why a linear collider now? s. dawson, bnl october, 2002 asian, european, and american communities...
TRANSCRIPT
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Why a Linear Collider Now?
S. Dawson, BNL
October, 2002Asian, European, and American communities
all agreeHigh Energy Linear Collider is next large
accelerator WHY???
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Where are we going?
• US high energy community just completed long range planning process
• 20 year roadmap for the future
• HEPAP subpanel:
We recommend that the highest priority of the U.S. program be a high-energy, high-luminosity, electron-positron linear collider, wherever it is built in the world
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Linear Collider Basics
• Initial design, e+e- at s=500 GeV• Luminosity 1034 cm2/sec
300 fb-1/yr• 80% e- polarization• Energy upgrade to .8-1.2 TeV in future• Physics in 2012
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• The international accelerator community believes that a TeV-scale linear collider can be successfully built
NLCHigh Power Klystron
TESLA Superconducting Cavity
JLCAcceleratorTest Facility
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TESLA
Preliminary designs for Linear Colliders
NLC
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?????• What are the big questions we want to
answer?• Why do we think we can predict where
we want to go?– What do we know now?– What do we expect to learn from the
Tevatron and LHC?– What questions will remain unanswered?
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What is particle physics?
Study of Space, Time, MatterBagger/Barish report
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The Big Questions?
• What is the origin of mass?
• Do protons decay?• Do forces unify at a
large scale?• Are there more than
four dimensions?• Why are there 4
forces?No unification of couplings in SM
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Cosmic Connections
• What is dark matter?
• How are particle physics & cosmology connected?
• What is dark energy?
• Where did the anti-matter go?
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Planning for the Future Based on Success of last 20 years….
• Model of electroweak physics verified at .1% level
• The problem of mass remains• W and Z bosons discovered at CERN
in 1983
• Masses not zero….or even small
GeVM
GeVM
Z
W
91
80
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Why is Mass a Problem?• Lagrangian for gauge field (spin 1):
L=-¼ FF
F=A-A
• L is invariant under transformation:
A (x) A(x)-(x)• Gauge invariance is guiding principle• Mass term for gauge boson
½ m2 AA
• Violates gauge invariance• So we understand why photon is massless
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Simplest possibility for Origin of Mass is Higgs Boson
• Higgs mechanism gives gauge invariant masses for W, Z
• Requires physical, scalar particle, H, with unknown mass
• Observables predicted in terms of:– MZ=91.1875.0021 GeV
– GF=1.16639(1) x 10-5 GeV-2
=1/137.0359895(61)
– Mh
• Higgs and top quark enter into quantum corrections, Mt
2, log(Mh)
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Precision Measurements sensitive to top quark before it was discovered!
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Large number of measurements fit
electroweak predictions
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Indirect Indications for Light Higgs Mass
• Direct measurements of MW, Mt agree well with indirect measurements
• Prefer Higgs in 100-200 GeV range
• ASSUMES no new physics
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Where is the Higgs boson?
• Higgs couplings of fixed
• Production rates at LEP, Tevatron, LHC fixed in terms of mass
• Direct search limit from LEP:
• Higgs contributions to precision measurements calculable
WWWh
fffh
gMgv
mg
clGeVM h %95@114
clGeVM h %95@193
Precision measurements:
G. Mylett, Moriond02
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Tantalizingly close…..
Direct limit: Mh>114.1 GeV
Indirect limit: Mh<193 GeV
New Physics is just around the corner!Fits assume Standard Model….if
Standard Model incorrect, even more exciting new physics….
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Higgs mass and scale of new physics correlated…..
Sensible theory here
130 < Mh < 170 GeV
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Fermilab Tevatron
• at s=2 TeV• May discover Higgs if
very lucky• Requires light Higgs
and high luminosity• Physics in 2002-2008
pp
bbhWhpp ,
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CDF
D0
Upgraded Detectors for RunII
Enhanced capabilities for b tagging aid Higgs search
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CERN Large Hadron Collider (LHC)
• pp interactions ats =14 TeV
• LHC will discover Higgs boson if it exists
• Sensitive to Mh from 100-1000 GeV
• Higgs signal in just a few channels
• Physics circa 2008
ATLAS TDR
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Discovery isn’t enough….
