The next step for particle physics:

1
P. Grannis Stony Brook 11/2/04
The next step for particle physics The ee-
Linear Collider

• I will describe the dreams of many particle
  physicists for taking the next steps toward
  understanding constitution of matter at the
  smallest scale.
• The International Linear Collider (ILC)
• Are the physics questions important enough to go
  there?
• Why ee-, why linear, and how would you do it?
• How to organize to achieve it?
• Is it worth the cost?

2
A. The case for the next step in understanding
the fundamental construction of matter
I will use an analogy (strained, as analogies
often are) of the present state of high energy
physics to an expedition to climb a mountain peak
in a range not previously explored.
HEP is at present at the lowest base camp for the
assault, on a plateau with hidden peaks rising
above us. Our base camp is mostly in fog and we
have been here a long time, but we have
occasional clearings with glimpses at the ridges
above.
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Our base camp the Standard Model
Over the past 30 years, the SM has been assembled
and tested with 100s of precision measurements.
3 doublets of quarks and leptons 3 forces
(Strong and unified EM and Weak). Gravity is
left out. In my view, there are no clear
demonstrations that the SM is inadequate, though
we have tried hard to find departures for decades.
High energy lepton and hadron collisions,
neutrino studies, rare decays will all give
complementary views, enabling a real
understanding of the SM, and what lies beyond it.
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From our base camp, we see crevasses that suggest
that our SM is not fully correct and stable.
Some of these fissures
a) The SM is QCD x EW SU(3)xSU(2)xU(1) but
the pieces are just pasted together. We know
that unification (coupling constants coming
together) does not happen within the context of
the SM. And gravity stands outside the SM.
Wouldnt it be elegant to have unified strong and
electroweak forces, and to bring gravity into the
mix?
5

• The Weak and EM interactions are joined as the
  Electroweak (EW) force, but the symmetry is
  broken mg ?mW ? mZ in the SM. The
  mechanism for breaking the symmetry for the force
  carriers is transmitted to the matter particles
  (quarks and leptons) as well. In the SM, it is
  the Higgs field that does this

• Higgs field interactions reduce the quark/lepton
  momenta at a given energy (give them masses) in
  proportion to the coupling gHqq . Larger Higgs
  couplings ? larger mass. (So thats an
  explanation for anything??)
• Why is the scale of the Higgs mass and EW
  symmetry breaking O(100 GeV) so
  different from the Planck or force unification
  scale (1016 1018 GeV)? The disparity is highly
  unstable huge fine tuning needed.
• Is EW symmetry breaking really due to the SM
  Higgs boson ?

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c) CP is violated rate (K0 ? p e- n) ? rate
(K0 ? p- e n). See it also in decays of b-quark
mesons rates of B0 and B0 into J/y KS are not
equal. But the SM CP violation is not sufficient
to explain the asymmetry between baryons and
antibaryons in the universe. It may be that CP
violation in the neutrino sector could explain
some of this, but it seems that new physics is
required.
d) Dark matter and dark energy are observed, but
are not explained in the SM. Extensions to the
SM could provide DM. Higgs vacuum energy might
have been dark energy, but is 10120 times too
large !
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Occasionally the fog at our base camp clears a
little and we can glimpse pathways upwards from
our base camp.
Several classes of theoretical models offer some
guidance on the route we might take. But we very
much need new tools that will enable us to climb
off the SM plateau and see what is up there.
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B. Getting there tools and paths to see more
clearly
As with mountain climbing, we want belt and
suspenders backup complementary ways of
gaining new understanding.
Large Hadron Collider (LHC)
14 TeV (ECM) pp collisions (but lower constituent
(unknown) collision energy).
Reactions occur by strong interaction copious
backgrounds to rare processes. Not a well
specified initial state, and many spectator
particles.
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with ECM0.5 to 1 TeV. Lower
energy than LHC, but all energy in single
constituent. Initial state is fixed energy and
quantum (JP1-).
ee- linear collider
Cross sections for interesting signal events and
backgrounds are comparable.
Beam polarization (e-) allows enhancement or
suppression of reactions or backgrounds.
Events are clean little background.
Lepton and hadron collisions give the
complementary view that enables real
understanding. LHC will start first (2007-8)
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Other routes
There are other routes to understanding the GUT
scale besides studying EW symmetry breaking
other faces of the mountain we could attempt to
scale.
Neutrino masses, CP violation, proton decay, very
rare meson decays, neutron electric dipole moment
etc. all can offer some direction. But at
present, these remain indirect views of the
summit without any clear paths for actually going
there.
Increasing the CM energy and exploring EW
symmetry breaking seems the most direct and
accessible path to really understanding the new
terrain.
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Possible paths to the summit on the high energy
route
1) Supersymmetry boson-fermion symmetry with
new fermionic space-time degrees of freedom. The
symmetry is clearly broken how does that
happen? Susy allows unification of Strong, EW,
Weak couplings Susy can provide the dark matter
candidate, and many other new particles Four (at
least) Higgs states

