LPNHE 11/25/04
Richard Kass
TauTau Physics with Physics with BaBarBaBarRichard Kass
Ohio State University & University of Paris VI,VII
Outline of TalkIntroduction & some factsSearches for Lepton Flavor ViolationTau lifetimeHadronic decays of the tau
τ→ 7 hadronsτ→ 5 hadrons
Summary & Conclusions
LPNHE 11/25/04
Richard Kass
Fact #1Fact #1
Since the cross section for e+e-→Y(4S)→BB
is about the same as the cross section fore+e-→τ+τ−
a B factory is also a τ factory.
LPNHE 11/25/04
Richard Kass
Fact #2Fact #2BaBar is a great detector for τ physics!
1.5 T Solenoid Electromagnetic Calorimeter
(EMC)Detector of Internally
RecflectedCherenkov
Light (DIRC)
Instrumented Flux Return
(IFR)Silicon Vertex Tracker (SVT)
Drift Chamber (DCH)e- (9 GeV)
e+ (3.1 GeV)
SVT, DCH: charged particle tracking ⇒ vertex &mom. resolutionEMC: electromagnetic calorimetry ⇒ γ/π0/ηDIRC, IFR, DCH: charged particle ID ⇒ π/K/p
LPNHE 11/25/04
Richard Kass
BaBarBaBar K/K/π π IDID
D*+ → D0π+
D0→ K+ π-
BaBar DIRC
BaBar (and Belle) are the first e+e- experiments with:excellent charged particle tracking excellent EM calorimetryexcellent Particle ID
and LOTS of data.
LPNHE 11/25/04
Richard Kass
Fact #3PEP-II is a great* accelerator
PEP-II delivered ~254 fb-1
BaBar recorded ~244 fb-1
Total # of τ decays: ~220M
PEP-II delivered ~254 fb-1
BaBar recorded ~244 fb-1
Total # of Total # of ττ decays: decays: ~220M~220M
Peak Luminosity 9 x 1033 cm-2 s-1
LPNHE 11/25/04
Richard Kass
Fact # 4Fact # 4
*Boosting CM is not optimal for τ physicsτ+
9.1 GeV e- 3.0 GeV e+
τ−
Must boost from lab frame to CM frame for tau studieslike to work with one tau per hemisphereharder to unravel tau decay products than at symmetric machines:
for some analyses the penalty may be high!BaBar τ→µγ <2×10-6 56×106 τ-pairs (ICHEP 2002)CLEO τ→µγ <1×10-6 13×106 τ-pairs (PRD 2000)
Detector is not symmetricimpacts tracking systematic errors
CM boost at Y(4S) βγ=0.56
LPNHE 11/25/04
Richard Kass
Search for Lepton Flavor Search for Lepton Flavor Violation (LFV) in Violation (LFV) in tautau decaydecay
In the neutrino sector there is large LFV!neutrino oscillations ⇒ θ23 ≈450, θ12 ≈320
solar neutrinos: νe →νe/3+νµ/3+ντ/3But in the standard model charged LFV is heavily suppressed.
( ) ( )5240
2
2
2
1010)( −− −≈
∆⇒→ OO
m
mB
W
ijµγτ
SUSY models can bring B(τ→µγ) all the way to 10-7!
However LFV OK in many extensions to standard model.
τ µ
γ
ντ νµ
W W
•
LPNHE 11/25/04
Richard Kass
Theory Says…..Theory Says…..
color coding: nownext few yearsnever
LPNHE 11/25/04
Richard Kass
LFV & LFV & bb→→ss!!SUSY GUT models can relate CP asymmetry in B→φKs to B(τ→µγ)!
1 2( )2 223 23( ) ( )
R L
id lm m e ϕ ϕ−≈% %GUT relation:
b→s penguin processes and τ → µγ are related in SUSY GUTs.
