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Giulia Ricciardi

Publications and source records attributed to Giulia Ricciardi.

At least 55 records · Page 3Linked to original sources
arXiv2012

eta - eta prime Mixing -From electromagnetic transitions to weak decays of charm and beauty hadrons

It has been realized for a long time that knowing the eta and eta^prime wave functions in terms of quark and gluon components probes our understanding of non-perturbative QCD dynamics. Great effort has been given to this challenge -- yet no clear picture has emerged even with the most recent KLOE data. We point out which measurements would be most helpful in arriving at a more definite conclusion. A better knowledge of these wave functions will significantly help to disentangle the weight of different decay subprocesses in semi-leptonic decays of D+, Ds^+ and B+ mesons. The resulting insights will be instrumental in treating even non-leptonic B transitions involving eta and eta^prime and their CP asymmetries; thus they can sharpen the case for or against New Physics intervening there.

hep-ph↗
arXiv2011

Exploring CP Violation and $η$-$η'$ Mixing with the $B^0_{s,d} \to J/ψη^{(\prime)}$ Systems

The $B^0_{s,d} \to J/ψη^{(\prime)}$ decays provide new terrain for exploring CP violation. After briefly discussing $η$-$η'$ mixing, we analyse the effective lifetimes and CP-violating observables of the $B_s$ channels, which allow us to probe New-Physics effects in $B^0_s$-$\bar B^0_s$ mixing. We have a critical look at these observables and show how hadronic corrections can be controlled by means of the $B_d$ decays. Using measurements of the $B^0_{s,d}\to J/ψη^{(\prime)}$ branching ratios by the Belle collaboration, we discuss tests of the $SU(3)_F$ flavour symmetry of strong interactions, obtain the first constraints on the hadronic parameters entering the $B^0_{s,d} \to J/ψη$ system, and predict the $B^0_d\to J/ψη'$ branching ratio at the $5\times10^{-6}$ level. Furthermore, we present strategies for the determination of the $η$-$η'$ mixing parameters from the $B^0_{s,d} \to J/ψη^{(\prime)}$ observables. We also observe that the $B^0_{s,d} \to J/ψη$ and $B^0_{s,d} \to J/ψη'$ decays are - from a formal point of view - analogous to the quark-antiquark and tetraquark descriptions of the $f_0(980)$ in the $B^0_{s,d} \to J/ψf_0(980)$ channels, respectively.

hep-ph↗
arXiv2011

Anatomy of $B^0_{s,d} \to J/ψf_0(980)$

The $B^0_s\to J/ψf_0(980)$ decay offers an interesting experimental alternative to the well-known $B^0_s\to J/ψϕ$ channel for the search of CP-violating New-Physics contributions to $B^0_s$--$\bar B^0_s$ mixing. As the hadronic structure of the $f_0(980)$ has not yet been settled, we take a critical look at the implications for the relevant observables and address recent experimental data. It turns out that the effective lifetime of $B^0_s\to J/ψf_0(980)$ and its mixing-induced CP asymmetry $S$ are quite robust with respect to hadronic effects and thereby allow us to search for a large CP-violating $B^0_s$--$\bar B^0_s$ mixing phase $ϕ_s$, which is tiny in the Standard Model. However, should small CP violation, i.e. in the range $-0.1\lsim S\lsim 0$, be found in $B^0_s\to J/ψf_0(980)$, it will be crucial to constrain hadronic corrections in order to distinguish possible New-Physics effects from the Standard Model. We point out that $B^0_d\to J/ψf_0(980)$, which has not yet been measured, is a key channel in this respect and discuss the physics potential of this decay.

hep-ph↗
arXiv2011

Minimal prescription corrected spectra in heavy quark decays

The Minimal Prescription procedure is applied to tame the Landau pole singularities of resummed formulae for heavy quark decays. Effects of the final quark mass are taken into account. Explicit expressions are obtained for the t->b and b->c transitions for both the frozen coupling approximation and in the QCD running coupling case.

hep-ph↗
arXiv2009

Inclusive Measure of |V_ub| with the Analytic Coupling Model

By analyzing B -> X_u l nu_l spectra with a model based on soft-gluon resummation and an analytic time-like QCD coupling, we obtain |V_ub| = (3.76 +-0.13 +- 0.22)*10^(-3), where the first and the second error refers to experimental and theoretical errors, respectively. The V_ub value is obtained from the available measured semileptonic branching fractions in limited regions of the phase-space. The distributions in the lepton energy E_l, the hadron invariant mass m_X, the light-cone momentum P_+ = E_X - p_X, together with the double distributions in (m_X,q^2) and (E_l,s_h^max), are used to select the phase-space regions. The q^2 is the dilepton squared momentum and s_h^max is the maximal m_X^2 at fixed q^2 and E_l. The V_ub value obtained is in complete agreement with the value coming from exclusive B decays and from an over-all fit to the Standard Model parameters. We show that the slight disagreement (up to +2 sigma) with respect to previous inclusive measurements is not related to different choices for the b (and c) masses but to a different modelling of the threshold (Sudakov) region.

hep-ph↗
arXiv2007

Threshold Resummation in B --> X_c l nu_l Decays

We compute the QCD form factor resumming threshold logarithms in B --> X_c + l + nu_l decays to next-to-leading logarithmic approximation. We present an interpolation formula including soft as well as collinear effects softened by the non-vanishing charm mass.

