lepton flavor violating decays at b-factories: results and prospects giovanni marchiori università...
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Lepton Flavor Violating decays at B-factories: results and prospects
Giovanni Marchiori
Università di Pisa & I.N.F.N.
15/9/2006 2G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 3G. Marchiori - LFV tau decays at B-factories: results and prospects
Theoretical expectations for LFV decays
Also promising: (enhanced in Higgs-mediated models)
15/9/2006 4G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 5G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 6G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 7G. Marchiori - LFV tau decays at B-factories: results and prospects
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 )
15/9/2006 8G. Marchiori - LFV tau decays at B-factories: results and prospects
• 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
15/9/2006 9G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 10G. Marchiori - LFV tau decays at B-factories: results and prospects
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’
15/9/2006 11G. Marchiori - LFV tau decays at B-factories: results and prospects
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)
15/9/2006 12G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 13G. Marchiori - LFV tau decays at B-factories: results and prospects
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”
15/9/2006 14G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 15G. Marchiori - LFV tau decays at B-factories: results and prospects
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)
15/9/2006 16G. Marchiori - LFV tau decays at B-factories: results and prospects
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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*
*
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15/9/2006 17G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 18G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 19G. Marchiori - LFV tau decays at B-factories: results and prospects
Backup
15/9/2006 20G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 21G. Marchiori - LFV tau decays at B-factories: results and prospects
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
15/9/2006 22G. Marchiori - LFV tau decays at B-factories: results and prospects
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)
15/9/2006 23G. Marchiori - LFV tau decays at B-factories: results and prospects
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)
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