1. B s Mixing Results for Semileptonic Decays at CDF Vivek Tiwari Carnegie Mellon University on behalf of the CDF Collaboration

2. 2 B Meson Flavor Oscillations Neutral B mesons can oscillate into their corresponding antiparticles via 2nd order weak interactions, dominated by the exchange of a top quark Several theoretical uncertainties cancel in the ratio New Physics may affect  m s /  m d  New particles in the loop V ts  = 1.210 +0.047 -0.035 (hep/lat-0510113)

3. 3 Neutral B Oscillations Neutral B Meson system Mixture of two mass eigenstates: B H and B L may have different mass and decay width   m = m H – m L   =  H -  L In case of  = 0

4. 4 B Physics at the Tevatron All B hadrons produced:  B +, B d, B s, B c,  b … Large B cross section  Tevatron:  B Factories: However, the total inelastic cross section,  (total  is more than 1000 times bigger  Need to select B events with high purity  It’s all about triggers at hadron colliders

5. 5 Tevatron Performance Delivered luminosity ~ 2.0 fb -1 (~1.6 fb -1 on tape) Mixing measurements at CDF use ~ 1.0 fb -1 Tevatron regularly making new records  Peak initial luminosity ~2.3 x 10 32 sec -1 cm -2  Record weekly integrated luminosity ~ 33 pb -1 Delivered : 1983 pb -1 Collected : 1606 pb -1 Used in this analysis

6. 6 The CDF II Detector Excellent momentum resolution  (p)/p<0.1% Large B yields:  High rate trigger/DAQ Particle Identification:  TOF, dE/dX in COT  Calorimeter & muon chambers Proper time resolution  Silicon detectors: SVXII, L00

7. 7 B Physics Triggers at CDF Conventional di-muon (J/  ) trigger  p T (  ) > 1.5 GeV  Samples used for flavor tagging studies Lepton + displaced track (SVT)  Lepton = e,  with p T > 4.0 GeV  p T > 2 GeV displaced track (120  m < I.P. (track) < 1mm)  Large semileptonic samples for mixing and flavor tagging studies Two displaced tracks  Two p T > 2 GeV SVT tracks  Provides access to hadronic decays and large semileptonic samples with lower p T leptons

8. 8 Overview of the Measurement “same” side “opposite” side Reconstruct B s decays (determine decay flavor from decay products) Measure proper decay time of the B s mesons Infer B s flavor at production (flavor tagging) e,  e+e+ LTLT LTLT

9. 9  m s Measurement Significance B s mesons mix much faster than B d The measured asymmetry is diluted by mistags, since the initial state flavor is not perfectly known  Oscillation Amplitude: D=1-2w, w = mistag probability Signal/Background Proper time resolution Effective tagging power (  =tagging efficiency) Moser, Roussarie, NIM A384 (1997)

10. 10 Particle Identification at CDF Lepton Identification  Combine variables into a global likelihood to discriminate against fake leptons  Electron: Calorimeter, shower & pre-shower quantities and dE/dx  Muon: Track-muon matching and calorimeter variables Likelihood based id is used for semileptonic signal selection as well as opposite side flavor tagging

11. 11 Particle Identification at CDF (contd.) Charged Kaon Identification  Combine information from dE/dx and TOF  dE/dx provides ~ 1.5  separation for p > 2.0 GeV tracks with 100% efficiency  TOF provides ~ 2.0  separation for p < 1.5 GeV tracks with 60% efficiency Used for B s signal selection and in the same side & opposite side kaon tagging algorithms

12. 12 B s Signal Reconstruction in Semileptonic Decays Semileptonic B s decays  B s  l D s X reconstructed in three final D s states: D s   / K*K /   l = e,  collected via the two- track and l +SVT triggers  Characterized by large branching ratios  Incomplete reconstruction (missing neutrino and other neutral particles)

13. 13 B s Signal Reconstruction (contd.) In D s   (  K  K  ) & D s  K*K (K*  K    ) modes, kaon identification is used  Helps suppress combinatorial background composed largely of pions  Helps reduce reflection from D   K       in D s  K*K mode Physics backgrounds contamination ~ 20-25%  Depends on lepton momentum  Split sample into cases when lepton is a trigger track Total B s  l D s X signal yield is 61,500 l D s : D s   29.6 K l D s : D s  K*K 22.0 K l D s : D s   9.9 K