• Strong coupling composite models new strong
  interactions with new heavy particles. In
  these, Higgs boson is a fermion condensate, like
  Cooper pairs for superconductivity

• Bring the mountain to Mohammed large extra
  spatial dimensions in which some or all SM
  particles can move. The real Planck scale is not
  so far above our current base camp at the TeV
  scale.

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C. Main elements of LC physics case
(1) Study the SM (or other) Higgs
SM Higgs bounded by expt 114 lt mH lt
230 GeV
LHC will see it if it exists up to gt 1 TeV. But
for mH lt 160 GeV, hard to observe the dominant
decay H ? bb. LHC will not measure Higgs
quantum numbers (JP). Will measure branching
ratios to only 20.
LC gives precise profile of Higgs. Determines JP
and GH. Measures BRs to few . These BRs
distinguish between SM and other kinds of Higgs.
Allowed MA
Possible BR measurements
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Higgs
At the ILC, it is possible to measure Higgs self
coupling in ee- ? ZHH, which determines the
Higgs potential directly self-consistency
check.
Can see all Higgs decays, including invisible,
from ee- ? ZH, using only the Z decay products.
The LC is the pathway to understanding the nature
of the Higgs boson.
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(2) Explore SUSY
LHC will discover SUSY if it exists, seeing
excess energy transverse to beam line due to SUSY
particle decays. LHC mainly produces partners
of strongly interacting quarks and gluons, but
partners of leptons or (W,Z,g,H) are rare. LHC
measures mainly mass differences (Q values in
decays).
LC makes the partners of leptons, neutrinos,
gauge bosons copiously. Measures the lowest mass
supersymmetric particle (partner of Z,g) best
candidate for dark matter. Simple production
processes like ee- ? m m-. Can measure masses
accurately and determine SUSY particle quantum
s. Can measure mixing matrices of the partners
of the (W,Z,g,H) and CP violating phases.
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SUSY symmetry breaking
The LC can make accurate mass measurements from
threshold scans (order of 0.1). Mapping out the
supersymmetry spectrum would by of itself be a
great achievement.
But this also opens the way to map the SUSY
symmetry breaking terrain itself, and to get a
clear glimpse into the grand unification scale.
LC can measure the CP violating parameters for
partners of top quark, or (Z, g). CP violation
in Supersymmetry could offer the explanation for
baryon-baryon asymmetry.
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(3) Large extra dimensions
LHC can sense extra dimensions through production
of jets that are unbalanced by other particles
(due to graviton flying into the new dimensions)
and measure the true Planck mass (ltlt 1019 GeV).
LC, using unbalanced photons, can tell us how
many extra dimensions there are (in models where
graviton only moves in extra dimensions)

• But if the extra dimension models are in general
  correct, there are many variations possible
• Metric in the extra dimensions
• Which fields live in the extra dimensions
• Size, number of the extra dimensons
• Information from both LHC and LC will be
  crucial

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D. Organizing the expedition
The physics case and its priority
In 2001, advisory panels in Europe (ECFA), US
(HEPAP) and Asia (ACFA) all recommended that the
next major project in High Energy Physics should
be a 500-1000 GeV Linear Collider. In 2002, a
subcommittee on science for the OECD concurred,
and noted that the LC should overlap with the LHC.
In 2003, the international community prepared a
15 page document that outlined the elements of
the scientific case for use by scientists.
Nearly 3000 physicists worldwide signed
it. http//blueox.uoregon.edu/lc/wwstudy/
Understanding Matter, Energy, Space and Time The
Case for the Linear Collider
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Guiding the international effort
2002 International Committee for Future
Accelerators (ICFA) formed a Steering Committee
(ILCSC) to guide the LC project. 2003 ILCSC
subcommittee report on the technical issues for
all potential designs of LC. The main contenders
were (1) room temperature copper
accelerating structures X-band, and (2)
superconducting Nb rf cavities SC. Alternate
lower frequency room temperature (C-band) as
backup if others wont work. RD on higher
energy LC using an intense beam to produce rf
power (CLIC) not ready for consideration
yet. This committee listed the critical RD steps
that remained before one could consider choosing
each technology. By mid 2004, X-band and SC both
satisfied these critical milestones.
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Guiding the international effort

• 2003 Subcommittee to draw up the scientific
  parameters and specification for the LC
• Baseline operate in range 200 lt ECM lt 500 GeV
  with 0.1 control of energy and ability to scan
  energy. Accumulate 500 fb-1 in 4 years 80 e-
  polarization
• Two experiments (push/pull)
• Upgrade to 1 TeV, acquire 1000 fb-1 in 4 years
• Options retain possibility for positron
  polarization, running at the Z boson mass (91
  GeV), gg collisions (backscatter laser from
  primary e- beam)
• These were set with the physics program goals in
  mind the Higgs boson physics, searches for
  Supersymmetry or other new physics, and the need
  to refine the precision of the Z, W boson and top
  quark properties.

