Speaker
Dr
Stefan Diehl
(stefan.diehl@exp2.physik.uni-gessen.de)
Description
For the first time, we have measured single beam spin asymmetries to extract $A_{LU}^{sin(\phi)}$ moments from the hard exclusive π$^{+}$ channel off the unpolarized hydrogen target in a wide range of kinematics from forward angles to backward angles in the center of mass frame.
While many experiments showed the QCD factorization mechanism in the "nearly forward region"
(large Q² and small |t|) can be divided into a hard part, described by perturbative QCD (pQCD) and in
two general structure functions, the GPDs for the nucleon and the pion distribution amplitudes (DAs),
describing the complex non perturbative structure of these particles. The recent measurement from
CLAS in the "nearly backward" kinematic region (large Q² and small |u|) provided the potentially
applicable collinear factorized description in terms of a convolution of the non-perturbative nucleonto-pion transitions (TDAs), the nucleon DAs and the hard interaction amplitude from pQCD.
The measured moment in forward angles is known to be sensitive to generalized parton distributions
(GPDs), while in backward angles, it is known to be sensitive to transition distribution amplitudes
(TDAs). Our results clearly show that the sign of forward beam spin asymmetry measurements is
positive whereas that of backward BSA measurements is negative, with the sign transition taking place
around 90 degrees. By performing accurate measurements over a wide range of Q², x$_{B}$ and -t, we can
explore the transition from hadronic to partonic reaction mechanisms.
Primary author
Dr
Stefan Diehl
(stefan.diehl@exp2.physik.uni-gessen.de)
