arXiv subjects

A. Strumia

Publications and source records attributed to A. Strumia.

At least 19 recordsLinked to original sources

arXiv2014

Graviton loops and brane observables

We discuss how to consistently perform effective Lagrangian computations in quantum gravity with branes in compact extra dimensions. A reparametrization invariant and infrared finite result is obtained in a non trivial way. It is crucial to properly account for brane fluctuations and to correctly identify physical observables. Our results correct some confusing claims in the literature. We discuss the implications of graviton loops on electroweak precision observables and on the muon g-2 in models with large extra dimensions. We model the leading effects, not controlled by effective field theory, by introducing a hard momentum cut-off.

hep-ph↗

arXiv2013

The cosmic-ray positron excess from a local Dark Matter over-density

We show that the cosmic-ray positron excess measured by PAMELA and AMS could be induced by Dark Matter annihilations in a local over-density. In such a context leptophilic DM is not needed and good fits to positron data, in agreement with antiproton and gamma-ray measurements, are obtained for DM annihilations to $WW$, $hh$, $ZZ$, $ t \bar t$, $b \bar b$, $q\bar q$ channels. The classic Dark Matter candidates, such as the pure supersymmetric Wino with standard thermal annihilation cross-section, can fit the positron excess, without invoking any additional assumption on Dark Matter properties.

hep-ph↗

arXiv2009

The likelihood for supernova neutrino analyses

We derive the event-by-event likelihood that allows to extract the complete information contained in the energy, time and direction of supernova neutrinos, and specify it in the case of SN1987A data. We resolve discrepancies in the previous literature, numerically relevant already in the concrete case of SN1987A data.

hep-ph↗

arXiv2006

The Minimal Set of Electroweak Precision Parameters

We present a simple method for analyzing the impact of precision electroweak data above and below the Z-peak on flavour-conserving heavy new physics. We find that experiments have probed about ten combinations of new physics effects, which to a good approximation can be condensed into the effective oblique parameters Shat, That, Uhat, V, X, W, Y (we prove positivity constraints W, Y >= 0) and three combinations of quark couplings (including a distinct parameter for the bottom). We apply our method to generic extra Z' vectors.

hep-ph↗

arXiv2005

Gravitational quantum corrections in warped supersymmetric brane worlds

We study gravitational quantum corrections in supersymmetric theories with warped extra dimensions. We develop for this a superfield formalism for linearized gauged supergravity. We show that the 1-loop effective Kahler potential is a simple functional of the KK spectrum in the presence of generic localized kinetic terms at the two branes. We also present a simple understanding of our results by showing that the leading matter effects are equivalent to suitable displacements of the branes. We then apply this general result to compute the gravity-mediated universal soft mass $m_0^2$ in models where the visible and the hidden sectors are sequestered at the two branes. We find that the contributions coming from radion mediation and brane-to-brane mediation are both negative in the minimal set-up, but the former can become positive if the gravitational kinetic term localized at the hidden brane has a sizeable coefficient. We then compare the features of the two extreme cases of flat and very warped geometry, and give an outlook on the building of viable models.

hep-th↗

arXiv2003

Towards a complete theory of thermal leptogenesis in the SM and MSSM

We perform a thorough study of thermal leptogenesis adding finite temperature effects, RGE corrections, scatterings involving gauge bosons and by properly avoiding overcounting on-shell processes. Assuming hierarchical right-handed neutrinos with arbitrary abundancy, successful leptogenesis can be achieved if left-handed neutrinos are lighter than 0.15 eV and right-handed neutrinos heavier than 2 10^7 GeV (SM case, 3sigma C.L.). MSSM results are similar. Furthermore, we study how reheating after inflation affects thermal leptogenesis. Assuming that the inflaton reheats SM particles but not directly right-handed neutrinos, we derive the lower bound on the reheating temperature to be T_RH > 2 10^9 GeV. This bound conflicts with the cosmological gravitino bound present in supersymmetric theories. We study some scenarios that avoid this conflict: `soft leptogenesis', leptogenesis in presence of a large right-handed (s)neutrino abundancy or of a sneutrino condensate.

hep-ph↗

arXiv2002

Predictions of the most minimal see-saw model

We derive the most minimal see-saw texture from an extra-dimensional dynamics. It predicts theta_13 = 0.078 \pm 0.015 and m_ee = 2.6 \pm 0.4 meV. Assuming thermal leptogenesis, the sign of the CP-phase measurable in neutrino oscillations, together with the sign of baryon asymmetry, determines the order of heavy neutrino masses. Unless heavy neutrinos are almost degenerate, successful leptogenesis fixes the lightest mass. Depending on the sign of the neutrino CP-phase, the supersymmetric version of the model with universal soft terms at high scale predicts BR(mu --> e gamma) or BR(tau --> mu gamma), and gives a lower bound on the other process.

