arXiv subjects
Publications and source records attributed to Alessandro Strumia.
We calculate bubble-nucleation rates for (2+1)-dimensional scalar theories at high temperature. Our approach is based on the notion of a real coarse-grained potential. The region of applicability of our method is determined through internal consistency criteria. We compare our results with data from lattice simulations. Good agreement is observed when the renormalized action of the simulated theory is known.
We obtain the bounds on all the gauge invariant, flavour symmetric, CP-even operators of dimension 6 that can affect the electroweak precision tests. For the preferred Higgs mass of about 100 GeV, their minimal scales range from 2 to 10 TeV. Depending on the individual operator, these limits are often significantly stronger than those quoted in the literature, when they exist at all. The impact, if any, of these bounds on the upper limit con the Higgs mass itself is discussed.
We present a consistent calculation of bubble-nucleation rates in theories of two scalar fields. Our approach is based on the notion of a coarse-grained free energy that incorporates the effects of fluctuations with momenta above a given scale k. We establish the reliability of the method for a variety of two-scalar models and confirm the conclusions of previous studies in one-field theories: Langer's theory of homogeneous nucleation is applicable as long as the expansion around the semiclassical saddle point associated with tunnelling is convergent. This expansion breaks down when the exponential suppression of the rate by the saddle-point action becomes comparable to the pre-exponential factor associated with fluctuations around the saddle point. We reconfirm that Langer's theory is not applicable to the case of weakly first-oder phase transitions. We also find that the same is true in general for radiatively induced first-order phase transitions. We discuss the relevance of our results for the electroweak phase transition and the metastability bound on the Higgs-boson mass.
Oscillations of three neutrinos with all squared mass splittings around 10^{-3} eV^2 are not firmly excluded by solar neutrino experiments. We carefully verify that they are also perfectly compatible with atmospheric neutrino experiments: due to accidental reasons the SuperKamiokande experiment is rather insensitive to `solar' nu_e/ν_mu oscillations, even if some characteristic small effects could become visible with more statistics. This pattern of oscillations can be excluded by new solar experiments, or cleanly discovered at KamLand. We also perform a fit of the most recent atmospheric SK data within the usual assumption that `solar' effects are negligible.
After presenting a simple procedure for testing naturalness (similar to Bayesian inference and not more subjective than it) we show that LEP2 experiments pose a naturalness problem for `conventional' supersymmetric models. About 95% of the parameter space of minimal supergravity MSSM is excluded by LEP2 experiments. Moreover in this model electroweak baryogenesis, or detectable supersymmetric corrections to mixing of K and B mesons, are possible only in very small corners of the parameter space. The naturalness problem is stronger in gauge mediation models, expecially with light messengers. We recall some possible explanations (different from an improbable numerical accident) of why supersymmetry has not (yet?) been found.
CP violation in hadronic B-decays is studied in a definite and well motivated framework of flavour physics and supersymmetry. Possible deviations from the standard model both in mixing and in decay amplitudes are discussed. An attempt is made to describe an experimental strategy for looking at these deviations and for measuring the relevant parameters.
In theories with three light neutrinos, a complete list of five zeroth-order textures for lepton mass matrices is found, which naturally yield: m_e = m_mu=0, m_tau \neq 0, Delta m^2_sun = 0, together with theta_{e tau} = 0, theta_{mu tau} approx 1. These textures provide suitable starting points for constructing theories which account for both atmospheric and solar neutrino fluxes. Using flavor symmetries, two schemes for such lepton masses are found, and example constructions exhibited.
We compute the next-to-leading order corrections to the gaugino masses $M_i$ in gauge-mediated models for generic values of the messenger masses M and discuss the predictions of unified messenger models. If M<100 TeV there can be up to 10% corrections to the leading order relations $M_i\propto α_i$. If the messengers are heavier there are only few % corrections. We also study the messenger corrections to gauge coupling unification: as a result of cancellations dictated by supersymmetry, the predicted value of the strong coupling constant is typically only negligibly increased.
