Lepton Flavor Violating decays at B-factories: results and prospects

Giovanni Marchiori

Università di Pisa & I.N.F.N.

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Introduction

• LFV in charged lepton sector in SM (via mixing) strongly suppressed (GIM) Observation of LFV is a clear signal of NP

• Most SM extensions predict enhanced LFV effects in decays (BR’s up to 10-610-7)

• Results reported in this talk:

l+l+l’l’

l+hh’ (h = ,K)

l+h0 (h0 = 0,,’,KS)l+V0 (V0 = ,K*,)

(l=e)

LFV as discriminating tool

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Theoretical expectations for LFV decays

Also promising: (enhanced in Higgs-mediated models)

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B-factories (ee,s10.58 GeV=M(4S))• A B-factory is also a -factory!

()~0.9nb, (bb)~1.1nb

~5.6x108 -pairs at Belle, ~3.5x108 -pairs at BaBar

BR ~ O(10-8) sensitivity!

~ 109 -pairs expected at each B-factory by 2008

• Detectors

BABAR:

90% tracking and e.m. calorimeter acceptance; 80% PID acceptance

e-ID: eff ~91%, misid ~0.2%; -ID: eff ~63%, misid ~4.8%

400fb-1

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Signal signature (example for )• e+e+

Event topology: thrust, number of charged tracks and neutral clusters in two (CM) hemispheres consistent with decay under study

Signal side: positive lepton-ID,

Tag side: pmiss within detector acceptance, pTmiss>0, Mmiss~0

ee

generic decay

signal event

(signal side)

1/3 prong(s) + missing (tag side)

)( 22 pEM −=CMbeam

CM EEE −=Δ

signal MC

3 ellipse: blinded region0~,~ EmM Δ

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Background suppression

• Radiative Bhabhas and dimuon events: mostly leptons, high-momentum (and small pT) tracks, jet-like

• Low-multiplicity hadronic events: more isotropic, more neutrals, Kaons

• Two-photon events: large Emiss, small pT

• SM decays: missing mass and momentum in both hemispheres

signaldata

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Analysis strategySignal extraction: M-ΔE plane (2 ellipse/box around (m,0) )

• Background (B) extrapolated from M -ΔE sidebands

• Signal (S) efficiency from full MC simulation

• Selection optimized for best BR upper limit when S=0

• Blind analysis

• Frequentist BR upper limits obtained from B, S, , N with systematic uncertainties included (NIM A320,331)

• B/B up to 100% (limited sidebands statistics)• / <~ 8% (tracks and photons reconstruction efficiency; momentum scale and resolution; PID efficiency)• (N)/N ~2.3% (luminosity and )

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• Belle(535fb-1, preliminary)

• BaBar(232fb-1, PRL 95, 041802)

– Beam-energy constrained mass– =7.4±0.5% (3-prong tag: 1.2%) – Nexpected= 6.2±0.5– Nobserved= 4

BR<6.8x10-8 at 90% CL

distribution in 2 band

signaldata

blinded region

signal region

– = 6.7% (only 1-prong tag) – Nexpected= 13.9– Nobserved= 10

BR<4.5x10-8 at 90% CL

Main BGs:– +ISR ()

– +ISR

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e Belle(535fb-1, preliminary)

BaBar(232fb-1 , PRL 96, 041801)

– Beam-energy constrained mass– = 4.7±0.3% – Nexpected= 1.9 ± 0.4– Nobserved= 1

BR<1.1x10-7 at 90% CL

– = 3.0% – Nexpected= ? – Nobserved= 5

BR<1.2x10-7 at 90% CL

Main BGs:– +ISR (e)

– ee+ISR

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ll’l’ (l,l’=e,)

BaBar(91.5fb-1 , PRL 92, 121801)

– only 1-prong tag decays – =7-11%– Nexpected= 3.41±0.22 in total– Nobserved= 3

BR<(1.1-3.3)x10-7 at 90% CL

~similar results from Belle (87.1fb-1, PLB 589, 103)

signal boxdata

Main BGs: higher order radiative e+e, +: e+elll’l’

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BaBar(315fb-1, hep-ex/0607045)

2 box

data

: 7.0±0.5 %• Nexp: 0.64±0.08• Nobs: 1

: 3.7±0.3 %• Nexp: 0.07±0.02• Nobs: 0

BR<1.6x10-7

70% ellipse (MC)

Belle(401fb-1, preliminary)

BR<6.5x10-8

: 6.4%• Nexpected: 0.40• Nobserved: 0

: 6.8%• Nexpected: 0.24• Nobserved: 0

signaldata

90% ellipse (MC)

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h0 / eh0(h0=,,’)

Belle(401fb-1, preliminary)• Based on previous analysis (154fb-1,

PLB622, 218)

• e’ / ’ : ’ mode is now included.– eff.: 4.3%(e’ ,’), 4.8%

(e’ ,’)

4.9%(’ ,’),

5.4%(’ ,’)

• Main BGs:– : – othersnegligibly small BGs

BR(e)<9.2x10

BR(e’)<1.6x10

BR(e)<8.0x10 BR()<1.2x10BR(’)<1.3x10

signaldata

3-6 times more sensitive results obtained

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lhh’ (l=e,; h,h’=,K)

– only 1-prong tag decays – ~3% for each mode– Nexpected= 11.30.7 in total (0.1-3.0 depending on channel)– Nobserved= 10

BR<(0.7-4.8)x10-7 at 90% CL~similar results from Belle (158fb-1, hep-ex/0603036)

BaBar(221fb-1 , PRL 95, 191801) signaldata

Main BGs:low-multiplicity qq evts

hh’h”

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KS / eKS (KS )

Belle(281fb-1, PLB639, 159)

