Transition distribution amplitudes and hard exclusive reactions with baryon number transfer

Abstract

Baryon-to-meson and baryon-to-photon transition distribution amplitudes (TDAs) arise in the collinear factorized description of a class of hard exclusive reactions characterized by the exchange of a non-zero baryon number in the cross channel. These TDAs extend the concepts of generalized parton distributions (GPDs) and baryon distribution amplitudes (DAs). In this review we discuss the general properties and physical interpretation of baryon-to-meson and baryon-to-photon TDAs. We argue that these nonperturbative objects are a convenient complementary tool to explore the structure of baryons at the partonic level. We present an overview of hard exclusive reactions admitting a description in terms of TDAs. We discuss the first signals from hard exclusive backward meson electroproduction at JLab with the 6 GeV electron beam and explore further experimental opportunities to access TDAs at JLab@12 GeV, PANDA and J-PARC.

Semi-leptonic three-body proton decay modes from light-cone sum rules

“…Here the proton spinor is understood to be on-shell, µνρσ is the fully antisymmetric Levi-Civita tensor with 0123 = 1 and we have introduced the abbreviations σ µp ≡ σ µν p ν , σ pq ≡ σ µν p µ q ν and µνpq ≡ µνρσ p ρ q σ . We add that after making use of the Dirac equation and algebraic identities the result (3.5) matches the decomposition provided in [24,25].…”

“…Here the upper (lower) line in each formula corresponds to the upper (lower) border of the corresponding envelope shown in Figure 2, while the middle line represents the central value of our LCSR form factor prediction. We add that the form factors w n are related to the off-shell form factors of the decomposition of the more general matrix element [24,25], where α, β and γ are Dirac indices. Certain combinations of the form factors w n therefore yield the form factors W k RR with k = 0, 1 that are relevant for GUT-like proton decay.…”

“…The most general decomposition of H αβγ (p p , q) in terms of a set of form factors for an off-shell proton is provided in [24,25] -see in particular (4.64) of the current arXiv version of [25]. Hence, both sets of on-shell form factors w n and W k RR can be related to these off-shell form factors upon using the equations of motion for the proton.…”

Unitarity constraints on meson electroproduction at backward angles

“…Also as the virtuality Q 2 increases, the process becomes able to distinguish quarks inside the hadrons, and QCD drives the direct coupling of the photon to these quarks. Such a partonic approach has been proposed [34][35][36][37]. It relies on the factorization of the non perturbative Transition Distribution Amplitudes (TDA), which describe at the quark level the transition between the nucleon and the meson, and the direct perturbative coupling of the virtual photon to the three quarks which are exchanged in the u-channel.…”