What can an e e Linear Collider teach us

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P. Grannis/PHENO 2001 May 7, 2001
What can an e e- Linear Collider teach us ?

• Understanding the source of electroweak
  symmetry breaking is the most pressing issue in
  high energy physics for the coming decade.
• The LHC (or the Tevatron) seems assured of
  discovering new phenomena related to EWSB but
  will leave critical questions unanswered.
• An ee- linear collider at 500 GeV can
  discover new phenomena and make precision
  measurements that illuminate the nature of EWSB,
  and point the way toward higher energy phenomena.
  There is likely to be a need for evolution of
  the collider.
• The e e- linear colliders are well developed
  technically and it is likely that we will make
  decisions a linear collider within the next few
  years. It is imperative that we understand the
  physics case as clearly as possible.

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Linear ee Colliders
TESLA JLC-C
NLC/JLC-X Ldesign (1034)
3.4 5.8 0.43 2.2
3.4 ECM (GeV) 500 800
500 500 1000 Eff. Gradient
(MV/m) 23.4 35 34
70 RF freq. (GHz) 1.3
5.7 11.4 Dtbunch
(ns) 337 176 2.8
1.4 bunch/train
2820 4886 72
190 Beamsstrahlung () 3.2 4.4
4.6 8.8
US and Japanese X-band RD cooperation,
but machine parameters may differ
L 1x1034 cm-2s-1 for 107 sec. year gives 100
fb-1 per year
TESLA Design report March 2001 German
Science Council recommendation mid-2002 NLC
aim complete RD for Design Rept 2003 JLC set
milestones end 2000 Design Rept 2003 CLIC
multi-TeV, 30 GHz, 150 MV/m gradient with drive
beam power source in RD phase
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NLC - 2001
3
NLC baseline 2001 26 km site (2 in CA, 2 in
IL). Two 10 km linacs sized for 1 TeV final
focus, injector for 1.5 TeV. Two IRs Hi E IR
with no bend (crossing angle 20 mrad) can work at
multi-TeV Lo E IR requires bend maximum energy
500 GeV ( 1 TeV?)
Recent work Improved klystrons and SS modulators
give x3-4 efficiency gain. New compact final
focus region. Optimum cost for gradient 70 MV/m
but deterioration of accelerating structure
surfaces seen (at high group velocity). Active
RD this year to understand. If need to reduce
to 50 MV/m, cost penalty is 5-10.
Cost reduction to date from 5.1B in Lehman
review (FY00, no escalation, contingency,
detectors) to 3.7B (30 reduction). Another 10
15 from possible scope reductions.
Injectors 19 Linacs 39
beam delivery 11 global
costs 17 management/business 14
4
TESLA
4
TESLA site length 33 km (15 km linacs).
Operates with superconducting RF cavities
design for 500 GeV is 22 MV/m. Bunches are
separated by 337 ns, allowing for head-on
collisions without satellite crossings.
spec
Cost 3.16B (using 0.93/Euro). Includes 1
IR, 1 detector (233M). xFEL added cost is
495M. Cost in 2000 prices no contingency
(HERA was on budget) no escalation no second
detector/IR exclusive of manpower at
collaborating institutes (6933 man-yrs estimated
700M)
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There are two fundamental questions before
experimental high energy physics at present

• What is the origin of the symmetry breaking
  observed in the electroweak interaction?
• What gives the W/Z (and fermions) their mass?
• Is there unification of forces, and if so, at
  what scale? Can gravity be incorporated?
• Are there new phenomena or new particles
  associated with the physics responsible for EWSB?

• What is the origin of flavors?
• Why three generations and the peculiar fermion
  mass patterns?
• Why is there CP violation, and why is it
  insufficient to give the matter/antimatter
  asymmetry in the universe?
• Does flavor physics (neutrino mass) imply
  something about physics at the GUT scale?