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Guiding the international effort
2004 Panel formed to make a recommendation on
which of the two primary technologies should be
selected recognizing that continuing on both
paths was wasteful of manpower and money.
12 people 4 Asians, 4 Europeans, 4 from US
like a trial by jury. Meetings in/near Oxford,
Hamburg, Palo Alto, Tokyo, Los Angeles and
Pohang, Korea from Jan. Aug. 2004
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E. How does one build a Linear Collider?
Why a linear collider

• Particle physics colliders to date have all been
  circular machines (with one exception SLAC
  SLC).
• Highest energy ee- collider was LEP2 ECM200
  GeV
• Synchrotron light sources are circular

As energy increases at given radius DE E4/r
(synchrotron radiation) e.g.
LEP DE4 GeV/turn P20 MW Going higher in energy
in a circular machine becomes prohibitively
expensive large power or huge tunnels. But LC
needs long single pass linacs to reach desired
energy.
we are here
cost
Circular Collider
Linear Collider
Energy
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The basic linear collider elements

• Source of polarized electrons (create positrons
  in secndry target)
• Damping rings to cool the beams create small
  emittance
• Compress bunch length to 100-300 mm (keeping low
  emittance!)
• Long acceleration section to full energy
  (keeping low emittance!)
• Final focus to squeeze beams transversely (sx
  /sy 300/3 nm) and bring into collision (flat
  beams max L, min. DE)

½ of Linear Collider
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Two choices rf frequency dictates the details
L-band (SC) 1.3 GHz l 3.7 cm
TESLA X-band (Rm Temp) 11.4 GHz 0.42 cm
NLC/GLC

• Power lost in structure walls gradient
  squared/shunt impedence
• Large Q means energy dissipates slowly large Q
  allows long bunch trains
• SC wants low frequency for high shunt
  impedence, high Q (1010)
• X-band wants high freq. for high shunt
  impedence, reasonable Q (104)

• Accelerating structure size is dictated by
  wavelength of the accelerating Electric field.
  Disruptive wakefields are inversely related to
  structure size thus so also is the difficulty in
  controlling emittance growth and final
  luminosity.
• Bunch spacing, train length related to rf
  frequency (and Q)
• Damping ring design depends on bunch train
  length, hence frequency
• So the frequency dictates many of the design
  issues for LC

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Important parameters
Many of design parameters follow from the
frequency choice
Parameter X-band Superconducting Bunch
spacing 1.4 ns 337 ns Bunch train length 267
ns 950 ms Train rep rate 120 Hz 5 Hz Accel.
Gradient 65 MV/m 35 MV/m (52 MV/m bm
loaded) 2 linac length (1 TeV) 27.6 km 37.5
km Wall plug effic. 2.5 23 Site power (500
GeV) 200 MW 140 MW
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I. Getting the energy rf acceleration
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Accelerating structures
Travelling wave structure need phase velocity
velectron c Circular waveguide mode TM01 has
vpgt c no good for acceleration! Need to slow
wave down (phase velocity c) using irises.
Bunch sees constant field EzE0 cosf Group
velocity lt c, controls the filling time in cavity.
Ez
c
z
SC cavity
Room temp Cu structure
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rf system

• Modulator converts wall ac power to HV dc pulse
  for klystron
• Klystron tube is rf pulse power amplifier
  --accelerate electrons through bunching cavities
  to generate microwave pulse
• For X-band, must shorten the pulse train use
  dual delay lines and rf switching to chop and
  fold rf pulse on itself
• rf pulse injected into set of rf structures on
  the beam line to accelerate particles