• Is this a Higgs or something else?
• Linear Collider can answer critical questions– Does the Higgs generate mass for the W,Z
bosons?– Does the Higgs generate mass for fermions?– Does the Higgs generate its own mass?
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Is it a Higgs?• How do we know what we’ve
found? • Measure couplings to
fermions & gauge bosons
• Measure spin/parity
• Measure self interactions
2
2
3)(
)(
m
m
h
bbh b
0PCJ
42
23
22
822h
v
Mh
v
Mh
MV hhh
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Coupling Constant Measurements
Zeppenfeld, hep-ph/0203123
• LHC measures combinations of coupling constants
• Typical accuracy, 10-20%
• Only some subset of couplings
• Assumptions necessary to get couplings
L=200 fb-1
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Linear Collider is Higgs Factory!
• e+e-Zh produces 40,000 Higgs/year
• Clean initial state gives precision Higgs mass measurement
Mh2=s-2sEZ+MZ
2
• Model independent Higgs branching ratios
WWh vertex
ZZH vertex
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Higgs mass measurements
• LC:
• LHC:
Direct reconstruction of
LC @ 350 Gev
Conway, hep-ph/0203206
MeVM
fbGeVM
h
h
50
500,120 1
h
MeVM
fbGeVM
h
h
100
300,150 1
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Precision Measurements of Higgs Couplings
• Dots are experimental error
• 1-2% measurement• Measure ALL Higgs
couplings• Bands are theory error
– Larger than experiment
– Largest error from mb
Battaglia & Desch, hep-ph/0101165
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Higgs measurements test model!
• Couplings to fermions very different in SUSY models
• LC can distinguish SM from SUSY up to MA=600 GeV
Standard Model
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• Angular correlations of decay products distinguish scalar/pseudoscalar
Miller, hep-ph/0102023
Threshold behavior measures spin
[20 fb-1 /point]
Higgs spin/parity in e+e-Zh
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Measuring Higgs Self Couplings
• ghhh, ghhhh completely predicted by Higgs mass
• Must measure e+e- Zhh
• Small rate (.2 fb for Mh=120 GeV), large background
• Large effects in SUSY
%24
1000 1
hhh
hhh
g
g
fb
Lafaye, hep-ph/0002238
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Problem with this picture…
• Fundamental Higgs is not natural
• Quantum corrections to Mh are quadratically divergent
Mh22
• So enormous fine-tuning needed to keep Higgs light
Mh2\Mh
2MW2\Mpl
210-32
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Solution is Supersymmetry
• Quadratic contributions to Higgs mass cancel between scalars and fermions
• To make cancellation hold to all orders need symmetry
• Bose-Fermi symmetry….supersymmetry
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Do the forces unify?
• Coupling constants change with energy
• Coupling constants unify in supersymmetric models
Hint for new physics?
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New particles in SUSY Theory
• Spin ½ quarks spin 0 squarks• Spin ½ leptons spin 0 sleptons• Spin 1 gauge bosons spin ½ gauginos• Spin 0 Higgs spin ½ Higgsino Experimentalists dream….many particles to search for! What mass scale?Supersymmetry is broken….no scalar with mass of
electron
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Supersymmetry
• Can we find it?
• Can we tell what it is?
• Masses of new particles depend on mechanism for breaking Supersymmetry
• Couplings of new particles predicted in terms of few parameters
• Simplest version has 105 new parameters
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Simplifying Assumption:
• Assume masses unify at same scale as couplings
• Everything specified in terms of scalar/fermion masses at high scale and 3 parameters
• Predictive anzatz…..
