arXiv subjects

L. Silvestrini

Publications and source records attributed to L. Silvestrini.

At least 73 records · Page 4Linked to original sources

arXiv2003

b to s transitions: where to look for SUSY

We present a systematic study of the CP conserving and violating SUSY contributions to b to s processes in a generic MSSM, considering gluino exchange contributions. Experimental information on B to Xs gamma, B to Xs l+ l- and the Bs-Bsbar mass difference Delta Ms have been taken into account. We study the induced correlations among several observables: Delta Ms and the amount of CP violation in B to phi Ks, Bs to J/psi phi, B to Xs gamma. Our results show that b to s transitions represent a splendid opportunity to constrain different MSSM realizations and offer concrete prospects to exhibit SUSY signals at B factories and hadron colliders.

hep-ph↗

arXiv2003

Grand Unification of Quark and Lepton FCNCs

In the context of Supersymmetric Grand Unified theories with soft breaking terms arising at the Planck scale, it is generally possible to link flavor changing neutral current and CP violating processes occurring in the leptonic and hadronic sectors. We study the correlation between flavor changing squark and slepton mass insertions in models à la SU(5). We show that the constraints coming from lepton flavor violation exhibit a strong impact on CP-violating B decays.

hep-ph↗

arXiv2002

B Physics and Supersymmetry

In this talk, I briefly review a selection of SUSY effects in B physics. First, I consider models with Minimal Flavour Violation. Then I discuss SUSY models with new sources of flavour violation in squark mass matrices, analyzing present constraints and possible developments with forthcoming data on b -> s and b -> d transitions.

hep-ph↗

arXiv2002

Charming Penguins Saga

We briefly recall the main formulae for computing the B -> K pi branching ratios within the "charming penguin" approach, present an updated fit to the data, and explain why we believe that, in general, these fits can hardly be used to extract gamma.

hep-ph↗

arXiv2002

Direct CP Violation in B->phi K_s and New Physics

In the presence of large New Physics contributions to loop-induced b->s transitions, sizable direct CP violation in B-> phi K decays is expected on general grounds. We compute explicitly CP-violating effects using QCD factorization and find that, even in the restricted case in which New Physics has the same penguin structure as the Standard Model, the rate asymmetry can be of order one. We briefly discuss a more general scenario and comment on the inclusion of power-suppressed corrections to factorization.

hep-ph↗

arXiv2001

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

We present a next-to-leading order determination of the gluino-mediated SUSY contributions to B_d-Bbar_d mixing and to the CP asymmetry a_{J/psi K_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.

hep-ph↗

arXiv2001

Anomalies in orbifold field theories

We study the constraints on models with extra dimensions arising from local anomaly cancellation. We consider a five-dimensional field theory with a U(1) gauge field and a charged fermion, compactified on the orbifold S^1/(Z_2 x Z_2'). We show that, even if the orbifold projections remove both fermionic zero modes, there are gauge anomalies localized at the fixed points. Anomalies naively cancel after integration over the fifth dimension, but gauge invariance is broken, spoiling the consistency of the theory. We discuss the implications for realistic supersymmetric models with a single Higgs hypermultiplet in the bulk, and possible cancellation mechanisms in non-minimal models.

hep-th↗

arXiv2001

Non-local symmetry breaking in Kaluza-Klein theories

Scherk-Schwarz gauge symmetry breaking of a D-dimensional field theory model compactified on a circle is analyzed. It is explicitly shown that forbidden couplings in the unbroken theory appear in the one-loop effective action only in a non-local way, implying that they are finite at all orders in perturbation theory. This result can be understood as a consequence of the local gauge symmetry, but it holds true also in the global limit.

hep-ph↗

arXiv2001

Two Body B Decays, Factorization and LambdaQCD/mb Corrections

By using the recent experimental measurements of B -> pi pi and B -> K pi branching ratios, we find that the amplitudes computed at the leading order of the LambdaQCD/mb expansion disagree with the observed BRs, even taking into account the uncertainties of the input parameters. Beyond the leading order, Charming and GIM penguins allow to reconcile the theoretical predictions with the data. Because of these large effects, we conclude, however, that it is not possible, with the present theoretical and experimental accuracy, to determine the CP violation angle gamma from these decays. We compare our results with those obtained with the parametrization of the chirally enhanced non-perturbative contributions by BBNS. We also predict large asymmetries for several of the particle--antiparticle BRs, in particular BR(B+ -> K+ pi0), BR(Bd -> K+ pi-) and BR(Bd -> pi+ pi-).

hep-ph↗

arXiv2001

Charming Penguins Strike Back

By using the recent experimental measurements of B to pi pi and B to K pi branching ratios, we find that factorization is unable to reproduce the observed BRs even taking into account the uncertainties of the input parameters. Charming and GIM penguins allow to reconcile the theoretical predictions with the data. Because of these large effects, we conclude, however, that it is not possible, with the present theoretical and experimental accuracy, to determine the CP violation angle gamma from these decays. Contrary to factorization, we predict large asymmetries for several of the particle-antiparticle BRs, in particular BR(B^+ to K^+ pi^0), BR(B_d to K^+ pi^-) and BR(B_d to pi^+ pi^-). This opens new perspectives for the study of CP violation in B systems.

hep-ph↗

arXiv2001

Signals of Supersymmetric Flavour Models in B Physics

If the mechanism of Supersymmetry breaking is not flavour blind, some flavour symmetry is likely to be needed to prevent excessive flavour changing neutral current effects. We discuss two flavour models (based respectively on a U(2) and on a SU(3) horizontal symmetry) providing a good fit to fermion masses and mixings and particularly constraining the supersymmetry soft breaking terms. We show that, while reproducing successfully the Standard Model fit of the unitarity triangle, it is possible to obtain sizable deviations from the Standard Model predictions for three very clean B-physics observables: the time dependent CP asymmetries in $B_d \to J/ψK^0$ and in $B_s \to J/ψϕ$ and the $B_s-\bar{B}_s$ mass difference. Our analysis exhibits with two explicit realizations that in supersymmetric theories with a new flavour structure in addition to the Yukawa matrices there exist concrete potentialities for revealing supersymmetry indirectly in theoretically clean $B$-physics observables.