rf pulse compression needed for high power, short
bunch X-band, not for SC
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klystrons
X-band PPM klystrons small but need 4500 with
75 MW pulse power 1.6 ms pulse length permanent
magnet focussing to reduce power
SC (TESLA) multibeam klystron Need 572 10 MW
pulse power 1.4ms pulse length Klystron is
larger, but simpler than X-band
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Accelerating structures
X-band Iris size 4.5 mm 106 Cu structures
SC Nb 9-cell cavity Iris size 3.5 cm 20,000
Cavities
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Issues for X-band accelerating structures
There have been problems with breakdown of Cu
structures at high power. Improved design, going
to lower group velocity has helped. Recent
structures have met the specified breakdown rates.
X-band specification on gradient and breakdown
rate (controls linac up time).
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Issues for SC accelerating structures
Learning how to prepare smooth pure Nb surfaces,
to get the design gradient was a decade long
effort. It has now been achieved. Recent
advance is using electropolishing instead of
chemical polishing. One still worries about
field emission from imperfections on the surface
that lead to current draw, and unacceptable load
on cryogenic systems.
SC specification on gradient and Q value. Now
exceeding spec.
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II. The problem of herding the electrons Must
transmit low emittance through the accelerator to
get small beams at collision for high luminosity
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Damping rings
Prepare the very small emittances needed for
small beams at collision in the damping rings.
Send few GeV e- or e beams through wiggler
magnets to radiate energy (lose energy in x, y
and z directions) and then restore pz with rf
acceleration. Must keep very careful control of
optics, vacuum, instabilities to avoid emittance
dilution.
Damping Ring for X-band has been built in KEK and
achieved necessary emittance.
34
Damping rings
Long train in SC ( 300 km) means that one must
fold the train on itself, and then kick out a
selected bunch without disturbing the neighboring
bunches.
Need special optics to transform flat round
beams. Need very fast kicker magnet with no
residual B field at succeeding bunch.
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Wakefields
Wakefields Off axis beam gives currents in
cavity walls these cause deflections of tail of
same bunch and on subsequent bunches. Within a
single bunch, betatron oscillation in head of
bunch creates a wakefield that resonantly drives
the oscillation of the tail. Can be cured by
reducing tail energy quads oversteer.
tail
head eqn
head
tail eqn
36
Wakefields
Wakefields also generate deflect subsequent
bunches. For X-band with small bunch spacing,
this is a problem.
Long range wakefield (on subsequent bunches)
cured for X-band by tuning the structures
differently. Destructive interference at
following bunches. Ultimately the wakefield
currents die out. For SC with 337 ns bunch
interval, this effect is small.
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Alignment
Beam alignment cavities to a few mm, quads to
100 nm, and final lens to few nm! First
survey the linac elements to 100 m level (harder
in SC since dont see cavities inside
cryomodules, and have to allow for thermal
contraction). Beam Position Monitors in all
magnets measure beam position to 10 m. Fast
feedback to correct orbit in real time. Steer the
beam to the gold orbit by reducing quadrupole
field and measuring deflections then move
cavities to gold orbit. Ground motion takes the
beam out of alignment must re-establish the
gold orbit daily non-invasive tuning.
Getting and keeping alignment is big deal
harder for X-band linac than SC owing to larger
wakefield effects. But the biggest problem is
final focus similar for both.
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F. Making the technology decision
Panel analyzed technology choice with matrix
having six general categories with specific items
for each technology

• Scope and parameters
• Technical issues
• Costs
• Schedule
• Operation for physics
• General considerations reflecting the impact on
  other sciences, technology and society

Evaluated each of these categories with the help
of answers to questions to the proponents.
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Technology decision

• Both technologies meet desired scope cost and
  schedule issues do not discriminate the two
  technologies at current level of precision.

• Technical issues
• SC cavities now being built with major industrial
  involvement. The same cavities are the basis for
  the DESY free electron laser project, but need
  more effort on dark currents. DR is complex.
• X-band damping rings have been demonstrated, but
  few klystrons and only a small section of linac
  shown to operate at full power with acceptable
  breakdown. Still worry about industrialization
  of klystrons, rf bunch compressor.
• Larger bunch Dt, smaller wakefields make SC
  linacs less risky
• Positron production scheme is awkard for SC.
• ? Main risk for X-band is in high cost linac.
  Main risks for SC is in lower cost subsystems.

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Technology decision

• Experimental conditions are slightly better in
  SC due to long time between bunches (event
  pileup) and slightly smaller energy spread. Not
  a major discriminant.
• Lower power costs for SC.

