1. Preliminary Measurement of the BF(   → K -  0  ) using the B A B AR Detector Fabrizio Salvatore Royal Holloway University of London for the B A B AR Collaboration

2. TAU04, September 2004 Fabrizio Salvatore, RHUL 2 Outline Motivation PEP-II and the B A B AR Experiment Event Selection: K -  0 selection and efficiency Systematic errors Preliminary Result Conclusions and Outlook

3. TAU04, September 2004 Fabrizio Salvatore, RHUL 3 Motivation Hadronic  decays provide a clean laboratory for studying the hadronic weak current For decays with overall net strangeness, SU(3) f symmetry breaking can be used to determine the absolute value of the CKM matrix element V us, the strong coupling constant  s and m s The uncertainty in the extraction of |V us | and m s is dominated by the experimental measurement uncertainties The high luminosity provided by PEP-II, coupled with   =0.89 nb at B A B AR energies, provides a high statistics sample to study  – → K -  0  decays

4. TAU04, September 2004 Fabrizio Salvatore, RHUL 4 PEP-II Performance PEP-II delivered ~254 fb -1 BaBar recorded ~244 fb -1 For analysis used 1999-2003 data: On peak112.1 fb -1 Off peak 12.3 fb -1 Total Lumi: L = 124.4 fb -1 Total # of  decays: ~220M PEP-II delivered ~254 fb -1 BaBar recorded ~244 fb -1 For analysis used 1999-2003 data: On peak112.1 fb -1 Off peak 12.3 fb -1 Total Lumi: L = 124.4 fb -1 Total # of  decays: ~220M

5. TAU04, September 2004 Fabrizio Salvatore, RHUL 5 DIRC (PID) 144 quartz bars 11000 PMs 1.5 T solenoid Drift Chamber 40 stereo layers Silicon Vertex Tracker 5 layers, double sided strips e + (3.1 GeV) e - (9.0 GeV) EMC 6580 CsI(Tl) crystals Instrumented Flux Return iron / RPCs (muon / neutral hadrons) The B A B AR Detector

6. TAU04, September 2004 Fabrizio Salvatore, RHUL 6 Event topology Event divided in two hemispheres defined by the plane perpendicular to the thrust vector Only events with one charged track in each hemisphere are selected:  “Signal” hemisphere: track identified as Kaon  “Tag” hemisphere: track identified as electron or muon ++ -- e  + K-K- 00   e   

7. TAU04, September 2004 Fabrizio Salvatore, RHUL 7 Lepton identification and  mis-ID Efficiency and mis-id measured from data:  efficiency: e + e -  and  +  -  sample   mis-ID: D* + →D 0  +, D 0 →K +  -

8. TAU04, September 2004 Fabrizio Salvatore, RHUL 8 Kaon efficiency and  mis-ID Efficiency and mis-ID computed using D * + → D 0  + data sample K +  - Good K/  separation using the information from the DIRC

9. TAU04, September 2004 Fabrizio Salvatore, RHUL 9 Event selection Thrust ≥ 0.9 R 2 = 2 nd /0 th Fox-Wolfram moment ≥ 0.5 Missing momentum ≥ 0.5 GeV/c Events where both charged tracks are identified as electron are rejected Only 1  0 in the event  Only  0 s from two separated EMC energy deposits with E  > 50 MeV;  100 < m  0 < 160 MeV/c 2 reject Bhabha events Angle between K and  0 candidates  K  0 < 1.0 rad Energy in the “signal” hemisphere not associated with the charged track or  0 candidate ≤ 50 MeV reject qq background

10. TAU04, September 2004 Fabrizio Salvatore, RHUL 10 Selected events After all cuts and selections:  21678 events selected in 124.4 fb -1 of data 12377 with e-tag 9301 with  -tag signal MC scaled using the BF in the MC generator: 0.46% Signal Efficiency:  (1.61±0.01) % (0.92±0.01) % e-tag (0.69±0.01) %  -tag very small hadronic background

11. TAU04, September 2004 Fabrizio Salvatore, RHUL 11 Background evaluation Background evaluated using MC events  generic , Bhabha,  +  -, qq and BB samples Main contribution comes from non-signal  decays  for m K  0 ≤ 0.8 GeV/c 2 mainly KK 0  0, K 0   and K  0  0 In particular, BF has large measurement uncertainty for:  BF /BF(  →KK 0  0 ) ~ 13%  BF /BF(  →K  0  0 ) ~ 40%  for m K  0 ≥ 1.0 GeV/c 2 mainly  0 (  BF /BF ~ 0.5%) Total background:  6086 events (e+  tag) 3524 in the e-tagged sample 2562 in the  -tagged sample

12. TAU04, September 2004 Fabrizio Salvatore, RHUL 12 Systematic uncertainties Systematice-tag [%]  -tag [%] Combined [%] Trk. eff.1.40  0 eff. 3.26 Particle ID2.203.502.40 L *   2.30 MC signal stat.0.871.000.63 MC bkgnd stat.0.830.970.63  Bkgnds 0.90 Total5.05.75.0 All numbers are preliminary

13. TAU04, September 2004 Fabrizio Salvatore, RHUL 13 Branching fraction result Datae-tag  -tag Combined Real12377±1119301± 9421678 ± 147 +-+- 3494 ± 592512 ± 506006 ± 78 uds25 ± 548 ± 673 ± 8 ccbar5 ± 22 ± 17 ± 2 BBbar---  sig 0.92±0.01 %0.69±0.01 %1.61±0.01 % Sample B (  - → K -  0  ) [%] e-tag0.436±0.005(stat)±0.022(syst)  -tag 0.442±0.006(stat)±0.025(syst) Combined0.438±0.004(stat)±0.022(syst) Preliminary PDG: (0.45±0.03)%

14. TAU04, September 2004 Fabrizio Salvatore, RHUL 14 Comparison with other results In agreement with PDG average Improved uncertainty

15. TAU04, September 2004 Fabrizio Salvatore, RHUL 15 What next? Measure the branching fraction of the   → K 0  -  decay and combine it with the presented result Study the mass spectrum of   → (K  ) -  Use   → (K  ) -  studies as input into the total strange spectral function   → (K  ) -  branching fraction

16. TAU04, September 2004 Fabrizio Salvatore, RHUL 16 Conclusions A preliminary measurement of the   → K -  0  branching fraction has been obtained using 124.4 fb -1 of data taken at B A B AR : This analysis represents a significant improvement on previously published results Measurement consistent with world average (PDG 2004)    → ( K  ) -  is a clean laboratory for studying hadronic weak currents more results to follow !

17. TAU04, September 2004 Fabrizio Salvatore, RHUL 17 Backup Slides

18.  background systematic Decay Channelw/10 -2  (PDG)/ B (PDG) [%]  - → e- e  0.1590.34  - →  -   0.0200.35  - →  -  0.0620.99  - →  -  0  20.0630.55  - → a 1 -  0.4261.10  - → K-  0.2643.35  - → K 0   0.8214.49  - → 2  -    0.0602.05  - →  - 3  0  0.0039.30  - → K- K+   0.0099.14  - → K- K 0    4.45612.90  - → K-      0.79139.70  - → K-   0.01015.15  - →  -  0  K 0   0.02911.11  - →    0.00313.79  - → K- K 0  0.33810.38 Systematic error due to the uncertainty in the BF of  background decays used in generated MC