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Tau Physics near Threshold

Tau Physics near Threshold. Achim Stahl RWTH Aachen University. Beijing, June 2006. Basic Tau Properties. mass: 1.777 GeV. lifetime: 290.6 10 -15 sec c τ = 87.11 m m. approx. 100 known decays. υ τ. τ. f. W. f’. s in nb. √s in GeV. 4 p a 2 3 s. 3 – b 2 2.

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Tau Physics near Threshold

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  1. Tau Physics near Threshold Achim Stahl RWTH Aachen University Beijing, June 2006

  2. Basic Tau Properties mass: 1.777 GeV lifetime: 290.6 10-15 sec cτ= 87.11 mm approx. 100 known decays υτ τ f W f’

  3. s in nb √s in GeV 4 pa2 3 s 3 – b2 2 √s in GeV Ruiz-Femenía, Pich hep-ph/0210003 s = b Cross Section tau production near threshold forL= 1033 /cm2 s 1 year running @ s = 1 nb  107t-pairs

  4. Running Strategy set points 1. below threshold √s = 3.50 s = 0 nb t-pairs background 1 nb ≈ 107tt

  5. Running Strategy set points 1. below threshold √s = 3.50 s = 0 nb 2. at threshold √s = 3.55 s = 0.1 nb t-pairs background 1 nb ≈ 107tt

  6. Running Strategy set points 1. below threshold √s = 3.50 s = 0 nb 2. at threshold √s = 3.55 s = 0.1 nb 3. below Y(2s) √s = 3.68 s = 2.4 nb t-pairs background 1 nb ≈ 107tt

  7. Running Strategy set points 1. below threshold √s = 3.50 s = 0 nb 2. at threshold √s = 3.55 s = 0.1 nb 3. below Y(2s) √s = 3.68 s = 2.4 nb 4. max. cross section √s = 4.25 s = 3.5 nb t-pairs background 1 nb ≈ 107tt

  8. Running @ Threshold • 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

  9. mt2 + mhad2 2 mt mt2 - mhad2 2 mt Ehad = phad = Running @ Threshold Kinematics of 2-body decays t hadnt had phad (mhad) t nt for example: t  p ntpp = 883 MeV t K ntpK = 820 MeV pmeasured - phad (mhad) = 0 ? kinematic constraint

  10. Ecms = 4.5 GeV p in GeV Running @ Threshold kinematic decay identification t  p nt t K nt t  m nm nt

  11. Running @ Threshold kinematic decay identification Emeasured - Ehad (mhad) t  r nt p p0 nt t a1 nt p p0 p0 nt t  p nt p p0 nt • fast simulation: • finite p-resolution • finite E-resolution • realistic g efficiency • fake g from hadrons

  12. Running @ Threshold kinematic decay identification Emeasured - Ehad (mhad) t  r nt p p0 nt t K* ntK p0 nt • fast simulation: • finite p-resolution • finite E-resolution • realistic g efficiency • fake g from hadrons

  13. mt2 – mhad2 mt2 + mhad2 bhad = Detector Requirements Time-of-Flight ToF had tt most difficult decay: t  p nt vs. t K nt bp= 0.987  t = 3.34 nsec bK= 0.856  t = 3.88 nsec for 1m flight distance with 100 psec resolution  at least 5 s separation

  14. Detector Requirements low mass drift chamber t  p ntpp = 883 MeV t K ntpK = 820 MeV momentum resolution < 1% (BES-III design ≈ 0.5% @ 1 GeV) particle-ID through dE/dx (ex. BaBar)

  15. Detector Requirements Electromagnetic Calorimeter about 45% of all t-decays contain at least 1 p0 • hermeticity • minimal dead material • best resolution  CsI(Tl) crystals BELLE

  16. Detector Requirements Hadron Calorimeter about 1.5% of all t-decays contain a K0 K0S drift chamber K0L  hadron calorimeter • almost all physics can be done withK0S • some veto capability against K0L would be good • muon identification with hadron calorimeter  high granularity, medium resolution, no muon chambers

  17. Physics

  18. best result from BES: 1776.96 MeV • +0.18 +0.25 • 0.21 - 0.17 Bread & Butter Physics tau-mass • systematics limited! • beam-calibration • energy spread • efficiency • background

  19. Bread & Butter Physics PDG: 140 decay modes (excluding LFV) All have their own interesting aspects Examples: enn/ mnn  lepton universality pn / Kn  fp, fK pp0n  CVC, r, r’, r’’ hpn  2nd class current … …

  20. Spectral Functions describe the mass spectrum of hadrons produced in t-decays sensitive to: aS, mS, qC, many QCD tests example: running of aS t-decays

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

  22. Michel Parameters nt L or R t hadrons or leptons S,V,T L or R M = 4 G/√2 Sgielℓ | Gi | nℓ  nt | Gi | t (example: leptonic decays) derived from spectra and angular distributions

  23. Michel Parameters leptonic decays model independent interpretation: search for arbitrary new currents but …

  24. Michel Parameters … the LHC will probably tell us what to look for. wild guess: ~ Precise measurement of couplings at tau-charm-factory

  25. Physics Topics QCD tests + as: non-strange spectral function (much better resolution!) strange spectral function (real measurement, v/a, … ) 2nd class currents, Wess-Zumino anomaly cPT: test predictions Exclusive decays: many branching ratios can be improved light meson spectroscopy (i.e. r, r’, r0 vs.r±) 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 VUS from inclusive strange decays: theory under control? Exotics: CP-violation in tau-decays (g-2)t

  26. Physics Topics What you cannot do at tau-charm: • rare decays (i.e. lepton-flavor violation) • tau lifetime ( universality with m-decays) • CP-violation in t-production (needs high q2) • neutral current couplings • nt mass (once was a very hot topic) • …

  27. Running Strategy During the initial running period: • 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 a later stage: More running @ threshold Use high energy runs for some topics

  28. Thank you Conclusions Tau physics near threshold: Excellent experimental conditions for high precision measurements Needs an excellent accelerator, with luminosity ≈ 1033/cm2 s and a not too large energy spread Needs an excellent detector, but all requirements within today's possibilities Much to be done, despite CLEO, LEP, b-fact…

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