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Bs Mixing at D0 Tania Moulik (Kansas University)
presented by Andrei Nomerotski (Fermilab/Oxford) 33rd International Conference in High Energy Physics (Jul 26th – Aug 2nd, Moscow, Russia)
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B mixing Mass eigenstates are a mixture of flavor eigenstates:
BH and BL have a different mass and may have different decay width. Dm = MH – ML = 2|M12| , DG = GH - GL = 2|G12| Dominant Diagram for the transition : Time evolution follows the Schrodinger equation
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Oscillations with amplitude = 1.0 and
In an Ideal Scenario.. “Opposite sign” Oscillations with amplitude = 1.0 and Frequency = Dms. “Same sign”
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DZero Detector Spectrometer : Fiber and Silicon Trackers in 2 T Solenoid Energy Flow : Fine segmentation liquid Ar Calorimeter and Preshower Muons : 3 layer system & absorber in Toroidal field Hermetic : Excellent coverage of Tracking, Calorimeter and Muon Systems SMT H-disks SMT F-disks SMT barrels
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Analysis outline Identify e/m. Signal Selection μ+/e+
p - K+ K- φ D-S μ(e) B n X Identify e/m. PT (e/m) > 2.0 | h| (e/m) < 1.0/2.0 Signal Selection Look for tracks displaced from primary vertex in same jet as m/electron Two tracks should form a vertex and be consistent with f mass (fp K Kp) or K* mass (K*K KKp) KKp invariant mass should be consistent with Ds mass
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Signal Selection μ (e)+ π - K+ K- φ μ(e) ν
D-S μ(e) B ν X Muons were selected by triggers without lifetime bias = no online/offline Impact Parameter cuts Trigger muon can be used as tag muon : gives access to eDs sample with enhanced tagging purity
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Bs can decay at IP and be reconstructed
Signal Selection Eff=30% X μ+ μ(e) B PV D-S π - φ LT(DS) ν K- K+ Ds lifetime is used to have non-zero selection efficiency at Interaction Point Bs can decay at IP and be reconstructed
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Effect of Neutrino Need to correct Decay Length for relativistic contraction need to know Bs momentum Can estimate Bs momentum from MC (through so called k-factor) at expense of additional uncertainty Dk/k uncertainty causes additional smearing of oscillations Only few first periods are useful for semileptonic channels Sensitivity at DL=0 is crucial All above represents the main difference wrt hadronic channels 200 micron # of periods
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Flavor Tagging and dilution calibration
Identify flavor of reconstructed BS candidate using information from B decay in opposite hemisphere. Ds a) Lepton Tag : Use semileptonic b decay : Charge of electron/muon identifies b flavor n Bs e / m b) Secondary Vertex Tag : Search for secondary vertex on opposite Side and loop over tracks assoc. to SV. m cos f (l, Bs) < 0.8 c) Event charge Tag: All tracks opposide to rec. B Secondary Vertex
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Dilution in Δmd measurement
Combine all tagging variables using likelihood ratios Bd oscillation measurement with combined tagger Dmd= 0.5010.030±0.016ps-1 Input for Bs measurement Combined dilution: εD2=2.48±0.21±0.08 %
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Bs decay samples after flavor tagging
NBs( fp + m) = 5601 102 NBs(fp + e) = 1012 62 (Muon tagged) NBs(K*K + m) = 2997 146 BsDs mn X Ds fp BsDs mn X Ds K*K BsDs e n X Ds fp
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K*K Fit Components Difficult mode due to K* natural width and mass resolution – larger errors wrt fp mode (Cabibbo suppressed)
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Results of the Lifetime Fit
From a fit to signal and background region: Decay Mode ctBs (mm) ctbkg (mm) BsDs m n X, Ds fp 4049 6276 BsDs e n X, Ds fp 44429 64518 BsDs m n X, Ds K*K 40722 54910 BsDs mn X BsDs e n X Ds K*K Ds fp
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Amplitude Method Amplitude fit = Fourier analysis + Maximum likelihood fit often used in oscillation measurements Need to know dilution (from Δmd analysis) If A=1, the Δm’s is a measurement of Bs oscillation frequency, otherwise A=0
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Cross-check on BdXμD±()
