios in the framework of $SU(4)_c\times SU(2)_L\times SU(2)_R$ unification, with additional symmetries to explain fermion masses and neutrino oscillations, provide interesting benchmarks for the search of SUSY by correlating a possible manifestation of it in dark matter, rare lepton decays and LHC signals.

hep-ph↗

arXiv2023

Primordial black holes and gravitational waves from non-canonical inflation

Primordial black holes (PBHs) can generically form in inflationary setups through the collapse of enhanced cosmological perturbations, providing us access to the early Universe through their associated observational signatures. In the current work we propose a new mechanism of PBH production within non-canonical inflation, using a class of steep-deformed inflationary potentials compatible with natural values for the non-canonical exponents. In particular, by requiring significant PBH production we extract constraints on the non-canonical exponents. Additionally, we find that our scenario can lead to the formation of asteroid-mass PBHs, which can account for the totality of the dark matter, as well as to production of solar-mass PBHs within the LIGO/VIRGO detection band. Finally, we find that the enhanced cosmological perturbations which collapse to form PBHs can produce a stochastic gravitational-wave (GW) background induced by second-order gravitational interactions. Very interestingly, we obtain a GW signal detectable by future GW experiments, in particular by SKA, LISA and BBO.

astro-ph.CO↗

arXiv2021

Inflation with non-canonical scalar fields revisited

We revisit inflation with non-canonical scalar fields by applying deformed-steepness exponential potentials. We show that the resulting scenario can lead to inflationary observables, and in particular to scalar spectral index and tensor-to-scalar ratio, in remarkable agreement with observations. Additionally, a significant advantage of the scenario is that the required parameter values, such as the non-canonicality exponent and scale, as well as the potential exponent and scale, do not need to acquire unnatural values and hence can accept a theoretical justification. Hence, we obtain a significant improvement with respect to alternative schemes, and we present distinct correlations between the model parameters that better fit the data, which can be tested in future probes. This combination of observational efficiency and theoretical justification makes the scenario at hand a good candidate for the description of inflation.

gr-qc↗

arXiv2013

Neutrino textures and charged lepton flavour violation in light of theta13, MEG and LHC data

In light of recent results from the LHC, MEG and neutrino experiments, we revisit the issue of charged lepton flavour violation (LFV) in supersymmetric theories with massive neutrinos, where flavour-violating soft supersymmetry-breaking masses for sleptons are induced naturally by radiative corrections. We link our results to the expectations for neutrino mixing angles in SU(5) enhanced by an abelian flavour symmetry, with particular focus on $θ_{13}$. We focus on the radiative decays $\ell_i \rightarrow \ell_j γ$ and on detection prospects at the LHC and a linear collider (LC). We use supersymmetric parameters consistent with cosmological considerations and with LHC searches for supersymmetry and the Higgs mass. We find a class of scenarios where the LHC may be sensitive to LFV sparticle decays and LFV processes could be detectable at a LC with centre-of-mass energy above 1 TeV, whereas LFV lepton decays may be suppressed by cancellations in the decay amplitudes.

hep-ph↗

arXiv2011

Flavour Structure of R-violating Neutralino Decays at the LHC

We study signatures of R-parity violation in the production of supersymmetric particles at the LHC, and the subsequent decay of the lightest neutralino being the end product of a supersymmetric cascade decay. In doing so, we pay particular attention to the possible flavour structure of the operators, and how one may discriminate between different possibilities. A neutralino LSP would couple to all quarks and leptons and a comparative study of its decays provides an optimal channel for the simultaneous study of all 45 R-violating operators. By studying the expected signals from all these operators, we demonstrate the ability to understand whether more than one coupling dominates, and to map the experimental signatures to operator hierarchies that can then be compared against theoretical models of flavour. Detailed comparisons with backgrounds, including those from MSSM cascade decays are made, using the PYTHIA event simulator.

