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B s Mixing Results for Semileptonic Decays at CDF

B s Mixing Results for Semileptonic Decays at CDF. Vivek Tiwari Carnegie Mellon University on behalf of the CDF Collaboration. V ts. V ts. V ts.  = 1.210 +0.047. -0.035. V ts. B Meson Flavor Oscillations.

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B s Mixing Results for Semileptonic Decays at CDF

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

  2. Vts Vts Vts  = 1.210 +0.047 -0.035 Vts 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 affectDms/Dmd • New particles in the loop (hep/lat-0510113)

  3. Neutral B Meson system Mixture of two mass eigenstates: BH and BL may have different mass and decay width Dm = mH– mL DG = GH - GL In case of DG = 0 Neutral B Oscillations

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

  5. Tevatron Performance Delivered : 1983 pb-1 • 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 1032 sec-1 cm-2 • Record weekly integrated luminosity ~ 33 pb-1 Collected : 1606 pb-1 Used in this analysis

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

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

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

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

  12. Bs Signal Reconstruction in Semileptonic Decays • Semileptonic Bs decays • Bs lDsX reconstructed in three final Ds states: Ds p / K*K / ppp • l = e, m collected via the two-track and l +SVT triggers • Characterized by large branching ratios • Incomplete reconstruction (missing neutrino and other neutral particles)

  13. Bs Signal Reconstruction (contd.) • In Ds p (  K+K-) & Ds K*K (K* K+ p-) modes, kaon identification is used • Helps suppress combinatorial background composed largely of pions • Helps reduce reflection from D- K+ p-p-in Ds K*K mode • Physics backgrounds contamination ~ 20-25% • Depends on lepton momentum • Split sample into cases when lepton is a trigger track • Total Bs lDsX signal yield is 61,500

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

  15. Proper Decay Time Reconstruction “Trigger” turnon pattern limit |d0| < 1 mm • 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

  16. Proper Decay Time Resolution • Excellent decay time resolution critical for sensitivity at high Dms • Sensitivity in semileptonic decays is driven by low decay time or high Mass(l-Ds) candidates • Variation of k-factor with Mass(l-Ds) significantly improves decay time resolution • Exploited by using Mass(l-Ds) directly in the fit. • sct determined directly from data • Event-by-eventsct is used taking into account dependenceon kinematical variableslike isolation, opening angleas well as vertex2 (more details in Jeff Miles’ talk) Osc Freq 18 ps-1

  17. B Flavor Tagging (Opposite Side) • b quarks produced in pairs: use the other B to infer production flavor • Lepton (e/m)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. 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 Bd decays • Calibrate expected dilution • Cross-check of the complicated unbinned maximum likelihood fit framework • Combined tag • Measured value of Dmd consistent with PDG

  19. B Flavor Tagging (Same Side Kaon Tagging) • 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 Bs data • Rely on Pythia MC prediction • Tagging track identification based on a NN combination of kaon probability and kinematical variables • SSKT

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

  21. Semileptonic Amplitude Scan • Points: A±(A) from likelihood fit for different Dms • Green band: A±1.645(A) • Dashed line: 1.645(A) as function ofDms • Measurement sensitivity: 1.645 (A) = 1 • Combined sensitivity on 1 fb-1: 19.3 ps-1 • Amplitude is consistent with unity ~17.8 ps-1 (A/sA ~2)

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

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

  24. Slides for Reference

  25. Systematic Uncertainties

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