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Main themes for the Linear Collider physics
program
Experiments in the past decade (LEP, SLC,
Tevatron, n scattering) have made precision
measurements that clearly indicate the need for
something like the Higgs boson. LEP has
indication of 115 GeV state (2.9s ).
Study the Higgs boson (or its surrogate) and
measure its characteristics.
The SM Higgs mechanism is unstable vacuum
polarization contributions from the known
particles should drive its mass to the force
unification scale. We expect some new physics
entering at the TeV scale. BNL (g-2 )
experiment comparison with theory suggests new
physics (2.6s ).
Find and explore this new physics sector.
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Where is the Higgs ??
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(The Higgs is what the measurements tell us it
is! )
All Expts Bknd Signal exp. Evnts
4 jet 0.93 1.60
3 missing ET 0.30 0.46
1 leptons 0.35 0.68
0 taus 0.14 0.29
0 ALL chan. 1.72 3.03
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(SM) Mhiggs limit Mhiggs 113.5 GeV. Tevatron can
discover up to 180 GeV
LEP Higgs search Maximum Likelihood for Higgs
signal at mH 115.0 GeV with overall
significance (4 expts) 2.9s
Higgs self-coupling diverges
If LEP indication correct, there must be physics
beyond the SM before the GUT scale
Higgs potential has 2nd min.
LHC (Tevatron) will discover 1 Higgs LHC
will get mass accurately total width,
couplings poorly likely not the quantum
numbers will not do self couplings and may
well not see heavy Susy Higgs.
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Higgs studies -- Mass
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The key discovery question for LC is What is the
nature of the Higgs ? -- revealed by its
quantum nos, couplings, total width. The LHC is
unlikely to do these. LC produces Higgs in
association with Z allowing study of its decays
without bias -- even invisible decays of Higgs
are possible using the recoil Z (in ee, mm, qq
modes).
For 120 GeV Higgs, ZH production mode 30K
evts/yr at 350 GeV 15K evts/yr at 500
GeV dM/M 1.2x10-3
Width
GTOT Measuring the lightest Higgs coupling
tests whether there are additional higher mass
Higgs.
In MSSM or 2 doublet models
S g2(h ZZ)i (MZ gEW / cos
qW)2 GTOT to few at LC for mass
using measured GWW from WW fusion or sZH ,
BR(H WW). LHC can do GTOT to 10-20 in this
range.
Higgs spin parity
q cm production angle f
fermion decay angle in Z frame

• JP 0
  JP 0-
• ds/dcosq sin2q
  (1 - sin2q )
• ds/dcosf sin2f
  (1 /- cosf )2
• and angular dependence near threshold permits
  unambiguous determination of spin-parity
• Can produce CP even and odd states separately
  using polarized gg collisions. gg H or A
  (can reach higher masses than ee-)

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Higgs Couplings
We need to determine experimentally that Higgs
couplings are indeed proportional to mass.
Use vertex meas., jet mass, topology in
likelihood to get BRs for 500 fb-1 , 300 GeV LC
H bb 2.4 H cc
8.3 H gg 5.5 H t t
6.0 H WW 5.4
approx. errors
(Mh 120 GeV)
Measurement of BRs is powerful indicator of new
physics (e.g. in MSSM, these differ from the
SM in a characteristic way. Higgs BR
must agree with MSSM parameters from many other
measurements.)
Higgs self couplings
SM value (decoupling limit)
Study ZHH production and decay to 6 jets (4 bs).
Cross section is small premium on very good
jet energy resolution. Can enhance XS with
positron polarization. Dl/l 20 with
1000 fb-1.
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Physics beyond the Standard Model
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The defects of the SM are widely known
No gauge interaction unification occurs
Higgs mass is unstable to loop corrections Many
possible new theories proposed to cure these ills
and embed the SM in a larger framework
Supersymmetry -- fermion/boson partners,
extending the Poincare group to include fermionic
dimensions. Susy models come in many variants
with different mechanisms and scales of Susy
breaking (supergravity, gauge mediation, anomaly
mediation ) Each has a different spectrum of
particles, underlying parameters.
A new gauge interaction like QCD with mesons at
larger masses. (Techicolor/topcolor) These
interactions avoid introducing a fundamental
scalar. technipions play the role of Higgs
new particles to be observed, and modifications
to WW scattering.
String-inspired models with some extra dimensions
compactified at millimeter to femtometer scales.
These yield anomalous mono-photon or mono-jet
production, heavy Z/W states, modification to
ee/gg production.
LC must be able to sort out which is at work, and
make precision measurements. Examples exist
where LHC sees new phenomena, but mis-understands
the source
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Supersymmetry
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Fermion/boson symmetry stabilizes the Higgs mass
-- scale of new Susy particles is O (1 TeV).
Lightest higgs state is to measure the underlying model parameters and
deduce the character of the supersymmetry, energy
scale for supersymmetry breaking. There are
105 unknown parameters, all of which need to be
measured, and used to fix models. This can be
done through measurement of the masses, quantum
numbers, branching ratios, asymmetries -- and in
particular the pattern of mixing of states with
similar quantum numbers -- the 2 stops, sbottoms,
staus, and the 2 chargino and 4 neutralino
states (partners of the g/Z/W and supersymmetric
Higgs states). Susy may well be the next
frontier for flavor physics FCNC, CP violation
in the sparticles, generation patterns, etc.
The LHC should discover Susy if it exists.
But disentangling the information on the full
mass spectrum and particle quantum nos/couplings
and the mixings will be very difficult at LHC.
The LC can make these crucial measurements,
(e.g. sparticle masses to 0.1 few level)
benefitting from -- Polarization of
electron (positron) beam Known partonic
cm energy Known initial state (JP 1- )
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Supersymmetry studies at the Linear
Collider
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An example production of selectron pairs --
have two diagrams typically the t-channel
dominates and allows measurement of neutralino
couplings to lepton/slepton.

