arXiv subjects

A. Masiero

Publications and source records attributed to A. Masiero.

At least 19 recordsLinked to original sources

arXiv2020

New physics at the MUonE experiment at CERN

A confirmation of the long-standing muon $g$-2 discrepancy requires both experimental and theoretical progress. On the theory side, the hadronic corrections are under close scrutiny, as they induce the leading uncertainty of the Standard Model prediction. Recently, the MUonE experiment has been proposed at CERN to provide a new determination of the leading hadronic contribution to the muon $g$-2 via the measurement of the differential cross section of muon-electron scattering. The precision expected at this experiment raises the question whether possible new-physics (NP) could affect its measurements. We address this issue studying possible NP signals in muon-electron collisions due to heavy or light mediators, depending on whether their mass is higher or lower than ${\cal O} (1{\rm GeV})$. We analyze the former in a model-independent way via an effective field theory approach, whereas for the latter we focus on scenarios with light scalar and vector bosons. Using existing experimental bounds, we show that possible NP effects in muon-electron collisions are expected to lie below MUonE's sensitivity. This result confirms and reinforces the physics case of the MUonE proposal.

hep-ph↗

arXiv2016

Contributions of axion-like particles to lepton dipole moments

Contributions of a spin 0 axion-like particle (ALP) to lepton dipole moments, g-2 and EDMs, are examined. Barr-Zee and light-by-light loop effects from a light pseudoscalar ALP are found to be capable of resolving the long-standing muon g-2 discrepancy at the expense of relatively large ALP-gammagamma couplings. The compatibility of such large couplings with direct experimental constraints and perturbative unitarity bounds is discussed. Future tests of such a scenario are described. For CP violating ALP couplings, the electron EDM is found to probe much smaller, theoretically more easily accommodated ALP interactions. Future planned improvement in electron EDM searches is advocated as a way to not only significantly constrain ALP parameters but also, to potentially unveil a new source of CP violation which could have far reaching ramifications.

hep-ph↗

arXiv2012

Status of Supersymmetric Seesaw in SO(10) models

We report on the status of supersymmetric seesaw models in the light of recent experimental results on $μ\to e + γ$, $θ_{13}$ and the light Higgs mass at the LHC. SO(10)-like relations are assumed for neutrino Dirac Yukawa couplings and two cases of mixing, one large, PMNS-like, and another small, CKM-like, are considered. It is shown that for the large mixing case, only a small range of parameter space with moderate $\tan β$ is still allowed. This remaining region can be ruled out by an order of magnitude improvement in the current limit on BR($μ\to e + γ$). We also explore a model with non-universal Higgs mass boundary conditions at the high scale. It is shown that the renormalization group induced flavor violating slepton mass terms are highly sensitive to the Higgs boundary conditions. Depending on the choice of the parameters, they can either lead to strong enhancements or cancellations within the flavor violating terms. Such cancellations might relax the severe constraints imposed by lepton flavor violation compared to mSUGRA. Nevertheless for a large region of parameter space the predicted rates lie within the reach of future experiments once the light Higgs mass constraint is imposed. We also update the potential of the ongoing and future experimental searches for lepton flavor violation in constraining the supersymmetric parameter space.

hep-ph↗

arXiv2012

Flavour and Collider Interplay for SUSY at LHC7

The current 7 TeV run of the LHC experiment shall be able to probe gluino and squark masses up to values larger than 1 TeV. Assuming that hints for SUSY are found in the jets plus missing energy channel by the end of a 5 fb$^{-1}$ run, we explore the flavour constraints on three models with a CMSSM-like spectrum: the CMSSM itself, a Seesaw extension of the CMSSM, and Flavoured CMSSM. In particular, we focus on decays that might have been measured by the time the run is concluded, such as $B_s\toμμ$ and $μ\to eγ$. We also analyse constraints imposed by neutral meson bounds and electric dipole moments. The interplay between collider and flavour experiments is explored through the use of three benchmark scenarios, finding the flavour feedback useful in order to determine the model parameters and to test the consistency of the different models.

hep-ph↗

arXiv2011

SUSY Flavour at LHC7

The current 7 TeV run of the LHC experiment shall be able to probe gluino and squark masses up to values of about 1 TeV. Assuming that hints for SUSY are found by the end of a 2 fb$^{-1}$ run, we explore the flavour constraints on the parameter space of the CMSSM, with and without massive neutrinos. In particular, we focus on decays that might have been measured by the time the run is concluded, such as $B_s\toμμ$ and $μ\to eγ$. We also briefly show the impact such a collider--flavour interplay would have on a Flavoured CMSSM.