Ks eKs

Main BGs: D lKS, KS - but negligible Main syst. on : KS reco. (4.5%), tracking eff. (4%)

signaldata

signal region

: (13.5±0.9)%Nexpected=0.20.2Nobserved=0BR<4.9*10-8

: (11.8±0.8)%Nexpected=0.20.2Nobserved=0BR<5.6*10-8

previousresultCLEO:14fb-1

Ks:<9.5x10-7

eKs:<9.1x10-7

20 times improvement wrt previous UL (CLEO)

blind region

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V0 / eV0 (V0=,K*,)

e e

ee

Belle(158fb-1, hep-ex/0603036)

– ~3-5%– Nexpected= 2.55±1.04 in e; negligible for non- modes (V0 mass cut)– Nobserved= 5 in e0; 0 in all other channels

BR<(2.0-7.7)x10-7 at 90% CL

signal boxdata

KK

K*K

e e

lepton

h

hV0

generic decay

Main BGs: +qq (V0) +2photon (eV0)

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Summary of LFV searches

mode x10-7 mode x10-7

Belle 0.45 lll 1.13.5

e BaBar 1.1 +ee BaBar 1.1

Belle 0.65 ee+ Belle 1.9

e Belle 0.92 lhh BaBar 0.74.8

' Belle 1.3 + BaBar 0.7

e' Belle 1.6 lV0 Belle 2.07.7

Belle 1.2 0 Belle 2.0

e Belle 0.80 BaBar 0.59

Ks Belle 0.52 BaBar 0.58

eKs Belle 0.60 BaBar 0.72

BaBar 1.5

red: results shown in this talk* : bkg currently negligible

Starting to constrain the parameter space of some NP (SUSY/Higgs/mSUGRA) models

*

*

*

*

*

*

*

*

*

*

*

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ProspectsFor many modes at present the expected background is negligible.

• probably the case also at the end of the B-factories era (2ab-1 in total) UL 1/L (5-10 improvement) UL ~10-8

• may not be the case at a Super flavor factory (50 ab-1) UL improvement ~1/L (another x5 improvement AT LEAST)– detailed studies (simulations) to be done– detector design to be optimized: improve and bkg suppression:

• Increased hermiticity• Better particle-ID performance• Improved tracking/neutral resolution

Sensitivity to BR ~10-9-10-10 could be reached Potential for LFV observation in decays

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Conclusion

• Data sample of B-factories (-factories):>900*106 pairs now~2*109 by 2008

• Studies of LFV decays now reach the O(10-8) level:• No signals are found in any of the LFV decay modes• Many ULs for LFV updated by B-factories’ results

• A Super flavor factory (50x more data, more optimized detector) can set O(10-9-10-10) limits and has potential for LFV discovery in many NP scenarios

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Backup

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Extracting BR upper limits (NIM A320, 331)

• Cousins-Highland method: extension to frequentist method to include syst. uncertainties.

• MC samples generated according to Poisson distribution with mean (S+B)– B drawn randomly from Gaussian distribution with

mean = Nexpected , std. dev. = (Nexpected)– S drawn randomly from Gaussian distribution with

mean = L**2*BR* , std. dev. = (L**2*BR*)

• BR varied until 10% of the sample yields a # of events < then the # observed in data

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Nested subdetectors:Vertex detector (SVT)(5 Si layers)

Drift chamber (DCH)(gas based, 40 cell layers)

Cherenkov detector (DIRC)(fused silica bars)

Electromagnetic calorimeter (EMC) (CsI(Tl) crystals)

Superconducting Magnet (1.5 T solenoidal field)

Muon and neutral hadron calorimeter (IFR) (Iron + RPC/LST)

The BaBar detector• “Multi-purpose” detector with full azimuthal and ~90% polar angle coverage• Optimized for CP violation studies in B decays (energies in the range 100 MeV – 4 GeV)

– Vertex resolution– Material budget (momentum resolution, low-energy photon efficiency)– Lepton/Kaon identification (B flavor tagging)

e

SVT

DCH

EMC

Solenoid

IFR

DIRC

e+

B decay vertex,soft from D*

K-ID

K-ID

e-ID

-ID

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performances

•SVT: –90% acceptance in CM, 4% X0 thickness(vtx): 70 m fully reco, 150 m tag(d0), (z0) ~ 50 m–Low-pT trk. eff. : >80% at pT=70 MeV/c

•DCH:–90% acceptance in CM, 1.08% X0 thickness

–Trk. eff.: 98% for pT>200 MeV/c(pT)/pT=0.13%pT/(GeV/c) + 0.45%–3 K/ separation (dE/dx) up to 700 MeV/c

•DIRC:–78% acceptance in CM, 17% X0 thickness(C)2.5 mrad, (t)~1.7 ns–K/ separation: 4.2 @ 3GeV/c, 2.5 @ 4GeV/c

•EMC: –90% acceptance in CM, 17 X0 thickness(E)/E: 2.3%/(E/GeV)^(1/4) 1.85%(,) = 3.9 mrad/(E/GeV)^(1/2)(0 mass)=6.9 MeV/c2

–e-ID: eff=91%, misid=0.2%

•IFR–65 cm of steel (+brass)–18 plates of RPCs (being replaced by LSTs)-ID: eff=63%, misid=4.8% (eff. > 90% with LSTs)

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Typical selection variables• charged tracks:

– d0, z0

– momentum, # of hits

• photons:– Energy

– Lateral shower shape

– Not matched to tracks

• K/:– dE/dx

– Cherenkov angle

• Unstable particles:– invariant mass of daughters 2 probability of vertex fit

: mEC, ΔE

• KS: – Distance of flight

– Line of flight vs p direction

, K*, , D*0:– helicity angle (polarization)