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L. Silvestrini

Publications and source records attributed to L. Silvestrini.

At least 91 records · Page 5Linked to original sources
arXiv1999

Probing Supersymmetric Flavor Models with $ε'/ε$

We discuss the supersymmetric contribution to $ε'/ε$ in various supersymmetric flavor models. We find that in alignment models the supersymmetric contribution could be significant while in heavy squark models it is expected to be small. The situation is particularly interesting in models that solve the flavor problems by either of the above mechanisms and the remaining CP problems by means of approximate CP, that is, all CP violating phases are small. In such models, the standard model contributions cannot account for $ε'/ε$ and a failure of the supersymmetric contributions to do so would exclude the model. In models of alignment and approximate CP, the supersymmetric contributions can account for $ε'/ε$ only if both the supersymmetric model parameters and the hadronic parameters assume rather extreme values. Such models are then strongly disfavored by the $ε'/ε$ measurements. Models of heavy squarks and approximate CP are excluded.

hep-ph↗
arXiv1999

Connections between epsilon'/epsilon and Rare Kaon Decays in Supersymmetry

We analyze the rare kaon decays $K_L \to π^0 ν\bar ν$, $K^+ \to π^+ ν\bar ν$, $K_L \to π^0 e^+ e^-$ and $K_L \to μ^+ μ^-$ in conjunction with the CP violating ratio $ε'/ε$ in a general class of supersymmetric models in which $Z$- and magnetic-penguin contributions can be substantially larger than in the Standard Model. We point out that radiative effects relate the double left-right mass insertion to the single left-left one, and that the phenomenological constraints on the latter reflect into a stringent bound on the supersymmetric contribution to the $Z$ penguin. Using this bound, and those coming from recent data on $ε'/ε$ we find ${\rm BR}(K_L \to π^0 ν\bar ν)\lsim 1.2\cdot 10^{-10}$, ${\rm BR}(K^+ \to π^+ ν\bar ν)\lsim 1.7\cdot 10^{-10}$, ${\rm BR}(K_L \to π^0 e^+ e^-)_{\rm dir}\lsim 2.0\cdot 10^{-11}$, assuming the usual determination of the CKM parameters and neglecting the possibility of cancellations among different supersymmetric effects in $ε'/ε$. Larger values are possible, in principle, but rather unlikely. We stress the importance of a measurement of these three branching ratios, together with improved data and improved theory of $ε'/ε$, in order to shed light on the realization of various supersymmetric scenarios. We reemphasize that the most natural enhancement of $ε'/ε$, within supersymmetric models, comes from chromomagnetic penguins and show that in this case sizable enhancements of $BR(K_L \to π^0 e^+ e^-)_{\rm dir}$ can also be expected.

hep-ph↗
arXiv1999

Standard Model Confronting New Results for epsilon'/epsilon

We analyze the CP violating ratio \epe=epsilon'/epsilon in the Standard Model in view of the new KTeV results. We review the present status of the most important non-perturbative parameters B_6, B_8, B_K and of the strange quark mass m_s. We also briefly discuss the issues of final state interactions and renormalization scheme dependence. Updating the values of the CKM parameters, of m_t and Lambda (MSbar) and using Gaussian errors for the experimental input and flat distributions for the theoretical parameters we find \epe substantially below the NA31 and KTeV data: \epe= (7.7^{+6.0}_{-3.5}) 10^{-4} and \epe= (5.2^{+4.6}_{-2.7}) 10^{-4} in the NDR and HV renormalization schemes respectively. A simple scanning of all input parameters gives on the other hand 1.05 10^{-4} < \epe < 28.8 10^{-4} and 0.26 10^{-4} < \epe < 22.0 10^{-4} respectively. Analyzing the dependence on various parameters we find that only for extreme values of B_6, B_8 and m_s and suitable values of CKM parameters and Lambda(MSbar), the ratio \epe can be made consistent with data. We analyze the impact of these data on the lower bounds for Im(V_{td}V_{ts}^*), Br(K_L to pi^0 nu barnu), Br(K_L to pi^0e^+e^-)_{dir} and on tan(beta) in the Two Higgs Doublet Model II.

hep-ph↗
arXiv1998

Pre-LHC SUSY Searches: an Overview

We discuss the prospects for searches of low-energy supersymmetry in the time interval separating us from the advent of LHC. In this period of time "indirect" searches may play a very relevant role. We refer to manifestations of supersymmetry in flavour changing neutral current and CP violating phenomena and to signals of the lightest supersymmetric particle in searches of dark matter. In the first part of the talk we critically review the status of the minimal supersymmetric model to discuss the chances that direct and indirect supersymmetric searches may have before the LHC start. In the second part we point out what we consider to be the most promising grounds where departures from the standard model prediction may signal the presence of new physics, possibly of supersymmetric nature. We argue that the often invoked complementarity of direct and indirect searches of low-energy supersymmetry is becoming even more true in the pre-LHC era.

