arXiv subjects

Andrea Romanino

Publications and source records attributed to Andrea Romanino.

At least 37 records · Page 2Linked to original sources

arXiv2006

Flavour from accidental symmetries

We consider a new approach to fermion masses and mixings in which no special "horizontal" dynamics is invoked to account for the hierarchical pattern of charged fermion masses and for the peculiar features of neutrino masses. The hierarchy follows from the vertical, family-independent structure of the model, in particular from the breaking pattern of the Pati-Salam group. The lightness of the first two fermion families can be related to two family symmetries emerging in this context as accidental symmetries.

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arXiv2004

Charged lepton contributions to the solar neutrino mixing and theta_13

A charged lepton contribution to the solar neutrino mixing induces a contribution to theta_13, barring cancellations/correlations, which is independent of the model building options in the neutrino sector. We illustrate two robust arguments for that contribution to be within the expected sensitivity of high intensity neutrino beam experiments. We find that the case in which the neutrino sector gives rise to a maximal solar angle (the natural situation if the hierarchy is inverse) leads to a theta_13 close to or exceeding the experimental bound depending on the precise values of theta_12, theta_23, an unknown phase and possible additional contributions. We finally discuss the possibility that the solar angle originates predominantly in the charged lepton sector. We find that the construction of a model of this sort is more complicated. We comment on a recent example of natural model of this type.

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arXiv2003

Signatures of Supernova Neutrino Oscillations into Extra Dimensions

We consider the mixing of muon and tau neutrinos with sterile fermion fields propagating in extra dimensions in the context of core collapse supernova physics, extending the analysis of the electron neutrino case done in a previous work. We show that the potentially dramatic modifications to the supernova evolution are prevented by a mechanism of feedback, so that no severe bounds on the parameters of the extra dimensions need to be imposed. Nevertheless, the supernova core evolution is significantly modified. We discuss the consequences on the delayed explosion mechanism and the compatibility with the SN1987A signal. Then, for the cases of both nu_{mu,tau} and nu_e mixing with bulk fermions, we analyse the distinctive features of the signal on Earth.

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arXiv2003

Natural relations among physical observables in the neutrino mass matrix

We find all possible relations among physical observables arising from neutrino mass matrices that describe in a natural way the currently observed pattern (tan_23 and tan_12 large, dm^2_Sun/dm^2_Atm and tan_13 small) in terms of a minimum number of parameters. Natural here means due only to the relative smallness (vanishing) of some parameters in the relevant lagrangian, without special relations or accidental cancellations among them.

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arXiv2002

Effects of new physics in neutrino oscillations in matter

A new flavor changing electron neutrino interaction with matter would always dominate the nu_e oscillation probability at sufficiently high neutrino energies. Being suppressed by theta_{13}, the energy scale at which the new effect starts to be relevant may be within the reach of realistic experiments, where the peculiar dependence of the signal with energy could give rise to a clear signature in the nu_e --> nu_tau channel. The latter could be observed by means of a coarse large magnetized detector by exploiting tau --> mu decays. We discuss the possibility of identifying or constraining such effects with a high energy neutrino factory. We also comment on the model independent limits on them.

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arXiv2001

Electron and muon electric dipoles in supersymmetric scenarios

We study if a sizeable muon electric dipole can arise in supersymmetric frameworks able to account for the tight experimental bounds on sfermion masses, like an appropriate flavor symmetry, or like a flavor-blind mechanism of SUSY breaking (in presence of radiative corrections charchteristic of GUT models, or due to Yukawa couplings of neutrinos in see-saw models). In some cases it is possible to evade the naive scaling d_mu/d_e = m_mu/m_e and obtain a d_mu as large as 10^{-22--23} e cm. In most cases d_mu is around 10^{-24--25} e cm and (d_mu/d_e)/(m_mu/m_e) is only slightly different from one: this ratio contains interesting informations on the source of the dipoles and on the texture of the lepton Yukawa matrix. We also update GUT predictions for mu --> e gamma and related processes.

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arXiv2000

Neutrino Masses and Mixing in Brane-World Theories

We present a comprehensive study of five-dimensional brane-world models for neutrino physics based on flat compactifications. Particular emphasis is put on the inclusion of bulk mass terms. We derive a number of general results for such brane-world models with bulk mass terms. In particular, in the limit of small brane-bulk couplings, the electroweak eigenstates are predominantly given as a superposition of three light states with non-trivial small admixtures of bulk states. As a consequence, neutrinos can undergo standard oscillations as well as oscillation into bulk Kaluza-Klein states. We use this structure to construct a specific model based on Z_2 orbifolding and bulk Majorana masses which is compatible with all observed oscillation phenomena. The solar neutrino deficit is explained by oscillations into sterile bulk states while the atmospheric neutrino deficit is due to mu - tau oscillations with naturally maximal mixing. In addition, the model can accommodate the LSND result and a significant neutrino dark matter component. We also analyze the constraints from supernova energy loss on neutrino brane-world theories and show that our specific model is consistent with these constraints.

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arXiv2000

Solar neutrino oscillation from large extra dimensions

A plausible explanation for the existence of additional light sterile neutrinos is that they correspond to modulini, fermionic partners of moduli, which propagate in new large dimensions. We discuss the phenomenological implications of such states and show that solar neutrino oscillation is well described by small angle MSW oscillation to the tower of Kaluza Klein states associated with the modulini. In the optimal case the recoil electron energy spectrum agrees precisely with the measured one, in contrast to the single sterile neutrino case which is disfavoured. We also consider how all oscillation phenomena can be explained in a model including bulk neutrino states. In particular, we show that a naturally maximal mixing for atmospheric neutrinos can be easily obtained.