hep-ph↗

arXiv2008

Supersymmetric Grand Unification and Lepton Universality in K-> l νDecays

Motivated by the prospects for an improved test of lepton universality in K -> l νdecays by the NA62 experiment at CERN, we study predictions for the possible lepton non-universality in K -> l νdecays in supersymmetric models. Violations of μ-e universality in this process may originate from mixing effects in the right-handed slepton sector, providing a unique window into this aspect of supersymmetric flavour physics in the large-\tanβregion. Minimal unification scenarios with universal soft supersymmetry-breaking terms at the GUT scale would predict negligible violation of lepton universality. However, lepton non-universality may be observable in non-minimal grand unified models with higher-dimensional terms contributing to fermion masses, in which case renormalization effects above the GUT scale may enhance the mixing among the right-handed sleptons. This could leads to observable lepton non-universality in K -> l νdecays in specific regions of the parameter space with high \tan β, large A terms and small charged Higgs boson mass. Observable non-universality in K -> l νdecays would be correlated with a large value of BR(τ-> e γ). The experimental upper limit on the electric dipole moment of the electron could be reconciled with leptogenesis, if the latter occurs at a relatively low scale, which would also alleviate the cosmological gravitino problem. Even if lepton non-universality is not seen in the near future, one may nevertheless obtain significant constraints on the model parameters and unknown aspects of right-handed fermion and sfermion mixing.

hep-ph↗

arXiv2007

CP and Lepton-Number Violation in GUT Neutrino Models with Abelian Flavour Symmetries

We study the possible magnitudes of CP and lepton-number-violating quantities in specific GUT models of massive neutrinos with different Abelian flavour groups, taking into account experimental constraints and requiring successful leptogenesis. We discuss SU(5) and flipped SU(5) models that are consistent with the present data on neutrino mixing and upper limits on the violations of charged-lepton flavours and explore their predictions for the CP-violating oscillation and Majorana phases. In particular, we discuss string-derived flipped SU(5) models with selection rules that modify the GUT structure and provide additional constraints on the operators, which are able to account for the magnitudes of some of the coefficients that are often set as arbitrary parameters in generic Abelian models.

hep-ph↗

arXiv2001

CP Violation in the Minimal Supersymmetric Seesaw Model

We study CP violation in the lepton sector of the supersymmetric extension of the Standard Model with three generations of massive singlet neutrinos with Yukawa couplings $Y_ν$ to lepton doublets, in a minimal seesaw model for light neutrino masses and mixing. This model contains six physical CP-violating parameters, namely the phase $δ$ observable in oscillations between light neutrino species, two Majorana phases $ϕ_{1,2}$ that affect $ββ_{0 ν}$ decays, and three independent phases appearing in ${Y_ν}{Y_ν}^\dagger$, that control the rate of leptogenesis. Renormalization of the soft supersymmetry-breaking parameters induces observable CP violation at low energies, including T-odd asymmetries in polarized $μ\to eee$ and $τ\to \ell \ell \ell$ decays, as well as lepton electric dipole moments. In the leading-logarithmic approximation in which the massive singlet neutrinos are treated as degenerate, these low-energy observables are sensitive via ${Y_ν}^\dagger{Y_ν}$ to just one combination of the leptogenesis and light-neutrino phases. We present numerical results for the T-odd asymmetry in polarized $μ\to eee$ decay, which may be accessible to experiment, but the lepton electric dipole moments are very small in this approximation. To the extent that the massive singlet neutrinos are not degenerate, low-energy observables become sensitive also to two other combinations of leptogenesis and light-neutrino phases, in this minimal supersymmetric seesaw model.

hep-ph↗

arXiv2000

Extra Dimensions: A View from the Top

In models with compact extra dimensions, where the Standard Model fields are confined to a 3+1 dimensional hyperplane, the $t \bar t$ production cross-section at a hadron collider can receive significant contributions from multiple exchange of KK modes of the graviton. These are carefully computed in the well-known ADD and RS scenarios, taking the energy dependence of the sum over graviton propagators into account. Using data from Run-I of the Tevatron, 95% C.L. bounds on the parameter space of both models are derived. For Run-II of the Tevatron and LHC, discovery limits are estimated.

