Gavril Giurgiu, Carnegie Mellon 1 B s Mixing at CDF Seminar at Fermi National Accelerator Laboratory...

Gavril Giurgiu, Carnegie Mellon

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Bs Mixing at CDF

Seminar at Fermi National Accelerator Laboratory

Gavril Giurgiu

Carnegie Mellon University

August 16, 2005


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Outline• Introduction• B mixing phenomenology• CDF detector and B Physics triggers• B mixing analysis overview

– semileptonic and hadronic Bs signals

– decay time

– Bs lifetime

– flavor tagging– B0 mixing and tagging calibration

– Bs semileptonic and hadronic amplitude scans

• Conclusions and outlook


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Introduction In the Standard Model, the charged current interactions of quarks are described by the Lagrangean:

The weak eigenstates d’, s’ and b’ are linear combinations of the mass eigenstates d, s and b and the quark mixing is given by the CKM matrix:

The Standard Model does not predict the values of CKM elements.We have to measure them.

B0 and Bs oscillations provide information on Vtd and Vts


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B Mixing PhenomenologyNeutral B system:

Mass eigenstates:

Oscillation frequency of Bq mesons given by mq = MH - ML

Width (lifetime) difference HL

Neglecting , mixing probability after time t is give by:

Asymmetry:


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B Mixing Phenomenology (cont)

Although md is well measured (0.502 0.007 ps-1) determination of Vtd is affected by ~20% error due to large uncertainties on different parameters

In the ratio between md and ms

many common parameters cancel

B0/Bs oscillations are described by top quark exchange box diagrams mq and Vtq (q=d,s) are related by known parameters


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Unitarity Triangle

Knowledge of both Bd and Bs mixing frequencies would provide better constraints on one side of unitarity triangle:

In the complex plane, the unitarity relation is represented by a triangle

Re

Im

B → J/ψ Ks

b → u decays

B0/Bs mixing

The CKM matrix is unitary:

One of the six unitarity relations:


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Current Bs StatusBs mixing not observed yetBs oscillates more than 30 times faster than Bd experimental challengeAt 95% C.L. lower limit ms > 14.4 ps-1 with sensitivity of 17.8 ps-1

CKM triangle global fit:


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CDF Detector


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CDF Detector – Schematic View

Plug Calorimeter

1.3 < || < 3.5

Central Tracker (COT)

|| < 1.0 dE/dx for PID

Time of Flight for K/p separationplaced before 1.4 Tesla Solenoid

Electromagnetic andHadronic calorimeters

Silicon Detector

|| < 2.0

Muon Detectors

|| < 1.0


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B Physics at the Tevatron

The b cross section is 103 times larger than

at the e+e- machines

Bs mesons are only produced at Tevatron

The total inelastic cross section is about 60 mb while the b production cross section in the central region is:

b, |y|<0.6b

Need triggers to select the b events


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Silicon Vertex Trigger (SVT) Silicon Vertex Trigger is designed to select b events

- Implemented at Level 2- Uses silicon detector information and beamline position to

determine the track impact parameter

- Good impact parameter resolution ~ 47 m:

~33 m beam size ~30 m intrinsic SVT resolution

- Trigger on displaced track


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SVT Triggers for B PhysicsSemileptonic (partially reconstructed) decays: Bs lepton Ds X - large number of events - decay time resolution degraded due to missing neutrino - triggered by 4 GeV lepton and displaced track with impact parameter |d0|>120 m and |d0|<1 mm

Hadronic (fully reconstructed) decays:

Bs Ds - smaller number of events - good decay time resolution - triggered by two displaced tracks with impact parameter |d0|>120 m and |d0|<1 mm

d0

d0d0


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Mixing Analysis OverviewMixing analysis ingredients: - Signal reconstruction - Decay time - B flavor at decay - B flavor at production inferred through flavor tagging: - lepton tags - jet charge tags

Statistical significance of ms measurement:

Tagging

Signal Reconstruction

Decay time resolution

Sig


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Semileptonic Bs SignalsMissing neutrino cannot see Bs mass peakUse Ds mass peak and (lepton, Ds) charge correlation:

l+D- - right sign combination

l-D- - wrong sign combination

Decay modes:

