NOTE: This page is largely obsolete.
Up-to-date version of Gepard software is documented
here, and is available
as a standard Python package.
DVCS related cross-sections
Executables downloadable here provide cross-sections for
![[e p --> e p gamma]](images/ep2epgamma.png)
, as resulting from global fits to DVCS measurements
of CLAS, Hall A, HERMES, H1 and ZEUS collaborations.
Seven different models are presently available: two (KM09a and KM09b)
are described in
K. Kumerički and D. Müller, Nucl. Phys. B841 (2010) 1-58,
arXiv:0904.0458,
while the remaining five are based on the same hybrid
model framework, but they also include the LO
evolution of the parton sea, so only they are
appropriate for xB smaller than 10^(-3) or Q2 larger than 5 GeV^2.
Table with detailed comparison of all models can be found in
arXiv:1602.02763.
Download
These executables are completely self-contained. Because of that
they are not very fast. If they are too slow for you contact the
authors - we can in principle provide also a much faster library linkable into
your software, or even numerical grids for some specific kinematical points.
If you need executable for another architecture (e.g. MacOS),
contact the authors and be prepared to do some work. Also, if you have
quite old Linux machine (more than few years), executables above may
not work. Please, contact the authors in this case.
Here is an example showing how to use the executable from within python:
xs-demo.pdf,
xs-demo.ipynb.
Documentation
Executables are simply invoked like this:
xs.exe ModelID Charge Polarization Ee Ep xB Q2 t phi
returns cross section (in nb) for scattering of lepton of energy Ee on proton
of energy Ep. xB, Q2 and t is usual kinematics. Charge=-1 is for electron.
Polarization=+1 is for lepton polarization along the beam. Output is:
phi xs_unp xs_TPcos xs_TPsin xs_LP
where total cross section is
xs = xs_unp + sin(theta_S) cos(phi-phi_S) xs_TPcos
+ sin(theta_S) sin(phi-phi_S) xs_TPsin
+ cos(theta_S) xs_LP
and theta_S and phi_S are proton polarization polar and
azimuthal angles, while phi is angle between lepton
and reaction planes. All in radians and Trento conventions.
ModelID is one of
0 debug, always returns 42,
1 KM09a - arXiv:0904.0458 fit without Hall A data,
2 KM09b - arXiv:0904.0458 fit with Hall A harmonics ratio,
3 KM10 - arXiv:1105.0899 fit with Hall A harmonics
4 KM10a - arXiv:1105.0899 fit without Hall A data
5 KM10b - arXiv:1105.0899 fit with Hall A harmonics ratio
6 KMM12 - arXiv:1301.1230 fit with Hall A harmonics and polarized target
7 KM15 - arXiv:1512.09014 fit now includes 2015 CLAS and Hall A data
where models 1-5 are for unpolarized target only.
For convenience, if last argument (phi=n) is larger than 2pi, you get grid
of n equidistant points with phi=0..2pi.
Example:
./xs.exe 7 -1 1 5.75 0 0.36 2.3 -0.17 0.131
(Output:)
0.131 0.07584357734528 -0.03809893007524 0.00826740897951 -0.03819278821799
Note
If the phase space bounds, given by the minimal values of the Bjorken scaling variable xB
and negative momentum transfer squared -t, are exceeded, the cross section is set to zero.
Also, because of DVCS kinematics, the upper bound on -t is set by
max(Q^2/4, 1) GeV^2, and the lower bound for the photon virtuality
by 1.5 GeV^2 = Q^2.
Moreover, the cut xB <= 0.5 is implemented for all models and, in addition,
models 1 and 2 (which rely on the scaling hypothesis) are restricted to the
region Q^2 <= 5 GeV^2 and max(10^-3, xBmin) <= xB. Violating
the kinematical cuts will return error. Also note that for
convenience each positive value for Ep that is smaller than the proton mass is
internally interpreted as the proton mass itself. Finally, the electron mass is
set to zero in the the theoretical description.
Authors
Krešimir Kumerički [email] and
Dieter Müller [email]
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