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•LHC/Tevatron will find SUSY• Discovery of many SUSY
particles is straightforward• Untangling spectrum is
difficult all particles
produced together• SUSY mass differences from
cascade decays;eg
• M0 limits extraction of other masses
qll
llqqL
0
1
02
~
~~~
Catania, CMS
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Light SUSY consistent with Precision Measurements
• SUSY predicts light Higgs
• SUSY predicts 5 scalars
• For MA, SUSY Higgs sector looks like SM
• Can we tell them apart?• Higgs BR are different in
SUSY
HAHh ,,, 000
GeVM SUSYh 130
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LHC
Find all the Higgs Bosons
Carena, hep-ph/9907422
Tevatron
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Into the wedge with a LC
• s>2MH
e+e- H+H-, H0A0
observable to MH=460 GeV at s=1 TeV• s<2MH
e+e- H+, H+tb L=1000 fb-1, s=500 GeV,
3 signal for MH 250 GeV
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LC can step through Energy Thresholds Run-time Scenario for L=1000 fb-1
Year 1 2 4 5 6 7
L (fb-1) 10 40 150 200 250 250
• SUSY masses to .2-.5 GeV from sparticle threshold scans M0/M0 7% (Combine with LHC data)
• 445 fb-1 at s=450-500 GeV• 180 fb-1 at s=320-350 GeV (Optimal for Higgs BRs)
• Higgs mass and couplings measured, gbbh1.5%
• Top mass and width measured, Mt150 MeV
Battaglia, hep-ph/0201177
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How do we know it’s SUSY?• Need to measure masses,
couplings
• Observe SUSY partners, eg
• Polarization can help separate states
• Discovery is straightforward
• e energies measure massesRL ee ~,~0
,,
~~
~~
ee
eeee RLRL
2min,max,
min,max,22~
)( ee
eeCMe EE
EEEM
LC Study, hep-ex/0106056
me1 GeV
L=50 fb-1
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SUSY Couplings:
• Compare rates at NLO:
• Lowest order,
• Super-oblique corrections sensitive to higher scales
•
• Masses from endpoints
• Assume
• Tests coupling to 1% with 20 fb-1
XffffX gg ~~
gqqee
gqqee
gqqee
~~
~~
ss gg ~
m
mg
g
g ~ln
16
~
~
~ 2
2
RLRL eeee ,,~~
Bee~~
eeB~~
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What is the universe made of?
• Stars and galaxies are only 0.1%
• Neutrinos are ~0.1–10%
• Electrons and protons are ~5%
• Dark Matter ~25%
• Dark Energy ~70%
H. Murayama
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Supersymmetry provides understanding of dark matter?
• Lightest SUSY particle (LSP) could be dark matter candidate!
• LSP is weakly interacting, neutral, and stable
• LSP in range of LC/LHC• LC can determine LSP
mass; check dark matter predictions
LSP is dark matter
Mh=115 GeV
g-2
Drees, hep-ph/0210142
M0 (GeV)
M1/
2
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Standard Model Needs Top Quark
• Top quark completes 3rd generation– Why are there 3
generations, anyways?
• Theory inconsistent without top
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Top Quark discovery at Fermilab in 1995
CDF top eventD0 top event
Why is Mt(=175 GeV)>>Mb(=5 Gev)??
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Understanding the Top Quark
• Why is ?
• Kinematic reconstruction of tt threshold gives pole mass at LC
• Compare LHC
2
vM t
MeVM
fb
t 200
40 1
Groote , Yakovlov, hep-ph/0012237
QCD effects well understood
NNLO ~20% scale uncertaintyGeVM
fb
t 21
50 1
2Mt (GeV)
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Top Yukawa coupling tests models
• tth coupling sensitive to strong dynamics
• Above tth threshold
e+etth
• Theoretically clean s=700 GeV, L=1000 fb-1
• Large scale dependence in tth rate at LHC
• L=300 fb-1
%5.6tth
tth
g
g
Baer, Dawson, Reina, hep-ph/9906419
Juste, Merino, hep-ph/9910301
Reina, Dawson, Orr, Wackeroth
Beenacker, hep-ph/0107081
% 16 tth
tth
g
g
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Exciting physics ahead
• LHC/Tevatron finds Higgs LC makes precision measurements of
couplings to determine underlying model• LHC finds evidence for SUSY, measures mass
differences LC untangles spectrum, finds sleptons LC makes precision measurements of
couplings and masses• etc
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