Photon Selection Algorithm Ming Yang Mingshui Chen BESIII

Photon Selection Algorithm Ming Yang ， Mingshui Chen BESIII Meeting 2006. 01

Contents n n n Photon Spatial resolution of photons Selection of good photons Detection efficiencies of photons π0/η reconstruction Some physics analysis 0 J/ γ + X, X J/ γ η’ n Summary 0

Spatial resolution of photons radian Sample: ( ) distribution vs. energy e+e- 2 γ We use howl to generate 1000 events with photon’s momentum ranged from 0. 2 Ge. V to 3. 2 Ge. V with fine step at 0. 2 Ge. V. In this channel, the two photons are back to back in direction. ( ) distribution vs. energy ( ) and ( ) are the spatial resolution

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

Fake photons from charged tracks / between Ext and Emc shower 20 o Sample: J/ 4 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 20 o the concentrating zone is( <20&& <20)

Fake photons from other photons events Angle between γandγ 7 o degrees Sample: J/ π0 + ρ0 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 = 7 o

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>20 Me. V Eγ>30 Me. V Eγ>40 Me. V Energy threshold of photon = 40 Me. V Ge. V Eγ>50 Me. V Eγ>60 Me. V Eγ>70 Me. V In the right plots, the curves had been made unitarily. Ge. V

Good. Photon selection efficiencies n n We use tester to generate 10 k single photon events with momentum ranged from 0. 1 Ge. V to 2. 0 Ge. V with fine step at 0. 1 Ge. V Then let the photon energy threshold equal 10, 20, 30, 40 Me. V separately, to see the photon reconstruction efficiencies and the noises level

Noises at different E threshold E>10 Me. V noises ~ 6% E>20 Me. V noises ~ 1. 5% E>30 Me. V noises ~ 0. 6% E>40 Me. V noises ~ 0. 3%

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

π0 reconstruction Selection Criteria Good Photon Selection and >20 o(between Photon and Charged Track) n (Angle) >7 o(between two Photons) Energy Threshold = 40 Me. V π0 Selection cosθ< 0. 98 (π0 decay angle) chisq < 20. 0 (Chisq of mass constraint fit) masswindow of π0 is (0. 07, 0. 18)Ge. V

Reconstruction Efficiencies of π0 We use tester to generate 10 k single π0 events with momentum ranged from 0. 1 Ge. V to 2. 0 Ge. V with fine step at 0. 1 Ge. V. The reconstruction efficiencies are greater than 50%.

Invariant mass ofπ0 (inclusive) Sample: J/ ρ 0 0 0 mass = 0. 135354+/-0. 00081 Ge. V/c 2 0 width = 0. 007247+/-0. 00084 Ge. V/c 2 No 4 C fit In the above plots, we get the inclusive 0 ‘s invariant mass spectrum (before 4 C fit ), with its fit by ( Crystal ball + gauss + poly ).

J/ γ X, X 0 0 Invariant mass spectrum of X Events Mass of X = 1. 270 Ge. V In the generator of Howl, we set the resonance’s mass = 1. 270 Ge. V and its width = 0. As show in the above plot, the width of X is about 13 Me. V.

J/ γη’ from J/ Anything η’->γ + ρ0 , ρ0 -> + + _ _ η’-> + + +η , η ->2γ As show in the above plot, in 200 k events of J/ Anything , we can only get the _ _ η’ of (η’-> + + +η , η ->2γ) and (η’->γ + ρ0 , ρ0 -> + + ).

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

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