Following Chankowski, Ellis and Pokorski we quantify the amount of fine-tuning of input parameters of the Minimal Supersymmetric Standard Model that is needed to respect the lower limits on sparticle and Higgs masses imposed by negative searches so far, direct or indirect. By including the one loop radiative corrections to the effective potential, the amount of fine-tuning is reduced with respect to the results of CEP by a factor of 2--5, strongly increasing as tanβapproaches 1. A further reduction factor may come from a more appropriate, less restrictive, definition of the fine-tuning parameter itself.
After a general discussion about the quantitative meaning of the naturalness upper bounds on the masses of supersymmetric particles, we compute these bounds in models with gauge-mediated soft terms. We find interesting upper limits on the right-handed slepton masses that, unless the messenger fields are very light, disfavor minimal models with large messenger content. Deep unphysical minima, that however turn out to be not dangerous, are usually present in such models. The mu-problem can be solved by adding a light singlet only at the price of a large amount of fine tuning that gives also rise to heavy sparticles and large tanβ.
We show that the two scenarios able to explain the HERA anomaly --- a new leptoquark coupling or a new contact interaction --- predict new contributions to atomic parity violation. These corrections are sufficiently large and different that a feasible reduction in the dominant atomic theory uncertainty could give some hint in favour of one of the two scenarios.
We show that gauge-mediation and unified-supergravity give sufficiently firm and different predictions for the spectrum of supersymmetric particles to make it possible to discriminate the two scenarios even if the messenger mass is close to the unification scale.
We discuss an initial condition on the superpotential couplings of an SU(5) theory which allows the R-violating term L d^c Q but avoids the simultaneous presence of L L e^c + d^c d^c u^c. This same condition keeps under control the products of pairs of different couplings lambda_{ijk} L_i d^c_j Q_k, which are more strongly constrained by flavour-changing-neutral-current limits. In our view, this observation makes relatively more plausible the interpretation of the high-Q^2 Hera anomaly, if real, as caused by squark production.
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.
A right-handed stop not much heavier or even lighter than the Z boson has today desirable phenomenological consequences. We study how it can result within the usual radiative scenario of electroweak symmetry breaking. A restriction on the gaugino mass parameters, M_2 < 0.3 m_10, arises if soft terms satisfy relations suggested by unification theories. Moreover, requiring to get a light stop without unnatural fine-tunings below the per-cent level, we obtain another more interesting upper bound on the chargino mass, M_chargino < M_Z, and we derive interesting conclusions about the masses of the gluino, M_3 \sim (150-300) GeV, of the heavy stop, M_stop \sim (250-500) GeV, and of the left/right mixing angle in the stop sector, |θ_stop| < 0.3.
In supersymmetric~SO(10) with large $\tanβ$ and Yukawa coupling unification $λ_t=λ_b=λ_τ$, we compute the rates for lepton flavour violating processes. Experiments in progress or foreseen for $μ\to eγ$, $μ\to e$ conversion (and the dipole moment of the electron) are shown to provide very significant tests of the theory for all slepton masses up to about $1\TeV$.
The flavor changing and CP violating phenomena predicted in supersymmetric unified theories as a consequence of the large top quark Yukawa coupling, are investigated in the quark sector and compared with related phenomena in the lepton sector, considered previously. In particular we study $\varepsilon_K$, $\varepsilon_K'/\varepsilon_K$, $Δm_B$, $b\to sγ$, the neutron electric dipole moment, $d_n$, and CP violation in neutral $B$ meson decays, both in minimal~SU(5) and~SO(10) theories. The leptonic signals are generically shown to provide more significant tests of quark-lepton unification. Nevertheless, mostly in the~SO(10) case, a variety of hadronic signals is also possible, with interesting correlations among them.
We describe a supersymmetric Grand Unified Theory based on the gauge group $\SU(5)^3$, or $\SO(10)^3$, invariant under the interchange of any~SU(5), or~SO(10), with each family multiplet transforming non trivially under one different individual group factor. A realistic pattern of fermion masses and mixings is obtained as a result of an appropriate choice out of the many possible discretely degenerate vacua of the theory. In the~SO(10) case, we predict the three neutrino masses in terms of an overall scale and, within factors of order unity, their mixing angles. A $ν_e$-$ν_μ$ oscillation is suggested as a solution of the solar neutrino problem, implying a visible $ν_μ$-$ν_τ$ oscillation in the forthcoming experiments. Grand unified theories of this type could be obtainable in a string theory framework.