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DVCS related cross-sections

Executables downloadable here provide cross-sections for [e p --> e p gamma] , 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.


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.


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.                                           
    ./xs.exe  7 -1  1  5.75  0  0.36  2.3  -0.17  0.131                            
0.131  0.07584357734528  -0.03809893007524   0.00826740897951  -0.03819278821799   


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.


Krešimir Kumerički [email] and Dieter Müller [email]

Last update: 2017-09-10