# FCAL Reconstruction in b1pi events 03/14/2012

This is mostly an update of FCAL Reconstruction in b1pi events 03/06/2012, but using svn rev 8894, which includes changes to the cluster to charged track matching code.

50,000 b1pi events. No EM background.

## Contents

### Thrown photons

### Reconstructed photons

(after vetoing clusters 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

### 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?)

#### Unconverted photons

Blue/Red=

Above 1 GeV, 94% of unconverted photons are reconstructed. This is less than the 99.6% reported for single photon sample; this difference is due to photon showers rejected due to overlap 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: 70%. Could be higher after trying to recover photons that convert.

#### After conversion

For photons that do convert, we use different criteria to determine if a photon is reconstructed "successfully": (E_recon - E_thrown)/E_thrown < 3.5*sigma_E && fabs(theta_thrown - theta_recon) < .01 && fabs(phi_thrown - phi_recon) < .2), where sigma_E=0.062/sqrt(E) as calculated below.

Total FCAL efficiency: ~77%

More can be done here? Where do photons convert? Can split clusters be re-merged?

### Extra photon problem

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

Timing cut reduces average number of "photons" per event from 1.44 to 1.16. (actually 0.77 photons should hit the FCAL per event)

Timing cut has negligible effect on efficiency.

### Energy resolution

#### Unconverted photons

Look at distribution of (E_recon - E_thrown)/E_thrown as a function of E:

Sigma:

Fit is 0.062/sqrt(E). Resolution is worse than nominal value (~5%/sqrt(E)?). Why is this? Is this really better for a single photon sample?

Mean:

Some trouble at low energies?

#### All photons

This will be biased since we cut on (E_recon - E_thrown)/E_thrown before making a correspondence between a thrown and reconstructed photon, in the case that the photon has converted before the FCAL.

0.063/sqrt(E)

issues here...

### Single block "clusters"

There is a tunable parameter in the FCAL code:

`MIN_CLUSTER_BLOCK_COUNT = 2;`

At lower energies, reconstruction efficiency is lower because often only one block has energy deposited in it (above the 20 MeV threshold in mcsmear).

Try setting `MIN_CLUSTER_BLOCK_COUNT = 1`

and see what happens.

#### Efficiency

- red:
`MIN_CLUSTER_BLOCK_COUNT = 1`

- black:
`MIN_CLUSTER_BLOCK_COUNT = 2`

Clearly a big improvement in efficiency below ~500 MeV.

Overall efficiency increases by ~6%. (from 77% to 82%)

Total FCAL photon count (after timing cut) increases by 16% (1.16 photons/event to 1.34).

#### Energy resolution

Issues with energy resolution.

Energy is underestimated, especially at lower energies.

Energy resolution gets worse.

Solutions?

- Redo energy correction/calibration
- Deal with 1 block clusters as a special case?

Do hdgeant/mcsmear model FCAL correctly?

### Identifying hadronic clusters by shape?

Hadronic showers often "look" different that EM showers:

How can we distinguish between the two?

DFCALCluster computes several size parameters:

double fRMS; // cluster r.m.s. size (cm) double fRMS_x; // cluster r.m.s. size along X-axis (cm) double fRMS_y; // cluster r.m.s. size along Y-axis (cm) double fRMS_u; // cluster r.m.s. size in radial direction (cm) double fRMS_v; // cluster r.m.s. size in azimuth direction (cm)

Plot RMS size for three classes of reconstructed photon:

- Black: photons unmatched to thrown photon (more likely to be hadronic)
- Red: photons matched to a thrown photon (more likely to be EM)
- Blue: photons matched to a thrown photon that converted before reaching the FCAL

Can cut out photons with RMS size outside the green lines. This cuts out 3792/57850 of photons that survived the timing cut.

Look at size vs energy.