147 research outputs found
Form factors of heavy-to-light B decays at large recoil
General relations between the form factors of B decays to light mesons are
derived using the heavy quark and large recoil expansion. On their basis the
complete account of contributions of second order in the ratio of the light
meson mass to the large recoil energy is performed. Both ground and excited
final meson states are considered. It is shown that most of the known form
factor relations remain valid after the inclusion of quadratic mass
corrections. The validity of some of such relations requires additional
equalities for the helicity amplitudes. It is found that all these relations
and equalities are fulfilled in the relativistic quark model based on the
quasipotential approach in quantum field theory. The contribution of 1/m_b
corrections to the branching fraction of the rare radiative B decay is
discussed.Comment: 23 pages, revte
Weak Hyperon Decays: Quark Sea and SU(3) Symmetry Breaking
An explanation of the difference in the values of the apparent ratios
for the S- and P- wave amplitudes of nonleptonic hyperon decays is proposed.
The argument is formulated in the framework of the standard pole model with
ground-state and excited baryons as intermediate
states for the P- and S- waves respectively. Under the assumption that the
dominant part of the deviation of from is due to large
quark sea effects, symmetry breaking in energy denominators is shown to
lead to a prediction for which is in excellent agreement with
experiment. This corroborates our previous unitarity calculations which
indicated that the matrix elements of the parity
conserving weak Hamiltonian between the ground-state baryons are characterized
by or more. A brief discussion of the problem of the
relative size of S- and P- wave amplitudes is given. Finally, implications for
weak radiative hyperon decays are also discussed.Comment: 26 pages, LATEX, 1647/PH IFJ Krako
Heavy-to-Light Form Factors in the Final Hadron Large Energy Limit of QCD
We argue that the Large Energy Effective Theory (LEET), originally proposed
by Dugan and Grinstein, is applicable to exclusive semileptonic, radiative and
rare heavy-to-light transitions in the region where the energy release E is
large compared to the strong interaction scale and to the mass of the final
hadron, i.e. for q^2 not close to the zero-recoil point. We derive the
Effective Lagrangian from the QCD one, and show that in the limit of heavy mass
M for the initial hadron and large energy E for the final one, the heavy and
light quark fields behave as two-component spinors. Neglecting QCD
short-distance corrections, this implies that there are only three form factors
describing all the pseudoscalar to pseudoscalar or vector weak current matrix
elements. We argue that the dependence of these form factors with respect to M
and E should be factorizable, the M-dependence (sqrt(M)) being derived from the
usual heavy quark expansion while the E-dependence is controlled by the
behaviour of the light-cone distribution amplitude near the end-point u=1. The
usual expectation of the (1-u) behaviour leads to a 1/E^2 scaling law, that is
a dipole form in q^2. We also show explicitly that in the appropriate limit,
the Light-Cone Sum Rule method satisfies our general relations as well as the
scaling laws in M and E of the form factors, and obtain very compact and simple
expressions for the latter. Finally we note that this formalism gives
theoretical support to the quark model-inspired methods existing in the
literature.Comment: Latex2e, 25 pages, no figure. Slight changes in the title and the
phrasing. Misprint in Eq. (25) corrected. To appear in Phys. Rev.
