Transcript Rare Tau-Decay at Belle -- Search for Lepton Flavor Violation

Rare Tau Decay at Belle
– Search for Lepton Flavor Violation –
Takayoshi Ohshima
Nagoya University
Belle Collaboration
New data on
tmg & tmh
We, Belle collaboration, search for LFVing tau-decay at KEKB-factory experiment. I would like to present the latest results on t->mg and mh.
EPS2003, Aachen
KEKB & Belle spectrometer
KEKB asymmetric e+e– collider
- e+ / e– : 3.5 / 8 GeV
- CM energy: 10.58 GeV
- Design luminosity: 1034/cm2/s
KEKB is an asymmetric electron-positron collider in Japan which attained
the world highest peak luminosity of 1× 10^34 this May.
We now accumulate about 160/fb data, corrsponding to about 140 M tau-pairs
produced at an energy of 10.6 GeV, by this general purpose Belle detector.
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Physics of
tmg
Forbidden in SM, while New physics allows LFV decay.
SUSY predicts LFV ( tmg, eg, …, meg )
Enhancement due to high mass
Br(tmg) ~ 105-6Br(meg)
This LFVing decay is forbidden in SM but is allowed
in new physics beyond the SM. Some SUSY models
predict rather a large branching fraction accessible
at Belle. The best limit is so far achieved by CLEO and
Belle as 1×10^-6 in the branching fraction.
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Event selection
Search for
(mg)+(m + ng >0 + neutrino(s))
86.3/fb data analyzed (78.5 M tt)
From the previous studies, we know that
(1) gtt and gmm form the prominent BG;
(2) Non-zero candidate events are found to exist in
signal region.
Therefore,
BG reduction & knowledge of its distribution
are essential to extract the number of signal events.
In order to remove BG, we newly introduce a cut,
pmissing-Mmissing cut
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Event selection
(dominant BG)
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pmissing vs. Mmissing cut & Blind analysis
98% tt and 86% mm removed ,
76% signals survived.
Signal MC is indicated by yellow.
Signal yield is evaluated in DE-vs-Mmg
(DE=EmgCM-EbeamCM)
In order to avoid bias on analysis, we
Blind the signal region
1.70 GeV < Mmg < 1.85 GeV
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BG in the signal region
BG comprises (1)ttg and
(2) mmg (one m misidentified as m) and (3) small cont.
ttg and cont. are obtained by MC,
For mmg, mmg from data and multiplied by m-ID inefficiency k.
For BG,
BG probability density (Si) is expressed by Gaussian and Landau functions.
Thus obtained B spectrum at the blinded region is shown here by the curves. BG can be also obtained from actual data by averaging
their distributions at both side-bands, as indicated by the histogram. Curve and histogram agree very well. Finally, we open the blinded area.
Dots are the remaining data. It well agree with the expected BGs. Yellow shows the expected signal distribution.
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DE vs. Mmg
Event distribution where the blinded region is unveiled.
(1)
Due to initial radiation and energy
leakage of photon calorimeter, the
distribution has a long tail.
(2)
5s region (e = 11.0%)
Resolution
DE： 65.40.6 MeV
Mmg：20.30.9 MeV/c2
(3)
In order to evaluate the number of signal events, we take 5s region indicated here,
which provides 10.3 % of detection efficiency.
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Unbinned EML fit
S. Ahmed et. al.,
PR D61 071101 (200)
Probability densities for a sum of BG and signal are displayed by dark and bright pattern, and the data by dots.
From these figures it can be seen, the events observed are much more characteristic of BG than of signal.
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Br(tmg)
Systematic uncertainty
on s0
Continuum & k : +0.06/-0.11 ev.
BG function:
Fit region:
0.3 ev.
0.07 ev.
Br(tmg) < 3.2 x 10 -7
at 90% CL.
Systematic
uncertainty
on 2eNtt
Track rec. eff. :
2.0%
Photon rec. eff. :
2.8%
Cut selection :
2.2%
Luminosity :
1.4%
Trigger efficiency : 1.6%
MC statistics :
0.8%
-----------------------------------Total :
4.7%
(LFV interaction structure &
spin correlation: < 0.1%)
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Physics of LFV
tmh
The constrained MSSM Higgs-mediated model.
An attractive process to give the most strigent bound on Higgs-mediated LFV in MSSM.
Especially, large tanb would provide large Br.
M. Sher, PR D66 057301 (2002); K.S. Bubu and C. Kolda, PRL 89, 241802 (2002);
A. Dedes, J. Ellis, and M. Raidal, PL B549, 159 (2002)
Br(t  mh) is 8.4 times larger than Br(t  mmm)
color facor  3
Higgs coupling  (ms/mm)2
Current upper limit from CLEO （Ldt = 4.7fb-1）
Using h  gg mode.

Br(tmh) < 9.62×10-6 ; Br(teh) < 8.19×10-6
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Event selection
Essentially, very similar to tmg
Two h decay-modes
hgg (Br = 39.4%):
2 tracks + ng > 2 + missing
hp+p-p0 (Br = 22.6%):
4 tracks + ng > 2 + missing (m is not required)
84.3/fb data used
h gg mass
s12 MeV
h p+p-p0 mass
s5 MeV
a resolution normalized h-mass in gg mode, and p0 and h-mass for 3p modes.
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Backgrounds & Resolution
hgg
h3p
From MC
Resolutions:
DE:
60.4 2.6 MeV,
38.5  2.0 MeV
Mmh: 22.5  0.6 MeV/c2, 12.1  0.3 MeV/c2
Blind analysis is performed as same as the mg case,
and the signal region is defined, this time,
by an ellipse, as shown here,
which gives a 90% acceptance.
DE-Mmh plot
Signal-ellipse: 90% acceptance
BG:
from MC mostly tt, and uds cont. and mm.
From data
Remaining events:
(after kinematical cuts)
18 events
(within 10s but out of ellipse) 7events
60 events
2 events
(MC=3.72.4) (MC=0)
Open blind
(within region)
0 events
(MC=0.9)
0 events
(MC=0)
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Number of signals & Br evaluation
hp+p-p0
hgg
s0: upper limit of signal events = 2.3
These are the events distributions in DE vs mhMass plane.
Dots are the data, open circle is tt MC events and square
is continuum. The ellipses are the signal region with
an acceptance of 90%.
Branching fraction
hgg
e  Brh
Ntt
3.3%
Br (10-7)
< 4.5
3p
gg+3p
1.1%
4.4%
76.9  106
< 13.6
< 3.4
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Br(tmh)
Systematic
uncertainties(%)
hgg h3p
Br(tmh) < 3.4 107
at 90% CL.
Include systematic uncertainty into
an upper limit at 90% CL.
S: detection sensitivity, b: BG
R.Cousins and V.Highland, NIM A320, 331 (1992)
Luminosity
Brh
Beam BG
Trigger eff.
Tracking eff.
p0 veto
h/p0 recon. eff.
m ID eff.
MC stat.
1.4
0.7
2.3
1.4
2.0
5.5
2.0
4.0
1.3
Sum
8.1
1.4
1.8
2.1
1.4
2.0
-4.2
4.0
2.1
7.3
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Summary
1.
We attain upper limits of
Br(tmg) < 3.2  10-7 & Br(tmh) < 3.4  10-7
at 90% CL using 85/fb data.
2. For the first data sensitivity reaches 10-7 level.
3. They provide some constraint on physics beyond the SM.
Dedes, J. Ellis, and M. Raidal, PL B549, 159 (2002)
K.S. Bubu and C. Kolda, PRL 89, 241802 (2002)
4. Additional available data of 75/fb should improve these sensitivities soon.
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