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arXiv2010

Thermal Relics in Modified Cosmologies: Bounds on Evolution Histories of the Early Universe and Cosmological Boosts for PAMELA

Alternative cosmologies, based on extensions of General Relativity, predict modified thermal histories in the Early Universe during the pre Big Bang Nucleosynthesis (BBN) era, epoch which is not directly constrained by cosmological observations. When the expansion rate is enhanced with respect to the standard case, thermal relics typically decouple with larger relic abundances. The correct value of the relic abundance is therefore obtained for larger annihilation cross--sections, as compared to standard cosmology. A direct consequence is that indirect detection rates are enhanced. Extending previous analyses of ours, we derive updated astrophysical bounds on the dark matter annihilation cross sections and use them to constrain alternative cosmologies in the pre--BBN era. We also determine the characteristics of these alternative cosmologies in order to provide the correct value of relic abundance for a thermal relic for the (large) annihilation cross--section required to explain the PAMELA results on the positron fraction, therefore providing a "cosmological boost" solution to the dark matter interpretation of the PAMELA data.

astro-ph.CO↗

arXiv2010

Flavor Physics in the Quark Sector

One of the major challenges of particle physics has been to gain an in-depth understanding of the role of quark flavor and measurements and theoretical interpretations of their results have advanced tremendously: apart from masses and quantum numbers of flavor particles, there now exist detailed measurements of the characteristics of their interactions allowing stringent tests of Standard Model predictions. Among the most interesting phenomena of flavor physics is the violation of the CP symmetry that has been subtle and difficult to explore. Till early 1990s observations of CP violation were confined to neutral $K$ mesons, but since then a large number of CP-violating processes have been studied in detail in neutral $B$ mesons. In parallel, measurements of the couplings of the heavy quarks and the dynamics for their decays in large samples of $K, D$, and $B$ mesons have been greatly improved in accuracy and the results are being used as probes in the search for deviations from the Standard Model. In the near future, there will be a transition from the current to a new generation of experiments, thus a review of the status of quark flavor physics is timely. This report summarizes the results of the current generation of experiments that is about to be completed and it confronts these results with the theoretical understanding of the field.

hep-ph↗

arXiv2009

FCNC and CP Violation Observables in a SU(3)-flavoured MSSM

A non-Abelian flavour symmetry in a minimal supersymmetric standard model can explain the flavour structures in the Yukawa couplings and simultaneously solve the SUSY flavour problem. Similarly the SUSY CP problem can be solved if CP is spontaneously broken in the flavour sector. In this work, we present an explicit example of these statements with a SU(3) flavour symmetry and spontaneous CP violation. In addition, we show that it is still possible to find some significant deviation from the SM expectations as far as FCNC and CP violation are concerned. We find that large contributions can be expected in lepton flavour violating decays, as $μ\to e γ$ and $τ\to μγ$, electric dipole moments, $d_e$ and $d_n$ and kaon CP violating processes as $ε_K$. Thus, these flavoured MSSM realizations are phenomenologically sensitive to the experimental searches in the realm of flavor and CP vioation physics.

hep-ph↗

arXiv2009

SU(3) Flavour Symmetries and CP Violation

In order to satisfy current FCNC and CP violation bounds, SUSY flavour structures cannot be generic. An interesting solution to these SUSY Flavour and CP Problems lies on the use of an SU(3) family symmetry which spontaneously breaks CP symmetry. Typical observables of such a model are electric dipole moments and LFV processes. In addition, these models can give contributions to CP violation in neutral meson mixing. We show how the latter can be used to restrict the allowed SUSY parameter space, and induce correlations between LFV and EDM predictions.

hep-ph↗

arXiv2009

B -> K pi Puzzle and New Sources of CP Violation in Supersymmetry

The difference between the CP asymmetries of the $B^0 \to K^+ π^-$ and $B^+ \to K^+ π^0$ decays has been recently confirmed with an evidence larger than $5σ$'s. We discuss it as a possible signal of new physics associated with new (large) CP violation in the electroweak penguin contributions. We propose a supersymmetry breaking scheme where such new sources of CP violation occur in the flavor non-universal trilinear scalar couplings.

hep-ph↗

arXiv2008

Anatomy and Phenomenology of the Lepton Flavor Universality in SUSY Theories

High precision electroweak tests, such as deviations from the Standard Model expectations of the Lepton Flavor Universality breaking in K->lnu (with l= e or mu), represent a powerful tool to test the Standard Model and, hence, to constrain or obtain indirect hints of New Physics beyond it. We explore such a possibility within Supersymmetric theories. Interestingly enough, a process that in itself does not need lepton flavor violation to occur, i.e. the violation of mu-e non-universality in K_l2, proves to be quite effective in constraining not only relevant regions of SUSY models where lepton flavor is conserved, but even those where specific lepton flavor violating contributions arise. Indeed, a comparison with analogous bounds coming from tau lepton flavor violating decays shows the relevance of the measurement of R_K to probe Lepton Flavor Violation in SUSY. We outline the role and the interplay of the direct New Physics searches at the LHC with the indirect searches performed by LFU tests.

hep-ph↗

arXiv2007

Enlarging mSUGRA parameter space by decreasing pre-BBN Hubble rate in Scalar-Tensor Cosmologies