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arXiv2000

Are heavy scalars natural in minimal supergravity?

It has been recently claimed that very large values of a universal soft mass term m_0 for sfermions and higgs bosons become natural when M_top is close to 175 GeV if tanβ\approx 10. We show that very large values of m_0 require accidental cancellations not guaranteed by experimental data or theoretical assumptions, and consequently an unnatural fine-tuning of the parameters.

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arXiv2000

A Brane-World Explanation of the KARMEN Anomaly

Motivated by the anomaly in the KARMEN experiment, we study new possibilities for brane-world models of neutrino masses. We show that the KARMEN anomaly can be understood in the context of a six-dimensional brane-world model. The fine-tuning problem associated with the single-particle interpretation of the anomaly is thereby alleviated. Our model shows some interesting properties of general interest for brane-world neutrino physics. In particular, the size of the brane-bulk mixing decouples from the scales relevant for atmospheric and solar neutrino oscillations.

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arXiv1999

Measuring CP-violation with a neutrino factory

We discuss the prospects of observing leptonic CP-violation at a neutrino factory in the context of the standard three neutrino scenario. If the large mixing angle MSW effect turned out to account for the solar neutrino deficit, we show that observing an asymmetry between the $\reu$ and $\reub$ oscillation probabilities would represent an exciting experimental challenge. We determine the portion of the parameter space where an evidence could be found as a function of the intensity of the muon source and of the detector size for different baselines. We discuss the consequent requirements on a neutrino factory. We address the problems associated with asymmetries induced by the experimental apparatus and by matter effects. Finally, we emphasize the importance of measuring two CP-conjugated channels in order to precisely determine $θ_{13}$.

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arXiv1999

Neutrino mixings from a U(2) flavour symmetry

We extend a previously developed description of the flavour parameters in the charged fermion sector, based on a U(2) flavour symmetry, to include two main features of the neutrino sector seemingly implied by recent data: a large mixing angle $θ_{μτ}$ and a large hierarchy in the neutrino squared mass differences. A unified description of quark and lepton masses and mixings emerges. The neatest quantitative predictions are for elements of the unitary mixing matrix in the lepton sector: |V_{μ1}| = |V_{e 3} / V_{μ3}| = |V_{e 2} / V_{τ3}| = \sqrt{m_e / m_μ}, which go together with the analogous relations in the quark sector: |V_{ub} / V_{cb}| = \sqrt{m_u / m_c}, |V_{td} / V_{ts}| = \sqrt{m_d / m_s}

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arXiv1999

Precise tests of a quark mass texture

The relations $|V_{ub}/V_{cb}|=(m_u/m_c)^{1/2}$ and $|V_{td}/V_{ts}|=(m_d/m_s)^{1/2}$ between the CKM matrix elements and the quark masses are shown to imply a remarkably precise determination of the CKM unitarity triangle or of the Wolfenstein parameters $ρ$, $η$ consistent with the data so far. We view this as a clean test of a quark mass texture neatly arising from a hierarchical breaking of a U(2) flavour symmetry.

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arXiv1998

Fermion masses and symmetry breaking of a U(2) flavour symmetry

We show how a specific sequential breaking pattern of a U(2) flavour symmetry occurs automatically in a broad framework. The relative orientation in U(2) space of the spurion fields that breaks the U(2) symmetry is uniquely fixed, thus determining the form of the fermion mass matrices in a predictive way.

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arXiv1997

Naturalness constraints on gauge-mediated supersymmetry breaking models

The question of naturalness is addressed in the context of gauge-mediated supersymmetry breaking models. Requiring that $M_Z$ arises naturally imposes upper limits on the right-handed selectron mass in these models that are stronger than in the Minimal Supersymmetric Standard Model and are interesting from the point of view of searches at the current and future colliders.

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arXiv1997

Consequences of a U(2) Flavour Symmetry

While solving the supersymmetric flavour problem, a U(2) flavour symmetry might be at the origin of the pattern of fermion masses and mixings. The consequences of this hypothesis are spelled out concerning the parameters of the CKM matrix and their observability in B- and K-physics. The relevance of searching for lepton flavour violation and for the electric dipole moments of the electron and the neutron is also emphasized.

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arXiv1997

Unified theories of flavour with U(2) as horizontal group

An unified flavour model based on U(2) as flavour group is described. Besides to explain the main characteristics of the fermion spectrum, the model is predictive and agrees quantitatively with experimental data in flavour physics.

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arXiv1996

Electric dipole moments from Yukawa phases in supersymmetric theories

We study quark and electron EDMs generated by Yukawa couplings in supersymmetric models with different gauge groups, using the EDM properties under flavour transformations. In the MSSM (or if soft terms are mediated below the unification scale) the one loop contributions to the neutron EDM are smaller than in previous computations based on numerical methods, although increasing as tan^3β. A neutron EDM close to the experimental limits can be generated in SU(5), if tanβis large, through the u-quark EDM d_u, proportional to tan^4β. This effect has to be taken into account also in SO(10) with large tanβ, where d_u is comparable to the d-quark EDM, proportional to tanβ.

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