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 mτ, the life time tτ and B(τ→ντeνe) provides the μτ-universality test :

,)(

52

mt

eBG

e

F

Experimental methods:• τ-threshold experiment (BES, DELCO, KEDR)- this method is direct and the most precise one in principle• Pseudomass method (CLEO, OPAL, BELLE)

m

m

G

G

F

F 52

2

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

tτ, fs B(τ→ντeνe), %

mτ, MeV Comments

0.9405

305.6 ± 6.0 ± 0.0185

17.93 ± 0.26 ± 0.0136

PDG 1992

-2.4σ

0.9609

295.7 ± 3.2 ± 0.0104

17.76 ± 0.15 ± 0.0081

W.A., October 1992

-2.4σ

0.9800

295.7 ± 3.2 ± 0.0106

17.76 ± 0.15

± 0.0083 1777.1±0.5

± 0.012

With BES preliminary

-1.5σ, 1996

0.9999

291.0 ± 1.5 ± 0.0052

17.83 ± 0.08 ± 0.0045

± 0.0008

PDG 1996

-0.01σ

1.0020

290.6 ± 1.1 ± 0.0038

17.84 ± 0.06 ± 0.0034

± 0.0008

PDG 2002, 2004

+0.4 σ

)(

)(2

2

F

F

G

G

7.26.31.1784

•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

7.26.31.1784

30.027.00.1777

29.026.099.1776

0095.00071.0

0097.00073.0

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 )

revdepspin n0/'1/ revspin

e-

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): 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

Scenario of τ-threshold experimentMeasurement of the threshold behavior of the cross section is the most direct method of - mass determination.

Narrow region Ebeam mE ~ 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)

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

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.

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: 0+0.57 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 determination 30 keV

Detection efficiency variations 100 keV

Energy spread determination accuracy 25 kev

Background dependence on the beam energy 15 keV

Luminosity measurement instability 15 keV

Beam energy spread variation 90 keV

Radiative corrections 30 keV

Sum in quadrature 145 keV

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

Pseudomass method

Pτ

Px

Pν

Mmin M

Mmin2 =Mx

2 + 2( E – Ex )( Ex – Px )

E = Ebeam: beam energy, run dependenceis corrected

Ex : hadron system energyPx : hadron system momentumMx : mass of the hadron system

All in CMS

MC for input masses:

1.767 GeV1.777 GeV1.787 GeV

MC input massF

it r

esul

t

We take threshold position a1 as 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.

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 , 30

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

–

Real Dataτ → 3π ντ

a1 =1777.41 ± 0.25 MeV

Fit by function:

(a3+a4·x)·arctg((x-a1)/a2)+a5+a6·x

Mmin, GeV

Analyzing the pseudomass spectrum for decay τ → 3π ντ 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 MeV2. Choice of the fit range and the shape of the

threshold function 0.18 MeV3. -decay misidentification and non +– negligible

events Total: 0.32 MeV

Mτ = 1776.77 ± 0.13(stat.) ± 0.32(sys.) MeV (BELLE, preliminary)

|Mτ+ – Mτ–|/Mτ < 2.8 · 10-4 @ 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 = 1776.71 ±0.13(stat.) ± 0.32(sys.) MeV (BELLE)

Mτ = 1776.80 (stat.) ± 0.15(sys.) MeV (KEDR)

Mτ =1776.99 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

+0.25-0.22+0.29-0.26