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Review of τ -mass measurements at e + e - - colliders

Review of τ -mass measurements at e + e - - colliders. Introduction Current status of τ -mass measurements and μ τ -universality test New results from e+e- -colliders KEDR: τ -threshold experiment BELLE: the pseudomass method Conclusion. Yury Tikhonov (Budker INP)

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Review of τ -mass measurements at e + e - - colliders

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  1. Review of τ-mass measurements at e+e- -colliders • Introduction • Current status of τ-mass measurements and μτ-universality test • New results from e+e- -colliders • KEDR: τ-threshold experiment • BELLE: the pseudomass method • Conclusion Yury Tikhonov (Budker INP) Contents IHEP06,Moscow

  2. Introduction • Mass is a fundamental characteristic of a particle interesting by itself. • In case of τ-lepton the measurement of the mass mτ, the life time tτ and B(τ→ντeνe) provides the μτ-universalitytest : • Experimental methods: • τ-threshold experiment (BES, DELCO, KEDR)- this method is direct and • the most precise one in principle • Pseudomass method (CLEO, OPAL, BELLE)

  3. Current status of τ -mass measurements (PDG-2005) • The world average is dominated by the BES results • KEDR collaboration made analysis of the BES data and it was shown that the confidence interval quoted by BES was underestimated by factor 2÷1.5 (Nucl. Phys. B 144(2005)20) • BES collaboration did not agree with this statement (published in the same issue)

  4. Progress in μτ-universality test • Accuracy δmτ≈ 0.3 MeV seems sufficient now, but progress in precision • measurement of tτ and B (τ→ντeνe) will require more precise τ mass • This test based on only BES results

  5. KEDR / VEPP-4M τ-threshold experiment KEDR detector: • Vacuum chamber • Vertex detector • Drift chamber • TOF scintillation counters • LKr barrel calorimeter • Superconducting coil (6 kGs) • Muon tubes • Magnet yoke • CsI and cap calorimeter • Aerogel Cherenkov counters In operation since 2002

  6. Energy Calibration by resonant depolarization e- • Polarization beams prepared at VEPP-3 • Touschek (intra-beam scattered) electron pairs are detected with 2x2 scintillation counters • Two bunches compensation technique: scattering rates from unpolarized and polarized beams are compared Δ=(fpol –funpol )/fpol • The precision of the single energy measurement is ~2 keV (10-6 ) e-

  7. Measurement of J/ψ, ψ’, ψ’’ –masses at KEDR/VEPP-4M • During preparation of τ-threshold experiment the measurement of J/ψ, ψ’, ψ’’ meson masses has been performed ( PLB 573 (2003) 63, ICHEP2005): MJ/ψ= 3096.917 ± 0.010 ± 0.007 MeV Mψ’ = 3686.117 ± 0.012 ± 0.015 MeV Mψ’’= 3773.5 ± 0.9 ± 0.6 MeV Obtained relative precision ~(3÷5)·10-6 The precision was improved by 3-6 times compared to PDG value

  8. Measurement of the threshold behavior of the cross section is the most direct method of - mass determination. Scenario of τ-threshold experiment Narrow region is most sensitive to mass value high requirements on Ebeam, E accuracy and stability  Optimal luminosity distribution for 3-parameter fit: • 15% below the threshold (backround) • 70% ± 0.5 MeV around the threshold (mass) • 15% well above the threshold (detection efficiency)

  9. Problems with resonant depolarization in τ -energy region: • Integer spin resonance Ωspin=4ωrev at E=1763 MeV • Below 1780 MeV the polarization life time is very short Reasonable energy calibration rate is once per a few days

  10. Energy monitoring using Compton scattering of laser radiation • The method uses head-on interaction of CO2 laser radiation (λ=10.591 μm) with the electron beam. The energy spectrum of backscattered photons is measured by large-volume germanium detector(R.Klein at al.: NIM A384(1997)293, BESSY-I and BESSY-II) Accuracy of the energy determination is 80 keV for 20-minutes run.

  11. Beam energy spread can be measured by CBS(energy spread and its stability are very important for τ –threshold experiment!)

  12. VEPP-4M energy vs time CBS an RD difference Energy reconstruction accuracy ~80 keV

  13. Event selection • Events: ττ→eX with X =e,μ,π,K,ρ • Data: 3.9 pb-1 • Efficiency: ~ 2.25% • Background: 0+0.57 pb The measured crossections ψ’-peak

  14. The result of τ -massmeasurements at KEDR Free parameters: τ –mass, efficiency, background

  15. Systematic error estimates Next steps(end 06): -improving analysis The goals: -statistical error  ≤0.2 MeV -take more data -systematic  ≤0.1 Mev

  16. Measurement of τ-lepton mass by BELLE MC for input masses: Pτ Px 1.767 GeV1.777 GeV1.787 GeV  Pν Pseudomass method All in CMS Mmin M Mmin2 =Mx2 + 2( E – Ex )( Ex – Px ) E = Ebeam: beam energy, run dependence is correctedEx : hadron system energyPx : hadron system momentumMx : mass of the hadron system Fit result MC input mass We take threshold position a1as an estimator of the  mass.The shift a1 = a1– M was taken from the Monte Carlo.a1 = 0.70 ± 0.26 MeV.We apply this correction (0.7 MeV) to obtain the  mass from the fit of the data.

  17. Event selection • Events : e+e–+– • Data : 250 fb-1 (exp 7-37) • MC statistics  ½ of the data for   3 decay mode • Used decay modes :   3 ,   30Br(τ→ 3π ντ) = (9.47±0.10)%Br(τ→ 3π π0 ντ)= (4.37±0.09)% • The other  decays leptonically : τ  ℓ νℓ ντ • Event selection criteria: • Total charge = 0 • Number of leptons = 1 • Number of charged  = 3 • Number of 0 = 0 / 1 –

  18. Real Dataτ → 3π ντ Fit by function: (a3+a4·x)·arctg((x-a1)/a2)+a5+a6·x Mmin, GeV a1 =1777.41 ± 0.25 MeV

  19. Analyzing the pseudomass spectrum for decay τ → 3π ντ with the Belle detector at KEK gives the following results: Mτ = 1776.77 ± 0.13(stat.) ± 0.32(sys.) MeV (BELLE, preliminary) |Mτ+ – Mτ–|/Mτ < 2.8 · 10-4 @ 90% CL Systematic uncertainties • Calibration of the tracking detectors and knowledge • of the beam energy 0.26 MeV • 2. Choice of the fit range and the shape of the threshold function 0.18 MeV • 3. -decay misidentification and non +–negligible events • Total:0.32 MeV

  20. Conclusion • There is a real progress in τ-mass measurements at e+e- -colliders during last year(2 different methods were used by KEDR and BELLE): Mτ = 1776.71 ±0.13(stat.) ± 0.32(sys.) MeV (BELLE) Mτ= 1776.80 (stat.) ± 0.15(sys.) MeV(KEDR) Mτ =1776.99 MeV(PDG) +0.25 -0.22 +0.29 -0.26 • The improvement of accuracy is expected from KEDR soon • The combination of all data will significantly improve the • accuracy of PDG τ-mass value

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