3,474 research outputs found
K* vector and tensor couplings from Nf = 2 tmQCD
The mass m_K* and vector coupling f_K* of the K*-meson, as well as the ratio
of the tensor to vector couplings fT/fV|_K*, are computed in lattice QCD. Our
simulations are performed in a partially quenched setup, with two dynamical
(sea) Wilson quark flavours, having a maximally twisted mass term. Valence
quarks are either of the standard or the Osterwalder-Seiler maximally twisted
variety. Results obtained at three values of the lattice spacing are
extrapolated to the continuum, giving m_K* = 981(33) MeV, f_K* = 240(18) MeV
and fT(2 GeV)/fV|_K* = 0.704(41).Comment: 1+11 page
Layered Higgs Phase as a Possible Field Localisation on a Brane
So far it has been found by using lattice techniques that in the anisotropic
five--dimensional Abelian Higgs model, a layered Higgs phase exists in addition
to the expected five--dimensional one. The exploration of the phase diagram has
shown that the two Higgs phases are separated by a phase transition from the
confining phase. This transition is known to be first order. In this paper we
explore the possibility of finding a second order transition point in the
critical line which separates the first order phase transition from the
crossover region. This is shown to be the case only for the four--dimensional
Higgs layered phase whilst the phase transition to the five--dimensional broken
phase remains first order. The layered phase serves as the possible realisation
of four--dimensional spacetime dynamics which is embedded in a
five--dimensional spacetime. These results are due to gauge and scalar field
localisation by confining interactions along the extra fifth direction.Comment: 1+15 pages, 12 figure
O(a^2) cutoff effects in lattice Wilson fermion simulations
In this paper we propose to interpret the large discretization artifacts
affecting the neutral pion mass in maximally twisted lattice QCD simulations as
O(a^2) effects whose magnitude is roughly proportional to the modulus square of
the (continuum) matrix element of the pseudoscalar density operator between
vacuum and one-pion state. The numerical size of this quantity is determined by
the dynamical mechanism of spontaneous chiral symmetry breaking and turns out
to be substantially larger than its natural magnitude set by the value of
Lambda_QCD.Comment: 38 pages, 1 figure, 2 table
Comment on ``Neutrino masses and mixing angles in a predictive theory of fermion masses''
In the extension of the Dimopoulos--Hall--Raby model of the fermion mass
matrices to the neutrino sector, there is an entry in the up-quark and neutrino
Dirac mass matrices which can be assumed to arise from the Yukawa coupling of a
{\bf 120}, instead of a {\bf 10} or a {\bf 126}, of SO(10). Although this
assumption leads to an extra undetermined complex parameter in the model, the
resulting lepton mixing matrix exhibits the remarkable feature that the does not mix with the other two neutrinos. Making a reasonable
assumption about the extra parameter, we are able to fit the large-mixing-angle
MSW solution of the solar-neutrino problem, and we obtain eV, the right mass range to close the Universe. Other possibilities for
explaining the solar-neutrino deficit are also discussed.Comment: standard LATEX, 6 pages, 2 figures available from the authors, report
No. CMU-HEP93-20 and DOE-ER/40682-4
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
Is There a Peccei-Quinn Phase Transition?
The nature of axion cosmology is usually said to depend on whether the
Peccei-Quinn (PQ) symmetry breaks before or after inflation. The PQ symmetry
itself is believed to be an accident, so there is not necessarily a symmetry
during inflation at all. We explore these issues in some simple models, which
provide examples of symmetry breaking before and after inflation, or in which
there is no symmetry during inflation and no phase transition at all. One
effect of these observations is to relax the constraints from isocurvature
fluctuations due to the axion during inflation. We also observe new
possibilities for evading the constraints due to cosmic strings and domain
walls, but they seem less generic.Comment: 14 pages. Several references adde
- …