hep-ph↗
arXiv1998

Model independent determination of the light-cone wave functions for exclusive processes

We present a method to compute, by numerical simulations of lattice QCD, the light-cone wave functions which enter exclusive processes at large momentum transfer, such as electromagnetic elastic scatterings, or exclusive semi-leptonic decays as B -> pi (B -> rho) and radiative decays as B -> K* gamma. The method is based on first principles and does not require any model assumption.

hep-ph↗
arXiv1998

Generalized Factorization in Non-leptonic Two-Body B-Decays: a Critical Look

We reanalyze critically the generalized factorization hypothesis in non-leptonic two-body B-decays discussed recently by several authors. In particular we address the determination of the factorization scale $μ_f$ and of the non-perturbative parameters $ξ^{NF}_1 (m_b)$ and $ξ^{NF}_2 (m_b)$ which are supposed to measure non-factorizable contributions to hadronic matrix elements with $ξ^{NF}_i (μ_f) = 0$. We emphasize that both $μ_f$ and $ξ^{NF}_i (m_b)$ are renormalization scheme dependent and we demonstrate analytically and numerically that for any chosen scale $μ_f=\ord(m_b)$ it is possible to find a renormalization scheme for which $ξ^{NF}_1 (μ_f) = ξ^{NF}_2 (μ_f)=0$. The existing data indicate that such "factorization schemes" differ from the commonly used schemes NDR and HV. Similarly we point out that the recent extractions of the effective number of colours $N^{eff}$ from two-body non-leptonic B-decays while $μ$ and renormalization scheme independent suffer from gauge dependences and infrared dependences.

hep-ph↗
arXiv1998

Model independent determination of the shape function for inclusive B decays and of the structure functions in DIS

We present a method to compute, by numerical simulations of lattice QCD, the inclusive semileptonic differential decay rates of heavy hadrons and the structure functions which occur in deep inelastic scattering. The method is based on first principles and does not require any model assumption. It allows the prediction of the differential rate in B semileptonic decays for values of the recoiling hadronic mass W ~ sqrt(M_B Lambda_QCD), which is in the relevant region to extract |V_ub| from the end-point of the lepton spectrum in inclusive decays.

hep-ph↗
arXiv1998

$K \to πν\bar ν$: A Model Independent Analysis and Supersymmetry

We present a model independent analysis of new-physics contributions to the decays K^+ \to π^+ ν\bar νand K_L \to π^0 ν\bar ν. We parameterize the effects of new physics in these decays by two parameters: r_K and θ_K, with r_K=1 and θ_K=0 in the Standard Model (SM). We show how these parameters can be extracted from future data. This analysis offers some insight in a possible violation of a "golden relation" between K \to πν\bar νdecays and the CP asymmetry in B \to ψK_S in the SM pointed out some time ago. We illustrate these ideas by considering a general class of supersymmetric models. We find that in the "constrained" MSSM, in which θ_K=0, the measurements of Br(K^+ \to π^+ ν\bar ν) and Br(K_L \to π^0 ν\bar ν) directly determine the angle β. Moreover, the "golden relation" remains unaffected. On the other hand, in general SUSY models with unbroken R-parity the present experimental constraints still allow for substantial deviations from r_K=1 and θ_K=0. Typically 0.5 < r_K < 1.3 and -25^0 < θ_K < 25^0. Consequently, in these models the violation of the "golden relation" is possible and values for Br(K^+ \to π^+ ν\bar ν) and Br(K_L \to π^0 ν\bar ν) departing from the SM expectations by factors 2-3 cannot be excluded. Our conclusions differ in certain aspects from the ones reached in previous analyses. In particular, we stress the possible importance of left-right flavour-violating mass insertions that were not considered before.

hep-ph↗
arXiv1997

Two Lectures on FCNC and CP Violation in Supersymmetry

These two lectures constitute a reappraisal and an update of the status of the Flavour Changing Neutral Current (FCNC) and CP violation issues in supersymmetry. The first lecture discusses these points in the framework of the Minimal Supersymmetric Standard Model (MSSM), while the second one provides an analysis in a generic low-energy supersymmetric extension of the Standard Model. The goal of these lectures is twofold: on one hand we present a qualitative and quantitative discussion of the threat that FCNC and CP violation represent on supersymmetry model building; on the other hand, we point out how precious FCNC and CP violation may be in obtaining some signals of the presence of supersymmetry at low energy in the years that separate us from the advent of LHC physics. In particular, concerning this latter point, we emphasize and thoroughly analyze the role of experimental searches for rare B decays and CP violation in B physics.

