Photon Selection Algorithm Ming Yang ， Mingshui Chen BESIII Meeting

Contents Photon Spatial resolution of photons Selection of good photons Detection efficiencies of photons π 0 / η reconstruction Some physics analysis J/  γ + X, X  0  0 J/  γ η’ Summary

We use howl to generate 1000 events with photon’s momentum ranged from 0.2 GeV to 3.2 GeV with fine step at 0.2 GeV. In this channel, the two photons are back to back in direction.  (  ) and  (  ) are the spatial resolution Spatial resolution of photons radian Sample: e + e -  2 γ  (  ) distribution vs. energy  (  ) distribution vs. energy

Selection of good photons Eliminate the fake photons associated with charged tracks Eliminate the fake photons split from other photons with higher energy Reduce the noises

In this channel, the detected photons are called fake photons. In the above plots,the   and  are the delta angle between the detected photons and the extrapolated track directions Fake photons from charged tracks     20 o  /  between Ext and Emc shower the concentrating zone is(   <20&&  <20) Sample:J/  4 

Fake photons from other photons 7o7o Angle between γandγ Sample:J/  π 0 + ρ 0 In this channel, the detected true photons are from π 0, we get the delta angle between any two photons. As show in the above plot, we can see that the fake photons could be easily eliminated from the true photons by setting the cut of delta angle = 7 o degrees events

Detection efficiencies of photons with low E Energy threshold of photon = 40MeV E γ >20MeV E γ >30MeV E γ >40MeV E γ >50MeV E γ >60MeV E γ >70MeV Events GeV Detection efficiencies of photons with low energy in the channel J/  ρ 0  0. Let the threshold equal different energy to get the different efficiencies plots. In the right plots, the curves had been made unitarily.

GoodPhoton selection efficiencies We use tester to generate 10k single photon events with momentum ranged from 0.1 GeV to 2.0 GeV with fine step at 0.1 GeV Then let the photon energy threshold equal 10,20,30,40MeV separately, to see the photon reconstruction efficiencies and the noises level

Noises at different E threshold E>10MeV noises ~ 6% E>30MeV noises ~ 0.6% E>20MeV noises ~ 1.5% E>40MeV noises ~ 0.3%

Selection efficiencies of photons E>10MeV efficiencies ~ 99% E>30MeV efficiencies ~ 99% E>20MeV efficiencies ~ 99% E>40MeV efficiencies ~ 99%

π 0 reconstruction Selection Criteria Good Photon Selection  and  >20 o (between Photon and Charged Track)  (Angle) >7 o (between two Photons) Energy Threshold = 40 MeV π 0 Selection cosθ< 0.98 ( π 0 decay angle) chisq < 20.0 (Chisq of mass constraint fit) masswindow of π 0 is (0.07,0.18)GeV

Reconstruction Efficiencies of π 0 We use tester to generate 10k single π 0 events with momentum ranged from 0.1 GeV to 2.0 GeV with fine step at 0.1 GeV. The reconstruction efficiencies are greater than 50%.

Invariant mass of π 0 (inclusive) Sample: J/  ρ 0  0 In the above plots, we get the inclusive  0 ‘s invariant mass spectrum (before 4C fit ), with its fit by ( Crystal ball + gauss + poly ). No 4C fit  0 mass = / GeV/c 2  0 width = / GeV/c 2

J /  γ X, X  0  0 Invariant mass spectrum of X Events GeV In the generator of Howl, we set the resonance’s mass = 1.270GeV and its width = 0. As show in the above plot, the width of X is about 13 MeV. Mass of X = 1.270GeV

Run 50k events of J/  γη’, then we got the number of events withη’ of each decay mode ， η’ are reconstructed as following plots: J /  γη ’ η’->2  0 + η, η ->2 γ η’->2  0 + η, η ->3  0 η’->2  0 + η, η ->  + +  _ +  0 η’->  + +  _ +η,η ->2γ η’->  + +  _ +η,η ->3  0 η’->  + +  _ +η,η ->  + +  _ +  0 η’->γ + ρ 0, ρ 0 ->  + +  _

Invariant mass spectrum of η ’ η’->γ + ρ 0, ρ 0 ->  + +  _ η’->2  0 + η, η ->2 γ η’->  + +  _ +η,η ->2γ η’of all decay modes

Invariant mass of η ’ (cont.) η’->  + +  _ +η,η ->3  0 η’->  + +  _ +η,η ->  + +  _ +  0 η’->2  0 + η, η ->  + +  _ +  0 η’->2  0 + η, η ->3  0

J /  γη ’ from J /  Anything As show in the above plot, in 200k events of J/  Anything, we can only get the η’ of ( η’->  + +  _ +η,η ->2γ ) and ( η’->γ + ρ 0, ρ 0 ->  + +  _ ). η’->  + +  _ +η,η ->2γ η’->γ + ρ 0, ρ 0 ->  + +  _

Invariant mass spectrum of η ’ η’of all decay modes η’->γ + ρ 0, ρ 0 ->  + +  _ η’->  + +  _ +η,η ->2γ

Summary Photon Selection Criteria  and  >20 o (between Photon and Charged Track)  (Angle) >7 o (between two Photons) Energy Threshold = 40 MeV In η’ channel, the above two decay modes have best efficiencies η’->γ + ρ 0, ρ 0 ->  + +  _ η’->  + +  _ +η,η ->2γ Many work should be done in the future Thank you !