2
3
14
5 10 ,
1/ 2,
5 10 ,
tan 30N
m eV
U
M GeV
τ
τ
ν
µ
β
−= ×
=
= ×
=CP asymmetry in Bd →φKs
From J. HisanoTAU 2004
LPNHE 11/25/04
Richard Kass
Current published 90%CL limits on BR(τ → µγ)
11 x 10-7 @90%CL CLEO, 13.8/fb, Phys. Rev. D 61, 071101(2000)
3.2 x 10-7 @90%CL BELLE, 86.3/fb, Phys. Rev. Lett. 92, 171802 (2004)
Search strategy: Use the known beam energy as a constraint
Search for Search for ττ →→ µγµγ
τ → µγMC
Fit for beam-energyconstrained mass: mEC = √(Ebeam
2 – pµγ2)
∆Ε = Ε µγ −Εbeam ∼ 0in absence of radiation
τ → µγMC
LPNHE 11/25/04
Richard Kass
τ τ →→ µγµγ SimulationSimulation
BLINDED
ISR
γ at edgeof acceptance
Signal box: ±2σ of expected mτ±2σ of expected ∆E
Blinding box: ±3σ of expected mτ±3σ of expected ∆E
LPNHE 11/25/04
Richard Kass
PreselectionPreselection StrategyStrategy- Restrict sample to 1-prong Vs 1 or 3-prong events- mEC fit & require γ with Eγ
CM>200MeV- ID track as a µ using a NN selector optimal for this search- keep events with: 1.5 < mEC < 2.1 GeV -1.0 < ∆E < 0.5 GeV
13Mµ ID1.6MmEC, ∆E region
380MmEC fit, Eγ> 200MeV734M1 Vs N prong827Mskimevents preselection
Still lots of background
e+e- → µ+µ- γe+e- → τ+ τ− (γ)e+e- → qq
τ → µ(γ) ν ν (~82%)τ → π(K)(γ) ντ → π(K)π0 ν
Backgrounds surviving preselection
LPNHE 11/25/04
Richard Kass
Signal and TagsSignal and TagsSignal has partner tau decaying to generic modes:
“tag-side”To maintain high efficiency, many tag modes used.Different backgrounds targeted in each mode.
9.1 GeV e- 3.0 GeV e+
τsignal→ µγ
τtag
τtag → eνντtag → eγνντtag → µνντtag → hντtag → h≥1π0ντtag → 3h≥0π0ν
LPNHE 11/25/04
Richard Kass
TagTag--mode dependent selectionmode dependent selection
Global cuts reduce backgrounds:thrust, θmiss, pµCM, ptagCM, tag-side mass signal γ not merge π0
<200MeV of extra γs on signal sideDifferent backgrounds contribute to the different tag-modes:
⇒ optimize selection criteria separately for each tag-mode.
Employ combination of cuts and Neural Net (NN)
NN is trained using data in GSB outside blinded region
Placement of cuts on NN output are optimized using MC (optimized for best expected Feldman&Cousins limit)
LPNHE 11/25/04
Richard Kass
Five well modeled variables before NN, all modes
cosθH(cuts at ±0.8)
-ln(2 pmissT/Ecm)
(cuts at 4 for 3h2 for rest)
-ln(Mmiss/ECM)mν
2
cuts at±0.4 for h, 3h±0.8 for hg
PtagCM
Mmiss=√(Ecm2-Pmiss
2)
Neural NetNeural Net
→ → →
En=(Ecm/2-Ehad)
pν=pτ-phad
LPNHE 11/25/04
Richard Kass
-ln(2 pmissT/Ecm)
-ln(Mmiss/ECM) mν2
PtagCM
After NN (all modes): Still Good agreementNeural NetNeural Net
cosθH
LPNHE 11/25/04
Richard Kass
Results on DataResults on Data
Signal Efficiency = (7.45±0.65)%
Background expected from data using SB:6.2±0.5 eventsUnblind the signal region: 4 events observed
LPNHE 11/25/04
Richard Kass
Results by Tag ModeResults by Tag ModeGood agreement between data and MC for each mode before and after NN in GSB, in predicted background and in selected events.
