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.