rare tau decay at belle – search for lepton flavor violation –

Rare Tau Decay at Belle– Search for Lepton Flavor Violation –

Takayoshi OhshimaNagoya University

Belle Collaboration

New data on

EPS2003, Aachen

We, Belle collaboration, search for LFVing tau-decay at KEKB-factory experiment. I would like to present the latest results on and

EPS03, Aachen; T. Ohshima, Belle Collaboration

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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 Forbidden in SM, while New physics allows LFV decay.

SUSY predicts LFV ( , e, …, e )

Enhancement due to high mass

Br() ~ 105-6Br(e)

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

()+(+ n>0 + neutrino(s))

86.3/fb data analyzed (78.5 M )From the previous studies, we know that

(1) and 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

Signal yield is evaluated in E-vs-M (E=E

CM-EbeamCM)In order to avoid bias on analysis, weBlind the signal region 1.70 GeV < M< 1.85 GeV

98% and 86% removed , 76% signals survived.

Signal MC is indicated by yellow.


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BG in the signal region BG comprises (1) and (2) (onemisidentified as )and (3) small cont.

For BG, and cont. are obtained by MC,For from data and multiplied by -ID inefficiency . 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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E vs. M

Due to initial radiation and energy leakage of photon calorimeter, the distribution has a long tail.

ResolutionE ： 65.40.6 MeV M ： 20.30.9 MeV/c2

region ( = 11.0%)

Event distribution where the blinded region is unveiled.

In order to evaluate the number of signal events, we take region indicated here, which provides 10.3 % of detection efficiency.

(1)

()

()


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Unbinned EML fitS. 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()

Systematic uncertainty on s

Continuum & : +0.06/-0.11 ev.BG function: 0.3 ev.Fit region: 0.07 ev.

Br() < 3.2 x 10 -7 at 90% CL.

Systematic uncertainty on 2NTrack 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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The constrained MSSM Higgs-mediated model.

An attractive process to give the most strigent bound on Higgs-mediated LFV in MSSM. Especially, large tan would provide large Br. 　　 M. Sher, PR D66 057301 (2002); K.S. Bubu and C. Kolda, PRL 89, 241802 (200

2); A. Dedes, J. Ellis, and M. Raidal, PL B549, 159 (2002)

Br()is 8.4 times larger than Br()

color facor 3

Higgs coupling (ms/m)2

Current upper limit from CLEO 　（ Ldt = 4.7fb-1 ）

Using mode. Br() < 9.62×10-6 ; Br(e) < 8.19×10-6 　

Physics of LFV


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Event selection

+-0 mass mass

12 MeV 5 MeV

Essentially, very similar to Two decay-modes

(Br = 39.4%): 2 tracks + n > 2 + missing(Br = 22.6%): 4 tracks + n > 2 + missing ( is not required)

84.3/fb data used

a resolution normalized -mass in mode, and 0 and -mass for 3 modes.


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Blind analysis is performed as same as the case, and the signal region is defined, this time, by an ellipse, as shown here, which gives a 90% acceptance.

Backgrounds & Resolution

3 From MCResolutions: E: 60.4 2.6 MeV, 38.5 2.0 MeV M: 22.5 0.6 MeV/c2, 12.1 0.3 MeV/c2

Signal-ellipse: 90% acceptance BG: from MC mostly and uds cont. and .

From dataRemaining events: (after kinematical cuts) 18 events 60 events (within 10 but out of ellipse) 7events 2 events (MC=3.72.4) (MC=0) Open blind (within region) 0 events 0 events (MC=0.9) (MC=0)

plot


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Number of signalsBr evaluation

s0: upper limit of signal events = 2.3

Branching fraction+3Br 3.3% 1.1% 4.4%N76.9 106

Br (10-7) < 4.5 < 13.6 < 3.4

These are the events distributions in E vs Mass plane. Dots are the data, open circle is MC events and square is continuum. The ellipses are the signal region with an acceptance of 90%.


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Br()

Systematic uncertainties(%) Luminosity 1.4 1.4

Br 0.7 1.8Beam BG 2.3 2.1Trigger eff. 1.4 1.4Tracking eff. 2.0 2.00 veto 5.5 --/0 recon. eff. 2.0 4.2 ID eff. 4.0 4.0MC stat. 1.3 2.1

Sum 8.1 7.3

Include systematic uncertainty into an upper limit at 90% CL. S: detection sensitivity, b: BG

Br() < 3.4 10-7

at 90% CL.

R.Cousins and V.Highland, NIM A320, 331 (1992)


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Summary

K.S. Bubu and C. Kolda, PRL 89, 241802 (2002)

4. Additional available data of 75/fb should improve these sensitivities soon.

Dedes, J. Ellis, and M. Raidal, PL B549, 159 (2002)

1. We attain upper limits of

Br() < 3.2 10-7 & Br() < 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.

rare tau decay at belle – search for lepton flavor violation –

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