arXiv subjects

Luca Silvestrini

Publications and source records attributed to Luca Silvestrini.

41 records · Page 3Linked to original sources

arXiv2003

Electroweak symmetry breaking and fermion masses from extra dimensions

We study higher-dimensional non-supersymmetric orbifold models where the Higgs field is identified with some internal component of a gauge field. We address two important and related issues that constitute severe obstacles towards model building within this type of constructions: the possibilities of achieving satisfactory Yukawa couplings and Higgs potentials. We consider models where matter fermions are localized at the orbifold fixed-points and couple to additional heavy fermions in the bulk. When integrated out, the latter induce tree-level non-local Yukawa interactions and a quantum contribution to the Higgs potential that we explicitly evaluate and analyse. The general features of these highly constrained models are illustrated through a minimal but potentially realistic five-dimensional example. Finally, we discuss possible cures for the persisting difficulties in achieving acceptable top and Higgs masses. In particular, we consider in some detail the effects induced in these models by adding localized kinetic terms for gauge fields.

hep-ph↗

arXiv1999

Theory of Nonleptonic B Decays

I briefly review some recent progress in the theory of nonleptonic B decays. After introducing the operator product expansion and the relevant effective Hamiltonian, I discuss the domain of validity and the theoretical justification of the factorization approximation and of its generalizations. Furthermore, I review some general parameterizations of B decay amplitudes: the "diagrammatic" approach and the formalism based on Wick contractions in the matrix elements of four-fermion operators.

hep-ph↗

arXiv1999

Constraints on an enhanced sbar d Z vertex from epsilon'/epsilon

We analyze rare kaon decays in models in which the dominant new effect is an enhanced sbar d Z vertex Z_{ds}. We point out that in spite of large theoretical uncertainties the CP-violating ratio epsilon'/epsilon provides at present the strongest constraint on Im Z_{ds}. Assuming 1.5 10^{-3} < epsilon'/epsilon < 3 10^{-3} and Standard Model values for the CKM parameters we obtain the bounds BR(K_L -> pi^0 nu nubar) < 3.9 10^{-10} and BR(K_L -> pi^0 e^+ e^-)_{dir} < 7.9 10^{-11}. Using the bound on Re Z_{ds} from K_L -> mu^+ mu^- we find BR(K^+ -> pi^+ nu nubar) < 2.6 10^{-10}. We also discuss new physics scenarios in which in addition to an enhanced sbar d Z vertex also neutral meson mixing receives important new contributions. In this case our most conservative bounds are BR(K_L -> pi^0 nu nubar) < 1.8 10^{-9}, BR(K_L -> pi^0 e^+ e^-)_{dir} < 2.8 10^{-10} and BR(K^+ -> pi^+ nu nubar) < 6.1 10^{-10}. The dependence of the upper bound for BR(K_L -> pi^0 nu nubar) on the CKM parameters is also analyzed.

hep-ph↗

arXiv1998

Non-Leptonic Two-Body B Decays Beyond Factorization

We present a general scheme and scale independent parameterization of two-body non-leptonic B decay amplitudes which includes perturbative QCD corrections as well as final state interactions in a consistent way. This parameterization is based on the Next-to-Leading effective Hamiltonian for non-leptonic B decays and on Wick contractions in the matrix elements of the local operators. Using this parameterization, and making no dynamical assumption, we present a classification of two-body B decay channels in terms of the parameters entering in the decay amplitudes. This classification can be considered as the starting point for a model-independent analysis of non-leptonic B decays and of CP violating asymmetries. We also propose, on the basis of the large N expansion, a possible hierarchy among the different effective parameters. We discuss the strategy to extract the most important effective parameters from the experimental data. Finally, we establish a connection between our parameterization and the diagrammatic approach which is widely used in the literature.

hep-ph↗

arXiv1998

Upper Bounds on $K \to πν\bar ν$ and $K_L \to π^0 e^+ e^-$ from $ε^\prime/ε$ and $K_L \to μ^+ μ^-$

We analyze rare kaon decays in models in which the dominant new effect is an enhanced $\bar s d Z$ vertex $Z_{ds}$. We point out that in spite of large theoretical uncertainties the CP-violating ratio $ε^\prime/ε$ provides at present the strongest constraint on $\Im Z_{ds}$. Assuming $0 \le ε^\prime/ε\le 2 \cdot 10^{-3}$ and Standard Model values for the CKM parameters we obtain the bounds ${\rm BR}(K_L \to π^0 ν\bar ν) \le 2.4 \cdot 10^{-10}$ and ${\rm BR}(K_L \to π^0 e^+ e^-) \le 3.6 \cdot 10^{-11}$ (which are substantially stronger than the bounds found recently by Colangelo and Isidori, using $ε$ instead of $ε^\prime/ε$). We illustrate how these bounds can be improved with the help of the forthcoming data on $ε^\prime/ε$. Using the bound on $\Re Z_{ds}$ from $K_L \to μ^+ μ^-$ we find ${\rm BR}(K^+ \to π^+ ν\bar ν) \le 2.3 \cdot 10^{-10}$. In this context we derive an analytic upper bound on ${\rm BR}(K^+ \to π^+ ν\bar ν)$ as a function of ${\rm BR}(K_L \to π^0 ν\bar ν)$ and the short distance contribution to ${\rm BR}(K_L \to μ^+ μ^-)$. We also discuss new physics scenarios in which in addition to an enhanced $\bar s d Z$ vertex also neutral meson mixing receives important new contributions. In this case larger values of the branching ratios in question cannot be excluded.

hep-ph↗