The tau Lepton

1
The tau Lepton
and
Flavour Violation
Mass Origin and Supersymmetry March 6 -8
2006 Tsukuba
Achim Stahl RWTH Aachen University
2
Lepton Number
Li S LeptonsGen i AntileptonsGen i
conserved in the Standard Model
today many examples
Wolfgang Pauli
3
Neutrino Oscillations
neutrino oscillations violate lepton number !
4
Leptonic tau-Decay
nt
t-
nm
W-
m-
t- ? m-
forbidden nm ? nt
5
Leptonic tau-Decay
nt
t-
nm
W-
m-
t- ? m-
forbidden nm ? nt
but energy/momentum conservation
6
LFV-Decay
possible decay
nt
t-
nm
W-
m-
t- ? m-
g
forbidden nm ? nt
but energy/momentum conservation
7
LFV-Decay
possible decays
nt
t-
nm
m-
W-
g/Z
m
m-
t- ? m- m m-
8
LFV-Decay
possible decays
nt
r0
Z
t-
nm
W-
m-
t- ? m- r0
9
GIM-Mechanism
nt
t-
nm
W-
m-
t- ? m-
g
10
GIM-Mechanism
ni
t-
Uti
W-
Umi

m-
br 10-40
11
The Paper of X.-Y. Pham
Is this true ?
Euro Phys J C8 (1999) 513, hep-ph/9810484
for discussion see Tord Riemanns
home-page http//www-zeuthen.desy.de/riemann/
12
Beyond the S.M.
new physics breaks GIM
13
Beyond the S.M.
new physics breaks GIM
14
Experimental Searches
15
Experimental Searches
Signal
Background
ee- ? mm- g
ee- ? tt-
_
ee- ? q q
16
LVF _at_ b-factories
currently best limits !
BaBar
approaching 109 t-pairs
t- ? m- g
Belle 2004 lt 3.1 10-7 BaBar 2005 lt 6.8 10-8
17
LVF _at_ b-factories
currently best limits !
BaBar
approaching 109 t-pairs
prel. 03 final 05
t- ? m- g

• (r/e-tag) 5.2 3.8
• (total) 9.4
• exp. BGD
• (r/e) 7.8 ev. 2.8 ev.
• (total) 6.2 ev.
• limit (10-8) lt 200 lt 6.8

Belle 2004 lt 3.1 10-7 BaBar 2005 lt 6.8 10-8
b-factories reaching the limits at x 10-8
18
LVF _at_ LHC
t sources at the LHC
1 year running low luminosity
p p ? W ? t nt p p ? Z/g ? t t p p ? B0 ? t X p p
? B ? t X p p ? Bs ? t X p p ? Ds ? t X
1.7 108 8.0 108 4.0 1011 3.8 1011 7.9 1011 1.5
1012
plenty of taus !
no chance to see t- ? m- g
19
LVF _at_ LHC
? Try t- ? m- m m-
Simulation with underlying event low luminosity
20
LVF _at_ LHC
Source W ? t nt
Trigger Single-Muon L1 pT gt 14 GeV HLT
pT gt 19 GeV
Trigger Di-Muon L1 pT gt 3 GeV HLT pT gt
7 GeV
Looks okay ?1.7 108 t per year
21
LVF _at_ LHC
DS - Source
Trigger Single-Muon L1 pT gt 14 GeV HLT
pT gt 19 GeV
Trigger Di-Muon L1 pT gt 3 GeV HLT pT gt
7 GeV
Trigger-Eff. only 1 ?1.5 1012 t per year
22
LVF _at_ LHC
Background Problem with DS source
DS ? F m nm 1 br. F ? m m (g) 10-5 br.
Does anybody know of a generator for this process
?
23
LVF Z-Decays
m
g/Z
nt
m-
t-
nm
m-
W-
t- ? m- m m-
24
LVF Z-Decays
m-
g/Z
m
nt
t-
nm
m-
W-
m- m t- ? m-
25
LVF Z-Decays
m-
g/Z
m
nt
nm
m-
m- m ? m- t
t
26
LVF Z-Decays
p
g/Z
p
nt
nm
m-
p p ? Z ? m- t
t
27
LVF Z-Decays
m-
nm
g/Z
p
W
p
nt
t
p p ? Z ? m- t
28
Summary
29
Conclusions
LFV tau-decays ? Interesting search for
physics beyond the S.M. (SUSY and others
)
b-factories will approach 10-8
Can we improve a few channels at LHC ? Also look
for LVF in Z-decays at LHC !