1. 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

2. Since the cross section for e+e-®Y(4S)®BB
Fact #1
Since the cross section for
e+e-®Y(4S)®BB
is about the same as the cross section for
e+e-®t+t-
a B factory is also a t factory.
Cross Section
sBB nb
stt nb
July 2006
Richard Kass

3. BaBar is a great detector for t physics!
Fact #2
BaBar is a great detector for t physics!
1.5 T Solenoid
Electromagnetic Calorimeter (EMC)
Detector of Internally Recflected Cherenkov Light (DIRC)
e+ (3.1 GeV)
e- (9 GeV)
Drift Chamber (DCH)
Instrumented Flux Return (IFR)
Silicon Vertex Tracker (SVT)
SVT, DCH: charged particle tracking Þ vertex &mom. resolution, K0s, L’s
EMC: electromagnetic calorimetry Þ g/e/p0/h
DIRC, IFR, DCH: charged particle ID Þ p/m/K/p
Highly efficient trigger for t events
Richard Kass

4. The BaBar Detector BaBar (and Belle) has:
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

5. PEP-II is a great accelerator
Fact #3
PEP-II is a great accelerator
BaBar recorded data ~477 fb-1
Total # of t decays: ~8.7x108
Expect ~ 1.5x109 by Sept. 2008
Peak Luminosity 1.2 x 1034 cm-2 s-1
Richard Kass

6. t LFV Search Technique
All of the analyses to be discussed rely on the following:
i) only 2 t’s produced in the event, each t 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
t   MC
τ+→μ+μ+μ-
τ-→π- ν
iii) Rely heavily on kinematics for signal/bkgd separation
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

7. Introduction to Lepton Flavor Violation (LFV) in τ decay
In the neutrino sector there is large LFV!
neutrino oscillations Þ q23 »450, q12 »320
solar neutrinos: ne ®ne/3+nm/3+nt/3
But in the standard model charged LFV is heavily suppressed. t m g nt nm W
Lee-Shrock, Phys. Rev. D 16, 1444 (1977)
However LFV OK in
many extensions to
standard model.
SUSY models can
bring B(t®mg) all
the way to 10-7
Richard Kass

8. Theory Says….. (a small sampling of the models)
= within range
Observe LFV Þ Observe New Physics
Don’t Observe LFV Þ Rule out some models
Richard Kass

9. Search for t   & t  e
BaBar used 232 fb-1 data (~430·106 τ’s) to search for τ→μγ & τ→eγ
τ→eγ
Main backgrounds: Bhabha & t+t-
[t→enn with ISR, FSR t→ enng]
# bckd expected =1.9±0.4 evts
# of observed events = 1
efficiency=(4.7±0.3)%
BF(t -  e- g ) < 11 x10-8
t - m- g
Main backgrounds: dimuon (m+m-),
t+t- [t ->mnn with ISR, FSR]
PRL 96, (2006)
# bckd expected =6.2±0.5
# of observed events = 4
efficiency=(7.4±0.7)%
BF (t -  m- g ) < 6.8 x10-8
PRL 95, (2005)
Richard Kass

10. Search for t   & t  e τ l Upper Limits Vs time
S. Banerjee (Tau06) Nucl. Phys. Proc., Suppl. 169, 199 (2007)
Richard Kass

11. t  & b®s!
SUSY GUT models can relate CP asymmetry in B®fKs to B(t®mg)
SU(5), right handed neutrino
GUT relation:
b®s penguin processes and t   are related in SUSY GUTs.
J. Hisano & Y.Shimizu,
PLB 565 (2003), 182.
PDG 2000
scalar mass
gaugino mass
CP asymmetry in Bd ®fKs
Richard Kass

12. mSUGRA mixing at GUT scale
t  & SUSY!
BaBar (L=232 fb-1)
Belle (L=535 fb-1)
preliminary
Hisano & Y.Shimizu
PDG 2000
BaBar 2007
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
mSUGRA + Seesaw
Igonkina, hep-ex/ v1
Richard Kass

13. Searches for τ→l+(π0, η, η′)
Signal box
(±2 in DE and MEC)
Shaded region
covers 68% of
signal MC
● =data
Using 339 fb-1 data (~620·106 τ’s)
t decay mode
UL (10-7)
ep0
1.3
mp0
1.1
eh (h->gg)
2.5
mh (h-> gg)
1.9
eh (h->ppp0)
5.4
mh (h->ppp0)
4.5 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
Background events: depending on mode
Total expected events = 3.1
Total observed events = 2
NO Signals
PRL 98, (2007)
Richard Kass