hep-ph↗

arXiv2001

Penguin Contractions and Factorization in B -> K pi Decays

We study Lambda_{QCD}/m_B corrections to factorization in B -> K pi decays. First, we analyze these decay channels within factorization, showing that, irrespectively of the value of gamma, it is not possible to reproduce the experimental data. Then, we discuss Lambda_{QCD}/m_B corrections to these processes, and argue that there is a class of doubly Cabibbo enhanced non-factorizable contributions, usually called charming penguins, that cannot be neglected. Including these corrections, we obtain an excellent agreement with experimental data. Furthermore, contrary to what is obtained with factorization, we predict sizable rate asymmetries in B^\pm -> K^\pm π^0 and B -> K^\pm pi^\mp.

hep-ph↗

arXiv2000

epsilon'/epsilon and Rare K Decays in the Standard Model and Supersymmetry

After briefly reviewing the status of epsilon'/epsilon in the Standard Model, I discuss SUSY contributions to epsilon'/epsilon, K^+ to pi^+ nu bar nu, K_L to pi^0 nu bar nu and K_L to pi^0 e^+ e^-. While in the simplest case of the MSSM with Minimal Flavour Violation the main effect is a suppression of these transitions with respect to the Standard Model, large enhancements are possible in more general SUSY models, with interesting correlations among the different processes.

hep-ph↗

arXiv2000

epsilon'/epsilon and Rare K and B Decays in the MSSM

We analyze the CP violating ratio epsilon'/epsilon and rare K and B decays in the MSSM with minimal flavour and CP violation, including NLO QCD corrections and imposing constraints on the supersymmetric parameters coming from epsilon, B_{d,s}^0-\bar B_{d,s}^0 mixings, B to X_s gamma, Delta rho in the electroweak precision studies and from the lower bound on the neutral Higgs mass. We provide a compendium of phenomenologically relevant formulae in the MSSM. Denoting by T(Q) the MSSM prediction for a given quantity normalized to the Standard Model result we find the ranges: 0.53 < T(epsilon'/epsilon) < 1.07, 0.65 < T(K^+ to pi^+ nu nubar) < 1.02, 0.41 < T(K_L to pi^0 nu nubar) < 1.03, 0.48 < T(K_L to pi^0 e^+ e^-) < 1.10, 0.73 < T(B to X_s nu nubar) < 1.34 and 0.68 < T(B_s to mu^+ mu^-) < 1.53. We point out that these ranges will be considerably reduced when the lower bounds on the neutral Higgs mass and tan beta improve. Some contour plots illustrate the dependences of the quantities above on the relevant supersymmetric parameters. As a byproduct of this work we update our previous analysis of epsilon'/epsilon in the SM and find in NDR epsilon'/epsilon = (9.2^{+6.8}_{-4.0}), a value 15 % higher than in our 1999 analysis.

hep-ph↗

arXiv2000

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 (Delta M)_d/(Delta M)_s and sin(2 beta) measured first through the CP asymmetry in B_d^0 to psi K_S and later in K to pi nu nubar decays. Also suitable ratios of the branching ratios for B to X_{d,s} nu nubar and B_{d,s} to 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 non-universal triangles extracted in each model using epsilon, (Delta M)_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.

hep-ph↗

arXiv2000

Delta M_K and epsilon_K in SUSY at the Next-to-Leading order

We perform a Next-to-Leading order analysis of Delta S=2 processes beyond the Standard Model. Combining the recently computed NLO anomalous dimensions and the B parameters of the most general Delta S=2 effective Hamiltonian, we give an analytic formula for Delta M_K and epsilon_K in terms of the Wilson coefficients at the high energy scale. This expression can be used for any extension of the Standard Model with new heavy particles. Using this result, we consider gluino-mediated contributions to Delta S=2 transitions in general SUSY models and provide an improved analysis of the constraints on off-diagonal mass terms between the first two generations of down-type squarks. Finally, we improve the constraints on R-violating couplings from Delta M_K and epsilon_K.

hep-ph↗

arXiv2000

Final State Interactions and epsilon'/epsilon: A Critical Look

We critically analyze recent attempts to include final state interaction (FSI) effects in the calculation of the CP violating ratio epsilon'/epsilon. In particular the approach of using the Muskhelishvili-Omnes equation for the decay amplitudes is examined. We find that the values of the dispersive correction factors are very sensitive to initial conditions which are, at present, out of control, and demonstrate that the claimed large enhancement of epsilon'/epsilon through FSI is questionable. We propose, instead, a different implementation of this approach which may be useful to lattice determinations of weak amplitudes.

hep-ph↗

arXiv1999

B -> Xs l+ l- decays in supersymmetry

We study the semileptonic decays B -> Xs e+ e- and B -> Xs mu+ mu- in generic supersymmetric extensions of the Standard Model. SUSY effects are parameterized using the mass insertion approximation formalism and differences with the Constrained MSSM results are pointed out. Constraints on SUSY contributions coming from other processes (e.g. b -> s photon) are taken into account. Chargino and gluino contributions to photon and Z-mediated decays are computed and non-perturbative corrections are considered. We find that the integrated branching ratios and the asymmetries can be strongly modified. Moreover, the behavior of the differential Forward-Backward asymmetry remarkably changes with respect to the Standard Model expectation.

hep-ph↗