135 research outputs found
Lattice studies of non-leptonic kaon decays
No full textI review recent progress in the development of the theoretical framework necessary for computing K !pp decay amplitudes in Euclidean finite volumes. The status of the theory of finite volume effects is discussed and the rôle of chiral perturbation theory and the determination of low energy constants is reviewed. A proposal to use twisted boundary conditions is explained and the status of a suggestion to investigate the rôle of the charm quark in the DI = 1=2 rule is discussed
Selected topics in lattice phenomenology
No full textI discuss three topics in lattice phenomenology: the use of twisted boundary conditions to improve the momentum resolution; the evaluation of the amplitude for K semileptonic decays and the theory of finite-volume corrections in the computation of K decay amplitudes
Theoretical issues in lattice simulations of heavy quark physics
No full textI review a number of theoretical issues in the computation of quantities in heavy-quark physics on the lattice. Since, particularly for the b-quark, mba>1, it is necessary to use effective theories, such as the Heavy Quark Effective Theory (HQET). In order to be useful for flavour physics, power corrections in 1/mb must be calculated, leading to ultra-violet divergences which behave like inverse powers of the lattice spacing. I argue that the mixing coefficients between operators of different dimensions must be determined non-perturbatively if generic non-perturbative QCD effects are to be included. I briefly outline the Zeuthen approach for achieving such a non-perturbative renormalization. I also discuss the Fermilab formulation of heavy quark physics on the lattice and its generalizations, noting that non-perturbative determinations of the parameters of the theory are beginning
Factorization in two-body nonleptonic B-decays
No full textI review the theoretical foundations of ITALIC {QCD Factorization} in nonleptonic B-Decays. This approach, developed in collaboration with Martin Beneke, Gerhard Buchalla and Matthias Neubert, provides a rigorous framework for the analysis of these decays in the heavy-quark limit. The significance of power corrections, terms which are formally suppressed by powers of \Lambda QCD/mb and which are not calculable in perturbation theory but which may have a significant impact on phenomenologically important decays, is discussed
SU(2) chiral perturbation theory for Kl3 decay amplitudes
No full textWe use one-loop SU(2)L×SU(2)R chiral perturbation theory (SU(2) ChPT) to study the behaviour of the form-factors for semileptonic K?? decays with the pion mass at q2=0 and at View the MathML source, where q is the momentum transfer. At q2=0, the final-state pion has an energy of approximately mK/2 (for mKmuch greater-thanm?) and so is not soft, nevertheless it is possible to compute the chiral logarithms, i.e. the corrections of View the MathML source.We envisage that our results at q2=0 will be useful in extrapolating lattice QCD results to physical masses. A consequence of the Callan–Treiman relation is that in the SU(2) chiral limit (mu=md=0), the scalar form factor f0 at View the MathML source is equal to f(K)/f, the ratio of the kaon and pion leptonic decay constants in the chiral limit. Lattice results for the scalar form factor at View the MathML source are obtained with excellent precision, but at the masses at which the simulations are performed the results are about 25% below f(K)/f and are increasing only very slowly. We investigate the chiral behaviour of View the MathML source and find large corrections which provide a semi-quantitative explanation of the difference between the lattice results and f(K)/f. We stress the generality of the relation View the MathML source in the SU(2) chiral limit, where P=K, D or B and briefly comment on the potential value of using this theorem in obtaining physical results from lattice simulations
Twisted boundary conditions in lattice simulations
No full textBy imposing twisted boundary conditions on quark fields it is possible to access components of momenta other than integer
multiples of 2?/L on a lattice with spatial volume L3. We use chiral perturbation theory to study finite-volume effects with
twisted boundary conditions for quantities without final-state interactions, such as meson masses, decay constants and semileptonic form factors, and confirm that they remain exponentially small with the volume. We show that this is also the case for partially twisted boundary conditions, in which (some of) the valence quarks satisfy twisted boundary conditions but the sea quarks satisfy periodic boundary conditions. This observation implies that it is not necessary to generate new gluon configurations for every choice of the twist angle, making the method much more practicable. For K ??? decays we show that the breaking of isospin symmetry by the twisted boundary conditions implies that the amplitudes cannot be determined in general (on this point we disagree with a recent claim)
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
The kaon semileptonic form factor with near physical domain wall quarks
No full textWe present a new calculation of the K → π semileptonic form factor at zero momentum transfer in domain wall lattice QCD with N f = 2+1 dynamical quark flavours. By using partially twisted boundary conditions we simulate directly at the phenomenologically relevant point of zero momentum transfer. We perform a joint analysis for all available ensembles which include three different lattice spacings (a = 0.09 - 0.14 fm), large physical volumes (m π L > 3.9) and pion masses as low as 171 MeV. The comprehensive set of simulation points allows for a detailed study of systematic effects leading to the prediction f+Kπ(0)=0.9670(20) (-46+18), where the first error is statistical and the second error systematic. The result allows us to extract the CKM-matrix element | Vu|=0.2237(-8+13) and confirm first-row CKM-unitarity in the Standard Model at the sub per mille level. © 2013 SISSA, Trieste, Italy.RBC/UKQCD collaboration, P.A. Boyle, J.M. Flynn, N. Garron, A. Jüttner, C.T. Sachrajda, K. Sivalingam, and J.M. Zanott
- …
