994 research outputs found
Higher Order Corrections at Zero Recoil
The general structure of the corrections at zero recoil is studied. The
relevant matrix elements are forward matrix elements of local higher
dimensional operators and their time ordered products with higher order terms
from the Lagrangian. These matrix elements may be classified in a simple way
and the analysis at the non recoil point for the form factor of heavy quark
currents simplifies drastically. The second order recoil corrections to the
form factor of the axial vector current, relevant for the
determination from decays, are estimated to be .Comment: LaTeX, 25 pages, one figure, appended after \end{document} as
uu-encoded and compressed eps file, uses epsf, CERN-TH.7162/9
Heavy Mesons in Two Dimensions
The large mass limit of QCD uncovers symmetries that are not present in the
QCD lagrangian. These symmetries have been applied to physical (finite mass)
systems, such as B and D mesons.
We explore the validity of this approximation in the 't Hooft model
(two-dimensional QCD in the large-N approximation). We find that the large mass
approximation is good, even at the charm mass, for form factors, but it breaks
down for the pseudoscalar decay constant.Comment: 4 pages, 3 figures inc
Multi-mass solvers for lattice QCD on GPUs
Graphical Processing Units (GPUs) are more and more frequently used for
lattice QCD calculations. Lattice studies often require computing the quark
propagators for several masses. These systems can be solved using multi-shift
inverters but these algorithms are memory intensive which limits the size of
the problem that can be solved using GPUs. In this paper, we show how to
efficiently use a memory-lean single-mass inverter to solve multi-mass
problems. We focus on the BiCGstab algorithm for Wilson fermions and show that
the single-mass inverter not only requires less memory but also outperforms the
multi-shift variant by a factor of two.Comment: 27 pages, 6 figures, 3 Table
Unitarity of Quantum Theory and Closed Time-Like Curves
Interacting quantum fields on spacetimes containing regions of closed
timelike curves (CTCs) are subject to a non-unitary evolution . Recently, a
prescription has been proposed, which restores unitarity of the evolution by
modifying the inner product on the final Hilbert space. We give a rigorous
description of this proposal and note an operational problem which arises when
one considers the composition of two or more non-unitary evolutions. We propose
an alternative method by which unitarity of the evolution may be regained, by
extending to a unitary evolution on a larger (possibly indefinite) inner
product space. The proposal removes the ambiguity noted by Jacobson in
assigning expectation values to observables localised in regions spacelike
separated from the CTC region. We comment on the physical significance of the
possible indefiniteness of the inner product introduced in our proposal.Comment: 13 pages, LaTeX. Final revised paper to be published in Phys Rev D.
Some changes are made to expand our discussion of Anderson's Proposal for
restoring unitarit
Four-quark Operators Relevant to B Meson Lifetimes from QCD Sum Rules
At the order of 1/m_b^3, the B meson lifetimes are controlled by the hadronic
matrix elements of some four-quark operators. The nonfactorizable magnitudes of
these four-quark operator matrix elements are analyzed by QCD sum rules in the
framework of heavy quark effective theory. The vacuum saturation for
color-singlet four-quark operators is justified at hadronic scale, and the
nonfactorizable effect is at a few percent level. However for color-octet
four-quark operators, the vacuum saturation is violated sizably that the
nonfactorizable effect cannot be neglected for the B meson lifetimes. The
implication to the extraction of some of the parameters from B decays is
discussed. The B meson lifetime ratio is predicted as
\tau(B^-)/\tau(B^0)=1.09\pm 0.02. However, the experimental result of the
lifetime ratio \tau(\Lambda_b)/\tau(B^0) still cannot be explained.Comment: 20 pages, latex, 6 figures, discussion on non-factorizable effect of
the four-quark condensate added, to appear in Phys. Rev. D57 (1998
Operator Product Expansion for Exclusive Decays: B^+ ->Ds^+ e+e- and B^+ -> Ds^{*+} e+e-
The decays and proceed
through a weak and an electromagnetic interaction. This is a typical ``long
distance'' process, usually difficult to compute systematically. We propose
that over a large fraction of phase space a combination of an operator product
and heavy quark expansions effectively turns this process into one in which the
weak and electromagnetic interactions occur through a local operator. Moreover,
we use heavy quark spin symmetry to relate all the local operators that appear
in leading order of the operator expansion to two basic ones. We use this
operator expansion to estimate the decay rates for .Comment: 4 pages, 1 figure, Latex, published version in PR
Heavy Quarkonium and nonperturbative corrections
We analyse the possible existence of non-perturbative contributions in heavy
systems ( and need not have the same flavour) which
cannot be expressed in terms of local condensates. Starting from QCD, with well
defined approximations and splitting properly the fields into large and small
momentum components, we derive an effective lagrangian where hard gluons (in
the non-relativistic aproximation) have been integrated out. The large momentum
contributions (which are dominant) are calculated using Coulomb type states.
