The Observation of B0s – B0
s Oscillations:a Historical Perspective
23 Nov 2006, Music Pier, Ocean City, NJ, photo: Tom Welsh
Joseph KrollUniversity of Pennsylvania
CKM 2006Nagoya12-16 Dec 2006
15 Dec 2006 J. Kroll (Penn) 2
Our Story Begins 20 Years AgoUA1 1986: Evidence for B0 & B0
s mixing
From the Abstract:“Combined with the null resultfrom searches for B0 $ B0
oscillations at e+e- colliders,our results are consistent withwith transitions in the B0
s
system as favoured theoretically”
Phys. Lett. B 186, 247 (1987)
(the B0s had not even
been observed yet)
15 Dec 2006 J. Kroll (Penn) 3
Neutral Meson Flavor Oscillations (Mixing)
Due to phase space suppression:K0
L very long-lived: 5.2£ 10-8 s(K0
S: 0.0090£ 10-8 s)
1954: over 50 years ago
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Long-Lived Neutral Kaon
Discovered in 1956
Led to discovery of CP Violation in 1964 (Nobel Prize in 1980)BF(K0
L ! +-) = 0.2%Christenson, Cronin, Fitch, Turlay, Phys. Rev. Lett. 13, 138 (1964)
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Neutral Meson Mixing (Continued)
“If there is any place where we have achance to test the main principles of quantum mechanics in the purest way – does the superposition of amplitudeswork or doesn’t it – this is it.”
R. P. Feynman in Lectures on Physics Vol. III
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Two-State Quantum Mechanical System
Common decay modes ! 2-state QM system
Eigenstates of 2-state system (neglecting CP violation)
“Light” (CP-even)
“Heavy” (CP-odd)
mass & width
Antiparticleexists at time t!
Start (t=0) withparticle
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Importance of Neutral B Meson OscillationsCabibbo-Kobayashi-Maskawa Matrix
weak mass
fundamental parameters that must be measured
Oscillation frequencies (md, ms) determine poorly known Vtd, Vts
|Vtd| & |Vtd/Vts| measure one side of Unitary Triangle
New particles in loops alter expectations test Standard EWK Model
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Theoretical uncertainties reduced in ratio:
All factors well known except
from Lattice QCD calculations (Okamoto, hep-lat/0510113)
Limits precision on Vtd, Vts to ~ 10%
PDG 2006
~ 4%
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Some HistoryImportant prehistory: 1983: long B hadron lifetime 2006 APS Panofsky Prize
1986: 1st evidence of B mixing from UA1 C. Albajar et al., PLB, 186, 247 (1987)
1987: Definitive observation of B0 mixing by ARGUS - indicates UA1 must be Bs, heavy top (>50 GeV) - 1989 confirmed by CLEO
1990’s: LEP, SLC, Tevatron - time-integrated meas. establishes Bs mixes (maximally) - measure time-dependent B0 oscillations
- lower limits on Bs oscillation frequency
2000: B factories improve precision of B0 oscillation frequency
2006: Tevatron discovers Bs oscillations - two-sided 90% CL limit by DØ - 1st measurement of oscillation frequency by CDF - definitive observation of oscillation signal by CDF
H. Albrecht et al., PLB, 192, 245 (1987)
V. M. Abazov et al., PRL, 97, 021802 (2006)
A. Abulencia et al., PRL, 97, 021802 (2006) & PRL, 97, 242003 (2006)
M. Artuso et al., PRL, 62, 2233 (1989)
too many references to list individually
Belle: K. Abe et al., PRD 71, 072003 (2005) Babar: B. Aubert et al., PRD 73, 012004 (2006)
15 Dec 2006 J. Kroll (Penn) 10
1983: B Hadron Lifetime Measurement
2006 Panofsky Prize: Bill Ford, John Jaros, Nigel Lockyer
MAC: E. Fernandez et al., PRL 51 1022 (1983)
MarkII: N. Lockyer et al., PRL 51 1316 (1983)
Signed impact parameter (no Silicon)
c sample
b sample
background
e,
thru
st a
xis
500m
100
m
SLD:xy = 3.5mZ = 17m
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Methods to Measure m
Time Integrated (assuming =0)
Measure probability B decays as B:
Mixing first established with time integrated quantities
Time Dependent (required for m À )
Measure probability B decays as B as a function of proper decay time t
m = oscillation frequency
(LEP, LHC, SLC, Tevatron)
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First Time Integrated Measurements
Based on semileptonic B decay:
