Bs Mixing at CDF

1
Bs Mixing at CDF
Jónatan PiedraUniversity of Paris VI
January 26, 2006 Paris
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Introduction
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Symmetries in Particle Physics

• Three fundamental symmetries
• parity P reversal of spatial coordinates
• CAN BE BROKEN (ASYMMETRIC b -RAY SPECTRUM IN
  60Co)
• charge conjugation C swap particle and
  antiparticle
• CAN BE BROKEN (SEEN IN
  DECAYS)
• time reversal T reverse direction of time flow
• CAN BE BROKEN (SEEN AS DECAY
  RATES DIFFER)
• Combined symmetries, not as restrictive
• combined charge-parity CP universe of reflected
  antiparticles
• CAN BE BROKEN (IN DECAYS OF KAONS AND B MESONS)
• combined charge-parity-time CPT add backward
  time flow
• HOLDS (NO EXPERIMENT EVER OBSERVED ANY VIOLATION)

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Standard Model of Particle Physics
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Quark Interactions in the SM

• The eigenstates of the weak interaction are
  different from those of the strong interaction ?
  mixing in quark families
• Flavor changing interactions via W bosons
• Amplitude of transitions proportional to CKM
  elements Vq1q2

V IS THE CABIBBO-KOBAYASHI-MASKAWA (CKM) MATRIX
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CKM matrix elements

• CKM element values are not predicted by Standard
  Model
• Among the major free parameters along with quark
  masses
• Have to be measured
• How to measure Vq1q2
• Particle decays. Channels, probabilities,
  lifetimes
• Transitions between neutral flavor eigenstates
  mixing

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Unitarity Triangle

• Standard Model requires CKM matrix to be unitary
• UU ? 1 ? 4 independent parameters for a 3?3
  matrix
• 3 angles and 1 complex phase
• complex phase leads to CP violation
• Wolfenstein parameterization up to ?(l4)
• least known parameters are r and ?
• From unitarity constrains

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Present Experimental Knowledge

• Each colored region represents an experiment
• MIXING, DECAY RATES, CP EFFECTS
• Overcostrain
• ARE SIDES ANGLES CONSISTENT?
• Probe for New Physics

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Neutral Meson Mixing

• Quark mixing ? non-diagonal Hamiltonian for
• Diagonalizing the Hamiltonian results in
• two masses mH and mL, with Dm ? mH mL
• two decay widths GH and GL, with DG ? GH GL
• G ? 1/t

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B Oscillations

• Two-state mixing system
• Heavy and Light weak eigenstates
• and mass eigenstates

q QUARK s, d

• Solution in proper decay time

Dmd,s IS THE MIXING PARAMETER
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Goal at Fermilab

• SM prediction for the ratio of Bs and B0 mixing
  frequencies
• measure ? find to 2.5
• Dmd very precisely measured
• Dms has not been seen
• potential New Physics discovery

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Tevatron and CDF II
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The Tevatron Collider
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The Tevatron Collider

• Currently the highest energy particle accelerator
  in the world
• 980 980 GeV proton-antiproton collisions
• 900 900 GeV in Run I

• Underground ring with r 1 km
• Main Injector replaced Main Ring
• 150 GeV proton storage ring
• 2 multi-purpose detectors
• D? and CDF II

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The CDF II Detector
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The CDF II Detector

• Inherited from Run I
• 1.4 T Solenoid
• Partially new
• Muon system (up to ? 1.5)
• New
• Tracking System
• Silicon Tracker (up to ? 2)
• Faster Drift Chamber
• Time-of-Flight (particle ID)
• DAQ system, front end electronics

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B Physics at the Tevatron

• Production rates are orders of magnitude higher
  than at ee- ? ?(4S)
• Heavy hadron states produced (unlike B factories)
• B, B0, Bs, Lb0, Bc, ?b
• High energy ? long travel distance
• Proton-antiproton collision ? parton energy is
  unknown
• The other b-hadron is often out of the fiducial
  volume
• Contamination from the underlying event
• Backgrounds are 3 orders of magnitude higher
• huge inelastic cross section 100 mb ? 1 B decay
  103 QCD

A dedicated selective trigger is needed -
efficiency lt ?(1)
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B Triggers

