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-Femena, 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 | i | 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