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Luca Silvestrini

Publications and source records attributed to Luca Silvestrini.

At least 37 records · Page 2Linked to original sources
arXiv2017

The Global Electroweak and Higgs Fits in the LHC era

We update the global fit to electroweak precision observables, including the effect of the latest measurements at hadron colliders of the $W$ and top-quark masses and the effective leptonic weak mixing angle. We comment on the impact of these measurements in terms of constraints on new physics. We also update the bounds derived from the fit to the Higgs-boson signal strengths, including the observables measured at the LHC Run 2, and compare the improvements with respect to the 7 and 8 TeV results.

hep-ph↗
arXiv2017

On Flavourful Easter eggs for New Physics hunger and Lepton Flavour Universality violation

Within the standard approach of effective field theory of weak interactions for $ΔB = 1$ transitions, we look for possibly unexpected subtle New Physics effects, here dubbed "flavourful Easter eggs". We perform a Bayesian global fit using the publicly available HEPfit package, taking into account state-of-the-art experimental information concerning these processes, including the suggestive measurements from LHCb of $R_{K}$ and $R_{K^{*}}$, the latter available only very recently. We parametrize New Physics contributions to $b \to s$ transitions in terms of shifts of Wilson coefficients of the electromagnetic dipole and semi-leptonic operators, assuming CP-conserving effects, but allowing in general for violation of lepton flavour universality. We show how optimistic/conservative hadronic estimates can impact quantitatively the size of New Physics extracted from the fit. With a conservative approach to hadronic uncertainties we find nonzero New Physics contributions to Wilson coefficients at the level of $\sim 3σ$, depending on the model chosen. Furthermore, given the interplay between hadronic contributions and New Physics effects in the leptonic vector current, a scenario with nonstandard leptonic axial currents is comparable to the more widely advocated one with New Physics in the leptonic vector current.

hep-ph↗
arXiv2017

Electroweak precision constraints at present and future colliders

We revisit the global fit to electroweak precision observables in the Standard Model and present model-independent bounds on several general new physics scenarios. We present a projection of the fit based on the expected experimental improvements at future $e^+ e^-$ colliders, and compare the constraining power of some of the different experiments that have been proposed. All results have been obtained with the HEPfit code.

hep-ph↗
arXiv2016

$B\to K^*\ell^+\ell^-$ in the Standard Model: Elaborations and Interpretations

Disentangling New Physics effects from the Standard Model requires a good understanding of all pieces that stem from the latter, especially the uncertainties that might plague the theoretical estimations within the Standard Model. In the light of recent measurements made in the decay of $B\to K^*\ell^+\ell^-$, and accompanying possibilities of New Physics effects, we re-examine the hadronic uncertainties that come about in this exclusive $b \to s$ transition. We show that it is not trivial to distinguish New Physics effects from these hadronic uncertainties and we attempt to quantify the latter in its magnitude and kinematic shape from the recent LHCb measurements of the angular observables in this decay mode. We also update our fit with the more recent calculations of the form factors combined with the ones computed with Lattice QCD.

hep-ph↗
arXiv2016

$B\to K^* \ell^+ \ell^-$ decays at large recoil in the Standard Model: a theoretical reappraisal

We critically reassess the theoretical uncertainties in the Standard Model calculation of the $B \to K^* \ell^+ \ell^-$ observables, focusing on the low $q^2$ region. We point out that even optimized observables are affected by sizable uncertainties, since hadronic contributions generated by current-current operators with charm are difficult to estimate, especially for $q^2 \sim 4 m_c^2\simeq 6.8$ GeV$^2$. We perform a detailed numerical analysis and present both predictions and results from the fit obtained using most recent data. We find that non-factorizable power corrections of the expected order of magnitude are sufficient to give a good description of current experimental data within the Standard Model. We discuss in detail the $q^2$ dependence of the corrections and their possible interpretation as shifts of the Standard Model Wilson coefficients.

