1. Search for D 0 D 0 Mixing and Direct CP Violation in 2-body Hadronic Decays Y.Z. Sun, J.Y. Zhang IHEP Mar. 08, 2006

2. Outline D 0 D 0 Mixing –In SM, Mixing is very small (10 -6 ) –BESIII is sensitive to 10 -4 Direct CP Violation –SM predicts the A cp may be as big as 10 -3 –BESIII is sensitive up to 10 -2 Experimental search at Ψ(3770) –Challenge to detector and Accelerator performance –Good place to look for new physics Summary

3. D 0 D 0 Mixing D 0 decays as D 0 Key Point: Separate Mixing from DCSD

4. Mixing Phenomenology(1) D 0,D 0 not mass eigenstates but flavor eigenstates Two eigenstates D 0 1 and D 0 2 : their masses M 1, M 2 and widths Γ 1, Γ 2 Like the K 0 K 0 mixing, constructing D S and D L

5. Mixing Phenomenology(2) DCS Mixing CF Measuring the Asymmetry of CP eigen state (K + K - (+), K s ρ 0 (-) …) Supposing CP violation is small Possible to measure the phase shift

6. Experimental Situation x 0 0.1 0.2 0 -0.1 0.1 y x and y are in the orders of 10 -1 ─10 -2

7. Separate Mixing from DCSD D0D0 D0D0 K+π-K+π- DCSD mixing CF Two path to D 0  D 0 decay final state: D 0 － >K + π - (DCSD) D 0 － >D 0 － >K + π - (MIX) DCSD impossible : D 0  Kπ and D 0  Kπ require L even For Ψ(3770) decay, L = 1

8. Experimental Approach 2-body hadronic channel –Kπ vs Kπ –Big challenge to PID Quasi 2-body hadronic channel –K - ρ + vs K - ρ +, K *- π + vs K *- π +, K *0 π 0 vs K *0 π 0 –D 0  K - π + π 0 Dalitz Analysis (Both D together) –Sensitivity need detail study Semi-leptonic channel –Klν vs Klv, l= e, μ –Hard to reduce background Kπ channel is selected in this analysis

9. Analysis Technique Main background source –KK, π π channel –Double miss identification: K  π, π  K Kinematic information –P distribution –Δ E distribution –Kinematic Fit Particle Identification –Barrel TOF pid

10. D 0  K π sample ΔE Distribution K misidentified as π π misidentified as K K, π double misidentified

11. Big Challenge to PID(I) Learn from Δ E distribution –Background from KK,ππ can be removed easily –ΔE CUT can not totally remove the background due to double mis-PID PKPK PπPπ D0KπD0Kπ D0KKD0KK D 0  π π

12. Big Challenge to PID(II) K,π momentum distributes between 0.65-1.05 GeV –dE/dx is hard to separate –Barrel TOF has best K/π separation at BESIII R R>0.9 R>0.9 cut: 1)Lost ~20% eff 2)Remove >99% bg

13. Results Events pass ΔE cut and PID cut are subjected to 5C kinematicfit with additional equal mass constraint ~12.5% efficiency and ~1 background event from 100K Monte Carlo sample M Kπ (GeV) Mixing tag ~1 background event is expected Normal tag Efficiency:12.5%

14. Direct CP Violation at ψ(3770) Suppose Both D 0 decay to CP eigenstate f 1 and f 2. Thus if a final state such as (KK)( ππ ) observed, we immediately have evidence of CP violation

15. CP eigen states CP + –K + K - (3.89X10 -3 ) –π + π - (1.38X10 -3 ) –ρ 0 π 0 (<0.011) –π 0 π 0 (8.4X10 -4 ) –K S K S (7.1X10 -4 ) CP – –K S π 0 (0.023) –K s η (7.7X10 -3 ) –K S ρ 0 (0.0155) –K s ω (0.024) –K S φ (9.4X10 -3 ) –K S η’ (0.0188) D 0  K + K -, π + π - are selected to search for Direct CP Violation

16. KK vs KK and ππ vs ππ mode Same procedure as Mixing analysis M KK M ππ 100K D 0 (KK, ππ) vs D 0 (KK, ππ) Monte Carlo sample KK and ππ are mixed according to BF

17. KK vs ππ Mode Contamination due to the cross talk from Kπ vs Kπ mode Additional cuts on χ 2 (KKππ)<χ 2 (KπKπ) χ 2 (KKππ)-χ 2 (KπKπ) MDMD KK vs ππ Kπ vs Kπ Analysis maybe need improving

18. Summary Data sample –15fb -1  5.8x10 7 D 0 D 0 pair D 0 D 0 Mixing –1~2 background events leads to –r D < (3-4)x10 -4 @ 90%C.L. Direct CP Violation –~120 events will be expected in KK vs KK and ππ vs ππ modes if CP is violated completely –~100 events will be expected in KK vs ππ mode if CP is violated completely –A CP < 10 -2 @90% C.L. Further plan –Study other interesting channels –Apply new analysis technique Analysis based on BOSS 5.0.0, results may be updated in future