778 research outputs found

### New Physics Effects and Hadronic Form Factor Uncertainties in B -> K^* l^+ l^-

It is well known that New Physics can contribute to weak decays of heavy
mesons via virtual processes during its decays. The discovery of New Physics,
using such decays is made difficult due to intractable strong interaction
effects needed to describe it. Modes such as B -> K^* l^+ l^- offer an
advantage as they provide a multitude of observables via angular analysis. We
show how the multitude of "related observables" obtained from B -> K^* l^+ l^-,
can provide many new "clean tests" of the Standard Model. The hallmark of these
tests is that several of them are independent of the unknown universal form
factors in heavy quark effective theory. We derive a relation between
observables that is free of form factors and Wilson coefficients, the violation
of which will be an unambiguous signal of New Physics. We also derive relations
between observables and form factors that are independent of Wilson
coefficients and enable verification of hadronic estimates. We show how form
factor ratios can be measured directly from helicity fraction with out any
assumptions what so ever. We find that the allowed parameter space for
observables is very tightly constrained in Standard Model, thereby providing
clean signals of New Physics. We examine both the large-recoil and low-recoil
regions of the K^* meson and point out special features and derive relations
between observables valid in the two limits. In the large-recoil regions
several of the relations are unaffected by corrections to all orders in
\alpha_s. We present yet another new relation involving only observables that
would verify the validity of the relations between form-factors assumed in the
low-recoil region. The several relations and constraints derived will provide
unambiguous signals of New Physics if it contributes to these decays.Comment: 26 pages, 9 figures (20 pdf figure files). Several typos correcte

### Double charmonium production in exclusive bottomonia decays

This paper is devoted to the leading twist exclusive bottomonia decays with
double charmonium in the final state. Using models of the twist-2 charmonia
distribution amplitudes the widths of these decays are calculated within light
cone formalism. In addition, the processes under consideration are studied
within nonrelativistic QCD. In our analysis we have found that the production
of some of the $P$-wave charmonia mesons with $L_z \neq 0$ is allowed already
at the leading twist approximation. This means that the selection rules which
predict the suppression of such decays are violated. The mechanism which lies
behind this violation is discussed.Comment: Minor changes, some references adde

### Study of exclusive processes e^+ e^- \to VP

This paper is devoted to consideration of the hard exclusive processes
$e^+e^- \to VP$, where $V=\rho,\phi; P=\eta,\eta'$. Experimental measurement of
the cross section of the process $e^+ e^- \to \phi\eta$ at BaBar collaboration
at large center mass energy $\sqrt s=10.6$ GeV and some low energy experimental
data $\sqrt s \sim 2-4$ GeV give us the possibility to study the cross section
in the broad energy region. As the result, we have determined the asymptotic
behavior of the cross section of $e^+ e^- \to \phi\eta$ in the limit $s \to
\infty$, which is in agreement with perturbative QCD prediction. Assuming that
the same asymptotic behavior is valid for the other processes under
consideration and using low energy experimental data we have predicted the
cross sections of these processes at energies $\sqrt s=3.67,~10.6$ GeV. In
addition, we have calculated the cross sections of these processes at the same
energies within perturbative QCD. Our results are in agreement with available
experimental data.Comment: 10 pages, 2 tables, 6 figures; minor changes; article accepted to
publication is Phys. Rev.

### Probing Quark Distribution Amplitudes Through Generalized Parton Distributions at Large Momentum Transfer

In the large momentum transfer limit, generalized parton distributions can be
calculated through a QCD factorization theorem which involves
perturbatively-calculable hard kernels and light-cone parton distribution
amplitudes of hadrons. We illustrate this through the $H_q(x,\xi,t)$
distribution for the pion and proton, presenting the hard kernels at leading
order. As a result, experimental data on the generalized parton distributions
in this regime can be used to determine the functional form of the parton
distribution amplitudes which has thus far been quite challenging to obtain.
Our result can also be used as a constraint in phenomenological GPD
parametrizations.Comment: 7 pages, 4 figures; new references and figure added, errors correcte

