D 0 - D 0 Mixing at B A B AR Amir Rahimi The Ohio State University For B A B AR Collaboration

A. RahimiCharm20062 Outline –Introduction to mixing –Motivation for using this mode –Mixing formalism in a multibody decay –Lifetime fit and mixing results D 0 -D 0 Mixing with

A. RahimiCharm20063 Introduction The mass eigenstates and have different masses and lifetimes Signs of new physics: –Observation of CP violation – Time integrated mixing rate:

A. RahimiCharm20064 Decay Mode Obtain a pure sample by reconstructing: In cc-bar events –Obtain the flavor of D0/D0bar from charge of  slow –Right-sign Cabibo-favored (CF) D 0  K -  +  0 used for normalization –Wrong-sign D 0  K +  -  0 has contributions from doubly Cabibo- suppressed (DCS) decays and CF mixed decays –Separate signal from background by fitting to m(K  0 ) and  M = m(K  0  s ) – m(K  0 )

A. RahimiCharm20065 Branching Ratio In non-leptonic search for D mixing, DCS obscure the signs of mixing: –Consider –Belle –B A B A R –Compare to the standard decay D 0  K +  - preliminary hep-ex/

A. RahimiCharm20066 Resonance Contributions The resonance amplitudes are different for DCS and CF- there is more sensitivity to mixing –In D 0  K -  +  0 the main resonance is K -  + –In D 0  K +  -  0 the main resonance is K* +  - u u u uu u s d  +,  + -, --, -         s c c d ss W + CF DCS

A. RahimiCharm20067 Event-level tagging To do a Dalitz analysis need to reduce the large peaking background in DCS –Real D 0 ’s with uncorrelated slow pions Use an event-level tag Require a second tag in the opposite event hemisphere –Use K +,  ± s, e ±, and  ±, in the other side of the event –Provides consistency check on   tag K + tag  00  + tag D0D0 D0D0  - tag interaction point beamspot  - tag e - tag K-K-

A. RahimiCharm20068 Event-Level Tagging Using an event-level tag significantly reduces background –Use K +,  ± s, e ±, and  ±, in the other side of the event –Never done before in this type of analysis 

A. RahimiCharm20069 Event-Level Tagging With this second tag, we can now look at the resonance contributions 

A. RahimiCharm Resonance Contributions Event-level tagged Prominent K* peak in DCS Mode D 0  K -  +  0 D 0  K +  -  0

A. RahimiCharm Resonance Contributions Event-level tagged Prominent  peak in CF Mode D 0  K -  +  0 D 0  K +  -  0 

A. RahimiCharm Selection of Phase-Space Regions Based on inspection of the Dalitz plots, we exclude events in the regions: –850 < m(K  ) < 950 MeV/c 2 –850 < m   ) < 950 MeV/c 2 WS (3.8 ± 0.36) x 10 2 (3.79 ±0.36) x 10 2 RS (2.518 ±0.006) x 10 5 (2.512 ±0.006) x 10 5 WS (7.5 ± 0.5) x 10 2 (8.1 ±0.5) x 10 2 RS (3.648 ± 0.007) x 10 5 (3.646 ± 0.006) x 10 5 (a) (b) D 0 Cand. (a)entire allowed phase-space region (b)selected phase-space region for mixing analysis Preliminary

A. RahimiCharm Decay Time with Mixing At any particular point in phase space (Dalitz Plot): Integrating over an arbitrary region of phase space:  is the suppression factor)

A. RahimiCharm Consider CP Violation We account for possible CP violation by fitting D 0 and D 0 separately and making the substitutions: (  and  are the suppression factors)

A. RahimiCharm PDF Fit to Decay Times Data after a statistical background subtraction Decay times in a signal region

A. RahimiCharm Preliminary Mixing Results CP Conserved R M < 0.054% upper limit at 95% confidence level (determined using  log L ) Consistent with no mixing at 4.5% confidence level (determined using a frequentist method) Contours determined using  log L levels hep-ex/

A. RahimiCharm Preliminary Mixing Results CP Violation Allowed Contours determined using  log L levels

A. RahimiCharm Summary and Outlook Performed the first analysis of D 0  K -  +  0 Uncovered the DCS Dalitz plot Time-dependent Dalitz plot analysis of this mode is underway Additional B A B AR mixing results coming up soon: –Semi-leptonic mixing using doubly-tag analysis – D 0  K -   – D 0  K -       An observation of D mixing may be on the horizon

A. RahimiCharm Charm Mixing in The Standard Model Box Diagram SM Charm Mixing is expected to be very low Long distance SM predictions accommodate higher rates d, s, b V* ci V ui V uj V* cj D0D0 D0D0 c u W d, s, b W c u SM Mixing: box diagram SM Mixing: a long-range contribution D0D0 D0D0 c u c u u W+W+ d d u   u d d u W-W- (Plot courtesy of A. Petrov, hep/ph ) : x=  M/  : y=  /2  mixing rate = |amplitude| 2 SM Mixing Predictions

A. RahimiCharm Fit to the CF Events ML Fit and data projected in signal regions <  M < GeV/c < m(Kpp0)< 1.88 GeV/c 2

A. RahimiCharm Upper Limit on R M We use  log L to set an upper limit –Behavior near zero consistent with a frequentist method –Straight forward to compare with other experiments