473 research outputs found
Gauge Invariance and k_T-Factorization of Exclusive Processes
In the -factorization for exclusive processes, the nontrivial
-dependence of perturbative coefficients, or hard parts, is obtained by
taking off-shell partons. This brings up the question of whether the
-factorization is gauge invariant. We study the -factorization for
the case at one-loop in a general covariant gauge.
Our results show that the hard part contains a light-cone singularity that is
absent in the Feynman gauge, which indicates that the -factorization is
{\it not} gauge invariant. These divergent contributions come from the
-dependent wave function of and are not related to a special
process. Because of this fact the -factorization for any process is not
gauge invariant and is violated. Our study also indicates that the
-factorization used widely for exclusive B-decays is not gauge invariant
and is violated.Comment: Add some explanation, conclusion unchanged. Version to appear in PL
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We investigate \ovar{B^0} \to D_s^- \pi^+ decay in perturbative QCD
approach which has recently been applied to meson decays. \ovar{B^0} \to
D_s^- \pi^+ decay (and its charge conjugated mode) can be one of the hopeful
modes to determine since it occurs through transition
only. We estimate both factorizable and non-factorizable contribution, and show
that the non-factorizable contribution is much less than the factorizable one.
Our calculation gives {BR}(\ovar{B^0} \to D_s^- \pi^+) = (50 \sim 70) \times
f_{Ds}^2|{V_{ub}}{V_{cs}}|^2.Comment: 12 pages, 3 figures, LaTeX2e with graphics packag
Futher Study of CP Violation and Branching Ratios for and in the Standard Model and Beyond
In this work we study the CP violation for and
up to leading and next-to-leading order QCD corrections
in the standard model, two-Higgs-doublet model and the minimal supersymmetric
extension of the standard model. We also study the effect of new physics on the
branching ratios of these two decay modes. We find that within the parameter
space constrained by the observation of the decay , new physics
does not affect the CP asymmetries greatly, and the prediction of new physics
to the branching ratios of and is the same as that of the standard model up to a minor discrepancy as far
as the Yukama couplying constants are perturbative.Comment: some figures being reduced, accepted by J. Physics
Moderate Supersymmetric CP Violation
It is well known that supersymmetry (SUSY) gives neutron and electron
electric dipole moments ( and ) which are too large by about
. If we assume a SUSY model cannot contain fine-tunings or large mass
scales, then one must require that the SUSY breaking mechanism give real soft
breaking parameters, in which case the minimal SUSY model has no violation
other than from the CKM matrix (besides possible strong violating
effects). We show that in non-minimal SUSY models, a moderate amount of
violation can be induced through one loop corrections to the scalar potential,
giving an effective phase of order , and thus implying and
can be near their current experimental bounds . This moderate amount
of SUSY violation could also prove important for models of electroweak
baryogenesis. We illustrate our results with a specific model.Comment: 19pp plain LATEX, 1 fig (by EMAIL request), TRI-PP-93-86. (Some
clarifying comments about renormalizability added--version to appear in Phys.
Rev. D
Study of Bc --> J/psi pi, etac pi decays with perturbative QCD approach
The Bc --> J/psi pi, etac pi decays are studied with the perturbative QCD
approach. It is found that form factors and branching ratios are sensitive to
the parameters w, v, f_J/psi and f_etac, where w and v are the parameters of
the charmonium wave functions for Coulomb potential and harmonic oscillator
potential, respectively, f_J/psi and f_etac are the decay constants of the
J/psi and etac mesons, respectively. The large branching ratios and the clear
signals of the final states make the Bc --> J/psi pi, etac pi decays to be the
prospective channels for measurements at the hadron collidersComment: 21 pages, revtex
Evidence for two-quark content of in exclusive decays
Inspired by a large decay branching ratio (BR) of
measured by Belle recently, we propose that a significant evidence of the
component of in could be
demonstrated in exclusive decays by the observation of in
the final states and . We predict the BRs of to be () while
the unknown wave functions of () are chosen to fit the
observed decays of .Comment: 4 pages, 2 figures, Revtex4, version to appear in PR
CP Violation in Kaon System in Supersymmetric SU(5) Model with Seesaw-Induced Neutrino Masses
CP violations in the kaon system are studied in supersymmetric SU(5) model
with right-handed neutrinos. We pay a special attention to the renormalization
group effect on the off-diagonal elements of the squark mass matrices. In
particular, if the Yukawa couplings and mixings in the neutrino sector are
sizable, off-diagonal elements of the right-handed down-type squark mass matrix
are generated, which affect CP and flavor violations in decay processes of the
kaon. We calculate supersymmetric contributions to epsilon (as well as Delta
m_K), Br(K_L -> pi^0 nu \bar{nu}), and epsilon'/epsilon in this framework. We
will see that the supersymmetric contribution to the epsilon parameter can be
as large as (and in some case, larger than) the experimentally measured value.
