1 / 25

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.

mdonovan
Télécharger la présentation

B s Mixing Results for Semileptonic Decays at CDF

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related