Processes and observables

Processes

Presently, only Deeply virtual Compton scattering (DVCS) process can be considered fully implemented (using the so-called 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:

Processes

Proces	DataPoint process value	Generic process class
\(e p \to e p \gamma\)	ep2epgamma	DVCS
\(e n \to e n \gamma\)	en2engamma	DVCS
\(\gamma^* p \to \gamma p\)	gammastarp2gammap	DVCS
\(\gamma^* p \to \rho^{0} p\)	gammastarp2rho0p	DVMP
\(e p \to e p X\)	dis	DIS

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

Version of BMK formulas

Theory class	description
g.BMK	old original BMK formulas from 2005
g.hotfixedBMK	improved formulas that are better for JLab kinematics
g.BM10ex	best formulas from 2010
g.BM10	like BM10ex, but with some Q2-suppresed terms removed
g.BM10tw2	like BM10 but with higher twists set to zero

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.

DVCS observables

Identifier	Description
XS	cross-section for leptoproduction of real photon
XGAMMA	cross-section for production of real photon by virtual one
XUU	beam spin sum a.k.a helicity independent XS
XUUw	XUU weighted by BH propagator
XLU	beam spin difference a.k.a helicity dependent XS
XLUw	XLU weighted by BH propagator
XCLU	beam charge-spin difference (COMPASS)
XCUU	beam charge-spin sum (COMPASS)
AC	beam charge asymmetry
ALU	beam spin asymmetry
ALUI	beam spin asymmetry, interference part
ALUDVCS	beam spin asymmetry, DVCS part
AUL	longitudinal target spin asymmetry
AUT	transversal target spin asymmetry
AUTI	transversal target spin asymmetry, interference part
AUTDVCS	transversal target spin asymmetry, DVCS part
BTSA	beam (longitudinal) target double spin asymmetry
ALTI	beam transversal target double spin asymmetry, interference part
ALTBHDVCS	beam transversal target double spin asymmetry, BH-DVCS 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.

DVMP observables

Name	Description
XGAMMA	cross-section 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.