hep-ph↗
arXiv2016

Uncovering Mass Generation Through Higgs Flavor Violation

A discovery of the flavor violating decay h --> tau mu at the LHC would require extra sources of electroweak symmetry breaking (EWSB) beyond the Higgs in order to reconcile it with the bounds from tau --> mu gamma, barring fine-tuned cancellations. In fact, an h --> tau mu decay rate at a level indicated by the CMS measurement is easily realized if the muon and electron masses are due to a new source of EWSB, while the tau mass is due to the Higgs. We illustrate this with two examples: a two Higgs doublet model, and a model in which the Higgs is partially composite, with EWSB triggered by a technicolor sector. The 1st and 2nd generation quark masses and CKM mixing can also be assigned to the new EWSB source. Large deviations in the flavor diagonal lepton and quark Higgs Yukawa couplings are generic. If the muon mass is due to a rank 1 mass matrix contribution, a novel Yukawa coupling sum rule holds, providing a precision test of our framework. Flavor violating quark and lepton (pseudo)scalar couplings combine to yield a sizable B_s --> tau mu decay rate, which could be O(100) times larger than the SM B_s --> mu mu decay rate.

hep-ph↗
arXiv2015

Global Bayesian Analysis of the Higgs-boson Couplings

We present preliminary results of a bayesian fit to the Wilson coefficients of the Standard Model gauge invariant dimension-6 operators involving one or more Higgs fields, using data on electroweak precision observables and Higgs boson signal strengths.

hep-ph↗
arXiv2014

Update of the electroweak precision fit, interplay with Higgs-boson signal strengths and model-independent constraints on new physics

We present updated global fits of the Standard Model and beyond to electroweak precision data, taking into account recent progress in theoretical calculations and experimental measurements. From the fits, we derive model-independent constraints on new physics by introducing oblique and epsilon parameters, and modified $Zb\bar{b}$ and $HVV$ couplings. Furthermore, we also perform fits of the scale factors of the Higgs-boson couplings to observed signal strengths of the Higgs boson.

hep-ph↗
arXiv2014

Standard Model updates and new physics analysis with the Unitarity Triangle fit

We present here the update of the Unitarity Triangle (UT) analysis performed by the UTfit Collaboration within the Standard Model (SM) and beyond. Continuously updated flavour results contribute to improving the precision of several constraints and through the global fit of the CKM parameters and the SM predictions. We also extend the UT analysis to investigate new physics (NP) effects on $ΔF=2$ processes. Finally, based on the NP 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 NP that contributes to these low-energy effective interactions.

hep-ph↗
arXiv2014

Vacuum Insertion Approximation and the $ΔI=1/2$ rule:a lattice QCD test of the naïve factorization hypothesis for $K$, $D$, $B$ and static mesons

Motivated by a recent paper by the RBC-UKQCD Collaboration, which observes large violations of the naïve factorization hypothesis in $K \to ππ$ decays, we study in this paper the accuracy of the Vacuum Insertion Approximation (VIA) for the matrix elements of the complete basis of four fermion $ΔF=2$ operators. We perform a comparison between the matrix elements in QCD, evaluated on the lattice, and the VIA predictions. We also investigate the dependence on the external meson masses by computing matrix elements for $K$, $D_s$, $B_s$ and static mesons. In commonly used renormalization schemes, we find large violations of the VIA in particular for one of the two relevant Wick contractions in the kaon sector. These deviations, however, decrease significantly as the meson mass increases and the VIA predictions turn out to be rather well verified for B-meson matrix elements and, even better, in the infinite mass limit.

hep-lat↗
arXiv2013

Electroweak Precision Observables, New Physics and the Nature of a 126 GeV Higgs Boson

We 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 $ε_{1,2,3,b}$ ones, for modified $Zb\bar{b}$ 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.

hep-ph↗
arXiv2012

The Standard Model confronts CP violation in $D^0 \to π^+π^-$ and $D^0 \to K^+K^-$

The recently measured direct CP asymmetries in the processes $D^0\to π^+π^-$ and $D^0\to K^+K^-$ show a significant deviation from the naive Standard Model expectation. Using a general parameterization of the decay amplitudes, we show that the measured branching ratios imply large SU(3) breaking and large violations of the naive $1/N_c$ counting. Furthermore, rescattering constrains the I=0 amplitudes in the $ππ$ and $KK$ channels. Combining all this information, we show that, with present errors, the observed asymmetries are marginally compatible with the Standard Model. Improving the experimental accuracy could lead to an indirect signal of new physics.

