1. A reconstruction algorithm for photons converted in tracker
Photon Recovery
A reconstruction algorithm for photons converted in tracker
Yuxuan Zhang

2. motivation
A noticeable portion of photons convert into e+e- before reaching Ecal, which can result in failure in tagging. At most cases, a photon decays into an electron-positron pair due its interaction with materials in TPC. Thus a method is introduced to find ”missing“ photons: it takes Arbor PFO as input and returns converted photons.

3. Conversion Rate
The number of converted photons depends on the amount of material that photons travel through. This graph examines 100,000 single 10GeV photon events in random angles and it shows such relationship. the polar angle is defined to be the angle between photon’s track and z-axis, and the conversion rate is simply: # 𝑜𝑓𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 # 𝑜𝑓 𝑎𝑙𝑙 𝑝ℎ𝑜𝑡𝑜𝑛𝑠

4. Selection Criteria I: mass
We are interested in the rest mass of an electron-positron pair because its invariant mass should be 0 by special relativity. However, in reality, within certain radius, arbor PFO takes the origin as a electron’s vertex, which can result in a error in the electron’s momentum. Overall speaking, our reconstruction cannot be 100% accurate, So it’s important to find a proper cut in the pair’s mass.

7. Selection Criteria II: Distance
We are also interested in the information in tracker. FarMag is the distance from the origin to the starting point of the further track. DisFar_Neighbour is the smallest distance from starting point of the further track to the near track.

8. Results and comparison

9. Summary
Recovered photons are compared with converted photons at MC truth level. A photon is reconstructed ”correctly” if its direction is no more than 0.1 rad aparts from its MC particle, and energy error is within 20% range.
In 100,000 single photon samples, 4206 photons are found converted, 3874 are reconstructed, while 3680 are reconstructed correctly. 87.5% overall efficiency. This photon recovery process thus contributes to about 3.68% improvement in photon reconstruction.

10. Major limit: FSR
Final state radiation (FSR) is the major limit in our photon recovery process. A finial state electron/positron may emit a photon, changing its energy/momentum significantly. We are certainly able to fix FSR, but we will have to face another trade off.