### Two-photon exchange in elastic electron-proton scattering : QCD factorization approach

We estimate the two-photon exchange contribution to elastic electron-proton
scattering at large momentum transfer $Q^2$. It is shown that the leading
two-photon exchange amplitude behaves as $1/Q^4$ relative to the one-photon
amplitude, and can be expressed in a model independent way in terms of the
leading twist nucleon distribution amplitudes. Using several models for the
nucleon distribution amplitudes, we provide estimates for existing data and for
ongoing experiments.Comment: 4 pages, 4 figure

### The pion diffractive dissociation into two jets

The method used and results obtained are described for calculation of the
cross section for the pion diffractive dissociation into two jets. The main new
qualitative result is that the distribution of longitudinal momenta for jets is
not simply proportional to the profile of the pion wave function
$\phi_{\pi}(x)$, but depends on it in a complicated way. In particular, it is
shown that under conditions of the E791 experiment, the momentum distribution
of jets is similar in its shape for the asymptotic and CZ wave functions. It is
concluded therefore that, unfortunately, the process considered is really
weakly sensitive to the profile of the pion wave function, and the accuracy of
data is insufficient to distinguish clearly between different models of
$\phi_{\pi}(x)$. Comparison with the results of other papers on this subject is
given.Comment: Latex, 11 pages, 3 figures. Talk given at the International Workshop:
"Exclusive Processes at High Momentum Transfer", Jefferson Lab, Newport News,
VA, USA, May 15-18, 200

### Higher twist distribution amplitudes of the pion and electromagnetic form factor $F_{\pi}(Q^2)$

The pion electromagnetic form factor is calculated within the QCD light-cone
sum rule method and using a renormalon model for the higher twist distribution
amplitudes (DAs). The theoretical predictions are compared with the
experimental data and constraints on the pion leading and twist-4 DAs are
extracted. An upper bound on the twist-4 contribution to the form factor and
estimates of effects due to higher conformal spins in the pion twist-4 DAs are
obtained.Comment: 13 pages, 7 figures; the version to appear in Phys.Rev.

### Mass Spectrum in SQCD and Problems with the Seiberg Duality. Another Scenario

N=1 SQCD with SU(N_c) colors and N_F flavors of light quarks is considered
within the dynamical scenario which assumes that quarks can be in two different
phases only. These are: a) either the HQ (heavy quark) phase where they are
confined, b) or they are higgsed, at the appropriate values of parameters of
the Lagrangian.
The mass spectra of this (direct) theory and its Seiberg's dual are obtained
and compared, for quarks of equal or unequal masses. It is shown that in all
cases when there is the additional small parameter at hand (it is
0<(3N_c-N_F)/N_F << 1 for the direct theory, or its analog 0<(2N_F-3N_c)/N_F <<
1 for the dual one), the mass spectra of the direct and dual theories are
parametrically different. A number of other regimes are also considered.Comment: 30 pages, purely technical improvements for readers convenienc

### The leading-twist pion and kaon distribution amplitudes from the QCD instanton vacuum

We investigate the leading-twist light-cone distribution amplitudes for the
pion and kaon, based on the nonlocal chiral quark model from the instanton
vacuum. Effects of explicit flavor SU(3)-symmetry breaking are taken into
account. The Gegenbauer moments are computed, analyzed and compared with those
of other models. The one-loop QCD evolution of the moments is briefly
discussed. The transverse momentum distributions are also discussed for the
pion and kaon light-cone wave functions.Comment: 17 pages, 12 figures. Accepted for publication in Physical Review

### Light Cone Sum Rules for the pi0-gamma*-gamma Form Factor Revisited

We provide a theoretical update of the calculations of the pi0-gamma*-gamma
form factor in the LCSR framework, including up to six polynomials in the
conformal expansion of the pion distribution amplitude and taking into account
twist-six corrections related to the photon emission at large distances. The
results are compared with the calculations of the B-> pi l nu decay and pion
electromagnetic form factors in the same framework. Our conclusion is that the
recent BaBar measurements of the pi0-gamma*-gamma form factor at large momentum
transfers are consistent with QCD, although they do suggest that the pion DA
may have more structure than usually assumed.Comment: 20 pages, 14 figures, 5 table

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