hep-ph↗

arXiv2000

Renormalisation effects of neutrino masses and interactions

1. Data and implications. 2. Neutrino threshold effects. 3. Renormalisation of the neutrino mass operator and stability properties of neutrino textures. 4. Neutrino thresholds effects and Yukawa unification. 5. Renormalisation-induced lepton-flavour-violating processes from non-zero neutrino masses. 6. Summary.

hep-ph↗

arXiv2000

Three-body Supersymmetric Top Decays

We discuss three-body supersymmetric top decays, in schemes both with and without R-parity conservation, assuming that sfermion masses are larger than m_t. We find that MSSM top decays into chargino/neutralino pairs have a strong kinematic suppression in the region of the supersymmetric parameter space consistent with the LEP limits, with a decay width =< 10^{-5} GeV. MSSM top decays into neutralino pairs have less kinematical suppression, but require a flavour-changing vertex, and are likely to have a smaller rate. On the other hand, R-violating decays to single charginos, neutralinos and conventional fermions can be larger for values of the R-violating couplings still permitted by other upper limits. The cascade decays of the charginos and neutralinos may lead to spectacular signals with explicit lepton-number violation, such as like-sign lepton events.

hep-ph↗

arXiv1999

Can Neutrinos be Degenerate in Mass?

We reconsider the possibility that the masses of the three light neutrinos of the Standard Model might be almost degenerate and close to the present upper limits from Tritium beta decay and cosmology. In such a scenario, the cancellations required by the latest upper limit on neutrinoless double-beta decay enforce near-maximal mixing that may be compatible only with the vacuum-oscillation scenario for solar neutrinos. We argue that the mixing angles yielded by degenerate neutrino mass-matrix textures are not in general stable under small perturbations. We evaluate within the MSSM the generation-dependent one-loop renormalization of neutrino mass-matrix textures that yielded degenerate masses and large mixing at the tree level. We find that m_{nu_e} > m_{nu_mu} > m_{nu_tau} after renormalization, excluding MSW effects on solar neutrinos. We verify that bimaximal mixing is not stable, and show that the renormalized masses and mixing angles are not compatible with all the experimental constraints, even for tanbeta as low as unity. These results hold whether the neutrino masses are generated by a see-saw mechanism with heavy neutrinos weighing approx. 10^{13} GeV or by non-renormalizable interactions at a scale approx. 10^5 GeV. We also comment on the corresponding renormalization effects in the minimal Standard Model, in which m_{nu_e} < m_{nu_mu} < m_{nu_tau}. Although a solar MSW effect is now possible, the perturbed neutrino masses and mixings are still not compatible with atmospheric- and solar-neutrino data.

hep-ph↗

arXiv1999

Aspects of Neutrino Masses and Lepton-Number Violation in the light of the Super-Kamiokande data

We discuss aspects of neutrino masses and lepton-number violation, in the light of the observations of Super-Kamiokande. As a first step, we use the data from various experiments, in order to obtain a phenomenological understanding of neutrino mass textures. We then investigate how the required patterns of neutrino masses and mixings are related to the flavour structure of the underlying theory. In supersymmetric extensions of the Standard Model, renormalisation group effects can have important implications: for small tanb, bottom-tau unification indicates the presence of significant muon-tau flavour mixing. The evolution of the neutrino mixing may be described by simple semi-analytic expressions, which confirm that, for large tanb, very small mixing at the GUT scale may be amplified to maximal mixing at low energies, and vice versa. Passing to specific models, we first discuss the predictions for neutrino masses in different GUT models (including superstring-embedded solutions). Imposing the requirement for successful leptogenesis may give additional constraints on the generic structure of the neutrino mass textures. Finally, we discuss direct ways to look for lepton-number violation in ultra-high energy neutrino interactions.

