537 research outputs found
The Resummed Photon Spectrum in Radiative Upsilon Decays
We present a theoretical prediction for the photon spectrum in radiative
Upsilon decay including the effects of resumming the endpoint region, E_\gamma
-> M_\Upsilon/2. Our approach is based on NRQCD and the soft collinear
effective theory. We find that our results give much better agreement with data
than the leading order NRQCD prediction.Comment: 4 pages, 6 figure
On the Resummed Hadronic Spectra of Inclusive B Decays
In this paper we investigate the hadronic mass spectra of inclusive B decays.
Specifically, we study how an upper cut on the invariant mass spectrum, which
is necessary to extract V_{ub}, results in the breakdown of the standard
perturbative expansion due to the existence of large infrared logs. We first
show how the decay rate factorizes at the level of the double differential
distribution. Then, we present closed form expressions for the resummed cut
rate for the inclusive decays B -> X_s gamma and B -> X_u e nu at
next-to-leading order in the infrared logs. Using these results, we determine
the range of cuts for which resummation is necessary, as well as the range for
which the resummed expansion itself breaks down. We also use our results to
extract the leading and next to leading infrared log contribution to the two
loop differential rate. We find that for the phenomenologically interesting cut
values, there is only a small region where the calculation is under control.
Furthermore, the size of this region is sensitive to the parameter
\bar{\Lambda}. We discuss the viability of extracting V_{ub} from the hadronic
mass spectrum.Comment: 18 pages, 5 figures, minor change
Resumming the color-octet contribution to e+ e- -> J/psi + X
Recent observations of the spectrum of J/psi produced in e+ e- collisions at
the Upsilon(4S) resonance are in conflict with fixed-order calculations using
the Non-Relativistic QCD (NRQCD) effective field theory. One problem is that
leading order color-octet mechanisms predict an enhancement of the cross
section for J/psi with maximal energy that is not observed in the data.
However, in this region of phase space large perturbative corrections (Sudakov
logarithms) as well as enhanced nonperturbative effects are important. In this
paper we use the newly developed Soft-Collinear Effective Theory (SCET) to
systematically include these effects. We find that these corrections
significantly broaden the color-octet contribution to the J/psi spectrum. Our
calculation employs a one-stage renormalization group evolution rather than the
two-stage evolution used in previous SCET calculations. We give a simple
argument for why the two methods yield identical results to lowest order in the
SCET power counting.Comment: 27 pages, 7 figure
Approximate NNLO Threshold Resummation in Heavy Flavour Decays
We present an approximate NNLO evaluation of the QCD form factor resumming
large logarithmic perturbative contributions in semi-inclusive heavy flavour
decays.Comment: 16 pages, 3 figures, Latex; minor changes; 2 figures adde
Shape-Function Effects and Split Matching in B-> Xs l+ l-
We derive the triply differential spectrum for the inclusive rare decay B ->
Xs l+ l- in the shape function region, in which Xs is jet-like with . Experimental cuts make this a relevant region. The
perturbative and non-perturbative parts of the matrix elements can be defined
with the Soft-Collinear Effective Theory, which is used to incorporate alphas
corrections consistently. We show that, with a suitable power counting for the
dilepton invariant mass, the same universal jet and shape functions appear as
in B-> Xs gamma and B-> Xu l nu decays. Parts of the usual alphas(m_b)
corrections go into the jet function at a lower scale, and parts go into the
non-perturbative shape function. For B -> Xs l+ l-, the perturbative series in
alphas are of a different character above and below mu=mb. We introduce a
``split matching'' method that allows the series in these regions to be treated
independently.Comment: 33 pages; journal versio
Radiative decays of light vector mesons in a quark level linear sigma model
We calculate the P0 to gamma gamma, V0 to P0 gamma and V0to V'0 gamma gamma
decays in the framework of a U(3)xU(3) linear sigma model which includes
constituent quarks. For the first two decays this approach improves results
based on the anomalous Wess-Zumino term, with contributions due to SU(3)
symmetry breaking and vector mixing. The phi to (omega,rho) gamma gamma decays
are dominated by resonant eta' exchange . Our calculation for the later decays
improves and update similar calculations in the -closely related- framework of
vector meson dominance. We obtain BR(phi to rho gamma gamma)=2.5x10^{-5} and
BR(phi to omega gamma gamma)=2.8x10^{-6} within the scope of the
high-luminosity phi factories.Comment: 8 pages, submitted to Phys. Rev.
Remarks on Semileptonic B and D Decays into Orbitally Excited Mesons
We have obtained the differential decay rate and calculated the branching
ratios of the exclusive semileptonic decays , where is a
p-wave meson, using the nonrelativistic ISGW quark model. Our results are
compared with the predictions of the ISGW2 model. We have computed some
branching ratios that were not reported or were reported with 0.00 in this
model. For example, we find that , and , which seems to be
at the reach of forthcoming experiments. Furthermore, we have classified the
decays in two groups and compared the semileptonic and
nonleptonic decays including a tensor meson in the final state.Comment: 11 pages, LaTe
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