Experimental Review on Lepton Flavor Violating Tau decays 2008/4 K.Inami Nagoya university International workshop e + e - collisions from phi to psi PHIPSI08

Experimental Review on

Lepton Flavor ViolatingTau decays

2008/4 K.Inami Nagoya university

International workshop e+e- collisions from phi to psi PHIPSI08

2008/4 International workshop e+e- collisions from phi to psi 2

Lepton Flavor Violation (LFV) Standard Model (SM) exactly forbids LFV. Extended SM including neutrino mixing term includ

es LFV. Charged Lepton Flavor Violation

can occur through loop diagram.But, extremely tiny branching ratio

below experimental sensitivity Br()<O(10-40) (X.-Y. Pham, EPJ C8(1999)513)

Many new models include LFV vertex naturally. Observation of charged LFV

Clear signal of new physics!

Tau lepton has many possible LFV decay modes. Wide window to probe New Physics effect.


LFV tau decays Predicted by many new physics models

Normal NP models enhance For some NP parameters, other modes may be

enhanced instead.

Many models allow measurable BR of O(10-

8) within the parameter space.

Reference

SM+ mixing EPJ C8(1999)513 10-40 10-14

SM + heavy Maj R PRD 66(2002)034008 10-9 10-10

Non-universal Z’ PLB 547(2002)252 10-9 10-8

SUSY SO(10) PRD 68(2003)033012 10-8 10-10

mSUGRA+seesaw PRD 66(2002)115013 10-7 10-9

SUSY Higgs PLB 566(2003)217 10-10 10-7


Most famous SM extension LFV are generated through slepton mixing.

Independent parameter for e

SUSY seesaw (J.Hisano et. al.,PRD 60 (1999) 055008)

Achievable BR of O(10-7~-8) if tan~60 and mSUSY~1TeV/c2

SUSY


When sleptons are heavy (>weak scale),Br() is suppressed.

Higgs in SUSY enhances other LFV modes, such as 3l, l+hadrons, ...

Higgs-mediated MSSM 3 (A.Brignole, A.Rossi, PLB 566 (2003) 217)

Enhanced if tan is large and Higgs mass is small.

SUSY Higgs


SUSY Higgs Higgs-mediated MSSM (M.Sher, PRD 66 (2002) 057301)

Br() : Br() = 8.4 : 1 Phase space, color factor, mass

MSSM seesaw (E.Arganda, arXiv:0803.2039v1)

Large BR of O(10-7) for , ’,

Need to search for all possible LFV modes To probe unknown physics and discriminate models


Experiment Recent results obtained by B-factories

High intensity e+e- collider on (4S) KEKB/Belle; World highest luminosity of 1.7x1034/cm2/s

Similar cross-section with BB (~0.9nb) -factory Detectors; Good PID ability and vertex resolution

Lepton ID eff. : ~90% fake rate : ~2% e fake rate : ~0.3 %


Status of LFV search at spring 2007

Current accumulated luminosity;>700fb-1 at Belle and >500fb-1 at BaBar.

Searches for 3l, lhh modes should be updated.

BelleBelle BaBarBaBar BelleBelle BaBarBaBarUL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

0.5 535 0.7 232 e 0.8 401 1.3 339

e 1.2 535 1.1 232 3l 2-4 87 1-3 92

0.7 401 1.5 339 lhh 2-16 158 1-5 221

’ 1.3 401 1.3 339 V0 2.8-7.7 158

1.2 401 1.5 339 eV0 3.0-7.3 158

e 0.9 401 1.6 339 Ks 0.5 281

e’ 1.6 401 2.4 339 eKs 0.6 281


LFV analysis Signal extraction

Calc. Minv and E E=Erec-Ebeam

Blinded signal region Event selection study

Estimate backgroundusing sideband data

Open blind and estimate signal yield

Estimate upper limitsby Likelihood fit or number of events in signal region

BG reduction is important to improve the sensitivity.

Find effective ways to use all information

Signal MCSignal region

Background

Minv


LFV decays; Signal and Background

qq _

2photon process f=leptons,quarks

signal

Only tag side has neutrino(s).

Both sides have neutrino(s).

radiative Bhabha process

e+ e

eee

e

many tracks

e+e-

1 prong tau decay (BR~85%)


3 leptons; BG reduction

Signal topology

BG events

- conversion veto 　 (e-e+e- and -e+e-) - Electron-veto on the tag-side 　 (e-e+e- and e-+-)

Bhabha() ee(eeee)

Optimized event selection for each mode separately because the BG processes are different.


lll search; Belle Data; 535fb-1

Efficiency; (6.0-12.5)% NBG; 0.0-0.4 events Nobs; 0 events for all modes Br<(2.0-4.1)×10-8 at 90%C.L.