hep-ph↗

arXiv1999

Neutrino masses from U(1) symmetries and the Super-Kamiokande data

Motivated by the Super-Kamiokande data, we revisit models with U(1) symmetries and discuss the origin of neutrino masses and mixings in such theories. We show that, in models with just three light neutrinos and a hierarchy of neutrino masses, large (2-3) mixing fixes the lepton doublet U(1) charges and is thus related to the structure of the charged lepton mass matrix. We discuss the fermion mass structure that follows from the Abelian family symmetry with an extended gauge group. Requiring that the quark and lepton masses be ordered by the family symmetry, we identify the most promising scheme. This requires large, but not necessarily maximal, mixing in the mu-tau sector and gives small e-mu mixing in the range that is required for the small angle solution of the solar neutrino deficit.

hep-ph↗

arXiv1998

Violation of Time-Reversal Invariance and CPLEAR Measurements

Motivated by the recent CPLEAR measurement on the time-reversal non-invariance, we review the situation concerning the experimental measurements of charge conjugation, parity violation and time reversibility, in systems with non-Hermitean Hamiltonians. This includes in particular neutral meson systems, like K0-barK0, D0-barD0 and B0-barB0. We discuss the formalism that describes particle-antiparticle mixing and time evolution of states, paying particular emphasis to the orthogonality conditions of incoming and outgoing states. As a result, we confirm that the CPLEAR experiment makes a direct measurement of violation of time-reversal without any assumption of unitarity and CPT-violation. The asymmetry which signifies T-violation, is found to be independent of time and decay processes.

hep-ph↗

arXiv1998

Scale of Leptogenesis

We study the scale at which one can generate the lepton asymmetry of the universe which could then get converted to a baryon asymmetry during the electroweak phase transition. We consider the possibility that the Yukawa couplings are small but sufficiently large to generate enough lepton asymmetry. This forbids the possibility of the $(B-L)$ breaking scale being the electroweak scale.

hep-ph↗

arXiv1998

Hierarchies of R-violating interactions from Family Symmetries

We investigate the possibility of constructing models of R-violating LQD Yukawa couplings using a single U(1) flavour-symmetry group and supermultiplet charge assignments that are compatible with the known hierarchies of quark and lepton masses. The mismatch of mass and current eigenstates inferred from the known charged-current mixing induces the propagation of strong phenomenological constraints on some R-violating couplings to many others. Applying these constraints, we look for flavour-symmetry models that are consistent with different squark-production hypotheses devised to explain the possible HERA large-Q^2 anomaly. The e^+ d -> stop interpretation of the HERA data is accommodated relatively easily, at the price of postulating an extra parity. The e^+ s -> stop interpretation of the events requires models to have only small (2,3) mixing in the down quark sector. The e^+ d -> scharm mechanism cannot be accommodated without large violations of squark-mass universality, due to the very strong experimental constraints on R-violating operators. We display a model in which baryon decay due to dangerous dimension-five operators is automatically suppressed.

hep-ph↗

arXiv1995

Small scale structure predictions from discrete symmetry breaking - early quasar formation

We discuss the local density fluctuations which arise due to the topological defects that appear after the phase transition of light pseudo-Goldstone bosons. It has been found that in a post-inflationary universe the fluctuations of these defects at large scales may have led to galaxy formation, while being consistent with the measurements of the cosmic microwave background radiation. Here we show that, at the local level, the fluctuations may be sufficiently large to lead to the production of smaller structures (ie quasars) with the observed distribution, which peaks at $z=2$ and drops rapidly for higher redshifts. Moreover it may be possible that a limited number of quasars are produced at redshifts of order 10, much earlier than what hot and cold dark matter scenarios predict. Although in this letter we work in the parameter space which is optimal for the generation of large scale structure as well, these features are generic for a wide class of domain wall models.

hep-ph↗