6,570 research outputs found
Exclusive rare B -> K*e+e- decays at low recoil: controlling the long-distance effects
We present a model-independent description of the exclusive rare decays B->
K* e+e- in the low recoil region (large lepton invariant mass q^2 ~ m_b^2). In
this region the long-distance effects from quark loops can be computed with the
help of an operator product expansion in 1/Q, with Q={m_b, \sqrt{q^2}}.
Nonperturbative effects up to and including terms suppressed by Lambda/Q and
mc^2/mb^2 relative to the short-distance amplitude can be included in a
model-independent way. Based on these results, we propose an improved method
for determining the CKM matrix element |V{ub}| from a combination of rare and
semileptonic B and D decays near the zero recoil point. The residual
theoretical uncertainty from long distance effects in this |V{ub}|
determination comes from terms in the OPE of order alpha_s(Q)\Lambda/mb,
alpha_s^2(Q), mc^4/mb^4$ and duality violations and is estimated to be below
10%.Comment: 21 pages RevTex, 2 figures; v3: extensive numerical changes in the
NLL analysis, with improved stability under scale dependence. Typos fixed,
version to appear in Phys.Rev.
Heavy Meson Physics: What have we learned in Twenty Years?
I give a personal account of the development of the field of heavy quarks.
After reviewing the experimental discovery of charm and bottom quarks, I
describe how the field's focus shifted towards determination of CKM elements
and how this has matured into a precision science.Comment: This talk was presented during the ceremony awarding the Medalla 2003
of the Division of Particles and Fields of The Mexican Phsyical Society, at
the IX Mexican Workshop on Particles and Fields; submitted for proceedings; 9
pages, 9 figures; replacement: fix multiple typo
Light scalar at LHC: the Higgs or the dilaton?
It is likely that the LHC will observe a color- and charge-neutral scalar
whose decays are consistent with those of the Standard Model (SM) Higgs boson.
The Higgs interpretation of such a discovery is not the only possibility. For
example, electroweak symmetry breaking (EWSB) could be triggered by a
spontaneously broken, nearly conformal sector. The spectrum of states at the
electroweak scale would then contain a narrow scalar resonance, the
pseudo-Goldstone boson of conformal symmetry breaking, with Higgs-like
properties. If the conformal sector is strongly coupled, this pseudo-dilaton
may be the only new state accessible at high energy colliders. We discuss the
prospects for distinguishing this mode from a minimal Higgs boson at the LHC
and ILC. The main discriminants between the two scenarios are (1) cubic
self-interactions and (2) a potential enhancement of couplings to massless SM
gauge bosons. A particularly interesting situation arises when the scale f of
conformal symmetry breaking is approximately the electroweak scale v~246 GeV.
Although in this case the LHC may not be able to tell apart a pseudo-dilaton
from the Higgs boson, the self-interactions differ in a way that depends only
on the scaling dimension of certain operators in the conformal sector. This
opens the possibility of using dilaton pair production at future colliders as a
probe of EWSB induced by nearly conformal new physics.Comment: 7 pages, LaTe
A Precision Model Independent Determination of |Vub| from B -> pi e nu
A precision method for determining |Vub| using the full range in q^2 of B->
pi \ell nu data is presented. At large q^2 the form factor is taken from
unquenched lattice QCD, at q^2=0 we impose a model independent constraint
obtained from B-> pi pi using the soft-collinear effective theory, and the
shape is constrained using QCD dispersion relations. We find |Vub| =(3.54\pm
0.17\pm 0.44) x 10^{-3}. With 5% experimental error and 12% theory error, this
is competitive with inclusive methods. Theory error is dominated by the input
points, with negligible uncertainty from the dispersion relations.Comment: 4 pages, 3 figure
Composition-tuned smeared phase transitions
Phase transitions in random systems are smeared if individual spatial regions
can order independently of the bulk system. In this paper, we study such
smeared phase transitions (both classical and quantum) in substitutional alloys
AB that can be tuned from an ordered phase at composition to
a disordered phase at . We show that the ordered phase develops a
pronounced tail that extends over all compositions . Using optimal
fluctuation theory, we derive the composition dependence of the order parameter
and other quantities in the tail of the smeared phase transition. We also
compare our results to computer simulations of a toy model, and we discuss
experiments.Comment: 6 pages, 4 eps figures included, final version as publishe
Neutrino Masses in the Lee-Wick Standard Model
Recently, an extension of the standard model based on ideas of Lee and Wick
has been discussed. This theory is free of quadratic divergences and hence has
a Higgs mass that is stable against radiative corrections. Here, we address the
question of whether or not it is possible to couple very heavy particles, with
masses much greater than the weak scale, to the Lee-Wick standard model degrees
of freedom and still preserve the stability of the weak scale. We show that in
the LW-standard model the familiar see-saw mechanism for generating neutrino
masses preserves the solution to the hierarchy puzzle provided by the higher
derivative terms. The very heavy right handed neutrinos do not destabilize the
Higgs mass. We give an example of new heavy degrees of freedom that would
destabilize the hierarchy, and discuss a general mechanism for coupling other
heavy degrees of freedom to the Higgs doublet while preserving the hierarchy.Comment: 7 pages, 1 figur
Analytical Heavy Quark Expansion in the 't Hooft Model
We present a number of exact relations for the heavy quark limit and develop
an analytical 1/m_Q expansion for heavy mesons in the 't Hooft model. Among the
new results are relation 3\mu_\pi^2=\bar\Lambda^2 - m_{sp}^2+\beta^2, 1/m_Q
corrections to the decay constants, to the kinetic expectation values and
1/m_Q^2 nonperturbative corrections to the B->D amplitude at zero recoil. The
properties of the IW functions are addressed and the small velocity sum rules
are verified.Comment: 22 pages, plain LaTeX (no figures
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