Data: find 4 events in signal region, predict 6.2MC: 2.8±1.7 events (66% µ-pair; 30% τ-pair)
LPNHE 11/25/04
Richard Kass
ττ−−→→µµ−−γγ ResultResult
Total signal efficiency = (7.45±0.65)% Nbkg= 6.2±0.5 events → Nobs= 4 eventsNτ+τ−= 1.97 x 108 (full BaBar data set)
PreliminaryPreliminary
BR(τ−→µ−γ) < 0.9 x 10-7 @ 90%CL
Including systematic errors does not change limit to the level of significance quoted
BELLE: 3.2 x 10-7 @90%CL 86.3/fb Phys. Rev. Lett. 92, 171802 (2004)
LPNHE 11/25/04
Richard Kass
Search for LFV: Search for LFV: τ→τ→llllll & & τ→τ→lhhlhhSUSY Models: B(τ→lll) & B(τ→lhh) in range 10-17 to 10-6
h0τ-
µ-
µ-
µ+
Pre B-Factory measurements at CLEO: B(τ→µµµ) < 1.9 × 10−6 90% C.LB(τ→eee) < 2.9 × 10−6 90% C.LB(τ→e+π-π-) <1.9 × 10-6 90% C.L + many other modes….
LPNHE 11/25/04
Richard Kass
Search for 20 different decay modes 3 leptons: 3e, 3µ, µee (2), eµµ (2)ehh: eKK (2), eKπ (3), eππ (2)µhh: µKK (2), µKπ (3), µππ (2)
Tag Side: One well identified tagging track with missing momentumSignal Side: 3 tracks, identified by PID No missing momentum allowed on signal side
different charge statese+µ−µ− and e-µ+µ−
Analysis TechniqueAnalysis Technique
LPNHE 11/25/04
Richard Kass
Analysis TechniqueAnalysis Technique
Use ∆M = Mec- Mτ and ∆E = Eec(CM)- ECM/2Signal has ∆M, ∆E ≈ 0
∆M, ∆E smeared by detector and radiative effectsSignal box is optimised for each channel.
MC: τ→µeeSignal box in red
LPNHE 11/25/04
Richard Kass
Background Estimate from DataBackground Estimate from DataFor each mode fit the data in the (∆M, ∆E) sidebandto estimate background in signal region
ττhadronic
QED
∆M ∆E
LPNHE 11/25/04
Richard Kass
Published Results For τPublished Results For τ→→llllllPRL 92, 121801 (2004) (91.5 fb-1)
• DATA
LPNHE 11/25/04
Richard Kass
Results For Results For τ→τ→lhhlhh
(221.5 fb-1)
No signals, just upper limits • DATA
similar resultsfor 6 othermodes!
LPNHE 11/25/04
Richard Kass
Summary of 3 Body LFV DecaysSummary of 3 Body LFV Decays
No signals found Set limits O(10-7) for 20 LFV modes (6 lll and 14 lhh)Limits have met up with upper end of theoretical predictions
eg: SUSY with Higgs Triplet: B(τ→ lll) is 10-7
Can probe 10-8 (SUSY) region with higher statistics:⇒ backgrounds are under control:
For τ → lll analysis expect 3.41 events find 3.For τ → lhh analysis expect 11.1 events find 10.
LPNHE 11/25/04
Richard Kass
TauTau Lifetime MeasurementLifetime Measurement
Motivation: precision check of lepton universality
LPNHE 11/25/04
Richard Kass
Lifetime Measurement TechniqueLifetime Measurement TechniqueReconstruct decay length (λt) in transverse plane onlyConvert transverse decay length to 3D using: λ=λt/sinθ
approximate direction (θ) of tau using 3-prong momentum vectorDo not use decay length errors as weights
equalize acceptance in φ using 60 binsAverage the λ measurements to get <λ>
do not use maximum likelihood methodcalculate statistical error in <λ> using variance of 100 subsamples
Calculate the average lifetime using:
><>=<
τ
τττ λτ
PM
<Pτ> calculated using MC Koralb (1st order)KK2f (2nd order, exponentiation)
Subtract measurement bias using MC
LPNHE 11/25/04
Richard Kass
TauTau Lifetime AnalysisLifetime AnalysisPhilosophy: sacrifice statistics for puritySelect events where the tag tau decays via τtag → eνν
use electron IDSignal tau decays into 3-prong
no electrons or γ-conversions allowedno particle ID for the 3 hadrons
Reduce backgrounds due to bhabhas, 2-photon, hadrons using:thrustvisible charged energy (Ech)missing energy (√s-Ech-Eγ’s)min and max 1-prong momentumsinα=PT/(√s-Ech)
High quality charged particle tracking≥6 SVT hits on all 3-prong tracks3-prong vertex fix P(χ2)>1%cut on vertex error
BLINDED ANALYSIS
Analysis uses 80fb-1
Total efficiency: 0.44%~ 310k events
LPNHE 11/25/04
Richard Kass
Data Vs MCData Vs MCVery detailed MC-data studies.Excellent agreement between MC and data
LPNHE 11/25/04
Richard Kass
TauTau Decay Length MeasurementDecay Length Measurement
λτ=λt/sinθ
<λτ>=238.145±0.752 µm
Detailed MC study to measure the bias in this procedure: event selection: -0.029±0.127% tau momentum reconstruction: +0.016±0.010%decay length reconstruction: +0.350±0.179% Total: +0.336±0.220%
LPNHE 11/25/04
Richard Kass
TauTau BackgroundsBackgrounds
bhabha and 2-photon backgrounds: use data to estimatehadronic backgrounds: use combination of data and MC
Note: large systematic uncertainty for continuum events (u,d,s,c).But negligible effect since contamination is very small.
