1,460 research outputs found
Correlator of heavy-quark currents at small q^2 in the large-beta_0 limit
The correlator of vector heavy-quark currents at small q^2 is considered in
the large-beta_0 limit. The leading IR renormalon ambiguity of the sum of the
perturbative series is canceled by the UV renormalon ambiguity of the gluon
condensate. Asymptotic behaviour of the perturbative series is obtained in a
model-independent way, up to a single unknown normalization factor.
Gluon-virtuality distribution functions for the perturbative correction are
calculated.Comment: 8 pages, 3 figure
Towards pair production near threshold with unstable particle effective theory
We illustrate the use of effective theory techniques to describe processes
involving unstable particles close to resonance. First, we present the main
ideas in the context of a scalar resonance in an Abelian gauge-Yukawa model. We
then outline the necessary modifications to describe W-pair production close to
threshold in electron-positron collisions.Comment: Invited talk given at the 11th International Conference on QCD,
Montpellier, France (5--10th July 2004
Nonperturbative definition of the pole mass and short distance expansion of the heavy quark potential in QCD
We show that the O(Lambda) ambiguity in the pole mass can be fixed in a
natural way by introducing a modified nonperturbative V-scheme momentum space
coupling tilde-alphaV(q) where the confining contributions have been subtracted
out. The method used is in the spirit of the infrared finite coupling approach
to power corrections, and gives a non perturbative definition of the `potential
subtracted' mass. The short distance expansion of the static potential is
derived, taking into account an hypothetical short distance linear term. The
magnitude of the standard OPE contributions are estimated in quenched QCD,
based on results of Luscher and Weisz. It is observed that the expansion is not
yet reliable at the shortest distances presently measured on the lattice.Comment: 10 pages, JHEP3.cls style; a few misprints corrected. To appear in
Physics Letters
On The Difficulty of Computing Higher-Twist Corrections
We discuss the evaluation of power corrections to hard scattering and decay
processes for which an operator product expansion is applicable. The Wilson
coefficient of the leading-twist operator is the difference of two perturbative
series, each of which has a renormalon ambiguity of the same order as the power
corrections themselves, but which cancel in the difference. We stress the
necessity of calculating this coefficient function to sufficiently high orders
in perturbation theory so as to make the uncertainty of the same order or
smaller than the relevant power corrections. We investigate in some simple
examples whether this can be achieved. Our conclusion is that in most of the
theoretical calculations which include power corrections, the uncertainties are
at least comparable to the power corrections themselves, and that it will be a
very difficult task to improve the situation.Comment: 27 pages, uuencoded file containing latex source and axodraw.sty fil
CP Violation in B -> pi+ pi- and the Unitarity Triangle
We analyze the extraction of weak phases from CP violation in
decays. We propose to determine the unitarity triangle
by combining the information on mixing induced CP
violation in , , with the precision observable
obtained from the CP asymmetry in . It is then possible to write
down exact analytical expressions for and as simple
functions of the observables and , and of the penguin
parameters and . As an application clean lower bounds on
and can be derived as functions of and ,
essentially without hadronic uncertainty. Computing and within QCD
factorization yields precise determinations of and since
the dependence on and is rather weak. It is emphasized that the
sensitivity to the phase enters only at second order and is extremely
small for moderate values of this phase, predicted in the heavy-quark limit.
Transparent analytical formulas are further given and discussed for the
parameter of direct CP violation in . We also discuss
alternative ways to analyze and that can be useful if new physics
affects -- mixing. Predictions and uncertainties for and
in QCD factorization are examined in detail. It is pointed out that a
simultaneous expansion in and 1/N leads to interesting simplifications.
