1,721,030 research outputs found
Sudakov effects in B -> pi l nu/l form factors
No full textIn order to obtain fundamental information about the Standard Model of particle physics from experimental measurements of exclusive hadronic two-body B-decays we have to be able to quantify the non-perturbative QCD effects. Although approaches based on the factorization of mass singularities into hadronic distribution amplitudes and form factors provide a rigorous theoretical framework for the evaluation of these effects in the heavy quark limit, it is not possible to calculate the O(?QCD/mb) corrections in a model-independent way, because of the presence of non-factorizing long-distance contributions. It has been argued that Sudakov effects suppress these contributions and render the corresponding corrections perturbatively calculable in terms of the distribution amplitudes. In this paper we examine this claim for the simple and related example of semileptonic B?? decays (which have similar long-distance contributions) and conclude that it is not justified. The uncertainties in our knowledge of the mesons' distribution amplitudes imply that the calculations of the form factors are not sufficiently precise to be useful phenomenologically. Moreover, it appears that a significant fraction of the contribution comes from the non-perturbative region of large impact parameters, and is therefore incalculable. We also raise a number of theoretical issues in the derivation of the underlying formalism. Our conclusion is therefore a disappointing one. For B-decays it is not possible to invoke Sudakov effects to calculate amplitudes for decays which have long-distance divergences (end-point singularities) in the standard hard-scattering approach
Factorization, the light cone distribution amplitude of the b meson and the radiative decay B??Lv(L)
No full textWe study the radiative decay B??Lv(L) in the framework of QCD factorization. We demonstrate explicitly that, in the heavy-quark limit and at one-loop order in perturbation theory, the amplitude does factorize, i.e. that it can be written as a convolution of a perturbatively calculable hard-scattering amplitude with the (non-perturbative) light-cone distribution amplitude of the B-meson. We evaluate the hard-scattering amplitude at one-loop order and verify that the large logarithms are those expected from a study of the b?u transition in the Soft-Collinear Effective Theory. Assuming that this is also the case at higher orders, we resum the large logarithms and perform an exploratory phenomenological analysis. The questions addressed in this study are also relevant for the applications of the QCD factorization formalism to two-body non-leptonic B-decays, in particular to the component of the amplitude arising from hard spectator interactions
Universality of nonperturbative QCD effects in radiative B decays
No full textWe demonstrate, by an explicit one-loop calculation, that at leading twist the nonperturbative effects in B → γℓνℓ, B → γγ and B → γℓ+ℓ- radiative decays are contained in a common multiplicative factor (ΛB(Eγ), where Eγ is the energy of the photon). We argue that this result holds also at higher orders. Ratios of the amplitudes for these processes do not depend on scales below the mass of the B-meson (MB), and can be calculated as perturbative series in αs(MB)
Spectator interactions in B -> V gamma decays and QCD factorisation
No full textWe study the radiative decays B = Vγ (where V = ρ,K*) in the framework of QCD factorisation, and in particular the hard-spectator contributions to the decay amplitudes. For the phenomenologically significant chromomagnetic operator, we show by an explicit next-to-leading-order computation that the spectator interactions factorise in the heavy-quark limit, i.e., that they can be written as the convolution of a hard-scattering kernel, computable in perturbation theory, and of the light-cone distribution amplitudes for the B and V mesons which contain the soft physics. The presence of an intermediate scale of O(MBΛQCD) leads to the presence of Sudakov logarithms. We indicate how the demonstration of factorisation can be extended to other (four-quark) operators
A non-minimal model for Lepton Flavour Universality Violation in decays
Get PDF331 models constitute an extension of the Standard Model (SM) obtained by enlarging the SM gauge group to the group . We investigate how a non-minimal 331 model may embed lepton flavour universality violating contributions to processes without introducing lepton flavour violation, as suggested by the recent LHCb measurements of the ratios and . We discuss the model-independent scenarios of New Physics in currently favoured by the data that could be accommodated by this model and consider a few phenomenological constraints on this model
The trouble with – updated global fits and future directions
No full textI review the interpretation of the available data on transitions, including recent updates from LHCb and Belle collaborations on the ratios and assessing Lepton-Flavour Universality in these decays
Low-energy pi-pi and pi-K scatterings revisited in three-flavour resummed chiral perturbation theory
No full text30 pages, 2 figuresInternational audienceChiral symmetry breaking may exhibit significantly different patterns in two chiral limits: N_f=2 massless flavours (m_u=m_d=0, m_s physical) and N_f=3 massless flavours (m_u=m_d=0=m_s=0). Such a difference may arise due to vacuum fluctuations of s-bar{s} pairs related to the violation of the Zweig rule in the scalar sector, and could yield a numerical competition between contributions counted as leading order and next-to-leading in the chiral expansions of observables. We recall and extend Resummed Chiral Perturbation Theory (ReChPT), a framework that we introduced previously to deal with such instabilities: it requires a more careful definition of the relevant observables and their one-loop chiral expansions. We analyse the amplitudes for low-energy pi-pi and pi-K scatterings within ReChPT, which we match in subthreshold regions with dispersive representations obtained from the solutions Roy and Roy-Steiner equations. Using a frequentist approach, we constrain the quark mass ratio as well as the quark condensate and the pseudoscalar decay constant in the N_f=3 chiral limit. The results favour significant contributions of vacuum fluctuations suppressing the N_f=3 quark condensate compared to its N_f=2 counterpart
pi-pi and pi-K scatterings in three-flavour resummed chiral perturbation theory
No full textProceedings of the EPS-HEP 2007 Conference, Manchester (UK). 3 pages, 1 figureThe (light but not-so-light) strange quark may play a special role in the low-energy dynamics of QCD. The presence of strange quark pairs in the sea may have a significant impact of the pattern of chiral symmetry breaking : in particular large differences can occur between the chiral limits of two and three massless flavours (i.e., whether m_s is kept at its physical value or sent to zero). This may induce problems of convergence in three-flavour chiral expansions. To cope with such difficulties, we introduce a new framework, called Resummed Chiral Perturbation Theory. We exploit it to analyse pi-pi and pi-K scatterings and match them with dispersive results in a frequentist framework. Constraints on three-flavour chiral order parameters are derived
Connecting B_d and B_s decays through QCD factorisation and flavour symmetries
No full textProceedings of the EPS-HEP 2007 Conference, Manchester (UK). 3 pages, 1 figureWe analyse B_{d,s} -> K(*)0 K(*)0 modes within the SM, relating them in a controlled way through SU(3)-flavour symmetry and QCD-improved factorisation. We propose a set of sum rules for such penguin-mediated decays to constrain some CKM angles. We determine B_s -> KK branching ratios and CP-asymmetries as functions of A_{dir}(B_d -> K0 \bar{K}0). Applying the same techniques to B_{d,s}-> K*0 \bar{K}*0, we outline strategies to determine the B_s mixing angle
How far can you go ? Surprises and pitfalls in three-flavour chiral extrapolations
No full text7 pages, 4 figures, Presented at the XXV International Symposium on Lattice Field Theory, July 30 - August 4 2007, Regensburg, GermanyThe presence of strange sea quark pairs may have a significant impact of the pattern of chiral symmetry breaking : in particular large differences can occur between the chiral limits of two and three massless flavours (i.e., whether is kept at its physical value or sent to zero). We recall some indications of such a scenario in QCD, in relation with the peculiar dynamics of the scalar sector. We explain how this could affect the convergence of three-flavour chiral series, commonly used to extrapolate the results of lattice simulations. Finally, we indicate how lattice simulations with three dynamical flavours could unveil such an effect through the quark-mass dependence of light meson masses and decay constants
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