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L. Silvestrini

Publications and source records attributed to L. Silvestrini.

102 records · Page 6Linked to original sources
arXiv1996

The Beauty of SUSY

B physics represents a privileged place to look for supersymmetry (SUSY) through its virtual effects. Here we discuss rare B decays ($b \to s γ$, $b \to s g$, $b \to s l^{+} l^{-}$) and $B-\bar{B}$ oscillations in the context of low-energy SUSY. We outline the variety of predictions that arise according to the choice of the SUSY extension ranging from what we call the ``minimal" version of the MSSM to models without flavour universality or with broken R-parity. In particular, we provide a model-independent parameterization of the SUSY FCNC effects which is useful in tackling the problem in generic low-energy SUSY. We show how rare B physics may be complementary to direct SUSY searches at colliders, in particular for what concerns extensions of the most restrictive version of the MSSM.

hep-ph↗
arXiv1996

Fermion Virtual Effects in $e^+ e^- -> W^+ W^-$ Cross Section

We analyse the contribution of new heavy virtual fermions to the $e^+e^- \rightarrow W^+W^-$ cross section. We find that there exists a relevant interplay between trilinear and bilinear oblique corrections. The result strongly depends on the chiral or vector--like nature of the new fermions. As for the chiral case we consider sequential fermions: one obtains substantial deviation from the Standard model prediction, making the effect possibly detectable at $\sqrt{s}=500$ or $1000$ GeV linear colliders. As an example for the vector--like case we take a SUSY extension with heavy charginos and neutralinos: due to cancellation, the final effect turns out to be negligible.

hep-ph↗
arXiv1996

Probing SUSY in FCNC and CP Violating Phenomena

We analyze constraints on low-energy flavour-changing sfermion mass terms, coming from FCNC and CP violating processes, in the model-independent framework of the mass insertion method. We discuss the relevance of these constraints as tests of supersymmetric extensions of the standard model; in particular, we consider grand-unified models and models with non-universal soft breaking terms.

hep-ph↗
arXiv1996

A complete analysis of FCNC and CP constraints in general SUSY extensions of the standard model

We analyze the full set of constraints on gluino- and photino-mediated SUSY contributions to FCNC and CP violating phenomena. We use the mass insertion method, hence providing a model-independent parameterization which can be readily applied in testing extensions of the MSSM. In addition to clarifying controversial points in the literature, we provide a more exhaustive analysis of the CP constraints, in particular concerning $\varepsilon^\prime/\varepsilon$. As physically meaningful applications of our analysis, we study the implications in SUSY-GUT's and effective supergravities with flavour non-universality. This allows us to detail the domain of applicability and the correct procedure of implementation of the FC mass insertion approach.

hep-ph↗
arXiv1996

Two-body non-leptonic decays on the lattice

We show that, under reasonable hypotheses, it is possible to study two-body non-leptonic weak decays in numerical simulations of lattice QCD. By assuming that final-state interactions are dominated by the nearby resonances and that the couplings of the resonances to the final particles are smooth functions of the external momenta, it is possible indeed to overcome the difficulties imposed by the Maiani-Testa no-go theorem and to extract the weak decay amplitudes, including their phases. Under the same assumptions, results can be obtained also for time-like form factors and quasi-elastic processes.

hep-ph↗
arXiv1995

Constraints on supersymmetry from FCNC and CP violation

We consider FCNC and CP violating processes mediated by gluino exchange in generalized supersymmetric theories. We present the constraints on flavour (F) changing squark mass terms at the electroweak energy scale, focusing our analysis on $ΔF = 2$ transitions. Results are also given for $ΔF = 1$ CP conserving processes both in the hadronic and leptonic sectors, while we limit ourselves to some remarks on the relevance of box diagrams in the evaluation of $ε^{\prime}/ε$.

hep-ph↗
arXiv1995

Flavour changing neutral currents and CP violating processes in generalized supersymmetric theories

We consider supersymmetric extensions of the standard model with general non-universal soft breaking terms. We analyse in a model-independent way the constraints on these terms at the electroweak energy scale coming from gluino mediated flavour (F) changing neutral current and CP-violating processes. We have computed the complete $ΔF=1$ and $ΔF=2$ effective hamiltonian for gluino mediated processes, including for the first time the effect of box diagrams in the evaluation of $ε^{\prime}/ε$. We present numerical results for the constraints on these non-universal soft breaking terms for different values of the parameters, extending the analysis also to the leptonic sector. A comparison with previous results in the literature is given.

hep-ph↗
arXiv1995

An Upgraded Analysis of $ε\prime/ε$ at the Next-to-Leading Order

An upgraded analysis of $\ep$, $x_d$ and $\epp/\ep$, using the latest determinations of the relevant experimental and theoretical parameters, is presented. Using the recent determination of the top quark mass, $m_t=(174 \pm 17)$ GeV, our best estimate is $\epp/\ep= 3.1 \pm 2.5 $, which lies in the range given by E731. We describe our determination of $\epp/\ep$ and make a comparison with other similar studies. A detailed discussion of the matching of the full theory to the effective Hamiltonian, written in terms of lattice operators, is also given.

hep-ph↗
arXiv1994

$b \rightarrow s γ$ AND $b \rightarrow s g$: A THEORETICAL REAPPRAISAL

We present upgraded theoretical predictions for inclusive and exclusive radiative decays of {\it B} mesons. Our results include those next-to-leading order corrections that have already been computed. Our best estimates in the Standard Model are $BR(B \rightarrow K^* γ)=(4.3 \pm 0.9^{ +1.4}_{-1.0}) \times 10^{-5}$, $BR(B \rightarrow X_s γ) =(1.9 \pm 0.2\pm 0.5) \times 10^{-4}$, $Γ(B \rightarrow K^* γ)/ Γ(B \rightarrow X_s γ)=0.23 \pm 0.09$ and $BR(B \rightarrow X_s g)= (1.57 \pm 0.15 ^{+0.86}_{-0.59} \pm 0.23) \times 10^{-3}$. We also consider limits found with two-Higgs-doublet models.

hep-ph↗
arXiv1993

Leading Order QCD Corrections to $b\rightarrow s \, γ$ and $b \rightarrow s \, g$ Decays in Three Regularization Schemes

We discuss in detail the calculation of the leading order QCD corrections to the Effective Hamiltonian which governs $b \rightarrow s\, γ$ and $b \rightarrow s \, g$ transitions in three different regularization schemes (HV, NDR and DRED). We show that intermediate stages of the calculation do depend on the regularization, but the same scheme independent coefficients can be obtained in all the considered schemes. A detailed discussion of the results already present in the literature is also given.

hep-ph↗
arXiv1993

Scheme Independence of the Effective Hamiltonian for $b \rightarrow s \, γ$ and $b \rightarrow s \, g$ Decays

We present a calculation of the effective weak Hamiltonian which governs $b \rightarrow s\, γ$ and $b \rightarrow s \, g$ transitions in two different renormalization schemes (NDR and HV). In the leading logarithmic approximation, we show that the coefficients of the effective Hamiltonian are scheme independent only when one takes correctly into account the scheme dependence of one- and two- loop diagrams. We demonstrate that in NDR there are contributions which were missed in previous calculations. These contributions are necessary to obtain scheme independent coefficients in the final results.

hep-ph↗