6 research outputs found
A novel way to probe distribution amplitudes of neutral mesons in e^+e^- annihilation
We derive the amplitude for the process at large
invariant energy. The process goes through the two-photon exchange and its
amplitude is expressed in terms of the convolution integral which depends on
the shape of the pion distribution amplitude (DA) and the centre of mass
scattering angle. Remarkable feature of the integral is that it is very
sensitive to the end-point behaviour of the pion DA -- it starts to diverge if
pion DA nullifies at the end-point as or slower. That makes the
process unique probe of the shape of the meson DAs. The
estimated cross section is rather small, for GeV it ranges from a
fraction of femtobarn (for the asymptotic DA) to couple of femtobarn (for the
Chernyak-Zhitnitsky DA). The observation of the process
with the cross section higher as estimated here would imply very unusual form
of the pion DA, e.g. the flat one. The derived amplitude can be easily
generalized to other processes like , etc.Comment: 5 pages, 3 figure
Twist-three analysis of photon electroproduction with pion
We study twist-three effects in spin, charge, and azimuthal asymmetries in
deeply virtual Compton scattering on a spin-zero target. Contributions which
are power suppressed in 1/Q generate a new azimuthal angle dependence of the
cross section which is not present in the leading twist results. On the other
hand the leading twist terms are not modified by the twist three contributions.
They may get corrected at twist four level. In the Wandzura-Wilczek
approximation these new terms in the Fourier expansion with respect to the
azimuthal angle are entirely determined by the twist-two skewed parton
distributions. We also discuss more general issues like the general form of the
angular dependence of the differential cross section, validity of factorization
at twist-three level, and a relation of skewed parton distributions to spectral
functions.Comment: 21 pages, LaTeX, 2 figures, text clarifications, an equation, a note
and references adde
Critical behavior of weakly-disordered anisotropic systems in two dimensions
The critical behavior of two-dimensional (2D) anisotropic systems with weak
quenched disorder described by the so-called generalized Ashkin-Teller model
(GATM) is studied. In the critical region this model is shown to be described
by a multifermion field theory similar to the Gross-Neveu model with a few
independent quartic coupling constants. Renormalization group calculations are
used to obtain the temperature dependence near the critical point of some
thermodynamic quantities and the large distance behavior of the two-spin
correlation function. The equation of state at criticality is also obtained in
this framework. We find that random models described by the GATM belong to the
same universality class as that of the two-dimensional Ising model. The
critical exponent of the correlation length for the 3- and 4-state
random-bond Potts models is also calculated in a 3-loop approximation. We show
that this exponent is given by an apparently convergent series in
(with the central charge of the Potts model) and
that the numerical values of are very close to that of the 2D Ising
model. This work therefore supports the conjecture (valid only approximately
for the 3- and 4-state Potts models) of a superuniversality for the 2D
disordered models with discrete symmetries.Comment: REVTeX, 24 pages, to appear in Phys.Rev.
On the calculation of RG functions in the Gross-Neveu model from from large- expansions of critical exponents
URL: http://www-spht.cea.fr/articles/T93/016 http://fr.arxiv.org/abs/hep-th/9302034International audienceA Proof of critical conformal invariance of Green's functions for a quite wide class of models possessing critical scale invariance is given. A simple method for establishing critical conformal invariance of a composite operator, which has a certain critical dimension, is also presented. The method is illustrated with the example of the Gross--Neveu model and the exponents at order and at order are calculated with the conformal bootstrap method.Les auteurs de l'article sont parmi les experts mondiaux du calcul des exposants critiques par le développement en où est le nombre de composantes du champ dans un modèle vectoriel. Les méthodes les plus efficaces utilisent la technique dite du \lq\lq bootstrap\rq\rq\ conforme. Cette technique repose sur l'invariance conforme d'une théorie des champs au point critique. Les auteurs étendent la preuve qu'une théorie des champs invariante d'échelle est également invariante conforme à des cas plus généraux. Comme illustration de la méthode ils donnent le développement de plusieurs exposants critiques du modèle de Gross--Neveu jusqu'aux ordres ou $ 1/N^3.