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Alessandro Strumia

Publications and source records attributed to Alessandro Strumia.

At least 163 records · Page 9Linked to original sources
arXiv2007

Thermal production of gravitinos

We reconsider thermal production of gravitinos in the early universe, adding to previously considered 2 -> 2 gauge scatterings: a) production via 1 -> 2 decays, allowed by thermal masses; b) the effect of the top Yukawa coupling; c) a proper treatment of the reheating process. Our final result behaves physically (larger couplings give a larger rate) and is twice larger than previous results, implying e.g. a twice stronger constraint on the reheating temperature. Accessory results about (supersymmetric) theories at finite temperature and gravitino couplings might have some interest.

hep-ph↗
arXiv2007

The smallest neutrino mass

We consider models where one Majorana neutrino is massless at tree level (like the see saw with two right-handed neutrinos), and compute the contribution to its mass m generated by two-loop quantum corrections. The result is m \sim 10^{-13}eV in the SM and m \sim 10^{-10} eV (tanBeta/10)^4 in the MSSM, compatible with the restricted range suggested by Affleck-Dine baryogenesis.

hep-ph↗
arXiv2006

Cosmology of neutrinos and extra light particles after WMAP3

We study how present data probe standard and non-standard properties of neutrinos and the possible existence of new light particles, freely-streaming or interacting, among themselves or with neutrinos. Our results include: sum m_nu < 0.40 eV at 99.9% C.L.; that extra massless particles have abundance Delta N_nu = 2 pm 1 if freely-streaming and Delta N_nu = 0 pm 1.3 if interacting; that 3 interacting neutrinos are disfavored at about 4 sigma. We investigate the robustness of our results by fitting to different sub-sets of data. We developed our own cosmological computational tools, somewhat different from the standard ones.

astro-ph↗
arXiv2006

Baryogenesis via leptogenesis

We discuss how leptogenesis can explain the observed baryon asymmetry and summarize attempts of testing leptogenesis. We first perform estimates and discuss the main physics, and later outline the techniques that allow to perform precise computations.

hep-ph↗
arXiv2006

Efficiency and maximal CP-asymmetry of scalar triplet leptogenesis

We study thermal leptogenesis induced by decays of a scalar SU(2)_L triplet. Despite the presence of gauge interactions, unexpected features of the Boltzmann equations make the efficiency close to maximal in most of the parameter space. We derive the maximal CP asymmetry in triplet decays, assuming that it is generated by heavier sources of neutrino masses: in this case successful leptogenesis needs a triplet heavier than 2.8 10^{10} GeV and does not further restrict its couplings, allowing detectable mu --> e gamma, tau --> mu gamma rates in the context of supersymmetric models. Triplet masses down to the TeV scale are viable in presence of extra sources of CP-violation.

hep-ph↗
arXiv2005

The Super-little Higgs

Supersymmetry combined with little-Higgs can render the Higgs vev super-little, providing models of electroweak symmetry breaking free from fine-tunings. We discuss the difficulties that arise in implementing this idea and propose one simple successful model. Thanks to appropriately chosen Higgs representations, D-terms give no tree-level mass term to the Goldstone. The fermion representations are anomaly free, generation independent and embeddable into an SU(6) GUT. A simple mechanism provides the large top quark mass.

hep-ph↗
arXiv2005

Low-scale standard supersymmetric leptogenesis

Strictly adhering to the standard supersymmetric seesaw mechanism, we present a neutrino mass model which allows successful standard thermal leptogenesis compatible with gravitino cosmology. At least some of the neutrino Yukawa couplings must be much larger than the naive estimates following from the seesaw formula. This leads to large BR(mu->e gamma), detectable in the next round of experiments. Ratios of mu->e gamma, tau->e gamma and tau->mu gamma branching ratios are predicted in terms of the measurable neutrino mass matrix.

hep-ph↗
arXiv2005

Little-Higgs corrections to precision data after LEP2

We reconsider little-Higgs corrections to precision data. In five models with global symmetries SU(5), SU(6), SO(9) corrections are (although not explicitly) of `universal' type. We get simple expressions for the Shat, That, W, Y parameters, which summarize all effects. In all models Shat > (W+Y)/2 and That, W, Y\ge 0. Results differ from previous analyses, which are sometimes incomplete, sometimes incorrect, and because we add LEP2 e ebar --> f fbar cross sections to the data set. Depending on the model the constraint on f ranges between 2 and 20 TeV. We next study the `simplest' little-Higgs model (and propose a new related model) which is not `universal' and affects precision data due to the presence of an extra Z' vector. By restricting the data-set to the most accurate leptonic data we show how corrections to precision data generated by a generic Z' can be encoded in four effective Shat, That, W, Y parameters, giving their expressions.

hep-ph↗
arXiv2005

Dark Energy and Right-Handed Neutrinos

We explore the possibility that a CP violating phase of the neutrino mass matrix is promoted to a pseudo-Goldstone-boson field and is identified as the quintessence field for Dark Energy. By requiring that the quintessence potential be calculable from a Lagrangian, and that the extreme flatness of the potential be stable under radiative corrections, we are led to an essentially unique model. Lepton number is violated only by Majorana masses of light, right-handed neutrinos, comparable to the Dirac masses that mix right- with left-handed neutrinos. We outline the rich and constrained neutrino phenomenology that results from this proposal.