hep-ph↗
arXiv2011

Theoretical uncertainty in sin 2beta: An update

The source of theoretical uncertainty in the extraction of sin 2beta from the measurement of the golden channel Bd -> J/psi K0 is briefly reviewed. An updated estimate of this uncertainty based on SU(3) flavour symmetry and the measurement of the decay Bd -> J/psi pi0 is also presented.

hep-ph↗
arXiv2007

Searching for New Physics in b to s Hadronic Penguin Decays

We review the theoretical status of b to s hadronic penguin decays in the Standard Model and beyond. We summarize the main theoretical tools to compute Branching Ratios and CP asymmetries for b to s penguin dominated nonleptonic decays, and discuss the theoretical uncertainties in the prediction of time-dependent CP asymmetries in this processes. We consider general aspects of b to s transitions beyond the Standard Model. Then we present detailed predictions in supersymmetric models with new sources of flavor and CP violation.

hep-ph↗
arXiv2005

QCD corrections to the electric dipole moment of the neutron in the MSSM

We consider the QCD corrections to the electric dipole moment of the neutron in the Minimal Supersymmetric Standard Model. We provide a master formula for the Wilson coefficients at the low energy scale including for the first time the mixing between the electric and chromoelectric operators and correcting widely used previous LO estimates. We show that, because of the mixing between the electric and chromoelectric operators, the neutralino contribution is always strongly suppressed. We find that, in general, the effect of the QCD corrections is to reduce the amount of CP violation generated at the high scale. We discuss the perturbative uncertainties of the LO computation, which are particularly large for the gluino-mediated contribution. This motivates our Next-to-Leading order analysis. We compute for the first time the order alpha_s corrections to the Wilson coefficients for the gluino contributions, and recompute the two-loop anomalous dimension for the dipole operators. We show that the large LO uncertainty disappears once NLO corrections are taken into account.

hep-ph↗
arXiv2005

Rare decays and CP violation beyond the Standard Model

We review the status of rare decays and CP violation in extensions of the Standard Model. We analyze the determination of the unitarity triangle and the model-independent constraints on new physics that can be derived from this analysis. We find stringent bounds on new contributions to K-Kbar and Bd-Bdbar mixing, pointing either to models of minimal flavour violation or to models with new sources of flavour and CP violation in b to s transitions. We discuss the status of the universal unitarity triangle in minimal flavour violation, and study rare decays in this class of models. We then turn to supersymmetric models with nontrivial mixing between second and third generation squarks, discuss the present constraints on this mixing and analyze the possible effects on CP violation in b to s nonleptonic decays and on Bs-Bsbar mixing. We conclude presenting an outlook on Lepton-Photon 2009.

hep-ph↗
arXiv2005

Upper Bounds on Rare K and B Decays from Minimal Flavour Violation

We study the branching ratios of rare K and B decays in models with minimal flavour violation, using the presently available information from the universal unitarity triangle analysis and from the measurements of Br(B -> X_s gamma), Br(B -> X_s l^+l^-) and Br(K^+ -> pi^+ nu nubar). We find the following upper bounds: Br(K^+ -> pi^+ nu nubar)< 11.9 10^{-11}, Br(K_L -> pi^0 nu nubar)< 4.6 10^{-11}, Br(K_L -> mu mubar)_{SD}< 1.4 10^{-9}, Br(B -> X_s nu nubar)< 5.2 10^{-5}, Br(B -> X_d nu nubar)< 2.2 10^{-6}, Br(B_s -> mu mubar)< 7.4 10^{-9}, Br(B_d -> mu mubar)< 2.2 10^{-10} at 95 % probability. We analyze in detail various possible scenarios with positive or negative interference of Standard Model and New Physics contributions, and show how an improvement of experimental data corresponding to the projected 2010 B factory integrated luminosities will allow to disentangle and test these different possibilities. Finally, anticipating that subsequently the leading role in constraining this kind of new physics will be taken over by the rare decays K^+ -> pi^+ nu nubar, K_L -> pi^0 nu nubar and B_{s,d} -> mu mubar, that are dominated by the Z^0 -penguin function C, we also present plots for several branching ratios as functions of C . We point out an interesting triple correlation between K^+ -> pi^+ nu nubar, B -> X_s gamma and B -> X_s l^+l^- present in MFV models.

hep-ph↗