We present a calculation of the width for χ cJ → e + e − decay. The amplitude of the process is computed within the NRQCD framework. The leading-order contribution is described by two terms associated with the two different integration domains in the electromagnetic loop describing two-photon annihilation of the heavy quark-antiquark pair. The corresponding operators are defined in the framework of NRQCD. The matrix element of one of these operators describes a configuration with an ultrasoft photon and can be associated with the higher Fock state of the heavy meson. In order to compute this contribution we use the heavy hadron chiral perturbation theory. We obtain that this contribution is numerically dominant. The obtained estimates for the decay widths of the χ c1 and χ c2 states are 0.09 eV and 0.07 eV, respectively.

Few typos and two mistakes has been found in the text of the published version of the paper. Unfortunately these errors provide a strong impact on the numerical results and therefore some of the discussed observations has been changed. The updated text includes the corrected formulas, updated numerical results and conclusions. It is found that the computed power suppressed contribution provides important numerical effect for the observable α but the absolute value of the power suppressed amplitude is considerably smaller than one obtained before. The obtained numerical estimates are more consistent with the effective field theory power counting and provide a better agreement with the existing data.

Due to multiple possible polarizations hard exclusive production of tensor mesons by virtual photons or in heavy meson decays offers interesting possibilities to study the helicity structure of the underlying short-distance process. Motivated by the first measurement of the transition form factor γ * γ → f 2 (1270) at large momentum transfers by the BELLE collaboration we present an improved QCD analysis of this reaction in the framework of collinear factorization including contributions of twist-three quark-antiquarkgluon operators and an estimate of soft end-point corrections using light-cone sum rules. The results appear to be in good agreement with the data, in particular the predicted scaling behavior is reproduced in all cases.

We derive the complete factorization formula for the leading power contribution in wide angle Compton scattering. It consists of the soft-and hard-spectator contributions. The hard-spectator contribution is well known and defined in the form of the convolution of a hard kernel with the nucleon distribution amplitudes. The soft-spectator contribution describes the scattering which involves the soft modes. We use the soft collinear effective theory in order to define this term in a field theoretical approach. Using the SCET framework we provide the proof of the factorization formula. We also compute the next-to-leading QCD corrections to the hard coefficient function of the soft spectator contribution and perform a phenomenological analysis of existing experimental data within the developed formalism.the confrontation of the theory with the data one has to implement a different picture of the scattering which describes also the dominant preasymptotic effects. To this extent, promising results have been obtained within the hand-bag model approach [13,14,15,16]. The main idea of this model is that the dominant contribution in the relevant kinematical region is provided by the soft-overlap contribution which is also known as the Feynman mechanism. In this model both photons interact with a single quark which couples to the soft spectators through the generalized parton distributions (GPDs). The GPDs describe the nonperturbative soft-overlap mechanism at small momentum transfer −t ∼ Λ 2 and their models can be constrained by using their relations to the form factors and usual parton distributions, see e.g. [17,18,19] and references therein. The extension of the GPD formalism for the description of the region with large −t Λ is the key assumption of the handbag model. There is also an alternative approach based on the constituent quark model [20]. All these models provide a satisfactory description of the cross section data and asymmetry K LL that demonstrates a strong support for the crucial role played by the soft-spectator scattering in WACS and possibly in other hard reactions.The further and more accurate experimental studies of the WACS cross sections and asymmetries can be performed at JLab after the 12 GeV Upgrade [21]. This provides a strong motivation to develop a systematic theoretical approach which accommodates consistently both hard-and soft-spectator reaction mechanisms and allows one to reduce the model dependence in the data analysis.Some steps in this direction were made in the framework of the GPD handbag model in Refs. [18,22]. However in the latter framework one is faced with the problem how to consistently perform the matching between the hard and the soft regions: it is not clear how to map systematically the infrared poles arising from the partonic diagrams with the ultraviolet structure of the GPD-based matrix elements. This difficulty does not allow to formulate a consistent theoretical approach.The importance of the soft modes and their specific role for the description of hard processes with ...