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TAU LEPTONS IN THE
QUEST FOR NEW
PHYSICS
Alexei Safonov
Texas A&M University
LIFE OF A TAU

Fairly typical life of a celebrity:
Michael Jackson
Tau Lepton
Unnoticed at birth
1958
14B yrs ago
Instant celebrity
when discovered
1970’s on: awards
and best selling
albums
1975: discovery
…Nobel Prize by
Martin Perl (1995)
People digging
through your
personal life
A lot! e.g. hundreds
publications in
Enquirer and such
1982-2004
measurements of tau
lifetime, branchings
…no lawsuits,
though
Greedy and heavy
exploration for profit
after death
e.g. $250M deal
signed for music
distribution rights
A tool for new great
discoveries
2
…heavily used to get
jobs and tenures
WHAT DOES A TAU LOOK LIKE?
 Unstable,

Lifetime: ct~87 mm
 Decay

undergoes weak decays
ne
channels:
Leptonic: t→enent, t →μnmnt (~36%) W
nt
t

Hadronic: t →πnt, t→ππ0nt, t→πππnt,
t→ππ0π0nt ... (~64%)

Nomenclature: 1-prong, 3-prong etc.
t
e
Kp,Np0,…
W
nt
3
TAU DISCOVERY (1975)

Discovered at MARK I using
e+e- beams at SLAC SPEAR

Stanford Positron Electron
Accelerating Ring


E<4 GeV per beam
The most “cost effective”
collider ever built
4
MARK I DETECTOR

Compared to CMS, almost
a table-top experiment


And not a very good one
First hard evidence was
the anomalous em events


Electron ID:


The main background was
some other meson or
hadron production
Pulses in 24 leadscintillator counters
extending full length with
PMTs on each end
Muon ID:

Spark chambers behind a
24 cm absorber
5
PROPERTIES OF ANOMALOUS EVENTS
Rate of events vs Ecm
 Simple mass estimate
m(t)=1.9±0.1 GeV/c2



A candidate em event
Event displays seem to have made a much bigger
progress since 1975 than the rest of our field
6
LEPTON OR BOSON?

Essentially trying to
distinguish between:
e+e-MMemnn
 e+e- LL(enn)(mnn)




Still a lot of disbelief
until in 1977 Pluto and
DASP (DORIS @ DESY)
confirm the discovery
The new lepton was
named t (triton = third)
Data used to measure
mass and B(tenn)≈
B(tmnn) ≈ 18%

Fraction of Ecm energy
carried by visible lepton

Data follows the 3-body
pattern consistent with a
lepton decay
7
SIGNIFICANCE OF TAU DISCOVERY

First evidence of the third generation

Many hoped this is just another one in a series of new
generations
Statistically significant confirmation of “V-A” versus
“V+A” nature of weak interactions
 First hints at large disparity in masses between
generations



m(t)=1.77 GeV/c2 vs m(e)=0.000511 m(m)=0.1057 GeV/c2
Also an amusing equality - Yoshio Koide (1981):
me  mm  mt
2
 0.666659 
3
me  mm  mt
8
LIFE AFTER DISCOVERY

Lifetime measurements
required better detectors

SLD decay length
measurement (1995)
using pixel vertex 9
detector
LEP: END OF TAU’S STORY OF LIFE

Ideal for high precision
measurements:
Ultra low backgrounds
 Fairly large boosts
 Precise reconstruction of
momentum and di-tau mass
via energy conservation

LEP performed exhaustive
studies of branching ratios,
rare decay modes, lifetime
etc.
 Tau: ready to be boxed and
put next to e and m

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IS THERE LIFE AFTER DEATH?
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TAUS IN SEARCHES FOR NEW PHYSICS
Two main reasons
 Many implications


Higgs boson:
Coupling to fermions
hff~mf
 Tau is the heaviest
lepton


Supersymmetry:
Third generation
SUSY particles could
be the lightest
 Even more Higgs
signatures with taus

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HIGGS LIKES TAUS
 Low mass Higgs:
 Taus: second highest
leptonic
Branching fractions after b’s
 Much cleaner signatures – can
potentially use ggH process

Low mass Higgs non-tau
signatures

Tevatron: relies on WH/ZH


5 times lower production rate
compared to gluon fusion
LHC: h:

