The Observation of B 0 s – B 0 s Oscillations: a Historical Perspective

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)


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


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)


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


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


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


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%


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)


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


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)


First Time Integrated Measurements

Based on semileptonic B decay:

Correct for:

Coherent incoherent


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


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


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


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


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


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


State of the Art: md


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)


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


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


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)


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


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


Example: Fully Reconstructed Signal

Cleanest decay sequence

Also use 6 body modes:

Add partially reconstructed decays:

Hadronic signal increased from 3600 to 8700


Semileptonic Signals

Semileptonic signal increased from 37000 to 61500


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


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)


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%)


Results: Amplitude Scan

A/A = 6.1 Sensitivity31.3 ps-1

Hadronic & semileptonic decays combined


Measured Value of ms

- log(Likelihood) Hypothesis of A=1 compared to A=0


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


Asymmetry (Oscillations) in Time Domain

Period0.35 ps

Aside: for B0 Period = 12.6 ps


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


Backup Slides


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


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


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


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


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


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


Compare PerformanceData and Simulation

Check prediction for kaon tag on B+, B0

Good agreement between data & MCSystematic based on comparisons

K

K


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)


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


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


Kaons Produced in Vicinity of B’sLarger fraction of Kaons near B0

s compared to B0, B+, as expected

Ph. D. Thesis, Denys Usynin


Example of Candidate

candidate

Same-side Kaon tag

Opposite-side Muon tag

Zoom in oncollision pt.


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


Proper Time & Lifetime Measurement

production vertex25m £ 25 m

Decay position

Decay time inB rest frame


Determination of |Vtd/Vts|

Previous best result: D. Mohapatra et al.(Belle Collaboration)PRL 96 221601 (2006)

CDF


Improvement from Neural Net Selection


Scans for Individual Signatures


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”


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

The Observation of B 0 s – B 0 s Oscillations: a Historical Perspective

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