• Conditions
• deadtimeless
• time between collisions 396 ns ? 80 events/s to
  tape
• B features
• travels 0.1 1 mm
• signatures e, m, high pT tracks, displaced
  tracks
• Upgrade for Run II
• trigger on displaced tracks ? d0
• online offline d0 resolution
• Main b, c hadron triggers
• di-muon B ? J/? X, J/? ? mm
• leptondisplaced track B ? ln X
• two-displaced tracks B ? Dp

1 mm
the trigger has to figure out which 80 of 2.5
million events it should save/s
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SVX II Detector and SVT Crates
intrinsic

• IP resolution 48 ?m 35 ?m ? 33 ?m

transverse beam size
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SVT

• Design and construction of the SVT have been a
  significant step forward in the technology of
  fast track finding
• Performance is as expected
• A trigger based on impact parameter
  
  allows data acquisition leading to
  significant physics results
• B Physics (but not only!) at hadron colliders
  substantially benefits from online tracking with
  offline quality

d0 cut at L2 Trigger ? CDF II is a new detector
in B Physics
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Oscillation Analysis Components
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Ingredients
Trigger Side
Opposite Side
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Proper Decay Time Reconstruction
semileptonic
hadronic
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Resolution Effect

• Resolving rapid oscillations is challenging

hadronic case
ideal
semileptonic case
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b-Flavor Tagging

• A flavor tagger determines the b-flavor at
  production time
• b quark pair production
• flavor tagging on the Trigger Side or the
  Opposite Side
• Soft Lepton Tagger
• look for B ? ln DX decay on the OS
• lepton charge indicates b-flavor
• Jet Charge Tagger
• look for a jet from OS b-hadron
• jet charge indicates b-flavor

Trigger Side
Opposite Side
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Dilution Effect

• A flavor tagger not always can be applied
• It can give a wrong answer. The dilution D is a
  measurement of the purity of the tagger ? a
  random (perfect) tagger has D 0 (1)
• The dilution attenuates
  the
  observed oscillations
• EVENT-BY-EVENT DILUTION IN THE FITS

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Bs Mixing

• Dms challenge, fast oscillations
• precise vertex
• precise momentum
• tagging essential
• many signal events
• low background
• Sensitivity to mixing
• NIM A 384 491, Moser Roussarie

Dmd 0.5 ps-1 Dms ? 14.4 ps-1
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Mass and Lifetime Analysis
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Sample Composition

• Two steps performed to select signal
• online (trigger) selection
• offline (using simulation and data) selection
• How do we know if a candidate is a real B meson?
• for a single candidate, we dont
• we can give the probability for a candidate to be
  signal

partially reconstructed b-hadrons decays with
particles lost by the tracking misreconstructed
b-hadrons when a particle has been wrongly
identified, e.g. B ? DK as B ? Dp combinatorial
background at least one track isnt from a
b-hadron
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Mass Spectrum Analysis

• Many B, B0 and Bs decays examined
• ONLY TWO EXAMPLES ARE SHOWN
• Bs ? Ds p, Ds ? ? p
• fit B mass
• B ? mD0X
• both B and B0 contribute
• fit D mass

355 pb-1
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Hadronic Modes Decay Time

• Determine proper decay time
• Expected distribution in nature for particle
  decay
• Measurement introduces two effects
• detector resolution Gauss(ct,sct)
• trigger/selection sculpting ?(ct)
• What we see in data

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Semileptonic Modes Decay Time

• X is not found ? missing pT
• Determine pseudo-ct from data
• ct ct K, estimate K from MC
• P(ct) ? P(ct)
• include K effect in signal PDF

F(K)
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Lifetime Fits

• Many fits for B, B0 and Bs
• ONLY TWO EXAMPLES ARE SHOWN
• Bs ? Ds p, Ds ? K K
• fit ct
• notice the log scale

• B ? mD0X
• fit ct
• several background components

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B0 Mixing and Taggers Calibration
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Flavor Analysis on B and B0

• Calibrate flavor taggers ? find D (event
  quantities)
• unbinned likelihood fit of m, ct, flavor
• simultaneous analysis of B and B0
• complex sample composition
• Measure B0 mixing
• cross-check for Bs mixing
• direct fit for Dmd
• slow oscillations resolved easily

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Fit for Bs Oscillations
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Fourier Analysis

• Two domains to fit for oscillations
• time ? fit for a cosine wave
• frequency ? examine f-spectrum
• Time domain approach
• fit for Dms in P(t) 1 ? D cos(Dmst)
• Frequency domain approach
• introduce amplitude, P(t) 1 ? AD cos(Dmst)
• fit for A at different Dms
• ? obtain frequency spectrum A(Dms)
• method is called amplitude scan

time domain
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Amplitude Scan on Dmd

