Photon Selection Algorithm

1. Photon Selection Algorithm Ming Yang ， Mingshui Chen BESIII Meeting 2006.01

2. Contents • Photon Spatial resolution of photons Selection of good photons Detection efficiencies of photons • π0/ηreconstruction • Some physics analysis J/γ + X, X0 0 J/γη’ • Summary

3. Spatial resolution of photons Sample: e+e-2 γ radian 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 () distribution vs. energy () distribution vs. energy

4. 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

5. Fake photons from charged tracks 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 • / between Ext and Emc shower   20o Sample:J/4 20o  the concentrating zone is(   <20&& <20)

6. Fake photons from other photons events In this channel, the detected true photons are from π0, 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 = 7o Angle between γandγ 7o degrees Sample:J/ π0 + ρ0

7. Detection efficiencies of photons with low E Detection efficiencies of photons with low energy in the channel J/ρ00. Let the threshold equal different energy to get the different efficiencies plots. Events Eγ>20MeV Eγ>30MeV Eγ>40MeV Eγ>50MeV Energy threshold of photon = 40MeV Eγ>60MeV Eγ>70MeV GeV GeV In the right plots, the curves had been made unitarily.

8. 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

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

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

11. π0reconstruction • Selection Criteria Good Photon Selection  and  >20o(between Photon and Charged Track) (Angle) >7o(between two Photons) Energy Threshold = 40 MeV π0 Selection cosθ< 0.98 (π0 decay angle) chisq < 20.0 (Chisq of mass constraint fit) masswindow of π0 is (0.07,0.18)GeV

12. Reconstruction Efficiencies ofπ0 We use tester to generate 10k single π0 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%.

13. Invariant mass ofπ0 (inclusive) Sample: J/ρ00 0 mass = 0.135354+/-0.00081GeV/c2 0 width = 0.007247+/-0.00084GeV/c2 No 4C fit In the above plots, we get the inclusive 0 ‘s invariant mass spectrum (before 4C fit ), with its fit by ( Crystal ball + gauss + poly ).

14. J/γX, X0 0 Invariant mass spectrum of X Events Mass of X = 1.270GeV 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.

15. J/γη’ Run 50k events of J/γη’, then we got the number of events withη’ of each decay mode， η’ are reconstructed as following plots: 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 + ρ0 , ρ0->+ + _

16. Invariant mass spectrum ofη’ η’of all decay modes η’->+ + _ +η ,η ->2γ η’->γ + ρ0 , ρ0->+ + _ η’->2 0 + η ,η->2 γ

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

18. J/γη’ fromJ/ Anything η’->γ + ρ0 , ρ0->+ + _ η’->+ + _ +η ,η ->2γ 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 + ρ0 , ρ0->+ + _).

19. Invariant mass spectrum ofη’ η’of all decay modes η’->γ + ρ0 , ρ0->+ + _ η’->+ + _ +η ,η ->2γ

20. Summary • Photon Selection Criteria  and  >20o(between Photon and Charged Track) (Angle) >7o(between two Photons) Energy Threshold = 40 MeV • In η’ channel , the above two decay modes have best efficiencies η’->γ + ρ0 , ρ0 -> + + _ η’-> + + _ +η ,η ->2γ • Many work should be done in the future Thank you !