We also discuss its implication to future tests of the unitarity triangle of
the Kobayashi-Maskawa matrix.Comment: 30 pages, 5 figue
Analysis of Supersymmetric Effects on B -> phi K Decays in the PQCD Approach
We study the effects of the MSSM contribution on B -> phi K decays using the
perturbative QCD approach. In this approach, strong phases can be calculated,
so that we can predict the values of CP asymmetries with the MSSM contribution.
We predict a large relative strong phase between the penguin amplitude and the
chromomagnetic penguin amplitude. If there is a new CP violating phase in the
chromomagnetic penguin amplitude, then the CP asymmetries may change
significantly from the SM prediction. We parametrize the new physics
contributions that appear in the Wilson coefficients. We maximize the new
physics parameters up to the point where it is limited by experimental
constraints. In the case of the LR insertion, we find that the direct CP
asymmetries can reach about 85% and the indirect CP asymmetry can reach about
-30%.Comment: 18 pages, 9 figures, REVTeX, Minor changes, Version to appear in
Phys. Rev.
Peptide exchange on MHC-I by TAPBPR is driven by a negative allostery release cycle.
Chaperones TAPBPR and tapasin associate with class I major histocompatibility complexes (MHC-I) to promote optimization (editing) of peptide cargo. Here, we use solution NMR to investigate the mechanism of peptide exchange. We identify TAPBPR-induced conformational changes on conserved MHC-I molecular surfaces, consistent with our independently determined X-ray structure of the complex. Dynamics present in the empty MHC-I are stabilized by TAPBPR and become progressively dampened with increasing peptide occupancy. Incoming peptides are recognized according to the global stability of the final pMHC-I product and anneal in a native-like conformation to be edited by TAPBPR. Our results demonstrate an inverse relationship between MHC-I peptide occupancy and TAPBPR binding affinity, wherein the lifetime and structural features of transiently bound peptides control the regulation of a conformational switch located near the TAPBPR binding site, which triggers TAPBPR release. These results suggest a similar mechanism for the function of tapasin in the peptide-loading complex
Linear and Second-order Optical Response of the III-V Mono-layer Superlattices
We report the first fully self-consistent calculations of the nonlinear
optical properties of superlattices. The materials investigated are mono-layer
superlattices with GaP grown on the the top of InP, AlP and GaAs (110)
substrates. We use the full-potential linearized augmented plane wave method
within the generalized gradient approximation to obtain the frequency dependent
dielectric tensor and the second-harmonic-generation susceptibility. The effect
of lattice relaxations on the linear optical properties are studied. Our
calculations show that the major anisotropy in the optical properties is the
result of strain in GaP. This anisotropy is maximum for the superlattice with
maximum lattice mismatch between the constituent materials. In order to
differentiate the superlattice features from the bulk-like transitions an
improvement over the existing effective medium model is proposed. The
superlattice features are found to be more pronounced for the second-order than
the linear optical response indicating the need for full supercell calculations
in determining the correct second-order response.Comment: 9 pages, 4 figures, submitted to Phy. Rev.
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