Constraining the S factor of 15N(p,g)16O at Astrophysical Energies
The 15N(p,g)16O reaction represents a break out reaction linking the first
and second cycle of the CNO cycles redistributing the carbon and nitrogen
abundances into the oxygen range. The reaction is dominated by two broad
resonances at Ep = 338 keV and 1028 keV and a Direct Capture contribution to
the ground state of 16O. Interference effects between these contributions in
both the low energy region (Ep < 338 keV) and in between the two resonances
(338 <Ep < 1028 keV) can dramatically effect the extrapolation to energies of
astrophysical interest. To facilitate a reliable extrapolation the 15N(p,g)16O
reaction has been remeasured covering the energy range from Ep=1800 keV down to
130 keV. The results have been analyzed in the framework of a multi-level
R-matrix theory and a S(0) value of 39.6 keV b has been found.Comment: 15 pages, 9 figure
Highly Pathogenic H5N1 Influenza Viruses Carry Virulence Determinants beyond the Polybasic Hemagglutinin Cleavage Site
Highly pathogenic avian influenza viruses (HPAIV) originate from avirulent precursors but differ from all other influenza viruses by the presence of a polybasic cleavage site in their hemagglutinins (HA) of subtype H5 or H7. In this study, we investigated the ability of a low-pathogenic avian H5N1 strain to transform into an HPAIV. Using reverse genetics, we replaced the monobasic HA cleavage site of the low-pathogenic strain A/Teal/Germany/Wv632/2005 (H5N1) (TG05) by a polybasic motif from an HPAIV (TG05poly). To elucidate the virulence potential of all viral genes of HPAIV, we generated two reassortants carrying the HA from the HPAIV A/Swan/Germany/R65/06 (H5N1) (R65) plus the remaining genes from TG05 (TG05-HAR65) or in reversed composition the mutated TG05 HA plus the R65 genes (R65-HATG05poly). In vitro, TG05poly and both reassortants were able to replicate without the addition of trypsin, which is characteristic for HPAIV. Moreover, in contrast to avirulent TG05, the variants TG05poly, TG05-HAR65, and R65-HATG05poly are pathogenic in chicken to an increasing degree. Whereas the HA cleavage site mutant TG05poly led to temporary non-lethal disease in all animals, the reassortant TG05-HAR65 caused death in 3 of 10 animals. Furthermore, the reassortant R65-HATG05poly displayed the highest lethality as 8 of 10 chickens died, resembling “natural” HPAIV strains. Taken together, acquisition of a polybasic HA cleavage site is only one necessary step for evolution of low-pathogenic H5N1 strains into HPAIV. However, these low-pathogenic strains may already have cryptic virulence potential. Moreover, besides the polybasic cleavage site, the additional virulence determinants of H5N1 HPAIV are located within the HA itself and in other viral proteins
Covariant quark model of form factors in the heavy mass limit
We show that quark models of current matrix-elements based on the
Bakamjian-Thomas construction of relativistic states with a fixed number of
particles, plus the additivity assumption, are covariant in the heavy-quark
limit and satisfy the full set of heavy-quark symmetry relations discovered by
Isgur and Wise. We find the lower bound of in such models to be
for ground state mesons, independently of any parameter. Another welcome
property of these models is that in the infinite momentum limit the wave
functions vanish outside the domain .Comment: 16 page
Direct CP Violation, Branching Ratios and Form Factors , in Decays
The and transitions involved in hadronic B decays are
investigated in a phenomenological way through the framework of QCD
factorization. By comparing our results with experimental branching ratios from
the BELLE, BABAR and CLEO Collaborations for all the B decays including either
a pion or a kaon, we propose boundaries for the transition form factors and depending on the CKM matrix element parameters and
. From this analysis, the form factors required to reproduce the
experimental data for branching ratios are and
. We calculate the direct CP violating asymmetry
parameter, , for and decays, in the case where mixing effects are taken into
account. Based on these results, we find that the direct CP asymmetry for
, ,
, and , reaches its maximum when the invariant mass is
in the vicinity of the meson mass. The inclusion of
mixing provides an opportunity to erase, without ambiguity, the phase
uncertainty mod in the determination of the CKM angles in case
of and in case of .Comment: 74 pages, 15 figures, 8 tables. A few misprints corrected, two
references adde
QCD Sum Rule Analysis of the Decays and
We use QCD sum rules to calculate the hadronic matrix elements governing the
rare decays and induced by
the flavour changing neutral current transition. We also study
relations among semileptonic and rare decay form factors. The
analysis of the invariant mass distribution of the lepton pair in and of the angular asymmetry in provides us with interesting tests of the Standard Model and its
extensions.Comment: 26 pages REVTEX + 7 figures. Some typos corrected, figure 5 and 7
modified. This version will appear on Physical Review
Model of the Quark Mixing Matrix
The structure of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is analyzed from
the standpoint of a composite model. A model is constructed with three families
of quarks, by taking tensor products of sufficient numbers of spin-1/2
representations and imagining the dominant terms in the mass matrix to arise
from spin-spin interactions. Generic results then obtained include the familiar
relation , and a less frequently
seen relation . The magnitudes of
and come out naturally to be of the right order. The phase in
the CKM matrix can be put in by hand, but its origin remains obscure.Comment: Presented by Mihir P. Worah at DPF 92 Meeting, Fermilab, November,
1992. 3 pages, LaTeX fil
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