Ds ( 4355 94 ) Ds K*K ( 1750 83 )

Ds 3 ( 1573 88 )

Total of 7000 Bs candidates but ~18% come from “Physics backgrounds”


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Semileptonic Bs Signals (cont)

Ds K*K ( 1750 83 ) Ds 3 ( 1573 88 )


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Physics BackgroundsOriginates from real B decays:

B0/+ Ds D (~13%)Bs Ds (~2%) (D / / D(s) lepton X)Bs Ds D(s) (~3%)

In each case we observe the same decay signature as in Bs Ds lepton

The decay time distributions and the reconstruction efficiencies are obtained from MC

Each decay time distribution is weighted in the maximum likelihood fit using the measured branching fractions and reconstruction efficiencies


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Hadronic Bs SignalsAll final state particles reconstructed observe Bs mass peak

Decay modes:

Ds ( 526 33 ) Ds K*K ( 254 21 )

Ds 3 ( 116 18 )

Total of 900 Bs candidates

Satellite peak:

Bs Ds* (Ds* Ds X)


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Hadronic Bs Signals (cont)Ds K*K ( 254 21 )

Ds 3 ( 116 18 )


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Decay Time Decay time:

In semileptonic modes missing neutrino is statistically corrected by:

Hadronic decays do not need momentum correction


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To resolve the fast Bs oscillations we need excellent decay time resolution

Hadronic decays Semileptonic decays (fully reconstructed): (partially reconstructed):

Decay Time Resolution


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Decay Time BiasBecause: (1) In both hadronic and semileptonic decays the triggers require displaced tracks (2) Bs events are selected based on decay distance cuts the Bs decay time distribution is biased

Efficiency as function of decay time obtained from Monte Carlo:


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Lifetime MeasurementAs a cross check of analysis framework measure Bs lifetimeLifetime fit projections in both hadronic and semileptonic modes

Semileptonic: c(Bs) = 443 10 (stat) xxx (syst) m

Hadronic: c(Bs) = 479 29 (stat) 5 (syst) m

Good agreement with PDG 2004: c(Bs) = 438 17 m


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Flavor TaggingFor Bs mixing analysis at CDF we used 5 opposite side flavor taggers Tag inferred from opposite side B in event: - muon and electron tag (semileptonic decay of opposite B) - three jet charge tag types:

- displaced vertex- displaced tracks- high pT

Tagging power given by D2 where is the tagging efficiency and

D = 1 – 2 Pmistag is the tagging dilution

Pmistag – mistag probability

Large dilution (D) means high tagging power

Knowledge of the dilution dependence on different quantities enhances tagging power

Trigger B meson


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Lepton IdentificationCombine different quantities in a global likelihood function which gives the probability that a lepton is realExample: for muon identification we use five quantities:

3 matching variables between extrapolated track and muon stub (X, , Z)

2 calorimeter variables (electromagnetic and hadronic energies)Obtain real muon templates from J/ψ→ and fake muon templates from →p where the proton matches to a muon stubX: Hadronic energy: Likelihood:


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Lepton TaggingWe use an inclusive lepton-SVT sample to determine the dilution of muon and electron taggers as function of: - lepton likelihood (probability that lepton is real)

- (transverse momentum of lepton w.r.t jet axis)relTp

Electron tagMuon tag


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Dilution of jet charge tagger is calculated as function of- the jet charge:

di - displacement of track i w.r.t the primary vertex - three jet charge tag types: displaced vertex, displaced

track and high momentum

Jet Charge Tagging

Combined tagging power of all five opposite side taggers (lepton + jet charge): D2 1.6 % calculated on inclusive lepton-SVT sample


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Measurement of md and Tagger Calibration

Perform measurement of md Since we observe B0 oscillations, we can also measure tag dilutions Analyze hadronic and semileptonic decays of B0 and B+:

B0 D+

B+ D0

B0 J/K*0

B+ J/K+

B0/+ D- l+ X

B0/+ D-* l+ X

B+/0 D0 l+ X

Event by event predicted dilution (D) Fit the dilution calibration factor (S) for each of 5 tag types

