TAU LEPTONS IN THE QUEST FOR NEW PHYSICS

Alexei SafonovTexas A&M University

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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…heavily used to get jobs and tenures

WHAT DOES A TAU LOOK LIKE? Unstable, undergoes weak

decaysLifetime: ct~87 mm

Decay channels:Leptonic: t→enent, t →μnmnt (~36%)

Hadronic: t →πnt, t→ππ0nt, t→πππnt, t→ππ0π0nt ... (~64%) Nomenclature: 1-prong, 3-prong etc. 3

t

W

nt

ne

e

t

Wnt

Kp,Np0,…

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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

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MARK I DETECTOR

First hard evidence was the anomalous em events The main background

was some other meson or hadron production

Electron ID: Pulses in 24 lead-

scintillator counters extending full length with PMTs on each end

Muon ID: Spark chambers behind

a 24 cm absorber

Compared to CMS, almost a table-top experiment And not a very good

one

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PROPERTIES OF ANOMALOUS EVENTS

A candidate em event

Rate of events vs Ecm Simple mass estimate

m(t)=1.9±0.1 GeV/c2

Event displays seem to have made a much bigger progress since 1975 than the rest of our field

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LEPTON OR BOSON?

Essentially trying to distinguish between: e+e-MMemnn 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(tenn)≈ B(tmnn) ≈ 18%

Fraction of Ecm energy carried by visible lepton Data follows the 3-body

pattern consistent with a lepton decay

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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):

3

2666659.0

mmm

mmm

e

e

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LIFE AFTER DISCOVERY

Lifetime measurements required better detectors

SLD decay length measurement (1995) using pixel vertex detector

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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

HIGGS LIKES TAUS

Low mass Higgs: Taus: second highest leptonic

Branching fractions after b’s Much cleaner signatures – can

potentially use ggH 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 bosonsNew “mirror”

particles

g,W,Z,h

e,n,u,d

SUSY partner

Particle

Dark Matter Candidate

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01

21

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due~,~,~,~

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HOW SUSY HELPS Resolves hierarchy

problem In SM, Higgs mass acquires

huge mass corrections Fine tuning needed (10-30)

SUSY: exact cancellation of diagrams with particles and sparticles

Unification of interactions Similar to EW unification Can include strong

interactions Dark Matter candidate

...]2[16

|| 22

22 UV

fHm

f

H H

f

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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: Tevatron discovers top: mt~175 GeV

Mixing sin2qW = mW/mZ - arbitrary in SM: 1980’s:

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: STAU CO-ANNIHILATION REGION

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

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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

Often enhanced cross-section Charged H+:

Another good use for taus

SUSY with Left-Right Symmetry: 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 h1a1a1 (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 tan :b

s(ggh/H/A)~tanb2

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HIGGS IN DI-TAUS AT THE TEVATRON

WHjj

MSSM H

Also an interesting interplay with the CDMS results

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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

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CHARGED HIGGS AT THE TEVATRON

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DOUBLY CHARGED HIGGSt

t

t

t

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NEXT-TO-MSSM

Adds a new singlet field to MSSM New decay mode for light higgs

haa For a large range of m(a)

dominant B(a) Sound as an abstract

theoretical exercise, but has its merits: Solves the ”m problem” in SUSY

(it is now generated by the new field)

Resolves many of the “naturalness” problems in SUSY

May explain the tension between direct and indirect higgs searches “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