Luca Silvestrini

Universal unitarity triangle and physics beyond the standard model

We make the simple observation that there exists a universal unitarity triangle for all models, like the SM, the Two Higgs Doublet Models I and II and the MSSM with minimal flavour violation, that do not have any new operators beyond those present in the SM and in which all flavour changing transitions are governed by the CKM matrix with no new phases beyond the CKM phase. This universal triangle can be determined in the near future from the ratio (DeltaM)(d)/(DeltaM)(s) and sin 2 beta measured first through the CP asymmetry in B-d(0) --> psi K-S and later in K --> pi nu )over bar> decays. Also suitable ratios of the branching ratios for B --> X(d,s)nu )over bar> and B-d,B-s --> mu (+)mu (-) and the angle gamma measured by means of CP asymmetries in B decays can be used for this determination. Comparison of this universal triangle with the nonuniversal triangles extracted in each model using epsilon, (DeltaM)(d) and various branching ratios for rare decays will allow to find out in a transparent manner which of these models, if any, is singled out by experiment. A virtue of the universal triangle is that it allows to separate the determination of the CKM parameters from the determination of new parameters present in the extensions of the SM considered here.

Model-independent constraints on ΔF = 2 operators and the scale of new physics

We update the constraints on new-physics contributions to ΔF = 2 processes from the generalized unitarity triangle analysis, including the most recent experimental developments. Based on these constraints, we derive upper bounds on the coefficients of the most general ΔF = 2 effective Hamiltonian. These upper bounds can be translated into lower bounds on the scale of new physics that contributes to these low-energy effective interactions. We point out that, due to the enhancement in the renormalization group evolution and in the matrix elements, the coefficients of non-standard operators are much more constrained than the coefficient of the operator present in the Standard Model. Therefore, the scale of new physics in models that generate new ΔF = 2 operators, such as next-to-minimal flavour violation, has to be much higher than the scale of minimal flavour violation, and it most probably lies beyond the reach of direct searches at the LHC.

The unitarity triangle fit in the standard model and hadronic parameters from lattice QCD: a reappraisal after the measurements of Δms and BR(B→τντ)

The recent measurements of the B0s meson mixing amplitude by CDF and of the leptonic branching fraction BR(B→τντ) by Belle call for an upgraded analysis of the Unitarity Triangle in the Standard Model. Besides improving the previous constraints on the parameters of the CKM matrix, these new measurements, combined with the recent determinations of the angles α, β and γ from non-leptonic decays, allow, in the Standard Model, a quite accurate extraction of the values of the hadronic matrix elements relevant for K0-0 and Bs,d0-s,d0 mixing and of the leptonic decay constant fB. In this paper we upgrade the UT fit, we determine from the data the kaon B-parameter K, the B0 mixing amplitude parameters fBs 1/2Bs and ξ, the decay constant fB, and make a comparison of the obtained values with lattice predictions. We also discuss the different determinations of Vub and show that current data do not favour the value measured in inclusive decays.

The 2004 UTfit collaboration report on the status of the unitarity triangle in the standard model

Using the latest determinations of several theoretical and experimental parameters, we update the Unitarity Triangle analysis in the Standard Model. The basic experimental constraints come from the measurements of |Vub/Vcb|, Δmd, the lower limit on Δms, K, and the measurement of the phase of the Bd–d mixing amplitude through the time-dependent CP asymmetry in B0→J/ψK0 decays. In addition, we consider the direct determination of α, γ, 2β+γ and cos 2β from the measurements of new CP-violating quantities, recently performed at the B factories. We also discuss the opportunities offered by improving the precision of the various physical quantities entering in the determination of the Unitarity Triangle parameters. The results and the plots presented in this paper can also be found at the URL http://www.utfit.org , where they are continuously updated with the newest experimental and theoretical results.

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.

Electroweak precision observables, new physics and the nature of a 126 GeV Higgs boson

A bstractWe perform the fit of electroweak precision observables within the Standard Model with a 126 GeV Higgs boson, compare the results with the theoretical predictions and discuss the impact of recent experimental and theoretical improvements. We introduce New Physics contributions in a model-independent way and fit for the S, T and U parameters, for the ϵ1,2,3,b ones, for modified

First evidence of new physics in b ↔ s transitions

Zb\overline{b}

B_d-Bbar_d Mixing and the B_d -> J/psi K_s Asymmetry in General SUSY Models

couplings and for a modified Higgs coupling to vector bosons. We point out that composite Higgs models are very strongly constrained. Finally, we compute the bounds on dimension-six operators relevant for the electroweak fit.

B → K∗ ℓ + ℓ − decays at large recoil in the Standard Model: a theoretical reappraisal

We combine all the available experimental information on Bsmixing, including the very recent tagged analyses of Bs→ J/Ψϕ by the CDF and DØ collaborations. We find that the phase of the Bsmixing amplitude deviates more than 3σ from the Standard Model prediction. While no single measurement has a 3σ significance yet, all the constraints show a remarkable agreement with the combined result. This is a first evidence of physics beyond the Standard Model. This result disfavours New Physics models with Minimal Flavour Violation with the same significance.PACS Codes: 12.15.Ff, 12.15.Hh, 14.40.Nb

Charming penguins strike back

Abstract We present a next-to-leading order determination of the gluino-mediated SUSY contributions to B d – B d mixing and to the CP asymmetry a J / ψK s in the framework of the mass-insertion approximation. Using hadronic matrix elements recently computed on the lattice, we obtain improved constraints on the squark-mass splittings.