Review of τ -mass measurements at e + e - - colliders Yury Tikhonov (Budker INP) Contents  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 IHEP06,Moscow

Introduction  Mass is a fundamental characteristic of a particle interesting by itself.  In case of τ-lepton the measurement of the mass m τ, the life time t τ and B(τ→ν τ eν e ) provides the μτ-universality test :  Experimental methods: τ -threshold experiment (BES, DELCO, KEDR)- this method is direct and the most precise one in principle Pseudomass method (CLEO, OPAL, BELLE)

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)

Progress in μτ-universality test t τ, fs B(τ→ν τ eν e ), % m τ, MeVComments ± 6.0 ± ± 0.26 ± PDG σ ± 3.2 ± ± 0.15 ± W.A., October σ ± 3.2 ± ± 0.15 ± ±0.5 ± With BES preliminary -1.5σ, ± 1.5 ± ± 0.08 ± ± PDG σ ± 1.1 ± ± 0.06 ± ± PDG 2002, σ Accuracy δm τ ≈ 0.3 MeV seems sufficient now, but progress in precision measurement of t τ and B ( τ →ν τ eν e ) will require more precise τ mass This test based on only BES results

KEDR / VEPP-4M τ -threshold experiment KEDR detector: 1.Vacuum chamber 2.Vertex detector 3.Drift chamber 4.TOF scintillation counters 5.LKr barrel calorimeter 6.Superconducting coil (6 kGs) 7.Muon tubes 8.Magnet yoke 9.CsI and cap calorimeter 10.Aerogel Cherenkov counters In operation since 2002

Energy Calibration by resonant depolarization 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-

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): M J/ψ = ± ± MeV M ψ’ = ± ± MeV M ψ’’ = ± 0.9 ± 0.6 MeV Obtained relative precision ~(3÷5)·10 -6 The precision was improved by 3-6 times compared to PDG value

Scenario of τ -threshold experiment Measurement of the threshold behavior of the cross section is the most direct method of  - mass determination. Narrow region is most sensitive to mass value  high requirements on E beam,  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)

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

Energy monitoring using Compton scattering of laser radiation The method uses head-on interaction of CO 2 laser radiation (λ= 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.

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

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

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

The result of τ -mass measurements at KEDR Free parameters: τ –mass, efficiency, background

Systematic error estimates Beam energy determination30 keV Detection efficiency variations100 keV Energy spread determination accuracy25 kev Background dependence on the beam energy15 keV Luminosity measurement instability15 keV Beam energy spread variation90 keV Radiative corrections30 keV Sum in quadrature145 keV Next steps(end 06): -improving analysis The goals: -statistical error  ≤ 0.2 MeV -take more data -systematic  ≤ 0.1 Mev

Pseudomass method PτPτ PxPx PνPν  M min  M  M min 2 =M x 2 + 2( E  – E x )( E x – P x ) E  = E beam :beam energy, run dependence is corrected E x :hadron system energy P x :hadron system momentum M x :mass of the hadron system All in CMS MC for input masses: GeV GeV GeV MC input mass Fit result We take threshold position a 1 as an estimator of the  mass. The shift  a 1 = a 1 – M  was taken from the Monte Carlo.  a 1 = 0.70 ± 0.26 MeV. We apply this correction (0.7 MeV) to obtain the  mass from the fit of the data. Measurement of τ-lepton mass by BELLE

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 ν τ ) = (9.47±0.10)% Br(τ → 3π π 0 ν τ )= (4.37±0.09)% The other  decays leptonically : τ  ℓ ν ℓ ν τ Event selection criteria: –Total charge= 0 –Number of leptons= 1 –Number of charged  = 3 –Number of  0 = 0 / 1 –

Real Data τ → 3π ν τ a 1 = ± 0.25 MeV Fit by function: (a 3 +a 4 ·x)·arctg((x-a 1 )/a 2 )+a 5 +a 6 ·x M min, GeV

Analyzing the pseudomass spectrum for decay τ → 3π ν τ with the Belle detector at KEK gives the following results: Systematic uncertainties 1.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 Mτ = ± 0.13(stat.) ± 0.32(sys.) MeV (BELLE, preliminary) |Mτ+ – Mτ–|/Mτ < 2.8 · 90% CL

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 = ±0.13(stat.) ± 0.32(sys.) MeV (BELLE) Mτ = (stat.) ± 0.15(sys.) MeV (KEDR) Mτ = MeV (PDG) The improvement of accuracy is expected from KEDR soon The combination of all data will significantly improve the accuracy of PDG τ-mass value