Amplitude Scan DØ Run II Preliminary EXACTLY the same sample & tagger Amplitude Scan shows Bd oscillations at correct place no lifetime bias with correct amplitude correct dilution calibration Same results for two other modes
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Measure Resolution Using Data
Ultimately Dms sensitivity is limited by decay length resolution – very important issue Use J/ψ→μμ sample Fit pull distribution for J/ψ Proper Decay Length with 2 Gaussians Resolution Scale Factor is 1.0 for 72% of the events and 1.8 for the rest Cross-checked by several other methods μ PV J/ψ vertex L±σL DØ Run II Preliminary
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Amplitude Scan of BsXμDs()
Deviation of the amplitude at 19 ps-1 2.5σ from 0 1% probability 1.6σ from 1 10% probability
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Log Likelihood Scan In agreement with the amplitude scan
Resolution K-factor variation BR (BsDsX) VPDL model BR (BsDsDs) Systematic Have no sensitivity above 22 ps-1 17 < Dms < % CL assuming Gaussian errors Most probable value of Dms = 19 ps-1
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Interpretation Results of ensemble tests: DZero result :
Combined with World (before CDF measurement): 17 21 Dms(ps-1) 15% 80% 5% 17 21 Dms(ps-1) 5% 90%
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Impact on the Unitarity Triangle
Before BS mixing
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Impact on the Unitarity Triangle
With D0
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Impact on the Unitarity Triangle
With CDF
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“Golden” Events for Visualization
Period of 19ps-1 DØ Run II Preliminary # of periods Weigh events using
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Can We See Bs Oscillations By Eye ?
Weighted asymmetry This plot does not represent full statistical power of our data # of periods
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More Amplitude Scans New results : Amplitude scans from two additional modes BsDs (fp) e n X BsDs mn X Ds fp Ds K*K
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Combination Amplitude is centred at 1 now, smaller errors
Likelihood scan confirms 90% CL Dms limits: ps-1 Data with randomized tagger : 8% probability to have a fluctuation (5% before for mfp mode) Detailed ensemble tests in progress
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Outlook Add Same Side Tagging
Add hadronic modes triggering on tag muon Add more data (4-8 fb-1 in next 3 years) with improved detector – additional layer of silicon between beampipe and Silicon Tracker (Layer0) – better impact parameter resolution Layer0 has been successfully installed in April 2006 S/N = 18:1 & no pickup noise First 50 pb-1 of data on tape, first tracks have been reconstructed
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Summary Established upper and lower limits on Dms using Bs Ds (fp) mn X mode Analysis published in PRL 97 (2006) Combined with two other channels Bs Ds (K*K) mn X Bs Ds (fp) en X considerable improvement in sensitivity 14.1 16.5 ps-1, no improvement for Dms interval Looking forward to a larger dataset with improved vertex detection If Dms is indeed below 19 ps-1 expect a robust measurement with the extended dataset
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BACKUP SLIDES
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Bu+ B0 Bs0 Bc+ B Mesons Matter b u b d b s b c b c b u b s b
A reminder : There are 4 B mesons with their respective anti-particles with the quark contents As shown here. b c b u b s b Anti-Matter d
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CKM matrix and B mixing Why are we interested to study B meson oscillations Wolfenstein parametrisation - expansion in l. complex
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B Mixing In general, probability for unmixed and mixed decays Pu,m(B) Pu,m(B). In limit, G12 << M12 (DG << DM) (Standard model estimate and confirmed by data), the two are equal. ~ 10-4 for Bs system ~ 10-3 for Bd system
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Constraing the CKM Matrix from Dms
CDF+D0 (2006) Dms inputs And similar expression for Dms (x2) x = 1.24 0.040.06 (from Lattice QCD calculations) Ratio suffers from lower theoretical Uncertainties – strong constraint Vtd Used URL :
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Excellent Tevatron Performance
Data sample corresponding to over 1 fb-1 of the integrated luminosity used for the Bs mixing analysis Full dataset is ready (85-90% DAQ efficiency)
