Abstract
In this talk the handbag approach to hard exclusive wide-angle processes is reviewed and applications, as for instance two-photon annihilations into pairs of mesons, are discussed.
Highlights
Handbag factorizationFactorization properties of QCD allow us to calculate exclusive processes provided a hard scale is available, either the three Mandelstam variables s, −t, −u are large as compared to a typical hadronic scale Λ2 where Λ is of order 1 GeV (wide-angle processes) or there is a highly virtual photon involved (deeply virtual processes)
Considering for instance γγ → MM with an active quark-antiquark pair (n = 2). In this case there is no spectator left and, the soft physics is encoded in two meson distribution amplitudes instead of a time-like generalized parton distributions (GPDs)
This is an example of the so-called ERBL factorization scheme which has been invented for γγ → MMin [6]
Summary
Factorization properties of QCD allow us to calculate exclusive processes provided a hard scale is available, either the three Mandelstam variables s, −t, −u are large as compared to a typical hadronic scale Λ2 where Λ is of order 1 GeV (wide-angle processes) or there is a highly virtual photon involved (deeply virtual processes). The K0K 0 (= 2KS KS ) cross section falls off rapidly with energy because of the relative strong contribution from the non-valence form factor in this case This is in agreement with experiment as can be seen from Tab. 1 where the effective powers of s obtained by BELLE from fits σ ∝ s−neff to its data are shown. As for the handbag approach the cross section behaves as 1/ sin θ but its energy dependence is predicted to be σ ∝ s−3, cf Tab. 1 While this is in rough agreement with experiment for charged mesons the magnitude of the corresponding cross section is underestimated when it is evaluated from a distribution amplitude that is close to the asymptotic form, 6x(1 − x) which is favored by current phenomenology and lattice results [14]. For these reasons it was suggested in [13] that at BELLE energies the production of neutral meson pairs is dominated by a contribution other the the ERBL mechanism but a quantitative study of the new contribution is lacking
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