46 research outputs found
Prediction of beauty particle masses with the heavy quark effective theory (II)
The effective theory for heavy quarks has additional symmetries with respect
to QCD, which relate charm and beauty hadron masses. Assuming the spectrum of
charmed particles, we predicted in a previous work the masses of some beauty
particles. The predictions of the Lambda_b mass, M(Lambda_b)=5630 MeV, and of
the B_s mass, M(B_s)=5379 MeV, are in agreement with present experimental data.
We continue this work using recent experimental data on charm hadron masses.
The results are: M(Sigma_b)=5822+-6 MeV, M(Sigma_b*)-M(Sigma_b)=33+-3 MeV,
M(Omega_b)=6080+-7 MeV, M(Omega_b*)-M(Omega_b)=32+-3 MeV, M(Lambda_b*)=5945+-3
MeV, M(Lambda_b**)-M(Lambda_b*)=15+-1 MeV. When experimental data for beauty
hadron masses are available, a comparison with the theoretical values allows a
quantitative estimate of the corrections to the static theory, which contain
informations on hadron dynamics at low energy.Comment: 8 pages, LaTex Version 2.0
Matching of the Shape Function
The shape function f(k_+) describes Fermi motion effects in inclusive
semi-leptonic decays such as B -> X_u+e+nu near the end-point of the lepton
spectrum. We compute the leading one-loop corrections to the shape function
f(k_+) in the effective theory with a hard cut-off regularization. The matching
constant onto full QCD is infrared safe, i.e. the leading infrared singularity
represented by the term log^2(k_+) cancels in the difference of integrals. We
compare the hard cut-off result with the result in dimensional regularization,
the latter containing an additional factor of two in the coefficient of the
log^2(k_+) term, and consequently requiring an oversubtraction.Comment: 11 pages, 1 figure added, minor changes in the tex
Long distance effects in semi-inclusive B decays
We discuss some issues on factorization of long distance effects for
semi-inclusive B decay spectra in full QCD and in the effective theory.Comment: Contribution to the Proceedings of 1st Workshop on Theory,
Phenomenology and Experiments in Heavy Flavour Physics, Anacapri, Italy,
29-31 May, 2006, to be published on Nucl. Phys. B Proceeding
Threshold resummed spectra in B -> Xu l nu decays in NLO (II)
We resum to next-to-leading order the distribution in the ratio of the
invariant hadron mass mX to the total hadron energy EX and the distribution in
mX in the semileptonic decays B -> Xu l nu. By expanding our formulas, we
obtain the coefficients of all the infrared logarithms at O(alphaS^2) and of
the leading ones at O(alphaS^3). We explicitly show that the relation between
these semileptonic spectra and the photon spectrum in the radiative decay B ->
Xs gamma is not a purely short-distance one. There are long-distance effects in
the semileptonic spectra which are not completely factorized by the structure
function as measured in the radiative decay and have to be modelled in some
way.Comment: 16 pages, no figure
Factorization and Effective Theories
We prove factorization in the decay of a B meson into a D* + jet using the
Large Energy Effective Theory.
The proof is non perturbative, does not require any gauge fixing and is exact
in the limit of a very narrow jet. On the other hand, it is shown that this
theory is unable to consistently describe completely exclusive processes such
as for example a B which decays into a D* + pion, due to an oversimplification
of transverse momentum dynamics.
Therefore we present a variant of the Large Energy Effective Theory, i.e. a
new effective theory for massless particles which properly takes into account
transverse degrees of freedom and is the natural framework to study exclusive
non-leptonic decays.Comment: 12 pages, no figures, LaTex correction
Threshold Resummed Spectra in B -> Xu l nu Decays in NLO (I)
We evaluate thresholds resummed spectra in B -> Xu l nu decays in
next-to-leading order. We present results for the distribution in E_X and in
m_X^2/E_X^2, for the distribution in E_X and E_l and for the distribution in
E_X, where E_X and m_X are the energy and the invariant mass of the final
hadronic state Xu respectively and E_l is the energy of the charged lepton. We
explicitly show that all these spectra (where there is no integration over the
hadronic energy) can be directly related to the photon spectrum in B -> Xs
gamma via short-distance coefficient functions.Comment: 33 pages, no figures. The section on the double distribution in the
hadron and electron energies has been largely rewritten with an improved
resummation scheme. Small stylistic changes in the remaining sections.
