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??????