LPNHE 11/25/04
Richard Kass
Everything looks great Everything looks great BUTBUT:
MC
:
Data
Measured decay length Vs φ is not flat!(but decay length Vs θ is!)
Believe azimuthal variations in data are related imperfect SVT alignmentDetailed studies using different SVT alignment sets show that thevariations in φ DONOT lead to a decay length bias at 0.1% level.
LPNHE 11/25/04
Richard Kass
Biases and Systematic ErrorsBiases and Systematic Errors
total bias total systematic
LPNHE 11/25/04
Richard Kass
TauTau Lifetime ResultsLifetime Results
ττ = 289.4 ± 0.91 ± 0.90 fs preliminary
Lifetime asymmetrypreliminaryno systematic error yet
LPNHE 11/25/04
Richard Kass
Lepton UniversalityLepton Universality
LPNHE 11/25/04
Richard Kass
Search for 7Search for 7--prong prong ττ decaysdecaysVery rare – no observation to date.
Previous Experimental Result:BR(τ → 7π(π0)ντ ) < 2.4 × 10-6 (CLEO, 1997, PRD 56, 5297)
Theory: Low BR a bit of a strong interaction puzzle. BR(1-prong): ~8.5×10-1
BR(3-prong): ~1.4×10-1
BR(5-prong): ~1×10-3
BR(τ→5hπ0ντ): ~2×10-4
BR(7-prong): <3×10-6
Perhaps can be explained…(S. Nussinov, M. Purohit, 2002, PRD 65)BR(τ → 7π(π0)ντ) < 6 × 10-11 (assumes no substructure)
BaBar’s Motivation:25 × CLEO’s statistics → possible observationLarge enough sample:
could help put bound on tau neutrino massinteresting channel to study resonant substructure
LPNHE 11/25/04
Richard Kass
e-
e+τtag
τrec
MC 1-7 event
1-prong side
7-prong side
Look for events satisfying 1-7 topology!Tag the 1-prong as:
electron +0 or 1 γmuon +0 or 1 γπ/K, 0 γρ, 0 γ
Develop criteria for the 7-prong that reject backgrounds:B mesonshadronic events (u, d, s, c)cross feed from tau events
τ−→(5π)−π0
γγe+e-
77--Prong Search StrategyProng Search Strategy
bhabhas, 2-photon, µ−pairs negligible backgroundsInvariant mass of 7-prong side is the key!
LPNHE 11/25/04
Richard Kass
77--Prong Search StrategyProng Search StrategyInvariant mass of 7-prong side must satisfy: m7 <mτ (mτ~1.78 GeV)
Signal region
BABARpreliminary
Mass (GeV/c2)
Analysis is “blinded”: events below 2 GeV/c2 are removed from the data.
LPNHE 11/25/04
Richard Kass
PseudomassPseudomass!!
Pseudomass was introduced by ARGUS in 1992 to measure the τ−lepton mass.
