1. Hisaki Hayashii (NWU, Japan) for the Belle collaboration
Measurement of the electroweak properties of t leptons in the Belle experiment (Precise measurement of t lepton lifetime )
Hisaki Hayashii
(NWU, Japan)
for the Belle collaboration
July 2014, Valencia, Spain

2. Contents Introduction: Lepton universality
Precise measurement of tau lifetime
Method
Systematics
Results
Discussion
2014/7/5
ICHEP 2014, Valencia, Spain

3. Introduction Motivation
Precise measurement of the t lifetime is necessary for the tests of lepton universality that is assumed in the SM.
W/o assuming lepton universality, leptonic branching fractions and lifetimes are given for L=t, m by
The ratios of the couplings are
t lifetime, ,plays an important role for lepton universality test.
2014/7/5
ICHEP 2014, Valencia, Spain

4. Tau lifetime, motivation
Present status
2.6 s deviation from the Standard Model
S. Schael et al. Phys. Rep. 532, 119 (2013)
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ICHEP 2014, Valencia, Spain

5. Measurement of tt at Belle
Belle vertex detector
Very small amount of material
2.7 % X0 : including beam pipe + SVD +
drift chamber inner cylinder
Impact parameter resolution
Two configurations:
Vertex detector : 1st 157/fb data
3 double side Si detector (SVD1)
Beam pipe: 2.0 cm diameter
Vertex detector 2 : remaining 554/fb data
4 double side Si detector (SVD2)
Beam pipe: 1.5cm diameter
Multiple scattering effects
Double side Si detector:
Each layer provides both r-f and z coordinates.
2014/7/5
ICHEP 2014, Valencia, Spain

6. Method of tt measurement (1)
We analyze events
These events allow to reconstruct t+ and t-
decay points in 3 dimensions (V1, V2)
Each t momentum direction in CMS is
determined from the energy and mass of 3p
system with two-fold ambiguity.
Using known boost factor, the momentum
directions in Lab for t+ and t- are determined.
In CMS
2014/7/5
ICHEP 2014, Valencia, Spain

7. Method of tt measurement (2)
Knowing the decay points and momentum directions for both t’s, asymmetric-energy layout allows to determine the t+t- production point (shown by V01, V02 ). Note dl=|V01-V02|=0 for ideal case. We use average points among two solutions .
The decay times for t+ and t- are reconstructed as
Lab. frame
where
No information about the beam spot position is needed in this approach !!
dl =0 for ideal case
2014/7/5
ICHEP 2014, Valencia, Spain

8. Data and selection Data sample L=711fb -1 (Y(4S) data )
Thrust >0.9, separate two hemispheres in CMS.
3 charged pion for both sides.
No p0, Ks, Ng<6 with Egtot<0.7GeV
Pt2(6p)>0.25GeV2, 4 GeV < M(6p) < GeV to
suppress 2g and other background .
Pseudomass
Good vertex-fit quality χ2<20
Minimum distance between t+ and t- in Lab.,
dl<0.02cm.
1,148,000 (3p)- nt, (3p)+ nt sample with small
background contamination. Main backgrounds
are from
Mmin dl
2014/7/5
ICHEP 2014, Valencia, Spain

9. Data model and linearity
Resolution function (MC)
Fitting form
R(t-t’) detector resolution function
Modified Gaussian, 4 parameters to parameterize s.
Auds: e+e-qq (q=uds) background: 2%
Bcb: e+e-qq (q=c,b) background: 0.2%
lt:estimator of ctt
The offset is determined by simulation from the true vs rec. relation.
2014/7/5
ICHEP 2014, Valencia, Spain

10. Fit result From the fit of the experimental data and using , we obtain
711 fb-1
Points: Data
Solid line: Fit
Red histogram: uds background
Magenta line: fit to uds backg.
Blue histogram: charm backg.
Blue line: fit to charm backg.
From the fit of the experimental data
and using , we obtain
uds
charm
1 mm
2014/7/5
ICHEP 2014, Valencia, Spain

11. Systematic uncertainty (1)
Source
Δτ (mm)
SVD alignment
0.090
Resolution fucntion
0.030
Beam energy and ISR description
0.024
Fit range
0.020
Background contribution
0.010
t- lepton mass
0.009
Total
(0.12 %)
Si vertex detector(SVD) alignment
SVD is aligned by using cosmic-rays. Effects from uncertainty of the both local and global alignment are taken into account.
local uncertainty: shifts (10 mm ), rotation(0.1mrad)
global uncertainty w.r.t. drift chamber: shift(20 mm), rotation(1mrad).
For cross check,
i) divide data into two different samples by azimuthal (f) angle.
ii) Results from two SVDs (SVD1 and SVD2):
-> These provide consistent results within stat. errors.
Resolution function
(0.12%)
2014/7/5
ICHEP 2014, Valencia, Spain

12. Systematic uncertainties (2)
Resolution functions:
Two different forms are tried for resolution functions
ISR effects.
Use KKMC. ISR simulation is cross checked by e+e-→　m+m- data
Background
-50% % uncertainties are assumed for e+e-qq
Dependence on the event selection.
It is found that the correction factor
depends slightly on the value of dl cut.
But the resultant value of t lifetime after correction
is quite stable for different cuts on dl.
Other selection　criteria are negligible effect.
(0.12%)
Resultant total systematic uncertainty is %
2014/7/5
ICHEP 2014, Valencia, Spain

13. Belle t life time results
Almost twice more precise
than the current world average.
This result is statistically limited yet. Repeated analysis in Belle II, increased statistics and better vertex detector, can improve the accuracy of the measurement.
2014/7/5
ICHEP 2014, Valencia, Spain

14. Discussion Test of the Lepton universality
Lepton universality is tested at 1.6 per mill level. SM
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15. Lifetime difference for t+ and t-
lifetime difference (CPT test)
t lifetime difference is measured
for the first time.
( Possible only at the asymmetric-energy
collider.)
Note that t+ t- mass difference is tested
at the 10-4 level by B-factory exp.
711 fb-1
t- : open points
t+ : solid points
2014/7/5
ICHEP 2014, Valencia, Spain

16. Summary
With 711fb-1 data, Belle measures t lifetime and the upper limit on the relative lifetime difference between t+ and t- .
New result is almost twice more precise than the current world average.
The lifetime difference has been measured for the first time.
See PRL 112, (2013) for details.
Other new results of EW properties of t lepton with full Belle statistics come soon
t → m g, e g lepton-flavor violating decays
t lepton Michel parameters
t lepton electric dipole moment (EDM) Stay tuned.
2014/7/5
ICHEP 2014, Valencia, Spain

17. Backup
2014/7/5
ICHEP 2014, Valencia, Spain

18. Tau mass measurement at Belle
PRL (2007)
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ICHEP 2014, Valencia, Spain

19. Precision studies of t properties at B-factories
Tau lifetime:
Belle: PRL 112, (2013)
Tau mass:
Belle: PRL (2007)
Babar: PRD 80, (2009)
Tau electric dipole moment(EDM)
Belle: PLB 551, 16 (2003) based on 29.5fb-1 data
Lepton universality
Babar: PRL 105, (2010)
Michel parameters in
This talk
With full data, coming soon
Coming soon
ICHEP 2014, Valencia, Spain
2014/7/5