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

Outline Motivation Single Tag of Charmed D Meson D0D0 Mixing Summary

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

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

Single tag event selection criteria Vertex cut Good charged track cut Good K0S cut Good photon cut and good π0 cut Kaon pion identified

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

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

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

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

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

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

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.

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

Mixing Phenomenology Two contributions to D0 D0 mixing:

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

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

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 < 2.5x10-4 @90% C.L. Normal tag Efficiency: 12.5% Inv. Mass

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

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 < 10-2 @90% C.L. Cut: ΔE<20MeV π π vs π π K K vs K K Inv. Mass Distribution

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)

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

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