490 research outputs found
Flavour symmetry restoration and kaon weak matrix elements in quenched twisted mass QCD
We simulate two variants of quenched twisted mass QCD (tmQCD), with
degenerate Wilson quarks of masses equal to or heavier than half the strange
quark mass. We use Ward identities in order to measure the twist angles of the
theory and thus check the quality of the tuning of mass parameters to a physics
condition which stays constant as the lattice spacing is varied. Flavour
symmetry breaking in tmQCD is studied in a framework of two fully twisted and
two standard Wilson quark flavours, tuned to be degenerate in the continuum.
Comparing pseudoscalar masses, obtained from connected quark diagrams made of
tmQCD and/or standard Wilson quark propagators, we confirm that flavour
symmetry breaking effects, which are at most 5%, decrease as we approach the
continuum limit. We also compute the pseudoscalar decay constant in the
continuum limit, with reduced systematics. As a consequence of improved tuning
of the mass parameters at , we reanalyse our previous
results. Our main phenomenological findings are and .Comment: 41 pages, figures included, one reference added. Final version as
accepted for publication on Nucl.Phys.
A precise determination of in quenched QCD
The parameter is computed in quenched lattice QCD with Wilson twisted
mass fermions. Two variants of tmQCD are used; in both of them the relevant
four-fermion operator is renormalised multiplicatively. The
renormalisation adopted is non-perturbative, with a Schroedinger functional
renormalisation condition. Renormalisation group running is also
non-perturbative, up to very high energy scales. In one of the two tmQCD
frameworks the computations have been performed at the physical -meson mass,
thus eliminating the need of mass extrapolations. Simulations have been
performed at several lattice spacings and the continuum limit was reached by
combining results from both tmQCD regularisations. Finite volume effects have
been partially checked and turned out to be small. Exploratory studies have
also been performed with non-degenerate valence flavours. The final result for
the RGI bag parameter, with all sources of uncertainty (except quenching) under
control, is .Comment: 54 pages, 11 figure
Non-perturbative scale evolution of four-fermion operators in two-flavour QCD
We apply finite-size recursion techniques based on the Schrodinger functional
formalism to determine the renormalization group running of four-fermion
operators which appear in the Delta S=2 effective weak Hamiltonian of the
Standard Model. Our calculations are done using O(a) improved Wilson fermions
with N_f=2 dynamical flavours. Preliminary results are presented for the
four-fermion operator which determines the B_K parameter in tmQCD.Comment: 7 pages, 2 figures, talk presented at Lattice2006 (Renormalization
Lattice renormalisation of O(a) improved heavy-light operators
The analytical expressions and the numerical values of the renormalisation
constants of improved static-light currents are given at one-loop
order of perturbation theory in the framework of Heavy Quark Effective Theory:
the static quark is described by the HYP action and the light quark is
described either with the Clover or the Neuberger action. These factors are
relevant to extract from a lattice computation the decay constants ,
and the set of bag parameters associated with
mixing phenomenology in the Standard Model and beyond.Comment: 16 pages, 2 figures, 4 tables; few comments and references added;
version to be published in Phys Rev
Non-perturbative renormalization of moments of parton distribution functions
We compute non-perturbatively the evolution of the twist-2 operators
corresponding to the average momentum of non-singlet quark densities. The
calculation is based on a finite-size technique, using the Schr\"odinger
Functional, in quenched QCD. We find that a careful choice of the boundary
conditions, is essential, for such operators, to render possible the
computation. As a by-product we apply the non-perturbatively computed
renormalization constants to available data of bare matrix elements between
nucleon states.Comment: Lattice2003(Matrix); 3 pages, 3 figures. Talk by A.
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