14. τ→lη and New Physics 2007 Exclusion regions in
tanβ=<Hu>/<Hd>, A0= pseudoscalar CPodd Higgs
A0= pseudoscalar CPodd Higgs
M.Sher, PRD 66, (2002)
2007
Exclusion regions in
the mA and tanb plane by
different 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

15. Search for τ→lw Mode Eff.(%) expected bckd observed bckd UL@90%CL τ→eω
Use fb-1 data (~710·106 τ’s) for this analysis
Use ω→π+π-π0
τ→ew
τ→mw
Signal box
±3 in DE-MEC
● =data
● =data
Mode
Eff.(%)
expected bckd
observed bckd
τ→eω
2.96±0.21
0.35±0.06
1.1·10-7
τ→μω
2.56±0.23
0.73±0.03
1.0·10-7
NO Signals
arXiv: [hep-ex]
submitted to PRL
Richard Kass

16. Search for τ→lll m- t- h0 m+
SUSY Models: B(τ→lll) & B(τ→lhh) in range to 10-6
Many Modes to search: t- -> e+e-e- , m+m-m-, e-m+m-, m-e+e- , m+e-e-, e+m-m- m- h0 t- m+
Babu, Kolda PRL (2002)
t
Signal Mode
m-m+m-
e-e+e-
μ-e+e-
e-m+m-
μ+e-e-
e+m-m-
Dominant
Background tt
uds
eemm
Bhabha
eeee mm
Event selection is optimized separately for each signal
mode accounting for different background compositions
Richard Kass

17. Results for τ→lll search
● =data
Used 376 fb-1 (~690·106 τ’s) of data
Efficiency: %
Background events: depends on mode
Total background = 4.20.8 events
Total number of observed events =6
90%CL
2-5 X lower limits than previous Babar results
NO Signals
arXiv: [hep-ex]
accepted by PRL.
Richard Kass

18. Results For τ→lhh′ (221.5 fb-1) 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, %
Number of background events estimated for each mode, events, data 0-3events
Upper limits calculated according to method of Cousins and Highland (NIM A 320 (1992) 331.)
BR’s < ( )10-7 (90% CL) Published in PRL 95 (2005)
Richard Kass

19. Summary of 3 Body LFV Decays
No signals found
Set limits O( ) 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:
Þ backgrounds are under control (~ 1 channel):
For t → lll analysis expect 4.2 events find 6.
For t → lhh′ analysis expect 11.1 events find 10.
Richard Kass

20. 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
t-→Lp-
pp-
t-→«p-
pp+ t- L p- B 1
B-L -1 +1 t- p- B -1 L 1
B-L
(B-L) violating
(B-L) conserving
Also searched for decays with a K- instead of π
Richard Kass

21. L & B # violation in τ Decay
t-→«p-
t-→Lp-
237 fb-1 BaBar data
( ~440·106 τ’s) used
for this analysis.
● =data
hep-ex:
Signal MC with
arbitrary normalization
t- →«K-
t- →LK-
Elliptical signal region
NO Signals
Mode
B-L
Eff.(%)
expected bckd
observed bckd
t-→«p-
conserve
12.28
0.42±0.42
5.9·10-8
t-→Lp-
violate
12.21
0.56±0.56
5.8·10-8
t-→«K-
10.63
0.26±0.26
7.2·10-8
t-→LK-
9.47
0.12±0.12 1
15·10-8
Richard Kass

22. Summary & Conclusions
►BaBar has performed many searches for LFV in τ decay
2-body decays: t  l, 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
Best chance to see LFV in τ decay is at a Super B-factory
produce ~1.5·1011 τ’s at Super B factory
M. Shapkin,
Nuc. PhysB 169 (2007) 363
Richard Kass

23. LFV in t decays: SuperB capability
t , hh’ : the sensitivity is not likely to be limited by irreducible backgrounds: the sensitivity scales as 1/L. For 75 ab-1 expect ~2x10-10
Principal backgrounds
From: Hitlin, Giorgi, SuperB workshops
Richard Kass 23

24. Extra slides
Richard Kass

25. BaBar K/p ID
D*+ → D0p+
D0→ K+ p-
BaBar DIRC
Richard Kass