At first order infrared divergences are absent, while the most important
effects are retained. Independent experimental tests of the factorization
framework are briefly discussed.Comment: 26 pages, 7 figure
On measuring alpha in B(t)-> rho^\pm pi^\mp
Defining a most economical parametrization of time-dependent B-> rho^\pm
pi^\mp decays, including a measurable phase alpha_{eff} which equals the weak
phase alpha in the limit of vanishing penguin amplitudes, we propose two ways
for determining alpha in this processes. We explain the limitation of one
method, assuming only that two relevant tree amplitudes factorize and that
their relative strong phase, delta_t, is negligible. The other method, based on
broken flavor SU(3), permits a determination of alpha in B^0-> rho^\pm pi^\mp
in an overconstrained system using also rate measurements of B^{0,+}-> K^* pi
and B^{0,+}->rho K. Current data are shown to restrict two ratios of penguin
and tree amplitudes, r_\pm, to a narrow range around 0.2, and to imply an upper
bound |alpha_{eff} - alpha| < 15 degrees. Assuming that delta_t is much smaller
than 90 degrees, we find alpha =(93\pm 16) degrees and (102 \pm 20) degrees
using BABAR and BELLE results for B(t)-> rho^\pm pi^mp. Avoiding this
assumption for completeness, we demonstrate the reduction of discrete
ambiguities in alpha with increased statistics, and show that SU(3) breaking
effects are effectively second order in r_\pm.Comment: 23 pages, 2 figures, data and references updated, to be published in
Phys. Rev.
Four-fermion production near the W pair production threshold
We perform a dedicated study of the four-fermion production process e- e+ ->
mu- nubar_mu u dbar X near the W pair-production threshold in view of the
importance of this process for a precise measurement of the W boson mass.
Accurate theoretical predictions for this process require a systematic
treatment of finite-width effects. We use unstable-particle effective field
theory (EFT) to perform an expansion in the coupling constants, GammaW/MW, and
the non-relativistic velocity v of the W boson up to next-to-leading order in
GammaW/MW ~ alpha_ew ~ v^2. We find that the dominant theoretical uncertainty
in MW is currently due to an incomplete treatment of initial-state radiation.
The remaining uncertainty of the NLO EFT calculation translates into delta MW ~
10-15 MeV, and to about 5 MeV with additional input from the NLO four-fermion
calculation in the full theory.Comment: 53 pages, v2: version to be published, inessential modification
Analytic structure in the coupling constant plane in perturbative QCD
We investigate the analytic structure of the Borel-summed perturbative QCD
amplitudes in the complex plane of the coupling constant. Using the method of
inverse Mellin transform, we show that the prescription dependent Borel-Laplace
integral can be cast, under some conditions, into the form of a dispersion
relation in the a-plane. We also discuss some recent works relating resummation
prescriptions, renormalons and nonperturbative effects, and show that a method
proposed recently for obtaining QCD nonperturbative condensates from
perturbation theory is based on special assumptions about the analytic
structure in the coupling plane that are not valid in QCD.Comment: 14 pages, revtex4, 1 eps-figur
Next-to-Leading Order QCD Corrections to the Lifetime Difference of Mesons
We compute the QCD corrections to the decay rate difference in the system, , in the next-to-leading logarithmic
approximation using the heavy quark expansion approach. Going beyond leading
order in QCD is essential to obtain a proper matching of the Wilson
coefficients to the matrix elements of local operators from lattice gauge
theory. The lifetime difference is reduced considerably at next-to-leading
order. We find in terms of the bag parameters in the
NDR scheme. As a further application of our analysis we also derive the
next-to-leading order result for the mixing-induced CP asymmetry in inclusive
decays, which measures .Comment: 14 pages, LaTeX, 1 figure; minor modifications of the text, improved
discussion of eq. (35), all results unchange
Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory
It is argued that soft-collinear effective theory for processes involving
both soft and collinear partons, such as exclusive B-meson decays, should
include a new mode in addition to soft and collinear fields. These
"soft-collinear messengers" can interact with both soft and collinear particles
without taking them far off-shell. They thus can communicate between the soft
and collinear sectors of the theory. The relevance of the new mode is
demonstrated with an explicit example, and the formalism incorporating the
corresponding quark and gluon fields into the effective Lagrangian is
developed.Comment: 22 pages, 5 figures. Extended Section 6, clarifying the relevance of
different types of soft-collinear interaction
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