• Consider example of B0 mixing
• amplitude values and error bars come from
  unbinned likelihood fit
• yellow band ? 1.645sA around data points
• DEFINES THE 95 CL REGION
• Dm values where A 1.645sA lt 1 are excluded at
  95 CL

• sensitivity is Dm where 1.645sA 1
• MIXING WITHIN SENSITIVITY EXPECTED
• simultaneous amplitude scan of B0 ? Dp and B0 ?
  J/? K
• blue band ? time domain fit

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Amplitude Scan on Dms

• Amplitude scan method ? easy to combine results

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World 2005 and CDF Combined

• SIGNIFICANT IMPACT ON WORLD AVERAGE

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Conclusions

• Probe CP violation at Tevatron
• Bs mesons are uniquely produced at the Tevatron
• will measure fundamental ratio
• has potential for New Physics effects
• Dms is a gold plated test of Standard Model
• CDF sensitivity alone, Dms gt 13 ps-1 (95 CL)
• Soon Tevatron taking over world average

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Improvements

• Better tagging
• add Same Side Kaon Tagger (SSKT)
• current eD 2 1.6
• SSKT will provide eD 2 3
• More decay channels
• add inclusive hadronic Bs
• More data
• here 355 pb-1
• on tape gt1.1 fb-1
• add other trigger paths
• Better vertex resolution

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Sensitivity Projections

• dark green this result
• (Fall 2005)
• current Winter 2005 result
• baseline
• expected improvements
• stretched
• stronger improvements
• Fall 2005 improves significantly Winter 2005

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Matter-Antimatter Asymmetry

• Universe appears to contain dominantly matter
• Very little antimatter observed
• Why not symmetric?
• Connection to Particle Physics via Sakharov
  Conditions
• NECESSARY TO EXPLAIN OUR CURRENT BARYON
  ASYMMETRICAL UNIVERSE
• Non-conservation of baryon number
• ? proton must decay
• CP symmetry is violated
• ? different behavior for particles and
  antiparticles
• Withdrawal from thermal equilibrium
• baryon-asymmetry generating reaction lt Universe
  expansion rate

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A Closer Look at Mixing

• Example becomes
• other mixing systems exist
• involves 6 of CKM matrix elements
• non-zero off-diagonal elements required for
  mixing
• quark transitions across quark generations

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How to observe CP violation

• Experimental point of view
• different behavior of particle and antiparticle ?
  different decay rate
• CP-violating interference need several paths to
  final state
• Direct CP violation. Decays of neutral/charged
  particles
• EXAMPLE
• Indirect CP violation. In mixing and decay of
  neutral particles
• EXAMPLE

meson
final state
antimeson
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Mixing in the Standard Model

• Oscillation frequency depends on Vq1q2 and mq in
  SM
• very slow oscillations of D0 difficult to see
• fast oscillations of Bs difficult to see
• for Bq (q d,s) in the SM
• Amount of CP violation also depends on Vq1q2 and
  mq in SM

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The Accelerator
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Why B Physics?

• Improves the Standard Model (SM) knowledge by
  constraining CKM matrix elements
• New Physics probe, by additional contributions in
  tree/loop diagramas
• Rare decays in the SM (tree-level suppressed)
• Penguin decays of B mesons
• Bs0 mixing
• We observe hadrons, not free quarks
• Strong interaction is non-perturbative at low
  energy scale
• Validation of theoretical methods applied on
  non-perturbative calculations
• Measurement of masses and lifetimes
• QCD probe at low energy scale

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Event Recorded by the COT
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Roadmap for the Data

• Samples
• use the displaced track trigger to collect data
• reconstruct B, B0 and Bs decays to ln DX and
  D(3)p
• Mass and lifetime analysis
• mass fits ? understand sample composition and S/B
• ct fits ? understand and measure ct
• b-Flavor Taggers
• calibrate opposite side taggers, measure D and
  Dmd
• calibration in large B and B0 samples eD2
  1.55
• use calibrated tagger dilution in fit for Bs
  mixing
• Fourier analysis of flavor oscillations
• find limits on Dms from amplitude scan

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Online Selection
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Decay Time Efficiency Curve

• SVT trigger and selection cuts sculpt the proper
  decay-length distribution

• Correct with an efficiency function ?(ct)
  determined in MC