hep-ph↗
arXiv2006

Semi-Inclusive B Decays and a Model for Soft-Gluon Effects

We compare experimental spectra of radiative and semileptonic B decays with the predictions of a model based on soft-gluon resummation to next-to-next-to leading order and on a ghost-less time-like coupling. We find a good agreement with photon spectra in the radiative decay and with hadron mass distributions in the semileptonic one: the extracted values for alpha_S(m_Z) are in agreement with the current PDG average within at most two standard deviations. The agreement is instead less good for the electron spectra measured by BaBar and Belle in semileptonic decays for small electron energies (< 2.2 GeV): our spectrum is harder. We also show that, in general, the inclusion of next-to-next-to-leading order effects is crucial for bringing the model closer to the data and that the non-power expansion introduced in the framework of analytic coupling studies does not accurately describe soft-gluon effects.

hep-ph↗
arXiv2006

Threshold resummed spectra in B ->Xu l nu decays in NLO (III)

We resum to next-to-leading order (NLO) the distribution in the light-cone momentum p+ = EX - |pX| and the spectrum in the electron energy, in the semileptonic decays B-> Xu l nu, where EX and pX are the total energy and three-momentum of the final hadron state Xu respectively. By expanding our formulas, we obtain the coefficients of all the infrared logarithms alpha^n L^k at order alpha^2 and at order alpha^3, with the exception of the alpha^3 L coefficient. We explicitly show that the relation between these semileptonic spectra and the hadron mass distribution in the radiative decay B -> Xs gamma is not a purely short distance one. By expanding our result for the p+ distribution and comparing it with the recent O(alpha^2 n_f) computation, we obtain an explicit and non trivial verification of our resummation scheme at the two-loop level. We also discuss the validity of the so-called Brodsky-Lepage-Mackenzie (BLM) scheme in the evaluation of the semileptonic spectra by comparing it with our results, finding that it is a reasonable approximation for the leading terms only. Finally, we show that long-distance phenomena, such as Fermi motion, are expected to have an effect on the electron spectrum smaller than on the hadron mass distribution.

hep-ph↗
arXiv2005

Threshold Resummed Spectra in B -> Xu l nu Decays in NLO (I)

We evaluate thresholds resummed spectra in B -> Xu l nu decays in next-to-leading order. We present results for the distribution in E_X and in m_X^2/E_X^2, for the distribution in E_X and E_l and for the distribution in E_X, where E_X and m_X are the energy and the invariant mass of the final hadronic state Xu respectively and E_l is the energy of the charged lepton. We explicitly show that all these spectra (where there is no integration over the hadronic energy) can be directly related to the photon spectrum in B -> Xs gamma via short-distance coefficient functions.

hep-ph↗
arXiv2005

Threshold resummed spectra in B -> Xu l nu decays in NLO (II)

We resum to next-to-leading order the distribution in the ratio of the invariant hadron mass mX to the total hadron energy EX and the distribution in mX in the semileptonic decays B -> Xu l nu. By expanding our formulas, we obtain the coefficients of all the infrared logarithms at O(alphaS^2) and of the leading ones at O(alphaS^3). We explicitly show that the relation between these semileptonic spectra and the photon spectrum in the radiative decay B -> Xs gamma is not a purely short-distance one. There are long-distance effects in the semileptonic spectra which are not completely factorized by the structure function as measured in the radiative decay and have to be modelled in some way.

hep-ph↗
arXiv2004

A model for next-to-leading order threshold resummed form factors

We present a model for next-to-leading order resummed threshold form factors based on a time-like coupling recently introduced in the framework of small x physics. Improved expressions for the form factors in N-space are obtained which are not plagued by Landau-pole singularities, as the included absorptive effects -- usually neglected -- act as regulators. The physical reason is that, because of faster decay of gluon jets, there is not enough resolution time to observe the Landau pole. Our form factors reduce to the standard ones when the absorptive parts related to the coupling are neglected. The inverse transform from N-space to x-space can be done directly without any prescription and we obtain analytical expressions for the form factors, which are well defined in all x-space.

hep-ph↗
arXiv2000

The structure function of semi-inclusive heavy flavour decays in field theory

We consider the decay of a heavy flavour into an inclusive hadronic state X of invariant mass m_X small with respect to its energy E_X, m_X << E_X. The electron spectrum and the hadronic mass distribution in semileptonic b -> u decays, or the photon spectrum in b -> s gamma decays, all require, close to their endpoints, a control over this region. This region is affected both by non-perturbative phenomena related to the Fermi motion of the heavy quark and by perturbative soft gluon radiation in the final state (Sudakov form factor).Fermi motion can be described by the shape function f (m*), which represents the distribution of the effective mass m* of the heavy quark at disintegration time. We perform a factorization with a simple technique in order to consistently separate perturbative from non-perturbative effects. We find that the shape function, contrary to naive expectations,is not a physical distribution, as it is affected by substantial regularization scheme effects, controlling even the leading, double-logarithmic term. It factorizes, however, the bulk of non-perturbative effects in lattice-like regularizations. Some non-perturbative effects are present in the coefficient function even at leading twist, but they are expected to be suppressed on physical grounds. Finally, we clarify a controversial factor of 2 in the evolution kernel of the shape function.

hep-ph↗
arXiv1998

Light masses in short distance penguin loops

Penguin diagrams for decays such as $b\to (s,d)γ$ involve virtual loops of $u$ or other light quarks. Logarithms of the virtual quark mass could, in principle, influence the phenomenological analysis of the decay. It is thus important to study these logarithms to all orders in QCD perturbation theory. In this paper we show that, at arbitrary order, the matrix elements of operators in the effective hamiltonian contributing to $b\to sγ$ are finite for the limit of $m_u \to 0$ in penguin loops.

hep-ph↗