hep-ph↗
arXiv1997

Next-to-Leading Order QCD Corrections to $ΔF =2$ effective Hamiltonians

The most general QCD next-to-leading anomalous-dimension matrix of all possible four-fermion, dimension-six $ΔF=2$ operators is computed. The results of this calculation can be used in many phenomenological applications, among which the most important are those related to theoretical predictions of $K^0-\bar{K}^0 $ and $B^0-\bar{B}^0$ mixing in several extension of the Standard Model (Supersymmetry, left-right symmetric, multi-Higgs models, etc.), to estimates of the $B^0_s$--$\bar B^0_s$ width difference, $ΔΓ_{B_s}$, and to the calculation of the $O(1/m_b^3)$ corrections for inclusive b-hadron decay rates.

hep-ph↗
arXiv1997

Charming-penguin enhanced B decays

Long-distance contributions of operators of the effective non-leptonic weak Hamiltonian containing charmed currents have been recently studied. Penguin-like contractions of these operators, denoted as charming penguins, have been shown to be relevant for several B decays into two pseudoscalar mesons. In particular, they are expected to give large enhancements to processes which would be otherwise Cabibbo suppressed. Their contributions easily lead to values of BR(B^+ \to K^0 π^+) and BR(B_d \to K^+ π^-) of about 1 \times 10^{-5}, as recently found by the CLEO collaboration. In this paper, we show that such large branching fractions cannot be obtained without charming penguins. This holds true irrespectively of the model used to compute the non-leptonic amplitudes and of the relevant parameters of the CKM mixing matrix. We use the experimental measurements of the B^+ \to K^0 π^+ and B_d \to K^+ π^- decay rates to constrain the charming-penguin amplitudes and to predict BRs for a large set of other two-body decay channels where their contributions are also important. These include several pseudoscalar-vector and vector-vector channels, such as B \to ρK and B \to ρK^*, which have not been measured yet.

hep-ph↗
arXiv1997

CP Violation in Low-Energy SUSY

We discuss CP violation in the context of the minimal SUSY extension of the standard model as well as in a generic low-energy SUSY model. After analyzing the SUSY contributions to the observed CP violation in the kaon system, we emphasize the prospects for disentangling SUSY loop contributions from pure SM effects in B physics.

hep-ph↗
arXiv1997

SUSY in Rare and CP-Violating B Decays

In this talk we discuss rare B decays (b -> s gamma, b -> s g, b -> s l^+ l^-), B-\bar{B} oscillations and CP violation in B physics 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 and CP-violating effects which is useful in tackling the problem in generic low-energy SUSY. We show how rare B decays and CP violation in B-decay amplitudes 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↗
arXiv1997

CP Violating B Decays in the Standard Model and Supersymmetry

We study the uncertainties of the Standard Model (SM) predictions for CP violating B decays and investigate where and how supersymmetric (SUSY) contributions may be disentangled. The first task is accomplished by letting the relevant matrix elements of the effective Hamiltonian vary within certain ranges. The SUSY analysis makes use of a formalism which allows to obtain model-independent results. We show that in some cases it is possible a) to measure the CP B-\bar{B} mixing phase and b) to discriminate the SM and SUSY contributions to the CP decay phases. The golden-plated decays to this purpose are the B \to ϕK_S and B \to K_S π^0 channels.

hep-ph↗
arXiv1997

Flavour changing top decays in supersymmetric extensions of the standard model

Flavour changing top decays $t \to c Z^0, t \to c g$ and $t \to c γ$ are predicted with invisible rates within the standard model and may represent a window on new physics. We consider these processes in supersymmetric extensions of the standard model and we show that observable rates can be obtained only if the SUSY breaking is non universal and flavour dependent.

hep-ph↗
arXiv1997

Charming Penguins in B decays

Full expressions of the $B^0_d \to π^+ π^-$ and $B^0_d \to π^0 π^0$ amplitudes, given in terms of matrix elements of operators of the effective weak Hamiltonian, are used to study the dependence of the relevant branching ratios on the different contributions. The uncertainty in the extraction of the weak phase $α$ from the measurement of the time-dependent asymmetry in $B^0_d \to π^+ π^-$ decays is also analyzed. We find that, among several effects which may enhance the $B^0_d \to π^0 π^0$ branching ratio, the most important is due to ``charming penguin" diagrams that have never been studied before. These diagrams easily increase $BR(B^0_d \to π^0 π^0)$ up to a value of $1-3 \times 10^{-6}$. The same effect produces, however, a large error in the extraction of $α$ from the measurement of the $B^0_d \to π^+ π^-$ time-dependent asymmetry. We show that it is possible to determine charming-penguin amplitudes from the experimental measurement of many decay rates. Their effect is impressive in $B^+ \to π^+ K^0$ and $B^0_d \to K^+ π^-$ decays, where charming-penguin contributions easily give values of $BR(B^+ \to π^+ K^0)$ and $BR(B^0_d \to K^+ π^-)$ of about $1 \times 10^{-5}$. Among other possibilities, we also suggest to use $B^0_d \to K^0 \bar K^0$, the BR of which can be as large as $2-3 \times 10^{-6}$, to determine the size of charming-penguin amplitudes.

hep-ph↗