hep-ph↗

arXiv2002

Minimal Flavour Violation: an effective field theory approach

We present a general analysis of extensions of the Standard Model which satisfy the criterion of Minimal Flavour Violation (MFV). We define this general framework by constructing a low-energy effective theory containing the Standard Model fields, with one or two Higgs doublets and, as the only source of SU(3)^5 flavour symmetry breaking, the background values of fields transforming under the flavour group as the ordinary Yukawa couplings. We analyse present bounds on the effective scale of dimension-six operators, which range between 1 and 10 TeV, with the most stringent constraints imposed by B -> X_s gamma. In this class of theories, it is possible to relate predictions for FCNC processes in B physics to those in K physics. We compare the sensitivity of various experimental searches in probing the hypothesis of MFV. Within the two-Higgs-doublet scenario, we develop a general procedure to obtain all tan(beta)-enhanced Higgs-mediated FCNC amplitudes, discussing in particular their impact in B -> l^+l^-, Delta M_B and B -> X_s gamma. As a byproduct, we derive some two-loop tan(beta)-enhanced supersymmetric contributions to B -> X_s gamma previously unknown.

hep-ph↗

arXiv2002

Old and new physics interpretations of the NuTeV anomaly

We discuss whether the NuTeV anomaly can be explained, compatibly with all other data, by QCD effects (maybe, if the strange sea is asymmetric, or there is a tiny violation of isospin), new physics in propagators or couplings of the vector bosons (not really), loops of supersymmetric particles (no), dimension six operators (yes, for one specific SU(2)-invariant operator), leptoquarks (not in a minimal way), extra U(1) gauge bosons (maybe: an unmixed Z' coupled to B-3L_mu also increases the muon g-2 by about 10^{-9} and gives a `burst' to cosmic rays above the GZK cutoff).

hep-ph↗

arXiv2002

SN1987A and the Status of Oscillation Solutions to the Solar Neutrino Problem (including an appendix discussing the NC and day/night data from SNO)

We study neutrino oscillations and the level-crossing probability PLZ in power-law potential profiles A(r)\propto r^n. We give local and global adiabaticity conditions valid for all mixing angles theta and discuss different representations for PLZ. For the 1/r^3 profile typical of supernova envelopes we compare our analytical to numerical results and to earlier approximations used in the literature. We then perform a combined likelihood analysis of the observed SN1987A neutrino signal and of the latest solar neutrino data, including the recent SNO CC measurement. We find that, unless all relevant supernova parameters (released binding energy, \barν_e and \barν_{μ,τ} temperatures) are near their lowest values found in simulations, the status of large mixing type solutions deteriorates considerably compared to fits using only solar data. This is sufficient to rule out the vacuum-type solutions for most reasonable choices of astrophysics parameters. The LOW solution may still be acceptable, but becomes worse than the SMA-MSW solution which may, in some cases, be the best combined solution. On the other hand the LMA-MSW solution can easily survive as the best overall solution, although its size is generally reduced when compared to fits to the solar data only.

hep-ph↗

arXiv2000

Comparison of two methods for calculating nucleation rates

First-order phase transitions that proceed via nucleation of bubbles are described by the homogeneous nucleation theory of Langer. The nucleation rate is one of the most interesting parameters of these transitions. In previous works we have computed nucleation rates with two different methods: (A) employing coarse-grained potentials; (B) by means of a saddle-point approximation, using dimensionally regularized field theory. In this article we compare the results of the two approaches in order to test their reliability and to determine the regions of applicability. We find a very good agreement.

cond-mat↗

arXiv1999

Vacuum oscillations of quasi degenerate solar neutrinos

The atmospheric neutrino oscillations and the vacuum oscillation solution of the solar neutrino problem can be consistently described by a doubly or triply degenerate neutrino spectrum as long as the high level of degeneracy required is not spoiled by radiative corrections. We show that this is the case for neutrino mass matrices generated by symmetries. This imposes a strong constraint on the mixing angles and requires the mixing should be close to bi-maximal. We briefly discuss the relevance of our results for the measurability of the neutrino spectrum.

