Update on BaBar Searches
for Lepton Flavor Violation
using Tau Decays
Richard Kass for the BaBar Collaboration
Outline of Talk
Introduction
some facts about BaBar & PEPII
Searches for Lepton Flavor Violation
2-body decays
3-body decays
Baryon number violating decays
Summary & Conclusions
Richard Kass
Miami 2007
1
Fact #1
Since the cross section for
e+e-Y(4S)BB
is about the same as the cross section for
e+e-+a B factory is also a  factory.
sBB
s
Richard Kass
Cross Section
1.05 nb
July 2006
0.92 nb
Miami 2007
2
Fact #2
BaBar is a great detector for  physics!
Electromagnetic
Calorimeter
(EMC)
1.5 T Solenoid
Detector of
Internally
Recflected
Cherenkov
Light (DIRC)
Drift Chamber
(DCH)
Instrumented
Flux Return
(IFR)
Silicon Vertex
Tracker (SVT)
SVT, DCH: charged particle tracking  vertex &mom. resolution, K0s, L’s
EMC: electromagnetic calorimetry  g/e/p0/h
DIRC, IFR, DCH: charged particle ID  p/m/K/p
Highly efficient trigger for  events
Richard Kass
Miami 2007
3
The BaBar Detector
r-f view
x-z view
BaBar (and Belle) has:
excellent charged particle tracking
excellent EM calorimetry
excellent Particle ID
and LOTS of data.
Richard Kass
Miami 2007
4
Fact #3
PEP-II is a great accelerator
BaBar recorded data~477 fb-1
Total # of  decays: ~8.7x108
Expect ~ 1.5x109 by Sept. 2008
Peak Luminosity 1.2 x 1034 cm-2 s-1
Richard Kass
Miami 2007
5
 LFV Search Technique
All of the analyses to be discussed rely on the following:
i) only 2 ’s produced in the event, each  has the beam energy
ii) divide event into 2 hemispheres; “Tag” and “Signal”
Tag side assumes the tau decays via standard model (e.g. τ-→π- ν, τ-→e νv)
Signal side contains the LFV decay.
Since the LFV decays are neutrinoless no missing mom/energy allowed on Signal side
  mg MC
τ+→μ+μ+μ-
τ-→π- ν
iii) Rely heavily on kinematics for signal/bkgd separation
E  Ebeam - Emeasured
2
2
M EC  Ebeam
- Pmeasured
signal box
For signal events: ΔE~0 and MEC~Mτ
iv) Use data (if possible) & MC to estimate backgrounds
Use MC to estimate signal efficiency
Richard Kass
Miami 2007
6
Introduction to Lepton Flavor
Violation (LFV) in τ decay
In the neutrino sector there is large LFV!
neutrino oscillations  q23 450, q12 320
solar neutrinos: ne ne/3+nm/3+n/3
But in the standard model charged LFV is heavily suppressed.
n  nm
3  m 

 sin 2q B(  mn mn  )  O10 
B
(


mg
)