Tiny branching fraction
 Taus

can come very handy:
Also we won’t know what we
found w/ just one measurement
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SUPERSYMMETRY (SUSY)
 New


symmetry:
fermions  bosons
New “mirror” particles
Particle
e,n,u,d
,W,Z,h
Dark Matter
Candidate
SUSY
partner
~
~
~
~
e ,n , u , d
~  , ~  ,
1
2
~10 ...~40
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HOW SUSY HELPS


In SM, Higgs mass acquires
huge mass corrections



H
Fine tuning needed (10-30)
SUSY: exact cancellation of
diagrams with particles and
sparticles
H
f
mH2 
|  f |2
16p 2
[22UV  ...]
Unification of interactions



f
Resolves hierarchy problem
Similar to EW unification
Can include strong
interactions
Dark Matter candidate
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AMUSING SUSY PREDICTIONS

Top quark mass:

1980’s:
Top quark mass was thought to be mt<~30 GeV, Tristan
collider is built to find top - no luck…
 SUSY prediction: top has to be heavy: mt>mW!


1995:


Mixing sin2qW = mW/mZ - arbitrary in SM:

1980’s:



Tevatron discovers top: mt~175 GeV
SUSY predicts sin2qW =mW/mZ= 0.231
1990: LEP sin2qW ~0.2309+/-0.0009
Could be a coincidence, but SUSY seems just too
good to not be true
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SEARCHES FOR SUSY
While we have been
setting boring limits,
the strongest
constraints on SUSY
came from some place
else
 WMAP measurements
of dark matter density


A handful of preferred
regions in SUSY
parameter space giving
the right amount of
dark matter
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
SUSY often over-produces the
dark matter


To solve it, need a mechanism to
destroy extra neutralinos
Stau co-annihilation:


If stau is slightly heavier than
lightest neutralino: mutual
annihilation
Can get the relic density right
~10
t
t~
~10
1
t
~10
t~
1
t
t

Mass of Squarks and Sleptons
SUSY: STAU CO-ANNIHILATION REGION
Mass of Gauginos
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FINDING SUSY
At the end, convincing discovery
of SUSY will likely require direct
detection at colliders
 SUSY in stau co-annihilation
region may be difficult to
discover



Complex cascades lead to busy
events
Can easily disguise as other
SUSY species:

If taus in the final state are not
recognized, you will discover
“wrong” SUSY
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HIGGS IN SUPERSYMMETRY

MSSM:
A more complex Higgs hierarchy:
 Three neutral higgs bosons h/H/A



Charged H+:


Another good use for taus
SUSY with Left-Right Symmetry:


Often enhanced cross-section
Doubly charged H++tt alongside right-handed W’s
and neutrinos
Next-to-MSSM:
More complex higgs sector, new light CP-odd higgs a1
 Can avoid standard searches via h1a1a1 (2t) (2t)

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SUSY: NEUTRAL HIGGS PRODUCTION


Additional
diagrams
and
modified
couplings
to quarks
Can be
right
around the
corner
Top row leads to enhanced production
at large tanb:
s(ggh/H/A)~tanb2
21
HIGGS IN DI-TAUS AT THE TEVATRON

MSSM Htt


WHttjj
Also an interesting interplay with the CDMS results
22
CHARGED HIGGS

If light enough, can be
produced in top
decays


Will modify top
branching fractions
due to preference for
taus
Or else can be
searched directly

Production reduced by
the coupling to light
quarks
23
CHARGED HIGGS AT THE TEVATRON
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DOUBLY CHARGED HIGGS
t
t
t
t
25
NEXT-TO-MSSM

Adds a new singlet field to
MSSM
New decay mode for light higgs
haa
 For a large range of m(a)
dominant B(att)


 May explain the tension
Sound as an abstract
theoretical exercise, but has its between direct and
merits:
indirect higgs searches
Solves the ”m problem” in SUSY
(it is now generated by the new
field)
 Resolves many of the
“naturalness” problems in SUSY



“Hiding” Higgs weakens
LEP limits
Experimental nightmare at a hadron collider!
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SUMMARY FOR TODAY

Many compelling arguments to look for new physics
in final states with taus
Almost always, taus are indispensible in understanding
the nature of the discovered phenomenon
 Frequently, taus hold keys to discoveries
 Sometimes, an “incorrect” discovery can be made if not
paying attention to taus


The bad news is that taus are challenging in hadron
collider environment


You saw some examples showing high backgrounds and
similar shapes
Tomorrow we will talk about experimental techniques
and challenges in searches for new physics with taus
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