14. 14 B s Signal Reconstruction (contd.) Mass ( l -D s ) distribution  Helps discriminate against physics, fake lepton & combinatorial background Obtain estimate of fake lepton background ~ 5-10% Mass ( l -D s ) distribution for fake leptons obtained via anti-selection on lepton likelihood  Quantifies missing momentum for signal B s  l D s X candidates Crucial for maintaining sensitivity at higher values of  m s

15. 15 Proper Decay Time Reconstruction Trigger distorts decay time distribution  Correct using efficiency function obtained from trigger simulation on Monte Carlo Missing decay products  Correct statistically using a missing momentum factor (k-factor) where distribution of is obtained from Monte Carlo “ Trigger” turnon pattern limit |d 0 | < 1 mm

16. 16 Proper Decay Time Resolution Excellent decay time resolution critical for sensitivity at high  m s Sensitivity in semileptonic decays is driven by low decay time or high Mass( l -D s ) candidates  Variation of k-factor with Mass( l -D s ) significantly improves decay time resolution  Exploited by using Mass( l -D s ) directly in the fit.  ct determined directly from data  Event-by-event  ct is used taking into account dependence on kinematical variables like isolation, opening angle as well as vertex   (more details in Jeff Miles’ talk) Osc Freq 18 ps -1

17. 17 B Flavor Tagging (Opposite Side) b quarks produced in pairs: use the other B to infer production flavor  Lepton (e/  Tagging: Semileptonic decay of OS B (high purity/low efficiency)  Kaon Tagging: Kaon from OS b  c  s transition (medium purity/medium efficiency)  Jet Charge Tagging: Weighted sum of fragmentation and decay products of OS B (low purity/high efficiency) Issues  OS B not always in acceptance  OS B mixing diminishes tagging performance

18. 18 B Flavor Tagging (Opposite Side contd.) Combine tagging algorithms using a Neural Net  Use dependence of expected tag purity on particle-id / kinematical variables Apply the combined tagging algorithm on samples of B + and B d decays  Calibrate expected dilution  Cross-check of the complicated unbinned maximum likelihood fit framework Combined tag  Measured value of  m d consistent with PDG

19. 19 Charge of closest fragmentation track correlated to B production flavor  Superior to OS tagging due to better acceptance and doesn’t suffer from OS mixing SSKT performance cannot be determined from B s data  Rely on Pythia MC prediction Tagging track identification based on a NN combination of kaon probability and kinematical variables SSKT B Flavor Tagging (Same Side Kaon Tagging)

20. 20 Fourier Analysis Technique Two domains to fit for oscillations:  Time: fit for cosine wave  Frequency: examine spectrum Time Domain Approach  Fit for  m s in p(t)~(1 ± D cos  m s t)  Good for measuring  m s Frequency Domain Approach  Fit for A(  m s ) in p(t)~(1 ± A D cos  m s t)  A = 1 for true  m s, else A=0  Good for exclusion, combining measurements Moser, Roussarie, NIM A384 (1997)

21. 21 Semileptonic Amplitude Scan Combined sensitivity on 1 fb -1 : 19.3 ps -1 Amplitude is consistent with unity ~17.8 ps -1 (A/    Points: A±  (A) from likelihood fit for different  m s Green band: A±1.645  (A) Dashed line: 1.645  (A) as function of  m s Measurement sensitivity: 1.645  (A) = 1

22. 22 Likelihood Profile Evidence of oscillations  Likelihood global minima at  m s = 17.9 ps -1 Strict Gaussian interpretation of the minima is not possible but ±1  around the minima gives an error on  m s ~ 0.3 ps -1 Can also set a 95% double bound:  m s  [16.9,19.5]

23. 23 Conclusions World’s best sensitivity in B s semileptonic decays =19.3 ps -1 Evidence of oscillations at  m s = 17.9 ps -1 95% double bound:  m s  [16.9,19.5] Details on mixing in hadronic decays at CDF and combination with semileptonic decays: see Jeff Miles’ talk

24. 24 Slides for Reference

25. 25 Systematic Uncertainties