m

128p m
W
W
g

2
23
2
W
-54
2

mix
Lee-Shrock, Phys. Rev. D 16, 1444 (1977)
However LFV OK in
many extensions to
SUSY models can
bring B(mg) all
the way to 10-7
standard model.
Richard Kass
2
Miami 2007
7
Theory Says…..
(a small sampling of the models)
= within range
Observe LFV  Observe New Physics
Don’t Observe LFV  Rule out some models
Richard Kass
Miami 2007
8
Search for   mg &   eg
BaBar used 232 fb-1 data (~430·106 τ’s) to search for τ→μγ & τ→eγ
τ→eγ
Main backgrounds: Bhabha & +[→enn with ISR, FSR → enng]
# bckd expected =1.9±0.4 evts
# of observed events = 1
efficiency=(4.7±0.3)%
BF( -  e- g ) < 11 x10-8
 - m- g
PRL 96, 041801 (2006)
Main backgrounds: dimuon (m+m-),
+- [ ->mnn with ISR, FSR]
# bckd expected =6.2±0.5
# of observed events = 4
efficiency=(7.4±0.7)%
BF ( -  m- g ) < 6.8 x10-8
PRL 95, 041802 (2005)
Richard Kass
Miami 2007
9
Search for   mg &   eg
τ lg Upper Limits Vs time
S. Banerjee (Tau06) Nucl. Phys. Proc., Suppl. 169, 199 (2007)
Richard Kass
Miami 2007
10
 mg & bs!
SUSY GUT models can relate CP asymmetry in BfKs to B(mg)
SU(5), right handed neutrino
(md2 ) 23  (ml2 ) 23 ei (1 -2 )
GUT relation:
R
L
bs penguin processes and   mg are related in SUSY GUTs.
J. Hisano & Y.Shimizu,
PLB 565 (2003), 182.
mn   5  10 -2 eV
PDG 2000
mn R  5  1014 eV
U 32  1 / 2
scalar mass
A0  0
trilinear coupling
gaugino mass
CP asymmetry in Bd fKs
Richard Kass
Miami 2007
11
 mg & SUSY!
Hisano & Y.Shimizu
PDG 2000
Belle (L=535 fb-1)
BaBar (L=232 fb-1)
BaBar 2007
preliminary
HFAG: SφKs=0.39±0.17
SUSY SO(10) + seesaw
Masiero et al. – NJP 6 (2004) 202
Excluded
mSUGRA mixing at GUT scale
Brignole, Rossi NJB701 (2004) 3
Richard Kass
mSUGRA + Seesaw
Igonkina, hep-ex/0606009v1
Miami 2007
12
Searches for τ→l+(π0, η, η′)
Signal box
(±2s in E and MEC)
Shaded region
covers 68% of
signal MC
data
●=
Using 339 fb-1 data (~620·106 τ’s)
 decay mode
UL (10-7)
 decay mode
UL (10-7)
Background events: 0.1-1.3
depending on mode
ep0
1.3
mp0
1.1
eh h->gg
2.5
mh h-> gg
1.9
Total expected events = 3.1
eh h->ppp0
5.4
mh h->ppp0)
4.5
Total observed events = 2
eh
1.6
mh
1.5
eh’ h’->pph
5.8
mh’ h’->pph
3.6
eh’ h’->rg
4.2
mh’ h’->rg
2.7
eh’
2.4
mh’
1.4
NO Signals
PRL 98, 061803 (2007)
Richard Kass
Miami 2007
13
τ→lη and New Physics
- 6  tan b   100 GeV 

B(  m h )  0.84 10  
 
 60   m A 
6
A0= pseudoscalar CPodd Higgs
4
tanβ=<Hu>/<Hd>, A0= pseudoscalar CPodd Higgs
M.Sher, PRD 66, 057301 (2002)
2007
Exclusion regions in
the mA and tanb plane by
different experiments @ 95%CL
Competitive with the direct
Higgs searches at CDF & D0
CDF: pp  H  τ+τ- (310 fb-1)
D0: pp  H  τ+τ- (325 fb-1)
D0: pp  H  bb (260 fb-1)
S.Banerjee (Tau 06) Nucl. Phys. Proc. Suppl. 169, 199 (2007)
Richard Kass
Miami 2007
14
Search for τ→lw
Use 384.1 fb-1 data (~710·106 τ’s) for this analysis
Use ω→π+π-π0
τ→ew
τ→mw
Signal box
±3s in E-MEC
data
●=
data
●=
Mode Eff.(%)
expected bckd observed bckd UL@90%CL
τ→eω 2.96±0.21
0.35±0.06
0
1.1·10-7
τ→μω 2.56±0.23
0.73±0.03
0
1.0·10-7
NO Signals
Richard Kass
Miami 2007
arXiv:0711.0980 [hep-ex]
submitted to PRL
15
Search for τ→lll
SUSY Models: B(τ→lll) & B(τ→lhh) in range 10-17 to 10-6
Many Modes to search: - -> e+e-e- , m+m-m-, e-m+m-, m-e+e- , m+e-e-, e+m-mm-
-
m-
h0
m+
Babu, Kolda PRL 89 241802 (2002)
4
6


