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M. Ciuchini

Publications and source records attributed to M. Ciuchini.

At least 91 records · Page 5Linked to original sources
arXiv1998

Next-to-Leading QCD Corrections to B -> X_s gamma: Standard Model and Two-Higgs Doublet Model

We present the QCD corrections to the matching conditions of the Delta B =1 magnetic and chromo-magnetic operators in the Standard Model and in two-Higgs doublet models. We use an off-shell matching procedure which allows us to perform the computation using Taylor series in the external momenta, instead of asymptotic expansions. In the Standard Model case, we confirm previous results derived on-shell and we obtain BR(B -> X_s gamma)=(3.62\pm 0.33) 10^{-4}. In the case of the usual two-Higgs doublet model, we show that going from the leading to the next-to-leading order result improves the CLEO bound on the charged-Higgs mass from 260 GeV to 380 GeV. This limit is very sensitive to the definition of errors and we carefully discuss the theoretical uncertainties. Finally, in the case of the two-Higgs doublet model in which both up- and down-type quarks couple to the same Higgs field, the theoretical prediction for BR(B -> X_s gamma) can be reduced by at most 20% with respect to the Standard Model value.

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 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

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.

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arXiv1996

The Chromomagnetic Dipole Operator and the B Semileptonic Branching Ratio

We consider the possibility of having a large branching ratio for the decay $b\to s g$ coming from an enhanced Wilson coefficient of the chromomagnetic dipole operator. We show that values of $BR(b\to s g)$ up to $\sim 10\%$ or more are compatible with the constraints coming from the CLEO experimental results on $BR(B\to X_sγ)$ and $BR(B\to X_sϕ)$. Such large values can reconcile the predictions of both the semileptonic branching ratio and the charm counting with the present experimental results. We also discuss a supersymmetric model with gluino-mediated flavour violations, which can account for such large values of $BR(b\to s g)$.

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

Estimates of $ε\prime/ε$

We review the latest calculations of the next-to-leading $\DSone$ effective Hamiltonian, relevant for $K\to ππ$ transitions. Numerical results for the Wilson coefficients are given for different regularization schemes. Predictions of $\epse$, obtained using different approaches to evaluate the relevant hadronic matrix elements, are compared. Given the present value of the top mass, $m_t=(174\pm 17)$ GeV, all the analyses, in spite of the large theoretical uncertainties, indicate that the value of $\epse$ is smaller than $1\times 10^{-3}$.

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

$B \rightarrow X_s\,γ$ and $B \rightarrow K^*\,γ$ in the standard and 2H models

Theoretical predictions for the branching ratios of the $B \rightarrow X_s\,γ$ and $B \rightarrow K^*\,γ$ decays are calculated in the Standard Model and in the (type II) two-Higgs-doublet model. Both the complete leading and the partially known next-to-leading order QCD corrections are included. The uncertainties due to the regularization scheme dependence introduced by the incomplete NLO terms are discussed. The results are compared with the recent CLEO II measurements and a new lower limit on the charged Higgs boson mass, $M_{H^\pm} > \sim 200$ GeV, is obtained.

hep-ph↗
arXiv1994

Next-to-leading prediction of $\epse$: an upgraded analysis

We present an updated theoretical prediction of $\epse$, using the next-to-leading $\DSone$ effective hamiltonian and lattice QCD matrix elements. The CP violating phase is costrained by using both the experimental values of $\ep$ and $x_d$, assuming the theoretical determination of $f_B$. Predictions of $\cosδ$ and $\sin 2β$ are also obtained in this way. For $\epse$, our estimate is $\epse=(2.8 \pm 2.4)\times 10^{-4}$.}

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↗
arXiv1994

Quark Masses from Lattice QCD at the Next-to-Leading Order

Using the results of several quenched lattice simulations, we predict the value of the strange and charm quark masses in the continuum at the next-to-leading order, $m^{\overline{MS}}_s(μ=2\,\, \rm{GeV})= (127 \pm 18)\,\, \rm{MeV}$ and $m^{\overline{MS}}_{ch}(μ=2 \,\,\rm{GeV})=(1.47 \pm 0.28)\,\, \rm{GeV}$. The errors quoted above have been estimated by taking into account the original statistical error of the lattice results and the uncertainties coming from the matching of the lattice to the continuum theory. A detailed presentation of the relevant formulae at the next-to-leading order and a discussion of the main sources of errors is also presented.

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↗
arXiv1993

The $ΔS=1$ Effective Hamiltonian Including Next-to-Leading Order QCD and QED Corrections

In this paper we present a calculation of the $ΔS=1$ effective weak Hamiltonian including next-to-leading order QCD and QED corrections. At a scale $μ$ of the order of few GeV, the Wilson coefficients of the operators are given in terms of the renormalization group evolution matrix and of the coefficients computed at a large scale $\sim M_W$. The expression of the evolution matrix is derived from the two-loop anomalous dimension matrix which governs the mixing of the relevant current-current and penguin operators, renormalized in some given regularization scheme. We have computed the anomalous dimension matrix up to and including order $\alphas^2$ and $\alphae \alphas$ in two different renormalization schemes, NDR and HV, with consistent results. We give many details on the calculation of the anomalous dimension matrix at two loops, on the determination of the Wilson coefficients at the scale $M_W$ and of their evolution from $M_W$ to $μ$. We also discuss the dependence of the Wilson coefficients/operators on the regularization scheme.

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