e
e

e e c10
e

e
c0
g,Z

e-
e-

e-
e-
Upper lower edges of decay electron energy
distribution from
gives masses of left and right handed
selectrons.

eL,R e c10
Angular distribution of decay electrons, using
both polarization states of beam e-, tell us
about quantum numbers, coupling of exchanged
neutralino and give information on neutralino
mixing, hence the underlying Susy mass parameters.
Similar studies for neutralino, chargino, stau
etc. production lead to independent measures of
similar parameters and enable constrained fit to
Susy model.
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Linear Collider Supersymmetry
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The Linear Collider can determine the Susy model,
and make progress to understand the higher energy
supersymmetry breaking scale. To do this, one
would like to see the full spectrum of sleptons,
gaugino/higgsino states.
Thresholds for selected sparticle pair
productions -- at LHC mSUGRA model points.
Point 1 2 3 4 5
6 GeV GeV GeV GeV GeV
GeV
reaction
c10 c10 336 336 90
160 244 92 c10 c20 494
489 142 228 355 233 c1 c1-
650 642 192 294 464
304 c1 c2- 1089 858
368 462 750 459 e e/ m m
920 922 422 1620 396 470 t
t 860 850 412
1594 314 264 Z h 186 207
160 203 184 203 Z H/A 1137
828 466 950 727 248 H H -
2092 1482 756 1724
1276 364 q q 1882 1896
630 1828 1352 1010
RED Accessible at 500 GeV GREEN added at 1 TeV








Operation in eg mode can increase mass reach
e.g. e- g e
c10 (g-2) result suggests relatively light
sfermions or charginos