References adde
Semi-Inclusive B Decays and a Model for Soft-Gluon Effects
We compare experimental spectra of radiative and semileptonic B decays with
the predictions of a model based on soft-gluon resummation to next-to-next-to
leading order and on a ghost-less time-like coupling. We find a good agreement
with photon spectra in the radiative decay and with hadron mass distributions
in the semileptonic one: the extracted values for alpha_S(m_Z) are in agreement
with the current PDG average within at most two standard deviations. The
agreement is instead less good for the electron spectra measured by BaBar and
Belle in semileptonic decays for small electron energies (< 2.2 GeV): our
spectrum is harder. We also show that, in general, the inclusion of
next-to-next-to-leading order effects is crucial for bringing the model closer
to the data and that the non-power expansion introduced in the framework of
analytic coupling studies does not accurately describe soft-gluon effects.Comment: 28 pages, 12 figure
Renormalons and confinement
We compute the renormalon ambiguity of the static potential, in the limit of
a large number of flavors. An extrapolation of the QED result to QCD implies
that the large distance behavior of the quark potential is arbitrary in
perturbation theory, as there are an infinite number of prescriptions to
assign. The shape of the potential at large distances is not only affected by
the renormalon pole closest to the origin of the Borel plane, but a resummation
of all renormalon contributions is required. In particular, confinement can be
accommodated, but it is not explained. At short distances there is no
indication of a linear term in the potential.Comment: 7 pages revtex; major changes: list of authors corrected, title,
abstract, body of paper changed
Factorization in Exclusive and Seminclusive Decays and Effective Theories for Massless Particles
We prove rigorously factorization in the seminclusive decay using the large energy effective theory. It is also shown that this
effective theory is unable to consistently describe completely exclusive
processes, such as the decay , and therefore also
related properties such as factorization. This is due to an oversimplification
of transverse momentum dynamics. We present a variant of the large energy
effective theory, i.e. a new effective theory for massless particles, which
properly takes into account transverse momentum dynamics and is therefore the
natural framework to study exclusive non-leptonic decays. Finally, it is shown
that the collinear instability of the large energy effective theory disappears
when seminclusive observables are considered.Comment: Latex file, 34 pages, no figures. There is a more detailed discussion
about pinch singularities; some typos are correcte
The structure function of semi-inclusive heavy flavour decays in field theory
We consider the decay of a heavy flavour into an inclusive hadronic state X
of invariant mass m_X small with respect to its energy E_X, m_X << E_X. The
electron spectrum and the hadronic mass distribution in semileptonic b -> u
decays, or the photon spectrum in b -> s gamma decays, all require, close to
their endpoints, a control over this region. This region is affected both by
non-perturbative phenomena related to the Fermi motion of the heavy quark and
by perturbative soft gluon radiation in the final state (Sudakov form
factor).Fermi motion can be described by the shape function f (m*), which
represents the distribution of the effective mass m* of the heavy quark at
disintegration time. We perform a factorization with a simple technique in
order to consistently separate perturbative from non-perturbative effects. We
find that the shape function, contrary to naive expectations,is not a physical
distribution, as it is affected by substantial regularization scheme effects,
controlling even the leading, double-logarithmic term. It factorizes, however,
the bulk of non-perturbative effects in lattice-like regularizations. Some
non-perturbative effects are present in the coefficient function even at
leading twist, but they are expected to be suppressed on physical grounds.
Finally, we clarify a controversial factor of 2 in the evolution kernel of the
shape function.Comment: 52 pages, 5 figures, LaTeX. More descriptive title. A few minor
changes. To be published in Nucl. Phys.