579 research outputs found
A Systematic Analysis of the Lepton Polarization Asymmetries in the Rare B Decay, B -> X_s\tau^+\tau^-
The most general model-independent analysis of the lepton polarization
asymmetries in the rare B decay, \Bstt, is presented. We present the
longitudinal, normal and transverse polarization asymmetries for the
and , and combinations of them, as functions of the Wilson coefficients
of twelve independent four-Fermi interactions, ten of them local and two
nonlocal. These procedures will tell us which type of operators contributes to
the process. And it will be very useful to pin down new physics systematically,
once we have the experimental data with high statistics and a deviation from
the Standard Model is found.Comment: 24 pages, 8 figures, LaTe
Higher twists in the pion structure function
We calculate the QCD moments of the pion structure function using Drell-Yan
data on the quark distributions in the pion and a phenomenological model for
the resonance region. The extracted higher twist corrections are found to be
larger than those for the nucleon, contributing around 50% of the lowest moment
at Q^2=1 GeV^2.Comment: 8 pages, 3 figures, to appear in Phys. Rev.
Higher moments of nucleon spin structure functions in heavy baryon chiral perturbation theory and in a resonance model
The third moment of the twist-3 part of the nucleon spin structure
function is generalized to arbitrary momentum transfer and is
evaluated in heavy baryon chiral perturbation theory (HBChPT) up to order
and in a unitary isobar model (MAID). We show how to link
as well as higher moments of the nucleon spin structure functions
and to nucleon spin polarizabilities. We compare our results with the
most recent experimental data, and find a good description of these available
data within the unitary isobar model. We proceed to extract the twist-4 matrix
element which appears in the suppressed term in the twist
expansion of the spin structure function for proton and neutron.Comment: 30 pages, 7 figure
Generalized parton distributions and Deeply Virtual Compton Scattering in Color Glass Condensate model
Within the framework of the Color Glass Condensate model, we evaluate quark
and gluon Generalized Parton Distributions (GPDs) and the cross section of
Deeply Virtual Compton Scattering (DVCS) in the small- region. We
demonstrate that the DVCS cross section becomes independent of energy in the
limit of very small , which clearly indicates saturation of the DVCS
cross section. Our predictions for the GPDs and the DVCS cross section at
high-energies can be tested at the future Electron-Ion Collider and in
ultra-peripheral nucleus-nucleus collisions at the LHC.Comment: 20 pages, 8 Figure
Modeling quark-hadron duality for relativistic, confined fermions
We discuss a model for the study of quark-hadron duality in inclusive
electron scattering based on solving the Dirac equation numerically for a
scalar confining linear potential and a vector color Coulomb potential. We
qualitatively reproduce the features of quark-hadron duality for all potentials
considered, and discuss similarities and differences to previous models that
simplified the situation by treating either the quarks or all particles as
scalars. We discuss the scaling results for PWIA and FSI, and the approach to
scaling using the analog of the Callan-Gross relation for y-scaling.Comment: 38 pages, 21 figure
Spin structure of the nucleon: QCD evolution, lattice results and models
The question how the spin of the nucleon is distributed among its quark and
gluon constituents is still a subject of intense investigations. Lattice QCD
has progressed to provide information about spin fractions and orbital angular
momentum contributions for up- and down-quarks in the proton, at a typical
scale \mu^2~4 GeV^2. On the other hand, chiral quark models have traditionally
been used for orientation at low momentum scales. In the comparison of such
model calculations with experiment or lattice QCD, fixing the model scale and
the treatment of scale evolution are essential. In this paper, we present a
refined model calculation and a QCD evolution of lattice results up to
next-to-next-to-leading order. We compare this approach with the Myhrer-Thomas
scenario for resolving the proton spin puzzle.Comment: 11 pages, 6 figures, equation (9) has been corrected leading to a
revised figure 1b. Revision matches published versio
Parton-Hadron Duality in Unpolarised and Polarised Structure Functions
We study the phenomenon of parton-hadron duality in both polarised and
unpolarised electron proton scattering using the HERMES and the Jefferson Lab
data, respectively. In both cases we extend a systematic perturbative QCD based
analysis to the integrals of the structure functions in the resonance region.
After subtracting target mass corrections and large x resummation effects, we
extract the remaining power corrections up to order 1/Q^2. We find a sizeable
suppression of these terms with respect to analyses using deep inelastic
scattering data. The suppression appears consistently in both polarised and
unpolarised data, except for the low Q^2 polarised data, where a large negative
higher twist contribution remains. Possible scenarios generating this behavior
are discussed.Comment: 17 pages, 9 figure
Search for Invisible Decays of and in and
Using a data sample of decays collected with the BES
II detector at the BEPC, searches for invisible decays of and
in to and are performed.
The signals, which are reconstructed in final states, are used
to tag the and decays. No signals are found for the
invisible decays of either or , and upper limits at the 90%
confidence level are determined to be for the ratio
and for . These are the first
searches for and decays into invisible final states.Comment: 5 pages, 4 figures; Added references, Corrected typo
Initial-State Interactions in the Unpolarized Drell-Yan Process
We show that initial-state interactions contribute to the
distribution in unpolarized Drell-Yan lepton pair production and , without suppression. The asymmetry is expressed as a
product of chiral-odd distributions , where the quark-transversity function
is the transverse momentum dependent, light-cone
momentum distribution of transversely polarized quarks in an {\it unpolarized}
proton. We compute this (naive) -odd and chiral-odd distribution function
and the resulting asymmetry explicitly in a quark-scalar diquark
model for the proton with initial-state gluon interaction. In this model the
function equals the -odd (chiral-even) Sivers
effect function . This suggests that the
single-spin asymmetries in the SIDIS and the Drell-Yan process are closely
related to the asymmetry of the unpolarized Drell-Yan process,
since all can arise from the same underlying mechanism. This provides new
insight regarding the role of quark and gluon orbital angular momentum as well
as that of initial- and final-state gluon exchange interactions in hard QCD
processes.Comment: 22 pages, 6 figure
A Model Independent Analysis of the Rare B Decay B -> X_s l^+ l^-
The most general model-independent analysis of the rare decay, \Bsll,
is presented. There are ten independent local four-Fermi interactions which may
contribute to this process. The branching ratio, the forward-backward
asymmetry, and the double differential rate are written as functions of the
Wilson coefficients of the ten operators. We also study the correlation between
the branching ratio and the forward-backward asymmetry by changing each
coefficient. This procedure tells us which types of operator contribute to the
process, and it will be very useful to pin down new physics systematically,
once we have the experimental data with high statistics and the deviation from
the Standard Model is found.Comment: 32 pages, 21 figures, LaTe
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