Dilution calibration factors are used for the Bs mixing analysis

Dilution Calibration Factor

DSeB

tmDSeBt

dt

1:

)cos(1:/

/0

Event by event dilution


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B0/B+ Semileptonic SignalsB mesons are identified by vertexing the trajectory of a D meson with a lepton trackEach lepton-D signature is a mixture of B0 and B+

B → l+D0X, D0 → K+- B → l+D-, D- → K+-- B → l+D*-, D*- → D0-

~100000 events ~52000 events ~25000 events B0/B+ ~ 20/80 B0/B+ ~ 85/15 B0/B+ ~ 85/15


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B0/B+ Hadronic SignalsB0 → D-+, D- → K+-- B+ → D0+, D0 → K+-

~6200 events ~5600 events

The “double horn” structures on the low mass sidebands comes from partially reconstructed B mesons. Example: B+ → D*0 + , D*0 → D0 0


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md ResultsHadronic: md = (0.503±0.063±0.015) ps-

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Dilution calibration factors: S(muon) = 0.83±0.10±0.03 S(electron) = 0.79±0.14±0.04 S(vertex) = 0.78±0.19±0.05 S(track) = 0.76±0.21±0.03 S(high pT) = 1.35±0.26±0.02

Total D2 ~ 1.1%

Semileptonic: md = (0.498±0.028±0.015) ps-1

Dilution calibration factors: S(muon) = 0.93±0.04±0.03 S(electron) = 0.98±0.06±0.03 S(vertex) = 0.97±0.06±0.04 S(track) = 0.90±0.08±0.05 S(high pT) = 1.08±0.09±0.09

Total D2 ~ 1.4%

Muon Tags

World average: md = 0.502 0.007 ps-1


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Amplitude Scan MethodIntroduce Fourier coefficient A (amplitude)Fix ms at different test values and fit for A: (Moser et.al., NIMA 384 491)

A 1 for true value of ms A 0 away from true value

Toy MC test with ms = 10 ps-1 and simulated sample 10x larger than real data - points: A 1 - yellow band: A 1.645 - dotted line: 1.645 - yellow band bellow 1 exclusion

at 95% CL


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Bs Analysis Performed “blind” analysis by randomizing the tag decision: tag = tag (-1)event number

Evaluate sensitivity and systematic uncertainties from “blind” analysis

Systematic errors evaluated using pseudo-experiments:- include all variables and distributions determined from data

- fit the toy sample with different Likelihood configurations

- use variations in Amplitude (A) and statistical error (A) to derive the systematic error:


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Semileptonic Amplitude Scan Measurement is statistics dominated Main systematic uncertainties from prompt background and from Physics background

Sensitivity: 7.4 ps-1 Limit: ms > 7.7 ps-1 at 95% C.L.


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Hadronic Amplitude ScanMeasurement is statistics dominatedMain systematic errors come from tagger calibration

Sensitivity: 0.4 ps-1 Limit: ms > 0.0 ps-1 at 95% C.L.


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Combined CDF result on msAfter combining semileptonic and hadronic modes:

Sensitivity: 8.4 ps-1 Limit: ms > 7.9 ps-1

With full Bs momentum reconstruction, hadronic mode will dominate the measurement at high ms


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Conclusions

95% C.L. ms limits from CDF:Semileptonic: 7.4 ps-1

Hadronic: 0.0 ps-1 (will become important at high ms with more statistics)

Combined limit: 7.9 ps-1, sensitivity: 8.4 ps-1

Results will substantially improve soon:- Same side Kaon tagger - Improve decay time resolution in hadronic modes- Add more data

Updated analyses expected soon


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Sensitivity ProjectionsUse analytical formula to predict sensitivity as a function of Bs yield

current: no improvement, baseline: +1% D2 and 10% improvement in decay time resolutionstretched: +3% D2 and 20% improvement in decay time resolution

25ps-1

only in semileptonic case


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Amplitude Scan Method (cont)Test amplitude method on B0 oscillations by scanning for md in hadronic modes

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Gavril Giurgiu, Carnegie Mellon 1 B s Mixing at CDF Seminar at Fermi National Accelerator Laboratory...

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