• Impact on other sciences, society

X-band linacs will benefit medical industry
(smaller systems), communications (airborne
radar) SC linacs now coming into use in a wide
range of scientific applications XFELs, ERLs,
Rare Isotope Accelerator, proton linacs for n
spallation sources, n production
41
Technology decision
Recommend superconducting technology This is a
choice of technology, not of the specific TESLA
design. Recommendation accepted by ICFA and
supported by major funding agencies as an
important demonstration that hard choices can be
made.
Retire the old names NLC, TESLA, GLC call the
new entity International Linear Collider ILC.
Recommend that the world HEP/Accelerator
community optimize a design of the
superconducting rf linear collider that
capitalizes on the best features of all RD to
date. Ask for capability to reach the highest
energy feasible (at least 1 TeV) through upgrades
to the baseline 500 GeV machine.
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G. Towards a fully international project
How do we organize the ILC so that all regions of
the world feel that they are full partners and
gain from participation?
Herding cats give value added for all
participants
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Next steps
1. Organize the Global Design
Initiative Central Team of about 20 people to
make the conceptual design, manage the project,
set cost estimate methodology, take technical
decisions, interact with governments. Regional
teams in Asia, Europe and North America to carry
out the subsystem RD, design and
industrialization. Keep accelerator science
strong in all three regions.

• Now choosing Central Team Director and site (by
  early 2005).
• Phase I Conceptual design and preliminary cost
  estimate by early 2006.
• Phase II Engineering design, ready for site
  and full agreement on international participation
  by 2008
• Final approval conditioned by early LHC results
  
• Start construction 2010 (if all goes well!!)

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Next steps

• Detector RD and experiment design
• Several different detector concepts (call for 2
  expts)
• large TPC gas-based tracking with conventional
  calorimeters
• Smaller silicon microstrip-based tracking with
  SiW compact calorimetry
• Choose experiments by time of project start

• Engage funding agencies in setting up project
• Meetings for past year with high level funding
  agency representatives from Canada, Europe/CERN,
  Germany, India, Japan, Korea, UK, US-DOE US-NSF
  to make plans for organizing, establishing
  procedure for site, etc. Funding agencies
  complimentary of decisions taken so far. They
  are basically supportive.

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Can we afford the ILC?
The ILC cost is not exactly understood. Lets
take the estimate for the 500 GeV TESLA project
which was 3.1B (4B) (not including salaries
of professionals). Increment to 6B to account
for energy increase to 1 TeV, operations
(250M/year), additional needs, remaining RD
etc. Divide by 3000 physicists (those signing the
consensus document) and by 20 years for building
initial operation project duration
Cost per physicist/year 100,000 Cost to be
shared across all three regions of the
world. ILC is a large total cost, but yearly
cost per physicist is not far out of line with
those for facilities in other areas.
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ILC in wider context

• Physics expands along lines with differing
  approaches
• Areas with increasingly clever experiments on
  very complex systems (brains, BEC, quantum
  computers, etc.), often with clear applications
  to society.
• Studies of the structure of matter and energy,
  space and time, at the simplest levels. Often
  with little practical benefit other than
  technology spinoffs, but with large impact on the
  way we view the world. And these often provide
  the magnet that attracts young people to physics.

Both approaches are critical to our field. Each
benefits from the health of the other. It is
important that we understand the potential for
advances in all areas, for we are truly
interconnected. Both types of physics need to be
sustained.
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ILC in wider context interconnectivity of
physics

• Nuclear, Astrophysics, Particle Physics
  connections
• NP/HEP ? Astro cross sections for ns, nuclear
  reactions as input to stellar evolution,
  supernovae nucleosynthesis etc.
• Astro ? HEP/NP number of neutrinos from
  primordial cosmology the wonderful puzzles of
  dark matter and dark energy. HEP may find what
  dark matter is.
• HEP ? Astro Inflation as a source of the
  cosmological isotropy and homegeneity
• HEP ? NP Quantum chromodynamics
• NP ? HEP chiral symmetry breaking, lattice
  gauge theory, instantons etc.
• NP ? HEP the field of particle physics itself !

48
ILC in wider context interconnectivity of
physics

• Interconnections between high energy and
  condensed matter physics
• CM ? HEP Cooper pairs for superconductivity,
  exploited in HEP and elsewhere as fermion
  condensates. (Maybe the Higgs boson is a fermion
  condensate?)
• HEP ? CM Field theories diagrammatic
  techniques, use of the renormalization group
• CM ? HEP Spontaneous symmetry breaking is seen
  in both disciplines insights in one field
  stimulate advances in the other.

• And HEP has developed accelerator technology,
  now pervasive for studies of nuclear and heavy
  ion physics, materials, biological systems,
  environment etc.

The totality of physics is greater than the sum
of its parts.
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Conclusions
The trek to understand physics at the highest
mass scales is scientifically worth making. The
ILC will allow us to make a significant next
stage of the trip. The new understanding will
affect all of physics.
No question that there will be major pitfalls in
trying to realize the ILC. But in the past few
years, we have made great progress.