Introduction

This study is the second iteration of studying the conversion method in MC, first tried on February 6th: drupal.star.bnl.gov/STAR/blog-entry/aliceb/2009/feb/06/conversions-qcd-mc.

In the first iteration of the study the number of photons in a jet cone of R=0.3 around the candidate photon in the GEANT record did not match the prediction of the conversion method.  This time I try adding a hadronic veto by requiring there be no energy in the postshower and repeat the study.

Sample

I'm using the MIT filtered QCD 2->2 monte carlo samples defined here:

http://drupal.star.bnl.gov/STAR/comp/grid/infrastructure/sites/mit/mit-simulation-productions

The samples are weighted and combined together, using a  little class written by Michael, which he kindly shared.

Event Selection

(1)Select standard candidates with the StGammaMaker.

(2)Candidate pt > 5 GeV.

(3)Ratio of pt in 1X1 cluster, corrected for leakage,  to pt in a cone of radius 0.3 > 0.9.  (Isolation cut).

(4)Energy in the postshower cone of radius 0.3 around the candidate is 0. (New: hadronic veto)

(5)Energy in the preshower cone of radius 0.3 around the candidate is 0. (CPV).

(6)Energy in the second preshower cone of radius 0.3 around the candidate is greater than 0 (analysis cut).

The efficiency, ε , is given by the ratio of the number of events passing stage 6 to the number of events passing stage 5.

The prediciton of the conversion method is that:

(1-ε_bg)= (1-ε_sig)^N where ε_sig = 1.0-e^{-0.678/cos(theta)} and N is the number of photons in your cone of radius 0.3.

Method

I implemented the event selection.  I divided the sample into 3 regions of pseudo-rapidity and found the efficiency of the analysis cut in each region.  Then I use the equation above to fit for N.  The result is shown below.

I fit 1.75+-0.16 photons.

Now I look in the GEANT record and find the number of photons matched in a cone of R=0.3 around the candidate.  A histogram of the number matched is in the figure below.

so I match, on average, 1.78+-0.02 photons.

Adding the hadronic veto greatly improves the agreement, but has the disadvantage of cutting out many more candidates.

p_T of the GEANT photons

I also looked at the pT spectrum of the GEANT photons.  The figure below shows all the photons matched to the candidate, divided by photon type in the StGammaPythiaEvent class:www.star.bnl.gov/cgi-bin/protected/cvsweb.cgi/offline/StGammaMaker/StGammaPythiaEvent.h

Most of the photons are "decay" photons - coming from pions.

Next steps:

1. Add additional MC that Michael generated.  It is located an /start/institutions/iucf.  IN PROGRESS.
2. Add trigger.  The code for this is done and this should be easy.
3. Optimize the hadronic veto.  This may include a more sophisticated cut where the tower the photon is in excluded because real photons do deposit on the postshower, but will have a very narrow shower.
4. A cross section?  This involves discussion with everyone.  I would like to know how to run the pion code at some point as well.