168 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
A non-minimal model for Lepton Flavour Universality Violation in decays
No full text331 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
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
Chiral order and fluctuations in multi-flavour QCD
No full textMulti-flavour (Nf greater than or equal to 3) chiral perturbation theory (chiPT) may exhibit instabilities due to vacuum fluctuations of sea q-bar q pairs. Keeping the fluctuations small would require a very precise fine tuning of the low-energy constants L-4(mu) and L-6(mu) to L-4(crit)(M-rho) = -0.51*10^(-3), L-6(crit)(M-rho) = -0.26*10^(-3). A small deviation from these critical values - like the one suggested by the phenomenology of OZI-rule violation in the scalar channel - is amplified by huge numerical factors inducing large effects of vacuum fluctuations. This would lead in particular to a strong Nf dependence of chiral symmetry breaking (chiSB) and a suppression of the multi-flavour chiral order parameters. A simple resummation is shown to cure the instability of N-f greater than or equal to 3 chiPT, but it modifies the standard expressions of some O(p(2)) and O(p(4)) low-energy parameters in terms of observables. On the other hand, for r = m(s)/m > 15, the two-flavour condensate is not suppressed, due to the contribution induced by massive vacuum ss pairs. Thanks to the latter, the standard two-flavour chiPT is protected from multi-flavour instabilities and could provide a well-defined expansion scheme in powers of non-strange quark masses
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
Resumming QCD vacuum fluctuations in three-flavour Chiral Perturbation Theory
No full text58 pages, 8 figuresDue to its light mass of order Lambda_QCD, the strange quark can play a special role in Chiral Symmetry Breaking (ChSB): differences in the pattern of ChSB in the limits N_f=2 (m_u,m_d->0, m_s physical) and N_f=3 (m_u,m_d,m_s->0) may arise due to vacuum fluctuations of s-bar s pairs, related to the violation of the Zweig rule in the scalar sector and encoded in particular in the O(p^4) low-energy constants L_4 and L_6. In case of large fluctuations, we show that the customary treatment of SU(3)xSU(3) chiral expansions generate instabilities upsetting their convergence. We develop a systematic program to cure these instabilities by resumming nonperturbatively vacuum fluctuations of s-bar s pairs, in order to extract information about ChSB from experimental observations even in the presence of large fluctuations. We advocate a Bayesian framework for treating the uncertainties due to the higher orders. As an application, we present a three-flavour analysis of the low-energy pi-pi scattering and show that the recent experimental data imply a lower bound on the quark mass ratio 2m_s/(m_u+m_d) > 14 at 95 % confidence level. We outline how additional information may be incorporated to further constrain the pattern of ChSB in the N_f=3 chiral limit
The role of strange sea quarks in chiral extrapolations on the lattice
No full text31 pages, 9 figures. A few comments added and typos correctedSince the strange quark has a light mass of order Lambda_QCD, fluctuations of sea s-s bar pairs may play a special role in the low-energy dynamics of QCD by inducing significantly different patterns of chiral symmetry breaking in the chiral limits N_f=2 (m_u=m_d=0, m_s physical) and N_f=3 (m_u=m_d=m_s=0). This effect of vacuum fluctuations of s-s bar pairs is related to the violation of the Zweig rule in the scalar sector, described through the two O(p^4) low-energy constants L_4 and L_6 of the three-flavour strong chiral lagrangian. In the case of significant vacuum fluctuations, three-flavour chiral expansions might exhibit a numerical competition between leading- and next-to-leading-order terms according to the chiral counting, and chiral extrapolations should be handled with a special care. We investigate the impact of the fluctuations of s-s bar pairs on chiral extrapolations in the case of lattice simulations with three dynamical flavours in the isospin limit. Information on the size of the vacuum fluctuations can be obtained from the dependence of the masses and decay constants of pions and kaons on the light quark masses. Even in the case of large fluctuations, corrections due to the finite size of spatial dimensions can be kept under control for large enough boxes (L around 2.5 fm)
QCD factorisation and flavour symmetries illustrated in B_d,s -> KK decays
No full text4 pages, 2 figures. Talk given at the 4th International Workshop on the CKM Unitarity Triangle (CKM2006), 12-16 December 2006, Nagoya, Japan, to appear in the proceedings (KEK Report)We present a new analysis of B_d,s -> KK 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 B_d,s -> K^0 \bar K^0 observables. We determine B_s -> KK branching ratios and CP-asymmetries as functions of A_dir(B_d -> K^0 \bar K^0), with a good agreement with current experimental measurements of CDF. Finally, we predict the amount of U-spin breaking between B_d -> pi+ pi- and B_s -> K+K-
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