Correct for:
Coherent incoherent
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UA1: 1st Evidence for B Mixing
C. Albajar et al., Phys. Lett. B 186, 247 (1987)
Measured:
Expected: for no mixing
Found:
no mixing disfavored at 2.9
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ARGUS: Observation of B0 Oscillations
H. Albrecht et al., PLB, 192, 245 (1987)
1 fully reconstructed eventwith two B0’s (two +)
Measured d using r from - dileptons (4.0) - lepton+B0’s (3.0)
unknown: B0 to B+ ratio
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B0s Mixing Established
Early ’90’s: Z-pole, hadron
R. Akers et al., ZfP, C60, 199 (1993)
Example: OPAL
Extracted:
Data favor maximal s
Bs mixing “discovered”
+ d from CLEO
Fraction of Bs mesons produced
Bs m
ixin
g p
rob
abili
ty
15 Dec 2006 J. Kroll (Penn) 16
1992: First Direct Evidence of Bs
Signal
Well knownbackground
poorly knownbackground(small)
P. Abreu et al. (Delphi) Phys. Lett. B 289, 199 (1992)
also:D. Buskulic et al. (Aleph) Phys. Lett. B 294, 145 (1992)P. D. Acton et al. (Opal) Phys. Lett. B 295, 357 (1992) Sample: 270 K hadronic Z
15 Dec 2006 J. Kroll (Penn) 17
Time Dependent Measurement of md
• Requires 3 pieces of information per event– Flavor of B at production
– Flavor of B at decay
– Proper decay time of B
• Flavor of B at production – several techniques• from other B hadron in event: leptons, jet-charge, kaons
• use associated particles produced in fragmentation
• Flavor of B at decay – depends on reconstruction– inclusive (reconstruct secondary vertex, use “jet-charge”)
– partial (lepton, charm)
– semi-exclusive or exclusive (lepton+charm, hadronic)
• Proper decay time– resolution depends on method of reconstructing B decay
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ALEPH: 1st Time Dependent Measurement
R. Akers et al., ZfP, C60, 199 (1993)
lepton tag from “other” B
Experimental Effects
perfect
proper time effects
prod. flavor mistag, backgnd
Measured Asymmetry
D0 Decay Length (cm)
D
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State of the Art: md
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How Do We Measure Oscillation Frequency?
Measure asymmetry A as a function of proper decay time t
“unmixed”: particle decays as particle
For a fixed value of ms, data should yieldAmplitude “A” is 1 @ true value of ms
Amplitude “A” is 0 otherwise
“mixed”: particle decays as antiparticle
Units: [m] = ~ ps-1, ~=1 then m in ps-1. Multiply by 6.582£ 10-4 to convert to eV
Amplitude method:H-G. Moser, A. Roussarie,NIM A384 p. 491 (1997)
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Start 2006: Published Results on ms
Results from LEP, SLD, CDF I ms > 14.4 ps-1 95% CL
source: http://www.slac.stanford.edu/xorg/hfag/osc/PDG_2006/index.html
Frequency ms (ps-1)15
Amplitude @ ms = 15 ps-1
Am
plit
ud
e
1
0
>3.4 cycles per lifetime
Average 0.48 § 0.43
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April 2006: Result from the CDF Collaboration
Probability that randomfluctuations mimic thissignal is 0.2% (3)
Assuming signal hypothesis: measure ms
A. Abulencia et al., Phys. Rev. Lett., 97, 062003 (2006)
Since then goal has been to observe signal with > 5 significance
likelihood ratio
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Ingredients in Measuring Oscillations
opposite-side K–
jet charge
Decay modetags b flavorat decay
2nd B tags production flavor
Dilution D = 1 – 2ww = mistag probability= efficiencyD2 = effective tagging power
Proper decay timefrom displacement (L)and momentum (p)
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Key Experimental Issues
Uncertainty onAmplitude
Signal size
Signal toBackground
Proper timeResolution
Production flavorTag performance
efficient tracking,displaced track trigger
excellent mass resolutionParticle ID: TOF, dE/dx
lepton id, Kaon id with TOF
Silicon on beampipe (Layer 00)
Fully reconstructed signal crucial
CDF’s strengths
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Improvements that led to Observation
• Same data set (1 fb-1)