hep-ph↗

arXiv1999

The Region of Validity of Homogeneous Nucleation Theory

We examine the region of validity of Langer's picture of homogeneous nucleation. Our approach is based on a coarse-grained free energy that incorporates the effect of fluctuations with momenta above a scale k. The nucleation rate I = A_k exp(-S_k) is exponentially suppressed by the action S_k of the saddle-point configuration that dominates tunnelling. The factor A_k includes a fluctuation determinant around this saddle point. Both S_k and A_k depend on the choice of k, but, for 1/k close to the characteristic length scale of the saddle point, this dependence cancels in the expression for the nucleation rate. For very weak first-order phase transitions or in the vicinity of the spinodal decomposition line, the pre-exponential factor A_k compensates the exponential suppression exp(-S_k). In these regions the standard nucleation picture breaks down. We give an approximate expression for A_k in terms of the saddle-point profile, which can be used for quantitative estimates and practical tests of the validity of homogeneous nucleation theory.

hep-ph↗

arXiv1999

Bounds on Kaluza-Klein excitations of the SM vector bosons from electroweak tests

Within a minimal extension of the SM in 4+1 dimensions, we study how Kaluza Klein excitations of the SM gauge bosons affect the electroweak precision observables. Asymmetries in Z decays provide the dominant bound on the compactification scale M of the extra dimension. If the higgs is so light that will be discovered at LEP2, we find the following 95% CL bounds: M > 3.5 TeV (if the higgs lives in the extra dimension) and M > 4.3 TeV (if the higgs is confined to our 4 dimensions). In the second case Kaluza Klein modes give "universal"corrections and a good fit of precision data can be obtained with an heavier higgs (up to 500 GeV) and with a smaller M > 3.4 TeV.

hep-ph↗

arXiv1999

Epsilon' from supersymmetry with non universal A terms?

In supersymmetric theories with a motivated flavour structure, we investigate the possibility that an epsilon' parameter as large as the measured value be generated without conflicting with the bounds from electron and neutron electric dipoles and mu --> e gamma.

hep-ph↗

arXiv1999

Natural ranges of supersymmetric signals

The LEP2 experiments pose a serious naturalness problem for supersymmetric models. The problem is stronger in gauge mediation than in supergravity models. Particular scenarios, like electroweak baryogenesis or gauge mediation with light messengers, are strongly disfavoured. Searching a theoretical reason that naturally explains why supersymmetry has not been found poses strong requests on model building. If instead an unlikely (p\approx 5%) numerical accident has hidden supersymmetry to LEP2, we compute the naturalness distribution of values of allowed sparticle masses and supersymmetric loop effects. We find that b to s gamma remains a very promising signal of minimal supersymmetry even if there is now a 20% (4%) probability that coloured particles are heavier than 1 TeV (3 TeV). We study how much other effects are expected to be detectable.

hep-ph↗

arXiv1998

A Consistent Calculation of Bubble-Nucleation Rates

We present a consistent picture of tunnelling in field theory. Our results apply both to high-temperature field theories in four dimensions and to zero-temperature three-dimensional ones. Our approach is based on the notion of a coarse-grained potential U_k that incorporates the effect of fluctuations with characteristic momenta above a given scale k. U_k is non-convex and becomes equal to the convex effective potential for k --> 0. We demonstrate that a consistent calculation of the nucleation rate must be performed at non-zero values of k, larger than the typical scale of the saddle-point configuration that dominates tunnelling. The nucleation rate is exponentially suppressed by the action S_k of this saddle point. The pre-exponential factor A_k, which includes the fluctuation determinant around the saddle-point configuration, is well-defined and finite. Both S_k and A_k are k-dependent, but this dependence cancels in the expression for the nucleation rate. This picture breaks down in the limit of very weakly first-order phase transitions, for which the pre-exponential factor compensates the exponential suppression.

hep-ph↗

arXiv1998

Oscillations of solar and atmospheric neutrinos

Motivated by recent results from SuperKamiokande, we study both solar and atmospheric neutrino fluxes in the context of oscillations of the three known neutrinos. We aim at a global view which identifies the various possibilities, rather than attempting the most accurate determination of the parameters of each scenario. For solar neutrinos we emphasise the importance of performing a general analysis, independent of any particular solar model and we consider the possibility that any one of the techniques --- chlorine, gallium or water Cerenkov --- has a large unknown systematic error, so that its results should be discarded. The atmospheric neutrino anomaly is studied by paying special attention to the ratios of upward and downward going nu_e and nu_mu fluxes. Both anomalies can be described in a minimal scheme where the respective oscillation frequencies are widely separated or in non-minimal schemes with two comparable oscillation frequencies. We discuss explicit forms of neutrino mass matrices in which both atmospheric and solar neutrino fluxes are explained. In the minimal scheme we identify only two `zeroth order' textures that can result from unbroken symmetries. Finally we discuss experimental strategies for the determination of the various oscillation parameters.

hep-ph↗