Besides the usual condensate corrections, we see the possibility of new
non-perturbative contributions. We parametrize them in terms of two low
momentum correlators with Coulomb bound state energy insertions . We
realize that the Heavy Quark Effective lagrangian can be used in these
correlators.
We calculate the corrections that they give rise to in the decay constant,
the bound state energy and the matrix elements of bilinear currents at zero
recoil. We study the cut-off dependence of the new contributions and we see
that it matches perfectly with that of the large momentum contributions.
We consider two situations in detail: i) () and ii) , and briefly discuss the
expected size of the new contributions in , and
systems.Comment: 28 pages, LaTeX. Minor changes, some comments and numerical results
added. To be published in Phys. Rev.
A Rigourous Treatment of the Lattice Renormalization Problem of F_B
The -meson decay constant can be measured on the lattice using a
expansion. To relate the physical quantity to Monte Carlo data one has to know
the renormalization coefficient, , between the lattice operators and their
continuum counterparts. We come back to this computation to resolve
discrepancies found in previous calculations. We define and discuss in detail
the renormalization procedure that allows the (perturbative) computation of
. Comparing the one-loop calculations in the effective Lagrangian approach
with the direct two-loop calculation of the two-point -meson correlator in
the limit of large -quark mass, we prove that the two schemes give
consistent results to order . We show that there is, however, a
renormalization prescription ambiguity that can have sizeable numerical
consequences. This ambiguity can be resolved in the framework of an
improved calculation, and we describe the correct prescription in that case.
Finally we give the numerical values of that correspond to the different
types of lattice approximations discussed in the paper.Comment: 27 pages, 2 figures (Plain TeX, figures in an appended postscript
file
Testing causality violation on spacetimes with closed timelike curves
Generalized quantum mechanics is used to examine a simple two-particle
scattering experiment in which there is a bounded region of closed timelike
curves (CTCs) in the experiment's future. The transitional probability is shown
to depend on the existence and distribution of the CTCs. The effect is
therefore acausal, since the CTCs are in the experiment's causal future. The
effect is due to the non-unitary evolution of the pre- and post-scattering
particles as they pass through the region of CTCs. We use the time-machine
spacetime developed by Politzer [1], in which CTCs are formed due to the
identification of a single spatial region at one time with the same region at
another time. For certain initial data, the total cross-section of a scattering
experiment is shown to deviate from the standard value (the value predicted if
no CTCs existed). It is shown that if the time machines are small, sparsely
distributed, or far away, then the deviation in the total cross-section may be
negligible as compared to the experimental error of even the most accurate
measurements of cross-sections. For a spacetime with CTCs at all points, or one
where microscopic time machines pervade the spacetime in the final moments
before the big crunch, the total cross-section is shown to agree with the
standard result (no CTCs) due to a cancellation effect.Comment: 28 pages, 8 figures, late
- …