(Y.Miyazaki et. al., PLB 660 (2008) 154)

eee

data signal

signal region


lll search; BaBar Data; 376fb-1

Efficiency; (5.5-12.4)% NBG; 0.3-1.3 events

Total BG; 4.2±0.8 Nobs; 6 events in total Br<(3.7-8.0)×10-8 at 90%C.L.

(B.Aubert et. al.,PRL 99 (2007) 251803)

=MinvM


lV0 search Belle; 543fb-1 data Ten modes are investigated,

l=e, and V0=,K*0,K*0, Main BG: SM decay; Signal Efficiency; 2.5 - 4.9 % Expected NBG; 0.0 - 1.0 events Observed number of events;

1 event for , e, eK*0

0 events for others Br<(5.9-18)×10-8

(Y.Nishio et. al., arXiv:0801.2475)

BaBar; 384fb-1 data Searched for , e

Br<(10-11)×10-8 (B.Aubert et. al., PRL 100;071802 2008)

data signal


lf0 search Scalar Higgs mediation

No search yet.

Belle; 671fb-1

Use f0

No event is observed. Preliminary result

Br(→lf0)xBr(f0→) f0 : <3.3X10-8, ef0 : <3.4X10-8

signal MC● data

f0

ef0

efficiency

NBG

f0 6.0% 0.11±0.08ef0 5.8% 0.10±0.07

Br(→f0):Br(→):Br (→) = 1.3 : 0.54 :1 　(C.H.Chen et.al, PRD74:035010,2006 )


Current status of LFV search

Sensitivity is improved by factors of 3-10. It comes from the luminosity increase and successful BG reduction.

Search for 3leptons sets most stringent upper limit.

BelleBelle BaBarBaBar BelleBelle BaBarBaBarUL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

UL90(10-7)

Lum(fb-1)

0.5 535 0.7 232 e 0.8 401 1.3 339

e 1.2 535 1.1 232 3l 2-40.2-0.4

87535

1-30.4-0.8

92376

0.7 401 1.5 339 lhh 2-16 158 1-5 221

’ 1.3 401 1.3 339 V0 2.8-7.70.6-1.3

158543 1.0() 384

1.2 401 1.5 339 eV0 3.0-7.30.6-1.8

158543 1.1(e) 384

e 0.9 401 1.6 339 f0 0.33 671

e’ 1.6 401 2.4 339 ef0 0.34 671


Current status of LFV search Experimental results have already ruled out some

parts of the parameter space. Exclude large tan, small SUSY/Higgs mass

Accessing other models and other parameter space

reference

SM+ mixing EPJ C8(1999)513 10-40 10-14

SM + heavy Maj R PRD 66(2002)034008 10-9 10-10

Non-universal Z’ PLB 547(2002)252 10-9 10-8

SUSY SO(10) PRD 68(2003)033012 10-8 10-10

mSUGRA+seesaw PRD 66(2002)115013 10-7 10-9

SUSY Higgs PLB 566(2003)217 10-10 10-7


Int. Luminosity at B-factory; >1300fb-1

~1.2x109 -pairs At super B-factory

>10 times more data BR sensitivity

It depends on background. l; scale as ~1/L

ee is irreducible BG. ~10-8 level at super B-factory

lll, lX0; scale as ~1/L BG can be reduced by lepton-ID

or mass requirement for X0

O(10-9) level at super B-factory

Future prospect


Summary A data sample of >109 -pairs has been successf

ully obtained by the B-factories.

New searches for 3-prong final state 3l, lV0, lf0 using 400~700 fb-1 of data Better sensitivity than previous analysis

Applied event selection optimized for each mode separately

LFV sensitivities are Br~O(10-8). Exploring some parameter space of New Physics

Many models predict possible BR of O(10-7~-8).

Super B-factory is planned to produce >1010 -pairs. LFV sensitivity will reach Br~O(10-9).


LFV decays; where we are.

Sensitivity of Br~O(10-8)


237 fb-1 BaBar data used for this analysis.

(hep-ex: 0607040)


3 leptons; Results

No signal events No BG events in sideband except for e-e+e-

Documents

Experimental Review on Lepton Flavor Violating Tau decays 2008/4 K.Inami Nagoya university International workshop e + e - collisions from phi to psi PHIPSI08