Assume neutrino is massless
τ direction is approximated by 7 ch. tracks
m*τ2=2(Ebeam – E7π)(E7π – P7π)+m7π
2
MC 7-prong Invariant and Pseudomass
BABARpreliminary
Mass (GeV/c2)
Eve
nts /
0.0
05 G
eV/c
2
Advantage of pseudomass:
Sharp cut-off at the τ mass (1.777 GeV/c2).
significant improvement of signal-background separation BR=2.4×10-6
BABARpreliminary
Invariant Mass (GeV/c2) Pseudomass (GeV/c2)
BABARpreliminary
Mass (GeV/c2)E
vent
s / 0
.01
GeV
/c2
Eve
nts /
0.0
1 G
eV/c
2
MC
MC MC
LPNHE 11/25/04
Richard Kass
Background EstimateBackground Estimate
extrapolate
integrate
fitµ, σ
Pseudomass (GeV/c2)Pseudomass (GeV/c2)
DATA After thrust cut DATA After all cutsBABAR
preliminary
Eve
nts /
0.0
25 G
eV/c
2
Eve
nts /
0.0
25 G
eV/c
2
Fit pseudomass distribution from 2 to 2.5 GeV/c2 after thrust cut with a Gaussian function
Use these fit parameters on the pseudo-mass spectrum after all cuts.
Extrapolate the fit below 2 GeV/c2
Integrate from 1.3 to 1.8 GeV/c2 (signal region)
Check procedure using data and MC
LPNHE 11/25/04
Richard Kass
Background Estimate ValidationBackground Estimate ValidationUse 1-8 prong DATA
12.0 ± 0.61-prong tag87.7 ± 2.3DOCAXY/pT
2219 ± 5pT
3229 ± 77-prong π ID5741 ± 10Thrust mag.
Observed Expected Cuts
Pseudo-Mass (GeV/c2)
1-8 Data after thrust cut 1-8 Data after all cutsBABAR
preliminary
Eve
nts /
0.0
25 G
eV/c
2
Eve
nts /
0.0
25 G
eV/c
2Pseudo-Mass (GeV/c2)
BABARpreliminary
Also do MC studyexpect: 1.8 ± 0.7observe: 1
LPNHE 11/25/04
Richard Kass
Preliminary ResultsPreliminary Results
Pseudo-Mass (GeV/c2)Pseudo-Mass (GeV/c2)
After all cutsAfter thrust cut
BABARpreliminary
BABARpreliminary
signal regionextrapolation of fit
Eve
nts /
0.0
25 G
eV/c
2
Eve
nts /
0.0
25 G
eV/c
2
Events in signal Region:observed events: 7expected background: 11.9 ± 2.2
No evidence for 7-prong τ decay
LPNHE 11/25/04
Richard Kass
Preliminary 7Preliminary 7--prong BR Upper Limitprong BR Upper Limit
Ν τ+τ− 1.1 × 108
τ+τ− background 0.6 ± 0.4
qq background 11.3 ± 2.2
Total expected background 11.9 ± 2.2
τ− → 4π− 3π+ ντ efficiency (8.05 ± 0.55) %
τ− → 4π− 3π+ π0 ντ efficiency (8.04 ± 0.55) %
BR (τ− → 4π− 3π+ (π0) ντ ) @ 90% CL < 2.7 × 10-7
B A
B A
R
p r
e l i
m i
n a
r y
systematicerror
Experiment CLEO (1997) BaBar
Luminosity (fb-1) 4.6 124.3
Observed (predicted) events 0 (2.8) 7 (11.9)
BR (τ− → 4π− 3π+ (π0) ντ ) @ 90% CL < 2.4 × 10-6 < 2.7 × 10-7
LPNHE 11/25/04
Richard Kass
τ→→ 5(hadron)5(hadron)νν final Statesfinal StatesTau semi-leptonic decays are an ideal place to study
strong interaction effectssum rules, spectral functions, Wess-Zumino anomaly……
The large sample of tau data from BABAR allows us to re-examine many of the well-known hadronic decays and to look in detail at the decays that were previously limited by statistics
(Previous 5-prong studies done with ~300 events)
This study focuses the 5-hadron final states: τ−→2π−π+2π0ντ (η, ω, φ→π−π+π0)
τ−→3h−2h+ντ (no particle ID for hadrons)
LPNHE 11/25/04
Richard Kass
Analysis StrategyAnalysis StrategyReduce non-tau backgrounds using standard techniques:
event variables: thrust, transverse momentum, missing energysplit event into signal and tag hemispheres (1 Vs 3 or 1 Vs 5)
require 1-prong tag to be either electron or muonτ−→2π−π+2π0ντ τ−→3h−2h+ντ
• Tracks required to be π’s• 2 π0 candidates• M(5pion) < 1.8 GeV
9900 candidates• Background 15%
Mainly tau decay background
• Efficiency 0.65%
• π or kaon tracks allowed• Veto electrons, γ
conversions and π0’s• M(5hadron) < 1.8 GeV
15869 candidates*• Background 21%
tau decays with π0’s or K0’s
• Efficiency 4.0%
*PDG result based on ~ 300 events
LPNHE 11/25/04
Richard Kass
Comparison with other measurementsComparison with other measurementssystematic errors:
τ−→2π−π+2π0ντ : 12.3%dominated by the background (9.3%) and π0 finding (6.5%)
τ−→3h−2h+ντ : 4.7%tracking (3.1%), luminosity×σ (2.3%), π0s (2%)
Will decrease soon!
