Achim Stahl RWTH Aachen University Beijing, June 2006

mass: GeV lifetime: sec c  =  m approx. 100 known decays   W f f’

√s in GeV  in nb Ruiz-Femenía, Pich hep-ph/  =  4   2 3 s 3 –  2 2 tau production near threshold for L = / cm 2 s 1 year s = 1 nb  10 7  - pairs √s in GeV

 -pairs background set points 1. below threshold √s = 3.50  = 0 nb 1 nb ≈ 10 7 

 -pairs background set points 1. below threshold √s = 3.50  = 0 nb 2. at threshold √s = 3.55  = 0.1 nb 1 nb ≈ 10 7 

 -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 ≈ 10 7 

 -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 ≈ 10 7 

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    had  E had = m  2 + m had 2 2 m  p had = m  2 - m had 2 2 m  p had (m had ) p measured - p had (m had ) = 0 ? kinematic constraint for example:      p  = 883 MeV    K   p K = 820 MeV had 

kinematic decay identification         K         p in GeV E cms = 4.5 GeV

kinematic decay identification               a                     E measured - E had (m had ) fast simulation:  finite p-resolution  finite E-resolution  realistic  efficiency  fake  from hadrons

kinematic decay identification               K*     K     E measured - E had (m had ) fast simulation:  finite p-resolution  finite E-resolution  realistic  efficiency  fake  from hadrons

 had ToF  had = m  2 – m had 2 m  2 + m had 2 most difficult decay:     vs.    K     =  t = 3.34 nsec  K =  t = 3.88 nsec for 1m flight distance with 100 psec resolution  at least 5  separation Time-of-Flight

low mass drift chamber      p  = 883 MeV    K   p K = 820 MeV momentum resolution < 1% (BES-III design ≈ 1 GeV) particle-ID through dE/dx (ex. BaBar)

Electromagnetic Calorimeter  hermeticity  minimal dead material  best resolution  CsI(Tl) crystals about 45% of all  -decays contain at least 1  0 BELLE

Hadron Calorimeter about 1.5% of all  -decays contain a K 0 K 0 S  drift chamber K 0 L  hadron calorimeter  almost all physics can be done with K 0 S  some veto capability against K 0 L would be good  muon identification with hadron calorimeter  high granularity, medium resolution, no muon chambers

tau-mass best result from BES: MeV 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 , f K  0  CVC, ,  ’,  ’’   2nd class current …

describe the mass spectrum of hadrons produced in  -decays sensitive to:  S, m S,  C, many QCD tests example: running of  S  -decays

OPAL Euro. Phys. J. C35 (’04) 437 non-strange vstrange v non-strange astrange a large uncertainties; especially in the strange sector approx. 500 ev bgd

ALEPH Eur. Phys. J. C11 (’99) 599 normalization of the spectral function: branching ratios

  hadrons or leptons M = 4 G/ √2  g i   ℓ |  i | ℓ    i   S,V,T L or R (example: leptonic decays) derived from spectra and angular distributions

model independent interpretation: search for arbitrary new currents but … leptonic decays

… 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 anomaly  PT: test predictions Exclusive decays: many branching ratios can be improved light meson spectroscopy (i.e. ,  ’,  0 vs.  ± ) Tau-mass: can you reduce calibration systematics compared to BES II? Michel parameters: substantial improvements possible you will probably know, what you are looking for V US from inclusive strange decays: theory under control? Exotics: CP-violation in tau-decays (g-2) 

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  -production (needs high q 2 ) o neutral current couplings o  mass (once was a very hot topic) o …

1 threshold: 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 threshold Use 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 requirements within today's possibilities Needs an excellent accelerator, with luminosity ≈ /cm 2 s and a not too large energy spread Much to be done, despite CLEO, LEP, b-fact…