81 research outputs found
Soft-collinear effective theory and heavy-to-light currents beyond leading power
An important unresolved question in strong interaction physics concerns the
parameterization of power-suppressed long-distance effects to hard processes
that do not admit an operator product expansion (OPE). Recently Bauer et al.\
have developed an effective field theory framework that allows one to formulate
the problem of soft-collinear factorization in terms of fields and operators.
We extend the formulation of soft-collinear effective theory, previously worked
out to leading order, to second order in a power series in the inverse of the
hard scale. We give the effective Lagrangian and the expansion of ``currents''
that produce collinear particles in heavy quark decay. This is the first step
towards a theory of power corrections to hard processes where the OPE cannot be
used. We apply this framework to heavy-to-light meson transition form factors
at large recoil energy.Comment: 46 pages, LaTeX; v2: two references added, eq. (52) correcte
Coherent control of enrichment and conversion of molecular spin isomers
A theoretical model of nuclear spin conversion in molecules controlled by an
external electromagnetic radiation resonant to rotational transition has been
developed. It has been shown that one can produce an enrichment of spin isomers
and influence their conversion rates in two ways, through coherences and
through level population change induced by radiation. Influence of conversion
is ranged from significant speed up to almost complete inhibition of the
process by proper choice of frequency and intensity of the external field.Comment: REVTEX, 13 pages + 6 eps figure
Towards an effective-action approach to fermion-loop corrections
We present a study of the effective action approach to incorporate
higher-order effects in e^+e^- -> n fermions. In its minimal version, the
effective action approach is found to exhibit problems with unitarity and
high-energy behaviour. We identify the origin of these problems by
investigating the zero-mode solutions of the Ward Identities. A numerical
analysis of the importance of the zero-mode solutions is presented for
four-fermion production processes.Comment: 43 pages, 1 figure. Accepted for publication in Nuclear Physics
Screened-Coulomb ansatz for the non-factorizable radiative corrections to the off-shell W^+ W^- production
We demonstrate that the results of the complete first order calculation of
the non-factorizable QED corrections to the single-inclusive cross-sections for
fermions could be very well reproduced by a simple
physically motivated ansatz. The latter allows to take into account effectively
the screening role of the non-Coulomb radiative mechanisms by introducing a
dampening factor in front of the width-dependent part of the known first-order
Coulomb correction, the so-called screened-Coulomb ansatz.Comment: 16 pages, LaTeX2e, 5 figure
Non-factorizable corrections to W-pair production: methods and analytic results
In this paper we present two methods to evaluate non-factorizable corrections
to pair-production of unstable particles. The methods are illustrated in detail
for W-pair-mediated four-fermion production. The results are valid a few widths
above threshold, but not at threshold. One method uses the decomposition of
-point scalar functions for virtual and real photons, and can therefore be
generalized to more complicated final states than four fermions. The other
technique is an elaboration on a method known from the literature and serves as
a useful check. Applications to other processes than W-pair production are
briefly mentioned.Comment: 45 pages, LaTeX2e, 4 postscript figures, uses axodraw.st
Non-factorizable corrections and effective field theories
We analyze the structure of higher-order radiative corrections for processes
with unstable particles. By subsequently integrating out the various scales
that are induced by the presence of unstable particles we obtain a hierarchy of
effective field theories. In the effective field theory framework the
separation of physically different effects is achieved naturally. In
particular, we automatically obtain a separation of factorizable and
non-factorizable corrections to all orders in perturbation theory. At one loop
this treatment is equivalent to the double-pole approximation (DPA) but
generalizes to higher orders and, at least in principle, to beyond the DPA. It
is known that one-loop non-factorizable corrections to invariant mass
distributions are suppressed at high energy. We study the mechanism of this
suppression and obtain estimates of higher-order non-factorizable corrections
at high energy.Comment: 45 page
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