Processes and observables
Processes
Presently, only Deeply virtual Compton scattering (DVCS) process can be considered fully implemented (using the socalled BMK formulas). There is also an initial implementation of Deeply virtual meson production (DVMP) of rho0 meson, valid for small Bjorken x kinematics.
Datapoints should have an attribute process
, which is one of
the following:
Proces 
DataPoint 
Generic process class 

\(e p \to e p \gamma\) 

DVCS 
\(e n \to e n \gamma\) 

DVCS 
\(\gamma^* p \to \gamma p\) 

DVCS 
\(\gamma^* p \to \rho^{0} p\) 

DVMP 
\(e p \to e p X\) 

DIS 
Formulas for generic theory class DVMP are implemented as
g.DVMP
, while those for DVCS have several slightly different
versions:
Theory class 
description 


old original BMK formulas from 2005 

improved formulas that are better for JLab kinematics 

best formulas from 2010 

like 

like 
Observables
Following observables are implemented in Gepard. Identifiers
from the table can be used either
as methods of theory objects, or as observable
attributes of datapoints.
Identifier 
Description 


crosssection for leptoproduction of real photon 

crosssection for production of real photon by virtual one 

beam spin sum a.k.a helicity independent XS 

XUU weighted by BH propagator 

beam spin difference a.k.a helicity dependent XS 

XLU weighted by BH propagator 

beam chargespin difference (COMPASS) 

beam chargespin sum (COMPASS) 

beam charge asymmetry 

beam spin asymmetry 

beam spin asymmetry, interference part 

beam spin asymmetry, DVCS part 

longitudinal target spin asymmetry 

transversal target spin asymmetry 

transversal target spin asymmetry, interference part 

transversal target spin asymmetry, DVCS part 

beam (longitudinal) target double spin asymmetry 

beam transversal target double spin asymmetry, interference part 

beam transversal target double spin asymmetry, BHDVCS part 
Many of these observables can be evaluated both as differential in azimuthal
angle \(\phi\) (if the DataPoint
argument has an attribute phi
),
or as “harmonic”, i. e., as Fourier integral over \(\phi\) (if the
DataPoint
argument has attribute FTn
).
Similary, XGAMMA
will be evaluated as differential in \(t\) if
DataPoint
has attribute t
, and as integrated over \(t\) if
it doesn’t.
Name 
Description 


crosssection for production of meson by longit. virtual photon 
Choice whether DVCS or DVMP XGAMMA
will be evaluated is dependent
on the value of pt.process
.