1,588 research outputs found
The nature of power corrections in large approximation
We investigate the nature of power corrections and infrared renormalon
singularities in large approximation. We argue that the power
correction associated with a renormalon pole singularity should appear at O(1),
in contrast to the renormalon ambiguity appearing at , and give
an explanation why the leading order renormalon singularities are generically
poles.Comment: 6 page
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.
Quenched and first unquenched lattice HQET determination of the Bs-Meson width difference
We present recent results for the prediction of the Bs lifetime difference
from lattice Heavy Quark Effective Theory simulations. In order to get a
next-to-leading order result we have calculated the matching between QCD and
HQET and the two-loop anomalous dimensions in the HQET for all the \Delta B=2
operators, in particular for the operators which enter the width difference. We
present results from quenched and, for the first time, from unquenched
simulations. We obtain for the Bs lifetime difference,
(\Delta\Gamma_Bs/\Gamma_Bs)^{(que.)}=(5.1+/- 1.9+/- 1.7)10^(-2) and
(\Delta\Gamma_Bs/\Gamma_Bs)^{(unq.)}=(4.3+/- 2.0+/- 1.9)10^(-2) from the
quenched and unquenched simulations respectivelyComment: Lattice 2000 (Heavy Quark Physics), 4 pages, LaTeX. Some misprints
corrected. No result change
Lepton flavour violation in RS models with a brane- or nearly brane-localized Higgs
We perform a comprehensive study of charged lepton flavour violation in
Randall-Sundrum (RS) models in a fully 5D quantum-field-theoretical framework.
We consider the RS model with minimal field content and a "custodially
protected" extension as well as three implementations of the IR-brane localized
Higgs field, including the non-decoupling effect of the Kaluza-Klein (KK)
excitations of a narrow bulk Higgs. Our calculation provides the first complete
result for the flavour-violating electromagnetic dipole operator in
Randall-Sundrum models. It contains three contributions with different
dependence on the magnitude of the anarchic 5D Yukawa matrix, which can all be
important in certain parameter regions. We study the typical range for the
branching fractions of mu -> e gamma, mu -> 3e, mu N -> e N as well as tau ->
mu gamma, tau -> 3 mu and the electron electric dipole moment by a numerical
scan in both the minimal and the custodial RS model. The combination of mu -> e
gamma and mu N -> e N currently provides the most stringent constraint on the
parameter space of the model. A typical lower limit on the KK scale T is around
2 TeV in the minimal model (up to 4 TeV in the bulk Higgs case with large
Yukawa couplings), and around 4 TeV in the custodially protected model, which
corresponds to a mass of about 10 TeV for the first KK excitations, far beyond
the lower limit from the non-observation of direct production at the LHC.Comment: 64 pages, 21 figures, LaTeX, v2: electron EDM analysis added, matches
published versio
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
Enhanced electroweak penguin amplitude in B-->VV decays
We discuss a novel electromagnetic penguin contribution to the transverse
helicity amplitudes in B decays to two vector mesons, which is enhanced by two
powers of mB/Lambda relative to the standard penguin amplitudes. This leads to
unique polarization signatures in penguin-dominated decay modes such as B-->rho
K* similar to polarization effects in the radiative decay B-->K*gamma, and
offers new opportunities to probe the magnitude and chirality of
flavour-changing neutral current couplings to photons.Comment: 4 pages, 1 figur
Next-to-next-to-leading soft-gluon corrections for the top quark cross section and transverse momentum distribution
I present results for top quark production in hadronic collisions at LHC and
Tevatron energies. The soft-gluon corrections to the differential cross section
are resummed at next-to-next-to-leading-logarithm (NNLL) accuracy via the
two-loop soft anomalous dimension matrices. Approximate
next-to-next-to-leading-order (NNLO) differential and total cross sections are
calculated. Detailed theoretical predictions are shown for the t tbar cross
section and the top quark p_T distribution at the Tevatron and the LHC.Comment: 23 pages, 14 figures; additional results and figure
Rare radiative exclusive B decays in soft-collinear effective theory
We consider rare radiative B decays such as B -> K^* gamma or B -> rho gamma
in soft-collinear effective theory, and show that the decay amplitudes are
factorized to all orders in alpha_s and at leading order in Lambda/m_b.By
employing two-step matching, we classify the operators for radiative B decays
in powers of a small parameter lambda(~ \sqrt{Lambda/m_b}) and obtain the
relevant operators to order lambda in SCET_I. These operators are constructed
with or without spectator quarks including the four-quark operators
contributing to annihilation and W-exchange channels. And we employ SCET_II
where the small parameter becomes of order Lambda/m_b, and evolve the operators
in order to compute the decay amplitudes for rare radiative decays in
soft-collinear effective theory. We show explictly that the contributions from
the annihilation channels and the W-exchange channels vanish at leading order
in SCET. We present the factorized result for the decay amplitudes in rare
radiative B decays at leading order in SCET, and at next-to-leading order in
alpha_s.Comment: v2: 31 pages, 11 figures. An appendix is added about the quark mass
effects on radiative B decay
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