6 8 10 126 8 10 12
BaBar 8.5 (0.4)
OPAL 9.1 (1.5)
ALEPH 8.7 (1.7)
CLEO 7.7 (1.0)
PDG 8.2 (0.6)
4( 3 2 ) (10 )B h h ττ ν− − + −→0 3
BABAR 4.51 0.07 0.56CLEO 5.2 0.5ALEPH 5.0 0.
( 2 2 ) (1
.7
0
7
)
0
B ττ π π π ν− − + −
±±
→± ±
±
Preliminary
Good Agreement!
LPNHE 11/25/04
Richard Kass
Structure in Structure in ττ−−→→2π2π−−ππ++2π2π00ννττ
0
20
40
60
80
100
120
140
0.5 0.52 0.54 0.56 0.58 0.6
BA BA RP R E L I M I N A R Y
M(3π) / GeV/c2
Entri
es /
2 M
eV/c2
BA BA RP R E L I M I N A R Y
0
200
400
600
800
1000
1200
0.7 0.72 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.88 0.9
BA BA RP R E L I M I N A R Y
M(3π) / GeV/c2
Entri
es /
5 M
eV/c2
BA BA RP R E L I M I N A R Y
η→π−π+π0
ω→π−π+π0
Β(τ−→ηπ−π0ντ)=(1.71±0.12±0.16)×10-3
Β(τ−→φπ−π0ντ)< 4.5×10-4
90% CL
0
50
100
150
200
250
0.9 0.95 1 1.05 1.1 1.15 1.2
M(3π) / GeV/c2
Ent
ries
/ 5
MeV
/c2
BA BA RP R E L I M I N A R Y
φ→π−π+π0
first upper limit!
3(1.74 0.24) 10 PDG−± ×
proceeds through Wess-Zumino anomaly
Β(τ−→ωπ−π0ντ)=(3.66±0.10±0.31)×10-3
3(4.3 0.5) 10 PDG−± ×
η’s and ω’s but No φ’s!
LPNHE 11/25/04
Richard Kass
The fun here is just beginningThe fun here is just beginningUnraveling all the substructure is a challenge
→many possible final states.states with charged K’smany resonances:
e.g. have hint of τ−→f1(1285)π−ντ (Β(f1(1285)→2π−2π+)=11% )understanding of 5 prong final states →improvement in ντ mass limit
m(5h)
τ−→3h−2h+ντ Also a challenge to QCDand model builders!
Present MCs (tauola/kk2f)use phase space which
does not represent data!
LPNHE 11/25/04
Richard Kass
Summary & ConclusionsSummary & ConclusionsBaBar is now turning out tau results!
new results in many areas: searcheslifetimeBRsresonant substructure
Many more results to come:more LFV searches, e.g. τ→eγsecond class currents (e.g. τ→ηπν, wrong G parity for vector current)
final states with kaons (BRs, spectral functions, Vus, ms)improved limit on mass of tau neutrino (~10MeV)searches for CPV??????