253 research outputs found
Light-by-light scattering in Double-Logarithmic Approximation
In the present paper we consider the elastic 2 -> 2 -scattering of virtual
photons at high energies in the forward kinematics at zero and non-zero values
of t. Accounting for both gluon and quark double-logarithmic (DL) contributions
to all orders in the QCD coupling, we obtain explicit expressions for
amplitudes of this process in Double-Logarithmic Approximation (DLA). First we
keep the QCD coupling fixed and then account for running coupling effects.
Applying the saddle-point method to the obtained expressions for the scattering
amplitude, we calculate the high-energy asymptotics of the amplitude, which
proved to be of the Regge form. The Reggeon bears the vacuum quantum numbers
and therefore it is a new, DL contribution to Pomeron. Comparison of the DL
Pomeron to the BFKL Pomeron shows that contribution of the DL Pomeron to the
high-energy asymptotics is of the same order as contribution of the BFKL
Pomeron, so the DL Pomeron should be taken into account together with the BFKL
Pomeron. We estimate the applicability region for the asymptotics of the
light-by-light scattering amplitude, where the the DL Pomeron can reliably
represent the parent amplitude.Comment: 16 pages, 4 figures. Multiple errors are corrected, 2 figs are added,
comparizon to BFKL is done in more detai
Comment on the recent COMPASS data on the spin structure function g_1
We examine the recent COMPASS data on the spin structure function g_1
singlet. We show that it is rather difficult to use the data in the present
form in order to draw conclusions on the initial parton densities. However, our
tentative estimate is that the data better agree with positive rather than
negative initial gluon densities.Comment: 8 pages, 1 figur
Comment on the frozen QCD coupling
The frozen QCD coupling is a parameter often used as an effective fixed
coupling. It is supposed to mimic both the running coupling effects and the
lack of knowledge of alpha_s in the infrared region. Usually the value of the
frozen coupling is fixed from the analysis of the experimental data. We present
a novel way to define such coupling(s) independently of the experiments. We
argue that there are different frozen couplings which are used in the double-
(DL) and single- logarithmic (SL) Approximations. We introduce four kinds of
the frozen couplings: the coupling used in DLA with a time-like argument (i.e.
the coupling present in the non-singlet scattering amplitudes and DIS structure
functions) which we find 0.24 approximately; the DLA coupling with a space-like
argument (in e+e- -annihilation, in DY processes and in any scattering
amplitude in the hard or backward kinematics) which is a factor two larger,
namely 0.48. We also show that the frozen coupling in the SL evolution
equations like BFKL has to be defined in a way less accurate compared to DLA,
and our estimate for this coupling is 0.1. Our estimates for the singlet and
non-singlet intercepts are also in a good agreement with the results available
in the literature.Comment: 11 pages, 3 figure
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