Alexei Safonov (Texas A&M University) For the EMU community.
T AU L EPTONS IN THE Q UEST FOR N EW P HYSICS Alexei Safonov Texas A&M University.
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Transcript of T AU L EPTONS IN THE Q UEST FOR N EW P HYSICS Alexei Safonov Texas A&M University.
TAU LEPTONS IN THE QUEST FOR NEW PHYSICS
Alexei SafonovTexas A&M University
2
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,…
4
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
5
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
6
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
7
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
8
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
9
LIFE AFTER DISCOVERY
Lifetime measurements required better detectors
SLD decay length measurement (1995) using pixel vertex detector
10
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
11
IS THERE LIFE AFTER DEATH?
12
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
13
14
SUPERSYMMETRY (SUSY) New symmetry:
fermions bosonsNew “mirror”
particles
g,W,Z,h
e,n,u,d
SUSY partner
Particle
Dark Matter Candidate
04
01
21
~...~,~,~
due~,~,~,~
15
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
16
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
17
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
18
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
t
t
t1~
c10~
c10~ t1
~
c10~
tt
gMass of GauginosM
ass
of
Sq
uar
ks a
nd
Sle
pto
ns
19
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
20
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)
21
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
22
HIGGS IN DI-TAUS AT THE TEVATRON
WHjj
MSSM H
Also an interesting interplay with the CDMS results
23
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
24
CHARGED HIGGS AT THE TEVATRON
25
DOUBLY CHARGED HIGGSt
t
t
t
26
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!
27
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