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 tag event 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 π 0 decay modes:
1.2－1.3 MeV
With π 0 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 and widths 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. K, π misidentified as each other
D0D0 mixing in experiment
100K K-π+ vs K+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 Cut: ΔE<20MeV Ratio>0.9
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 < 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 Cut: ΔE<20MeV ~200 (KK vs KK and π π vs π π)
Sample:100K D0->K K, ππ
D0->K K, ππmixed as B.F.
~200 (KK vs KK and π π vs π π)
Events will be expected in 15fb-1
Ψ(3770) data. if CP is totally violated,
It leads to ACP < C.L.
Cut: ΔE<20MeV
π π vs π π
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. rD is sensitive to about 10-4 for mixing
CP violation are studied. ACP is sensitive to about 10-2 for CP violation

22. THANK YOU