hep-ph↗
arXiv2005

Implications of neutrino data circa 2005

Adopting the 3 neutrino framework, we present an updated determination of the oscillation parameters. We perform a global analysis and develope simple arguments that give essentially the same result. We also discuss determinations of solar neutrino fluxes, capabilities of future experiments, tests of CPT, implications for neutrino-less double-beta decay, beta decay, cosmology.

hep-ph↗
arXiv2005

Frequentist analyses of solar neutrino data (updated including KamLAND and SNO data)

The solar neutrino data are analyzed in a frequentist framework, using the Crow-Gardner and Feldman-Cousins prescriptions for the construction of confidence regions. Including in the fit only the total rates measured by the various experiments, both methods give results similar to the commonly used Delta chi^2-cut approximation. When fitting the full data set, the Delta chi^2-cut still gives a good approximation of the Feldman-Cousins regions. However, a careful statistical analysis significantly reduces the goodness-of-fit of the SMA and LOW solutions. In the addenda we discuss the implications of the latest KamLAND, SNO and SK data.

hep-ph↗
arXiv2005

Supersymmetry and precision data after LEP2

We study one loop supersymmetric corrections to precision observables. Adding LEP2 e ebar --> f fbar cross sections to the data-set removes previous hints for SUSY and the resulting constraints are in some cases stronger than direct bounds on sparticle masses. We consider specific models: split SUSY, CMSSM, gauge mediation, anomaly and radion mediation. Beyond performing a complete one-loop analysis, we also develop a simple approximation, based on the Shat, That, W, Y `universal' parameters. SUSY corrections give W,Y > 0 and mainly depend on the left-handed slepton and squark masses, on M_2 and on mu.

hep-ph↗
arXiv2004

Probing oscillations into sterile neutrinos with cosmology, astrophysics and experiments

We perform a thorough analysis of oscillation signals generated by one extra sterile neutrino, extending previous analyses done in simple limiting cases and including the effects of established oscillations among active neutrinos. We consider the following probes: solar, atmospheric, reactor and beam neutrinos, Big-Bang Nucleosynthesis (helium-4, deuterium), Cosmic Microwave Background, Large Scale Structure, supernovae, neutrinos from other astrophysical sources. We find no evidence for a sterile neutrino in present data, identify the still allowed regions, and study which future experiments can best probe them: sub-MeV solar experiments, more precise studies of CMB or BBN, future supernova explosions, etc. We discuss how the LSND hint is strongly disfavoured by the constraints of (standard) cosmology.

hep-ph↗
arXiv2004

Electroweak symmetry breaking after LEP1 and LEP2

In a generic 'universal' theory of electroweak symmetry breaking, non fine-tuned heavy new physics affects the low-energy data through four parameters, which include and properly extend the generally insufficient S and T. Only by adding the LEP2 data to the global electroweak fit, can all these four form factors be determined and deviations from the SM be strongly constrained. Several of the recently proposed models (little Higgs, gauge bosons in extra dimensions or Higgsless models in 5D) are recognized to be 'universal' in a straightforward way after a proper definition of the effective vector boson fields. Among various applications, we show that proposed Higgsless models in 5D, when calculable, do not provide a viable description of electroweak symmetry breaking in their full range of parameters.

hep-ph↗
arXiv2004

Constraints on neutrino masses from leptogenesis models

Upper bounds on the CP asymmetry relevant for leptogenesis are reexamined and found weaker than in previous literature, both for hierarchical and for quasi-degenerate right-handed neutrinos. Successful leptogenesis implies the usual lower bound on right-handed neutrino masses, and an upper bound on left-handed neutrino masses (which we obtain to be 0.15eV at 3sigma) only if right-handed neutrinos are assumed to be much more hierarchical than left-handed neutrinos. Otherwise both bounds can be considerably relaxed. The constraint on light neutrino masses varies assuming different interpretations of why neutrinos should be quasi-degenerate. With conservative assumptions, we find that a mild quasi-degeneracy allows neutrinos heavier than an eV compatibly with leptogenesis. We also extend computations of thermal leptogenesis to an alternative model of neutrino mass mediated by fermion triplets which was never considered so far for leptogenesis. Leptogenesis can be successful despite the effect of gauge interactions, resulting in only slightly stronger constraints on neutrino masses.

hep-ph↗
arXiv2003

Baryogenesis through leptogenesis

Baryogenesis by heavy-neutrino decay and sphaleron reprocessing of both baryon and lepton number is reconsidered, paying special attention to the flavour structure of the general evolution equations and developing an approximate but sufficiently accurate analytic solution to the prototype evolution equation. Two different models of neutrino masses are examined, based on an Abelian U(1) or a non-Abelian U(2) family symmetry. We show that a consistent picture of baryogenesis can emerge in both cases, although with significant differences.

hep-ph↗
arXiv2003

Neutrino oscillations and signals in beta and 0nu2beta experiments

Assuming Majorana neutrinos, we infer from oscillation data the expected values of the parameters m_{nu_e} and m_{ee} probed by beta and 0nu2beta-decay experiments. If neutrinos have a `normal hierarchy' we get the 90% CL ranges |m_{ee}| = (0.7 - 4.6) meV, and discuss in which cases future experiments can test this possibility. For `inverse hierarchy', we get |m_{ee}| = (12 - 57) meV and m_{ν_e} = (40 - 57) meV. The 0nu2beta data imply that almost degenerate neutrinos are lighter than 1.05 h eV at 90% CL, competitive with the beta-decay bound. We critically reanalyse the data that were recently used to claim an evidence for 0nu2beta, and discuss their implications. Finally, we review the predictions of flavour models for m_{ee} and theta_{13}.

hep-ph↗