We determine under what conditions Scalar Tensor cosmologies predict an expansion rate which is reduced as compared to the standard General Relativity case. We show that ST theories with a single matter sector typically predict an enchanced Hubble rate in the past, as a consequence of the requirement of an attractive fixed point towards General Relativity at late times. Instead, when additional matter sectors with different conformal factors are added, the late time convergence to General Relativity is mantained and at the same time a reduced expansion rate in the past can be driven. For suitable choices of the parameters which govern the scalar field evolution, a sizeable reduction (up to about 2 orders of magnitude) of the Hubble rate prior to Big Bang Nucleosynthesis can be obtained. We then discuss the impact of these cosmological models on the relic abundance of dark matter is minimal Supergravity models: we show that the cosmologically allowed regions in parameter space are significantly enlarged, implying a change in the potential reach of LHC on the neutralino phenomenology.

hep-ph↗

arXiv2007

Flavour Physics and Grand Unification

In spite of the enormous success of the Standard Model (SM), we have strong reasons to expect the presence of new physics beyond the SM at higher energies. The idea of the Grand Unification of all the known interactions in nature is perhaps the main reason behind these expectations. Low-energy Supersymmetry is closely linked with grand unification as a solution of the hierarchy problem associated with the ratio M_GUT / M_Z. In these lectures we will provide a general overview of Grand Unification and Supersymmetry with special emphasis on their phenomenological consequences at low energies. We will analyse the flavour and CP problems of Supersymmetry and try to identify in these associated low-energy observables possible indications of the existence of a Grand Unified theory at high energies.

hep-ph↗

arXiv2007

Radiative B-L symmetry breaking in supersymmetric models

We propose a scheme where the three relevant physics scales related to the supersymmetry, electroweak, and baryon minus lepton (B-L) breakings are linked together and occur at the TeV scale. The phenomenological implications in the Higgs and leptonic sectors are discussed.

hep-ph↗

arXiv2007

Running $U_{e3}$ and BR($μ\to e +γ$) in SUSY-GUTs

In supersymmetric seesaw models based on SUSY-GUTs, it could happen that the neutrino PMNS mixing angles are related to the lepton flavour violating decay rates. In particular SO(10) frameworks, the smallest mixing angle would get directly correlated with the $μ\to e + γ$ decay amplitude. Here we study this correlation in detail considering $U_{e3}$ as a free parameter between 0 and $U_{e3}$(CHOOZ). Large radiative corrections to $U_{e3}$ present in these models, typically of the order ~$ΔU_{e3} ~\sim ~10^{-3}$ (peculiar to hierarchial neutrinos), can play a major role in enhancing the Br($μ\to e +γ$), especially when $U_{e3} \ler 10^{-3}$. For large tan$β$, even such small enhancements are sufficient to bring the associated Br($μ\to e + γ$) into realm of MEG experiment as long as SUSY spectrum lies within the range probed by LHC. On the other hand, for some (negative) values of $U_{e3}$, suppressions can occur in the branching ratio, due to cancellations among different contributions. From a top-down perspective such low values of $U_{e3}$ at the weak scale might require some partial/full cancellations between the high scale parameters of the model and the radiative corrections unless $U_{e3}$ is purely of radiative origin at the high scale. We further emphasize that in Grand Unified theories there exist additional LFV effects related to the running above the GUT scale, that are also independent on the low energy value of $U_{e3}$. These new contributions can become competitive and even dominant in some regions of the parameter space.

hep-ph↗

arXiv2007

Soft SUSY Breaking Grand Unification: Leptons vs Quarks on the Flavor Playground

We systematically analyze the correlations between the various leptonic and hadronic flavor violating processes arising in SUSY Grand Unified Theories. Using the GUT-symmetric relations between the soft SUSY breaking parameters, we assess the impact of hadronic and leptonic flavor observables on the SUSY sources of flavor violation.

hep-ph↗

arXiv2006

The B -- TAU FCNC connection in SUSY Unified Theories

In the context of SUSY grand unification a link is established between the hadronic and leptonic soft breaking sectors. Such relation is here exploited in particular for FCNC processes in B physics. It is shown how bounds on leptonic FCNC involving the third generation translate into constraints on FC B decays. In the second part of the contribution we show that tests of lepton universality in K and B decays can represent an interesting handle to obtain relevant information on the amount of FCNC in the second and third fermion generation.

hep-ph↗

arXiv2006

Lepton Flavour Violation from SUSY--GUTs: Where do we stand for MEG, PRISM/PRIME and a Super Flavour factory

We analyse the complementarity between Lepton Flavour Violation (LFV) and LHC experiments in probing the Supersymmetric (SUSY) Grand Unified Theories (GUT) when neutrinos got a mass via the see--saw mechanism. Our analysis is performed in an SO(10) framework, where at least one neutrino Yukawa coupling is necessarily as large as the top Yukawa coupling. Our study thoroughly takes into account the whole RG running, including the GUT and the right handed neutrino mass scales, as well as the running of the observable neutrino spectrum. We find that the upcoming (MEG, SuperKEKB) and future (PRISM/PRIME, Super Flavour factory) LFV experiments will be able to test such SUSY framework for SUSY masses to be explored at the LHC and, in some cases, even beyond the LHC sensitivity reach.

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