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Page 1: Tau Physics near Threshold

Achim StahlRWTH Aachen University Beijing, June 2006

Page 2: Tau Physics near Threshold

mass: 1.777 GeV

lifetime: 290.6 10-15 sec

cτ = 87.11 m

approx. 100 known decays

τυτ

Wf

f’

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√s in GeV

in

nb

Ruiz-Femenía, Pich hep-ph/0210003

= 4 2

3 s3 – 2

2tau production near threshold

for L = 1033 / cm2 s

1 year running@ s = 1 nb

107 τ-pairs√s in GeV

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τ-pairs

background

set points

1. below threshold √s = 3.50 = 0 nb

1 nb ≈ 107 ττ

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τ-pairs

background

set points

1. below threshold √s = 3.50 = 0 nb

2. at threshold √s = 3.55 = 0.1 nb

1 nb ≈ 107 ττ

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τ-pairs

background

set points

1. below threshold √s = 3.50 = 0 nb

2. at threshold √s = 3.55 = 0.1 nb

3. below (2s) √s = 3.68 = 2.4 nb

1 nb ≈ 107 ττ

Page 7: Tau Physics near Threshold

τ-pairs

background

set points

1. below threshold √s = 3.50 = 0 nb

2. at threshold √s = 3.55 = 0.1 nb

3. below (2s) √s = 3.68 = 2.4 nb

4. max. cross section √s = 4.25 = 3.5 nb

1 nb ≈ 107 ττ

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Taus are produced at rest (Tauonium atom) Highly efficient and clean tagging of taus Kinematic decay channel identification Excellent particle identification

Non-Tau background measured below threshold

Low cross section (0.1 nb)

Experimentally most favored situation

Not good for rare decays

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Kinematics of 2-body decays

τ

τ had τ Ehad = mτ

2 + mhad2

2 mτ

phad = mτ

2 - mhad2

2 mτ

phad (mhad)

pmeasured - phad (mhad) = 0 ?

kinematic constraint

for example:ττ p = 883 MeV

τKτ pK = 820 MeV

had

τ

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kinematic decay identification

ττ

τKτ

ττ

p in GeV

Ecms = 4.5 GeV

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kinematic decay identification

τττ

τaττ

τττ

Emeasured - Ehad (mhad)

fast simulation: finite p-resolution finite E-resolution realistic efficiency fake from hadrons

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kinematic decay identification

τττ

τK*τKτ

Emeasured - Ehad (mhad)

fast simulation: finite p-resolution finite E-resolution realistic efficiency fake from hadrons

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ττ

had

ToF

had = mτ

2 – mhad2

mτ2 + mhad

2

most difficult decay:ττ vs. τKτ

= 0.987 t = 3.34 nsec

K = 0.856 t = 3.88 nsec

for 1m flight distance

with 100 psec resolution at least 5 separation

Time-of-Flight

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low mass drift chamber

ττ p = 883 MeV

τKτ pK = 820 MeV

momentum resolution < 1%(BES-III design ≈ 0.5% @ 1 GeV)

particle-ID through dE/dx (ex. BaBar)

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Electromagnetic Calorimeter

hermeticity minimal dead material best resolution

CsI(Tl) crystals

about 45% of all τ-decays contain at least 1 0

BELLE

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Hadron Calorimeter

about 1.5% of all τ-decays contain a K0

K0S drift chamber

K0L hadron calorimeter

almost all physics can be done with K0S

some veto capability against K0L would be good

muon identification with hadron calorimeter

high granularity, medium resolution, no muon chambers

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tau-massbest result from BES:

1776.96 MeV+0.18 +0.25- 0.21 - 0.17

systematics limited! beam-calibration energy spread efficiency background

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PDG: 140 decay modes (excluding LFV)

All have their own interesting aspects

Examples:e / lepton universality / K f, fK

0 CVC, , ’, ’’ 2nd class current… …

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describe the mass spectrum of hadrons produced in τ-decays

sensitive to: S, mS, C, many QCD tests

example: running of S

τ-decays

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OPAL Euro. Phys. J. C35 (’04) 437

non-strange v strange vnon-strange a strange a

large uncertainties; especially in the strange sector

approx. 500 ev.+ 500 bgd

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τ

τ

hadronsor

leptons

M = 4 G/√2 giℓ | i | ℓτiτ

S,V,T

L or R

L or R

(example: leptonic decays)

derived from spectraand angular distributions

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model independent interpretation:search for arbitrary new currents

but …

leptonicdecays

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… the LHC will probably tell us what to look for.

wild guess:

Precise measurement of couplings at tau-charm-factory

~

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QCD tests + s:non-strange spectral function (much better resolution!)strange spectral function (real measurement, v/a, … )2nd class currents, Wess-Zumino anomalyPT: test predictions

Exclusive decays:many branching ratios can be improvedlight meson spectroscopy (i.e. , ’, 0 vs. ±)

Tau-mass:can you reduce calibration systematics compared to BES II?

Michel parameters:substantial improvements possibleyou will probably know, what you are looking for

VUS from inclusive strange decays:theory under control?

Exotics:CP-violation in tau-decays(g-2)τ

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What you cannot do at tau-charm:

o rare decays (i.e. lepton-flavor violation)

o tau lifetime ( universality with -decays)

o CP-violation in τ-production (needs high q2)

o neutral current couplings

o τ mass (once was a very hot topic)

o …

Page 27: Tau Physics near Threshold

1 month @ threshold:- 100.000 very clean tau pairs- enough to improve many existing measurements- understand background and efficiency for higher energy running

1 month below threshold- calibrate non-tau background- tune u,d,s Monte Carlos

During the initial running period:

During a later stage:

More running @ thresholdUse high energy runs for some topics

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Thank you

Tau physics near threshold:

Excellent experimental conditions for high precision measurements

Needs an excellent detector, but all requirementswithin today's possibilities

Needs an excellent accelerator, with luminosity ≈ 1033/cm2 s and a not too large energy spread

Much to be done, despite CLEO, LEP, b-fact…