1. CHARM MIXING and lifetimes on behalf of the BaBar Collaboration XXXVIIth Rencontres de Moriond  March 11th, 2002 at Search for lifetime differences in D 0 decays New BaBar limits on D 0 mixing parameter y: - method and event selection - lifetime extraction, systematics and results Alexis POMPILI Università and I.N.F.N. Bari

2. 2 MIXING PARAMETERS : ( flavour eigenstates ) Mass eigenstates: Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond INTRODUCTION: x, y masses: M 1, M 2 & widths:  1,  2 In the SM : …but … New Physics can significantly enhance x FSI and SU(3) breaking can enhance y Current sensitivy @ level of 10 -2 (few 10 -3 with 0.5ab –1 @ Belle & BaBar ) 3 types of experiments are sensitive to a combination of x & y : Wrong-sign searches : LIFETIME DIFFERENCE searches HADRONIC Semileptonic

3. 3 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Lifetime difference search strategy Rate asymmetry for neutral D decays into CP + (even) and CP  (odd) eigenstates : K  K + (    + ) is CP + eigenstate & K   + assumed an equal mixture of CP +, CP  NO CP CP in interference mixing-decay CP in mixing Extracting the rates by fitting to pure exp. the time-dependent rates:

4. 4 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond D 0 proper time reconstruction method Reconstructed DECAY CHAIN TAG 3 independent samples 1) allows flavor-tagging 2) reduces BKG  m = m(h + h   s ) - m(h + h  ) D * is primary vertex : use BS  constraint Refitting technique with BS  constraint  D in xy improves  m resolution DECAY LENGTH Beamspot

5. 5 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond D 0 candidates selection criteria REJECTION of D* from B decays: GOOD VERTEX FIT for BOTH D 0 and D* vertices : GOOD QUALITY TRACKS & VERTEXING RESOLUTION ( at least 6 SVT  hits for  s ) ONLY PARTICLE IDENTIFICATION applied on BOTH D 0 DAUGHTERS K : tight likelihood selection  : likelihood selection & muon veto REJECTION of combinatorial BKG due to low p pions : helicity-cut on  m -cut : ( pion contamination: <3% for p<3GeV/c ) This analysis uses 57.8 fb -1 (2000+2001 data) & selects events from continuum It uses also simulated (G4) data : Tracking [ DCH+SVT ], vertexing [ SVT ], PID [ DIRC +( DCH+SVT )] are crucial

6. 6 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond m(h+h)m(h+h) m=m(h+hs)-m(h+h)m=m(h+hs)-m(h+h) K+K+ ++ K+K+ Selected D 0 candidates after  m-cut K-+K-+ K-+K-+

7. 7 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Unbinned maximum likelihood fit [1  P sig (m i )] P sig (m i ) SIGNAL BKG proper-time RESOLUTION (1-f 2 )(1-f 3 )+ f 2 (1-f 3 )+ f 3 S1S1 S2S2 33 f0f4f0f4  4 fixed (1-f 0 ) + f 0 (1-f 4 )  NO - flying BKG flying BKG BUT with... for tails S BKG S 1 S BKG S 2 33  BKG DD

8. 8 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond K+K+ ++ K+K+ D 0 candidate signal probability in the lifetime fit Portion assigned to BKG LOW MASS TAIL BKG constrained in data “sidebands”candidates as part of likelihood fit K-+K-+ K-+K-+

9. 9 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Proper time distributions and lifetime fit results K+K+ K+K+ ++ Points : DATA |White histogram : FIT RESULT |Gray histogram : BKG fit portion

10. 10 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Lifetime ratio systematics - I D 0 lifetime measurement is an HIGH PRECISION measurement [ statistical uncertainty for our data sample is about 1.3fs namely 3/1000 ] At this level of precision wide studies of systematics sources (tracking, vertexing, alignment) are needed. At this stage we do not present an absolute D 0 lifetime value because the overall systematic uncertainty (about 3  4fs) can be reduced with further work Note:  1.5  m average bias in vertex position translates in 3fs lifetime bias  1MeV mass shift due to momentum bias means 0.3fs lifetime bias We checked, on a limited sub-sample, the full compatibility of the fit result with both our HF2001 check and PDG value Many systematics effects on lifetime cancel in the lifetime RATIO

11. 11 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Lifetime ratio systematics - II y systematics uncertainties calculated using large signal MC samples Simulated data has been modified by variation reflecting uncertainties in… - detector biases - event selection criteria - size and composition of BKG contribution - beamspot position and size SVT internal alignment systematic checks done using MC estimation shows about null bias within its statistical error which is taken as a fur- ther systematic uncertainty [zero lifetime control sample]

12. 12 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond RESULTS: limits on y PRELIMINARY EPS-2001

13. 13 Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond CONCLUSIONS AND NEAR TERM PROSPECTS Limits on x, y from hadronic wrong-sign analysis coming soon ! Our preliminary y measurement is consistent with zero but suggest a positive value not incompatible with FOCUS measurement It is crucial to get a new measurement over LARGER DATA SAMPLES It is important to get a result from D*  untagged KK sample : the statistical uncertainty is <1% but still working on systematics

14. BACKUP TRANSPARENCIES (NOT SHOWN)

15. 1+ Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Wrong-sign hadronic search strategy Look for deviation from exp in the time evolution of neutral D wrong-sign decay : x’x’ y’y’ DCSDINTERFERENCEMIXING HF9  : strong phase diff. between DCSD & CF D*+D*+ D 0 (cu)  + D0D0 MIXING CFD K+K+ DCSD NO CP Type of experiment sensitive to ( x cos  + y sin  ) cos , ( y cos  - x sin  ) sin , x 2 + y 2

16. 2+ Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Kaon ID performance

17. 3+ Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond Low mass tail MC study: possible explanation is internal bremsstrahlung Simulation by randomly selecting 5% of D 0 candidates for which one daughter’s momentum is corrected by the factor [1 -( E  /m D )] where energy E  is properly randomly generated.

18. 4+ Alexis Pompili (Univ. & INFN Bari) XXXVIIth Rencontres de Moriond MC check : lifetime pull MC indicates very little or null bias The resolution model is found to fit reasonably well to the MC (measured – true) proper time (meas. – true) pr. time / pr. time error