Post on 13-Jan-2016
description
Achim StahlRWTH Aachen University Beijing, June 2006
mass: 1.777 GeV
lifetime: 290.6 10-15 sec
cτ = 87.11 m
approx. 100 known decays
τυτ
Wf
f’
√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
τ-pairs
background
set points
1. below threshold √s = 3.50 = 0 nb
1 nb ≈ 107 ττ
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
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
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
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
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
kinematic decay identification
ττ
τKτ
ττ
p in GeV
Ecms = 4.5 GeV
kinematic decay identification
τττ
τaττ
τττ
Emeasured - Ehad (mhad)
fast simulation: finite p-resolution finite E-resolution realistic efficiency fake from hadrons
kinematic decay identification
τττ
τK*τKτ
Emeasured - Ehad (mhad)
fast simulation: finite p-resolution finite E-resolution realistic efficiency fake from hadrons
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
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)
Electromagnetic Calorimeter
hermeticity minimal dead material best resolution
CsI(Tl) crystals
about 45% of all τ-decays contain at least 1 0
BELLE
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
tau-massbest result from BES:
1776.96 MeV+0.18 +0.25- 0.21 - 0.17
systematics limited! beam-calibration energy spread efficiency background
PDG: 140 decay modes (excluding LFV)
All have their own interesting aspects
Examples:e / lepton universality / K f, fK
0 CVC, , ’, ’’ 2nd class current… …
describe the mass spectrum of hadrons produced in τ-decays
sensitive to: S, mS, C, many QCD tests
example: running of S
τ-decays
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
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
model independent interpretation:search for arbitrary new currents
but …
leptonicdecays
… the LHC will probably tell us what to look for.
wild guess:
Precise measurement of couplings at tau-charm-factory
~
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
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 …
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
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…