Ideally how should a photon and charged particle cluster look like ?

Ideally one can make the following comparative statements for clusters due to photon track and clusters due to charged particle track.

1. Photon cluster will deposit more energy in the sensitive medium of the detector than the cluster due to charged particle.

2. Photon cluster will hit more number of cells in the detector than the cluster due to charged particle.

What can go wrong ?

1. A low energy photon may have most of it's shower get absorbed in the convertor material. It may then deposit less energy and hit lower number of cells. So convertor thickness is important.

2. A charged particle may interact with the convertor material (for 3X0 the interaction probability is about 10%). It may then deposit more energy and hit larger number of cells. So convertor thickness is important.

3. The clustering algorithm in it's attempt to separate overlaping clusters may split a photon cluster into many small clusters. Small both in energy deposition and number of cells in a cluster.

What do we observe ?

We find that 28% of photon clusters have Ncell = 1. So if we we put a condition of Ncell > 1 to discriminate photon from hadron we loose 28% of the "converted" photons.

CLICK ON THE FIGURES

On the other hand we find that 37% of the clusters due to hadrons have Ncell > 1. So if we put only Ncell > 1 condition, 37% of total charged particles giving clusters will contribute to impurity of photon sample detected.

So it is important to put a energy deposition cut also.

This plot shows how the photon cluster Edep varies with various Ncell cuts. We expect to have most of the photons even with 2 MIP EDEP cut.

CLICK ON THE FIGURES

This plot shows how the charge cluster Edep varies with various Ncell cuts. We expect to get rid of most of the charge clusters even with 2 MIP EDEP cut.

Let us understand how does the photon clusters with Ncell = 1 and Ncell > 1 differ in transverse momentum and Energy of the photon.

This plot shows how the transverse momentum and Energy of the photon track giving a cluster with Ncell = 1 and Ncell > 1.