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Muon Triggers Limitation of data recording. Triggers are needed to select useful physics decay modes. 396 ns bunch crossing rate ~ 2.5 MHz ~50 Hz for data to be recorded. Single inclusive muon Trigger: |η|<2.0, pT > 3,4,5 GeV Muon + track match at Level 1 Prescaled or turned off depending on inst. lumi. We have B physics triggers at all lumi’s Extra tracks at medium lumi’s Impact parameter requirements Associated invariant mass Track selections at Level 3 Dimuon Trigger : other muon for flavor tagging e.g. at 50·10-30 cm-2s-1, L3 trigger rate : 20 Hz of unbiased single μ 1.5 Hz of IP+μ 2 Hz of di-μ No rate problem at L1/L2
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μ Sample Tagging efficiency 21.9±0.7% μDs: 5,601±102
Opposite-side flavor tagging μD±: 7,422 μDs: 26,710 μD±: 1,519 Tagging efficiency 21.9±0.7% μDs: 5,601±102
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check Using BdXμD±()
The Amplitude Scan shows Bd oscillations at 0.5 ps-1 no lifetime bias (A=1) : correct dilution calibration
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SM prediction - Dms ~ 20 ps-1
Detector Effects flavor tagging power, background Decay length resolution momentum resolution (p)/p = ? % sl = ? SM Prediction Dms = 20 ps-1, Tosc = 0.3 ps SM prediction - Dms ~ 20 ps-1 Trying to measure : Tosc~0.3 X s !
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Sample Composition Estimate using MC simulation, PDG Br’s, Evtgen exclusive Br’s Signal: 85.6%
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Flavor tag Dilution calibration
Bd mixing measurement using Bd D* m n X, D* D0 p, D0 K p, and evaluate dilution in various diution bins. Follows similar analysis outline as Bs mixing. Form measured asymmetry in 7 bins in visible proper decay length (xM) – Count OS and SS events (compare charge of reconstructed muon with tagger decision) Fit the c2: Also include B+ D0 m n X decay asymmetry.
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Dilution calibration : Results
For final fit, bin the tag variable |d| in 5 bins and do a simultaneuos fit c2 (i) where i=1,5. Parameters of the fit : Dm, fcc, 5 Dd, 5 Du = 12 B0 B+ Increasing dilution Increasing dilution Dm = 0.020 (stat.) ps-1 eD2 = (2.48 0.21) (%) (stat.) e = (19.9 0.2) (%) (stat.)
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Individual Taggers performance
Muon 6.61 0.12 0.473 0.027 1.48 0.17(stat) Electron 1.83 0.07 0.341 0.058 0.21 0.07 (stat) SV 2.77 0.08 0.424 0.048 0.50 0.11 (stat) Total OST 11.14 0.15 2.19 0.22 (stat) Note : To evaluate the individual tagger performance |dpr| > 0.3 This cut was not imposed for final combined tagger. Final eD2 is higher.
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Likelihood minimization to get Dms
Minimize Form Probability Density Functions (PDF) for each source dpr Dilution Calibration (From Dmd measurement) Signal selection function (y) Use variables that discriminate signal and background using Mfp distribution sidebands and signal region. Form yi = fs/fbkg for ith variable. y is product over all yi’s. Use the signal selection function in the likelihood Use the full information to weight the events
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Bs Signal and background
Signal PDF: Background PDF composed of long-lived and prompt components – Evaluated from a lifetime fit. Long Lived Background – Described by exponential convoluted with a gaussian resolution function. Non-sensitive to the tagging Non-oscillating Oscillating with Δmd frequency Prompt Background – Gaussian distribution with resolution as fit parameter.
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Combined flavor tag algorithm
Combine individual tag informations to tag the event. Get tag on opposite side and construct PDF’s for variables discriminating b (m- ) and b (m+) (Use B+ D0 m n X decays in data) Discriminating variables (xi): more pure Electron/Muon SV Tagger
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Ensemble Tests Using data Using MC
Simulate Δms=∞ by randomizing the sign of flavour tagging Probability to observe Δlog(L)>1.9 (as deep as ours) in the range 16 < Δms < 22 ps-1 is 3.8% 5% using lower edge of syst. uncertainties band Using MC Probability to observe Δlog(L)>1.9 for the true Δms=19 ps-1 in the range 17 < Δms < 21 ps-1 is 15% Many more parameterized MC cross-checks performed – all consistent with above
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