Search for D 0 D 0 Mixing at BESIII

1. Search for D0D0 Mixing at BESIII Jianyong Zhang, Yongzhao Sun, Kanglin He IHEP Jan. 11, 2006

2. Outline • Motivation • Single Tag of Charmed D Meson • D0D0 Mixing • Summary

3. Motivation • Charm physics at threshold D mesons pair produced at 3.773GeV low background • Take advantage of BEPCII and BESIII large data sample better mass resolution and particle identification • BESIII Offline Software test

4. Single tag • Single tagged event selection is basic for absolute branching ratios, semileptonic and pure leptonic decay • Single tagged event selection algorithm as a package

5. Single tagevent selection criteria • Vertex cut • Good charged track cut • Good K0S cut • Good photon cut and good π0 cut • Kaon pion identified

6. Single tag neutral D (I) Inv. Mass distribution D0->K-π+ D0->K-π+π+π- D0->K-π+π0 D0->K0Sπ+π+

7. Single tag neutral D(II) Inv. Mass distribution D0->K0Sπ0 400K DD sample Neutral D sigma: Without π0 decay modes: 1.2－1.3 MeV With π0 decay modes: 1.6－2.4 MeV D0->K0Sπ+π+π0 MDC reconstruction version based on Belle

8. Single tag neutral D(III) D0 Kπ Minv:1.863 GeV σ: 5.2MeV Without kinematic fit Cut: 1.8065<Minv<1.8865GeV 400K DD sample Inv. Mass

9. Single tag charged D (I) Inv. Mass distribution D+->K0Sπ+ D+->K-π+π+ D+->K0Sπ+π+π- D+->K0Sπ+π0

10. Single tag charged D(II) Inv. Mass distribution 400K DD sample Charged D sigma: Without π0 decay modes: 1.2－1.3 MeV With π0 decay modes: 1.6－1.7 MeV D+->K-π+π+π0 MDC reconstruction version are based on Belle

11. Single tag charged D(III) D+ K-π+π+ Minv: 1.867GeV σ: 3.5MeV Without kinematic fit Cut: 1.8065<Minv<1.8865GeV 400K DD sample Inv. Mass

12. D0D0 mixing • In standard model, D0D0 mixing are suppressed by CKM and GIM factors. A measurable signal would be a strong hint of new phisics. • D0－>K+π- and D0－>K-π+ are called wrong sign(WS) decay. D0－>K-π+ and D0－>K+π-are called right sign(RS) decay.

13. D0D0 mixing introduction D0,D0 not mass eigenstates but flavor eigenstates Two eigenstates D01 and D02: , their masses M1, M2 andwidths Γ1, Γ2 Two path to WS final state: D0－>K+π- (DCS) D0－>D0－>K+π- (MIX) mixing D0 DCS DCS impossible : D0->Kπand D0->Kπrequire L even For Ψ(3770) decay, L = 1 D0 K+π- CF

14. Mixing Phenomenology Two contributions to D0 D0 mixing:

15. D0D0 mixing in experiment 100K K-π+ vsK+π- sample π misidentified as K K, π misidentified as each other K misidentified as π ΔE distribution

16. ΔE and Ratio cut ΔE = |ED – E beam| Ratio>0.9 ΔE< 20MeV Ratio distribution Ratio(K/π) = ΔE distribution

17. rD estimate Mixing tag Cut: ΔE<20MeV Ratio>0.9 ~1 background event is expected ~18K Normal tags are expected in 15fb-1Ψ(3770) data, supposing no mixing event is observed, 1-2 background events will lead to rD < 2.5x10-4 @90% C.L. Normal tag Efficiency: 12.5% Inv. Mass

18. Direct CP Violation at ψ(3770) Suppose Both D0 decay to CP eigenstate f1 and f2 . Any oberservations of CP(f1)=CP(f2) at ψ(3770) are the direct evidence of CP Violation

19. CP violation Sample:100K D0->K K, ππ D0->K K, ππmixed as B.F. ~200 (KK vs KK and ππ vs π π) Events will be expected in 15fb-1 Ψ(3770) data. if CP is totally violated, It leads to ACP < 10-2 @90% C.L. Cut: ΔE<20MeV ππ vs ππ K K vs K K Inv. Mass Distribution

20. D0D0 mixing challenge • Big challenge to PID (Kπchannel) • Main backgrounds come from the double mis-PID • Searching in semi-leptonic decay modes are experimental difficulty with 2 missing neutrino (hard to reduce background contribution to 10-4)

21. Summary • BESIII offline software works smoothly • 6 neutral D and 5 charged D Single tagged channels are searched and will be packaged • D0D0 mixing are studied. rDis sensitive to about 10-4 for mixing • CP violation are studied. ACPis sensitive to about 10-2 for CP violation

22. THANK YOU