 tan b   100 GeV 
B( -  3m )  110-7  
 
60

  mh0 

Signal Mode
m-m+m-
e-e+e-
μ-e+ee-m+m-
μ+e-ee+m-m-
Dominant
Background

uds
eemm
Bhabha
eeee

eemm

mm

uds
Event selection is optimized separately for each signal
mode accounting for different background compositions
Richard Kass
Miami 2007
16
Results for τ→lll search
data
●=
Used 376 fb-1 (~690·106 τ’s) of data
Efficiency: 5.5-12.4%
Background events: 0.3-1.3 depends on mode
Total background = 4.20.8 events
Total number of observed events =6
BR()<(3.7-8.0)x10-8 @ 90%CL
2-5 X lower limits than previous Babar results
NO Signals
Richard Kass
arXiv:0708.3650 [hep-ex]
accepted by PRL.
Miami 2007
17
Results For τ→lhh′
Many Modes Searched
No signals, just upper limits
 DATA
(221.5 fb-1)
Used 221 fb-1 (~410·106 τ’s) of data
Selection criteria and signal region optimized for each mode for best Upper Limit
Signal efficiency varies by mode, 2.1-3.8%
Number of background events estimated for each mode, 0.1-3 events, data 0-3events
Upper limits calculated according to method of Cousins and Highland (NIM A 320 (1992) 331.)
BR’s < (0.7-4.8)10-7 (90% CL)
Richard Kass
Published in PRL 95 (2005) 191801
Miami 2007
18
Summary of 3 Body LFV Decays
No signals found
Set limits O(10-8-10-7) for 20 LFV modes (6 lll and 14 lhh′)
Limits at the upper end of theoretical predictions
eg: SUSY with Higgs Triplet: B(→ lll) is 10-7
Can probe 10-8 (SUSY) region with higher statistics:
 backgrounds are under control (~ 1 channel):
For  → lll analysis expect 4.2 events find 6.
For  → lhh′ analysis expect 11.1 events find 10.
Richard Kass
Miami 2007
19
Lepton & Baryon number
violation in τ Decay
Baryon asymmetry in the universe is an outstanding question.
Standard model (SM) has lepton (L) & baryon (B) number conservation built in.
Once you go beyond the SM B & L do not have to be conserved
But in some supersymmetric models B-L is conserved
-→«p-
-→Lp-
pp+
pp-
L
p-
-
«
p-
B
0
1
0
B
0
-1
0
L
1
0
0
L
1
0
0
B-L
-1
+1
0
B-L
-1
-1
0
(B-L) conserving
(B-L) violating
Also searched for decays with a K- instead of π
Richard Kass
Miami 2007
20
L & B # violation in τ Decay
-→«p-
-
-→Lp-
→«K-
237 fb-1 BaBar data
( ~440·106 τ’s) used
for this analysis.
● =data
hep-ex: 0607040
- →LK-
Signal MC with
arbitrary normalization
Elliptical signal region
NO Signals
Mode
B-L
-→«p-
conserve 12.28
0.42±0.42
0
5.9·10-8
-→Lp-
violate
12.21
0.56±0.56
0
5.8·10-8
-→«K-
conserve 10.63
0.26±0.26
0
7.2·10-8
0.12±0.12
1
15·10-8
-→LK- violate
Richard Kass
Eff.(%)
9.47
expected bckd
Miami 2007
observed bckd UL@90%CL
21
Summary & Conclusions
►BaBar has performed many searches for LFV in τ decay
2-body decays:   lg, τ→l+(π0, η, η′), τ→lω
3-body decays: τ→lll, τ→lhh′
baryon number violating
LNO evidence for LFV in τ decay
similar results from Belle
starting to limit the parameter space of models
►Since most channels are not background limited
expect better limits with more data
BaBar takes data until Sept. 2008
Expect ~2X improvement in UL’s with full data set
►What are the long(er) term chances of seeing LFV in τ decay?
Lots of τ’s produced at LHC but HUGE backgrounds:
BaBar B(τ→3m)<5.3·10-8 Vs 1 year of LHCb: B(τ→3m)<1.2·10-7
M. Shapkin,
Nuc. PhysB 169 (2007) 363
Best chance to see LFV in τ decay is at a Super B-factory
produce ~1.5·1011 τ’s at Super B factory
Richard Kass
Miami 2007
22
LFV in  decays: SuperB capability
 , hh’ : the sensitivity is not likely
to be limited by irreducible backgrounds:
the sensitivity scales as 1/L.
For SuperB @ 75 ab-1 expect ~2x10-10
Principal
backgrounds
From: Hitlin, Giorgi, SuperB workshops
Richard Kass
Miami 2007
23
Extra slides
Richard Kass
Miami 2007
24
BaBar K/p ID
D*+ → D0p+
D0→ K+ p-
BaBar DIRC
Richard Kass
Miami 2007
25