What is clustering efficiency ?

The photon after getting converted into electromagnetic shower particles, hits a group of cells of our detector. We then use a clustering algorithm to detect these clusters of photon.

Clustering efficiency is defined as the ratio of number of photons (tracks) that has converted and given a signal on our detector to the number of photon clusters (associated with converted photon tracks) obtained through the clustering algorithm in our detector.

The plot shows the clustering efficiency as a function of centrality of collision and it's event-by-event variation. We find the clustering efficiency is about 91%. And it does not have much a dependence on centrality.

So the maximum (upper limit) photon counting efficiency we can have is 0.77 * 0.91 = 0.70 ==> 70%.