FCAL Reconstruction in b1pi events 03/06/2012

svn rev. 8810. 50,000 b1pi events. No EM background.

Thrown photons

Reconstructed photons

(after vetoing cluster matched to charged tracks)

Questions

• Why are so many "photons" reconstructed? How can we reduce this?
• What is the actual reconstruction efficiency?
• How many photons reach the FCAL without converting?
  • Efficiency for non-converting photons?
  • Can we recover converted photons?
• Energy resolution, etc.

1 GeV photon events

Mihajlo in GlueX-doc-823 (2007) says for 1 GeV photons:

• 37% photon conversion before FCAL
• 99.6% efficiency for photons that reach FCAL
• Taking into account single clusters from photons that converted close to the FCAL face, the overall photon reconstruction efficiency was estimated to be 77%
• 4% energy resolution at 1 GeV

Has anything changed since 2007? How does b1pi sample differ from single photon sample?

Efficiency

Energy distribution of thrown photons:

• Black=all thrown photons
• Red=photons which reach the FCAL without converting (determined using DFCALTruthShower object)
• Blue=photons which reach the FCAL without converting AND are reconstructed successfully (fabs(theta_thrown - theta_recon) < .006 && fabs(phi_thrown - phi_recon) < .15)

Red/Black=% of photons that reach FCAL without converting:

Roughly 20% of photons convert before reaching FCAL. Relatively constant across wide range of energy. There is an angular dependence, not shown here. Big improvement since 2007 (geometry change?)

Blue/Red=

Above 1 GeV, 99.2% of uncoverted photons are reconstructed. This is less than the 99.6% reported for single photon sample, most likely this difference is due to photon showers overlapping with charged particle shower.

Below ~600 MeV, efficiency drops. This is due to showers contained within a single cell, which means no cluster is created.

Overall FCAL reconstruction efficiency considering only unconverted photons: 74%. Could be higher after trying to recover photons that convert.

Extra photon problem

To reduce number of extra "photons", consider a more aggressive matching of clusters to charged tracks:

• Only one matching FCAL cluster per track is allowed with the current code. Frequently hadronic showers are grouped into multiple clusters.
• The match radius is hardcoded as 5 cm, but in the calib/PID/photon_track_matching this parameter is set to 15 cm. Change this back to 15 cm.

Also use a timing cut (t_shower-t_flight) < 1.2 ns to cut out of out-of-time particles:

More aggressive cluster matching reduces average number of "photons" per event from 2.5 to 1.5. Timing cut on top of this reduces average to 1.2. (actually 0.77 photons should hit the FCAL per event)

This also reduces efficiency (as defined above) from 74% to 70%. Timing cut has negligible effect on efficiency.