[Halld-tagger] code for pair production

[Richard Jones]

Hovannes,

I have now got this code working, just this morning.  Here are some results for you to check out against known references.  If you want, I can just answer specific questions you have, or else I can give you the output from the program for you to play with yourself.

E(gamma) = 9.0 GeV, linear polarization 100%
pair plane asymmetry = 13.0 +/- 0.3 % without cuts on electron/positron energy
pair plane asymmetry = 24.5 +/- 0.5 % with cuts 4.0 < E(electron) < 5.0 GeV

http://zeus.phys.uconn.edu/halld/pairspectrometer/xsect-1-2011/allpairs.png
http://zeus.phys.uconn.edu/halld/pairspectrometer/xsect-1-2011/sympairs.png <http://zeus.phys.uconn.edu/halld/pairspectrometer/xsect-1-2011/allpairs.png>

Plots are found at the above links.  The code itself only computes the basic Feynman diagram, which is differential in 5 variables.  To get useful results you have to integrate over many of these, which I do using Monte Carlo integration.  This is why there are error bars on the cross section plots: they are the statistical errors coming from the Monte Carlo integration.  I have 10 million Monte Carlo events, saved with 5 kinematic variables E(electron), phi(electron), M(e+,e- pair mass), Q2 (recoil momentum squared), and phi(recoil).  Saved together with these 5 generated variables in a root Tree is the differential cross section (diffXS), the Monte Carlo weight factor (weight), and their product (weightedXS) that is most useful for making plots.  Let me know if you want access to these data.

-Richard J.




On 1/19/2011 11:53 AM, Hovanes Egiyan wrote:
> Hi Richard,
>
> during last Hall D beamline meeting you said that you could setup
> a calculator for me to evaluate the azimuthal asymmetries for
> pair production with linearly polarized photons. What you needed
> was an atomic formfactor of my choice. I would suggest to use the
> form factor for the case of complete screening :
> F(q) = 1/ ( 1 + ( 111*q*Z^(-1/3) )^2 )
>
> I did find some parameterizations of the atomic formfactors but those would
> need to be coded, and I think it would be useful to first check what we
> get with
> this formfactor.
>
> thanks in advance,
>                       Hovanes.
>
>

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