• Proper decay time resolution unchanged
• Signal selection– Neural network selection for hadronic modes
– add partially reconstructed hadronic decays
– use particle id (TOF, dE/dx) (separate kaons from pions)• looser kinematic criteria possible due to lower background
– additional trigger selection criteria allowed
• Production Flavor tag– opposite-side tags combined using neural network
• also added opposite-side kaon tag
– neural network combines kinematics and PID in same-side K tag
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Example: Fully Reconstructed Signal
Cleanest decay sequence
Also use 6 body modes:
Add partially reconstructed decays:
Hadronic signal increased from 3600 to 8700
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Semileptonic Signals
Semileptonic signal increased from 37000 to 61500
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Decay Time Resolution: Hadronic Decays
<t> = 86 £ 10-15 s¼ period for ms = 18 ps-1
Oscillation period for ms = 18 ps-1
Maximize sensitivity:use candidate specificdecay time resolution
Superior decay timeresolution gives CDFsensitivity at muchlarger values of ms
than previous experiments
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Semileptonics: Correction for Missing Momentum
Reconstructed quantity Correction Factor (MC) Decay Time
Reconstructed momentum fraction proper decay time (ps)
T = 2/ms
reso
luti
on (
fs)
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Same Side Flavor Tags
Need particle idTOF Critical(dE/dx too)
Charge of K tags flavorof Bs at production
Our most powerful flavor tag:D2 = 4-5%
(Opposite-side tags: D2 = 1.8%)
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Results: Amplitude Scan
A/A = 6.1 Sensitivity31.3 ps-1
Hadronic & semileptonic decays combined
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Measured Value of ms
- log(Likelihood) Hypothesis of A=1 compared to A=0
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Significance: Probability of Fluctuation
Probability ofrandom fluctuationdetermined from data
Probability = 8 £ 108(5.4)
Have exceededstandard thresholdto claim observation
28 of 350 millionrandom trialshave L < -17.26
-17.26
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Asymmetry (Oscillations) in Time Domain
Period0.35 ps
Aside: for B0 Period = 12.6 ps
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Summary of CDF Results on B0s Mixing
Observation of Bs Oscillations and precise measurement of ms
Precision: 0.7% Probability random fluctuation mimics signal: 8£10-8
Most precise measurement of |Vtd/Vts|
A. Abulencia et al., hep-ex/0609040, accepted by Phys. Rev. Lett.
( 2.83 THz, 0.012 eV)
20 year quest has come to a conclusion
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Backup Slides
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Key Features of CDF for B Physics
• “Deadtime-less” trigger system– 3 level system with great flexibility
– First two levels have pipelines to reduce deadtime
– Silicon Vertex Tracker: trigger on displaced tracks at 2nd level
• Charged particle reconstruction – Drift Chamber and Silicon– excellent momentum resolution: R = 1.4m, B = 1.4T
– lots of redundancy for pattern recognition in busy environment
– excellent impact parameter resolution
(L00 at 1.5cm, 25m £ 25m beam)
• Particle identification– specific ionization in central drift chamber (dE/dx)
– Time of Flight measurement at R = 1.4 m
– electron & muon identification
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Time of Flight Detector (TOF)
• 216 Scintillator bars, 2.8 m long, 4 £ 4 cm2
• located @ R=140 cm• read out both ends with fine mesh PMT (operates in 1.4 T B field – gain down ~ 400)• anticipated resolution TOF=100 ps• (limited by photostatistics)
Kaon ID for B physics
Measured quantities:s = distance travelledt = time of flightp = momentum
Derived quantities:v = s/tm = p/v
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B Flavor Tagging
We quantify performance with efficiency and dilution D
= fraction of signal with flavor tag
D = 1-2w, w = probability that tag is incorrect (mistag)
Statistical error A on asymmetry A (N is number of signal)
statistical error scales with D2
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Two Types of Flavor Tags
Opposite side
Same side Based on fragmentation tracks or B**
+ Applicable to both B0 and B0s
− other b not always in the acceptance
− Results for B+ and B0 not applicable to B0s
+ better acceptance for frag. tracks than opp. side b
Reminder: for limit on ms must know D
Produce bb pairs: find 2nd b, determine flavor,infer flavor of 1st b
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Types of Opposite Side Flavor Tags
Lepton tags
Jet charge tag
Kaon tag
mistags from
jet from b (b) has negative (positive) charge on average
low high D
high low D
Largest D2 @ B factoriesNot used in present analysis
TOF
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Calibrate with Large Statistics Samples of B+ & B0
Example: semileptonic signals
Results:D2 = 1.54 § 0.05[ md = 0.509 § 0.010 (stat) § 0.016 (syst)]
Hadronic signals:B+ (D0+) = 26,000B0 (D-+) = 22,000
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Compare PerformanceData and Simulation
Check prediction for kaon tag on B+, B0
Good agreement between data & MCSystematic based on comparisons
K
K
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Flavor Tagging Summary
Same-side kaon tag increases effective statistics £ 3 – 4
D2 Hadronic (%) D2 Semileptonic (%)
Muon 0.48 § 0.06 (stat) 0.62§ 0.03 (stat)
Electron 0.09 § 0.03 (stat) 0.10 § 0.01 (stat)
JQ/Vertex 0.30 § 0.04 (stat) 0.27 § 0.02 (stat)
JQ/Prob. 0.46 § 0.05 (stat) 0.34 § 0.02 (stat)
JQ/High pT 0.14 § 0.03 (stat) 0.11 § 0.01 (stat)
Total OST 1.47 § 0.10 (stat) 1.44 § 0.04 (stat)
SSKT 3.42 § 0.98 (syst) 4.00 § 1.02 (syst)
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Systematic Uncertainties on ms
• systematic uncertainties from fit model evaluated on toy Monte Carlo
• have negligible impact
• relevant systematic unc. from lifetime scale
Syst. Unc
SVX Alignment 0.04 ps-1
Track Fit Bias 0.05 ps-1
PV bias from tagging 0.02 ps-1
All Other Sys < 0.01ps-1
Total 0.07 ps-1
All relevant systematic uncertainties are common between hadronic and semileptonic samples
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Same Side Flavor Tags
Based on correlation betweencharge of fragmentation particleand flavor of b in B meson
TOF Critical(dE/dx too)Both due to PENN
15 Dec 2006 J. Kroll (Penn) 47
Kaons Produced in Vicinity of B’sLarger fraction of Kaons near B0
s compared to B0, B+, as expected
Ph. D. Thesis, Denys Usynin
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Example of Candidate
candidate
Same-side Kaon tag
Opposite-side Muon tag
Zoom in oncollision pt.
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Measuring Resolution in Data
Use large prompt D meson sample CDF II, D. Acosta et al., PRL 91, 241804 (2003)
Real prompt D+ from interaction point
pair with random trackfrom interaction point
Compare reconstructed decay point to interaction point
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Proper Time & Lifetime Measurement
production vertex25m £ 25 m
Decay position
Decay time inB rest frame
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Determination of |Vtd/Vts|
Previous best result: D. Mohapatra et al.(Belle Collaboration)PRL 96 221601 (2006)
CDF
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Improvement from Neural Net Selection
15 Dec 2006 J. Kroll (Penn) 53
Scans for Individual Signatures
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B0s Mixing Established
Early ’90’s: Z-pole, hadron
D. Decamp et al., PLB, 258, 236 (1991)
Example:
Extracted:
Data favor maximal s
Bs mixing “discovered”
15 Dec 2006 J. Kroll (Penn) 55
1992: First Direct Evidence of Bs
Signal
Well knownbackground
poorly knownbackground(small)
Signal: 16.0 § 4.3 () 17.0 § 4.5 (K*0K)
D. Buskulic et al. (Aleph) Phys. Lett. B 294, 145 (1992)
also:P. Abreu et al. (Delphi) Phys. Lett. B 289, 199 (1992)P. D. Acton et al. (Opal) Phys. Lett. B 295, 357 (1992)
Sample: 450K hadronic Z