It is likely that, in the case that supersymmetry
exists, one will want upgrades of energy to at
least 1 TeV.
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Susy breaking mechanism
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The LC complements the LHC ( LC will do sleptons,
sneutrinos, gauginos well). LHC will see those
particles coupling to color, some Higgs, lighter
gauginos if present in cascade decays of squarks
and gluinos. Electron polarization (positron?)
is essential for disentangling states and
processes at LC.
We really want understand the origin of Susy --
determine the 105 soft parameters from experiment
without assuming the model. (mSUGRA, GMSB,
anomaly, gaugino ) mediation. We want to
understand Susy breaking, gain insight into the
unification scale and illuminate string
theory. Mass spectra give some indications of
the model class.
LC mass, cross sect. as input to RGE evolution of
couplings reveal the model class without
assumptions. This study for 1000 fb-1 LC
operation and LHC meas. of gluinos and squarks
show dramatically different patterns of mSUGRA
and GMSB.
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Precision studies constrain ANY any new physics
generating the Higgs mechanism
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Standard Model (SU(2) x U(1) ST 0) agrees
with data
S T measure effect on W/Z vacuum polarization
amplitudes. S for wk isoscalar and T for
isotriplet All EW observables are linear
functions of S T and are presently measured to
0.01.
sin2 qW
Giga-Z samples at LC (20 fb-1) would improve
sin2qW by x10, WW threshold run improves dMW to
6 MeV, etc. Factor 8 improvement on S,T
Present 68 S,T limits
The chevron shows the change in S T as the
Higgs mass increases from 100 to 1000 GeV, given
the current top mass constraint. If the Higgs
is heavy ( 200 GeV), need some compensating
effects from new physics. Need a positive DT or
negative DS. Several classes of models to do
this all have observable consequences at LC.
68 S,T limits at LC
The precision measurement of ST at a linear
collider could crucial to understand the nature
of the new physics.
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Strong Coupling Gauge Models
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For many, fundamental scalars are unnatural.
We have a theory (QCD) in which pseudoscalars
(pions) arise as bound states of fundamental
fermions (quarks). Analogs of SU(3) color are
postulated with technicolor degrees of freedom,
but fermions at higher mass scale. The
technipions generate the Higgs mechanism.
Though inspired by QCD, the new model must differ
quantitatively (slow evolution of coupling) In
topcolor, the 3rd generation SM quarks (top in
particular) are singled out as being strongly
coupled to the new sector. Fermion pair (tt)
condensates play the role of higgs.
Observables in Strong coupling models New
technicolor particles should occur on the TeV
scale. Since the longitudinal components of W/Z
are primordial higgs particles, WW (ZZ)
scattering is modified. Also expect
modifications to WWg coupling and top VA form
factors, seen at LC (tough at LHC). Envisioned
ST constraints suggest that composite Higgs
state(s) should have mass
Allowed regions in Higgs mass and DT for W
mass error of 30 MeV and top mass error of
2 GeV Chivukula, Holbling, hep-ph/0002022
Seeing strong coupling effects may require LC
energy above 500 GeV. Better ST precision
will be crucial.
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Large Extra Dimensions
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String theories represent the only known avenue
for incorporating gravity and the microscopic
forces. Until recently, hope for any observable
effects from the compactification of extra
dimensions was dim. Recent suggestions that the
extra dimensions might be compactified on scales
larger than Planck length lead to observable
consequences for experiment and could explain
the heirarchy problem. If the effective Planck
mass TeV, gravity is modified at mm scale,
leading to anomalous g production (with missing
ET from gravitons), or modified cross sections
for fermion pairs. For compactification scale O
(TeV), Kaluza-Klein excitations of graviton or
gauge bosons should exist at TeV scale, and
observable as excited states at the LHC or LC.
If Susy in extra dimensions, gravitino towers
modify XSs. For compactification at the GUT
scale, new states are unobservable, but the
characteristic Susy pattern of these models
should remain, and the unification pattern of the
couplings should provide information. Models with
SM gauge field propagation in TeV sized extra
dimensions, can get scalars (Higgs) with SM
properties and EWSB. MH 165 230 GeV (and
other scalar composites). (Arkani-Hamed, Cheng,
Dobrescu, Hall hep-ph/0006238)
Large extra dimension models still being
developed. If this is our world, it is likely
that higher Linear Collider energies will be
desired.
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Scenarios for New Physics
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Although our experiments point to the Standard
Model, the Linear Collider should be capable of
illuminating the nature of physics beyond the SM.
We believe that some manifestation of the
equivalent to the Higgs mechanism should be seen
at 500 GeV or less.
Some Scenarios for Physics after few yrs LHC
1. Higgs-like stateSusy LHC/Tevatron discover Linear Collider
program is assured, exploring the Higgs and Susy
spectrum and determining their detailed
structures.
2. Higgs candidate seen but nothing else LC
studies all aspects of the Higgs (accessible
couplings, width etc.) to compare with SM.
Revisiting the Z-pole to refine the precision
measurements will be essential. Seek Z at
LHC/LC, anomalous VVV couplings, strong WW
scattering, etc.
3. No Higgs, No Susy seen Verify that no Higgs
to invisible modes. Measure anomalous W/Z
couplings and top anomalous form factors.
Increase the energy to seek new strong-coupling
or extra dimension physics. Return to the Z-pole
for precision S and T.
4. Multiple kinds of new phenomena seen at
LHC/TeV A wealth of new physics that needs
untangling -- Linear Collider has a field day!
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Options for beams / energies
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• There are several special operating conditions
  for the Linear Collider that may add important
  physics capabilities, but also create extra
  complexity or costs. How should we view these
  options?
• Positron polarization
• Polarized e probably required for improved
  precision EW measurements (ST) with Giga-Z
  provide increased XS for H iggs, useful for
  self-couplings allow improved measurements of
  Susy couplings/mixings. Obtain polarized e
  from intense polarized g beams (TESLA requires
  these anyway).
• gg , e-g, e-e- collisions
• Larger cross sections in gg offsets lower
  luminosity can separate Susy H/A, complementary
  triple gauge coupling info, lower threshold for
  selectrons in eg e-e- allows clean environment,
  high polarization, only one subprocess, good
  probes for new physics (KK towers, some Susy
  states )
• Low energy collisions (MZ , WW threshold, ZH
  cross section maximum)
• For any new physics whose origin is not
  immediately understood, return to the Z, WW
  threshold will greatly aid understanding.
  Operation at the maximum of the ZH cross section
  gives largest rates. Ideas exist to permit
  simultaneous operation at low (energy for NLC.
• X-ray Free Electron Laser
• Structural biology, plasma physics,
  materials science, chemical kinetics, surface
  science all benefit from short pulse angstrom
  level sources. There can be synergy between HEP
  and these communities through use of LC as XFEL.

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How does the world community proceed?
(a personal point of view)

• 1. Linear Collider Timelines
• Tesla design report in spring 2001 decision 2002
  
• Japan JLC proposal in few years
• US NLC RD over next 3 years leading to proposal
• All 3 regions conducting studies of physics
  priorities
• for next 20 years during coming year.
• LC decision likely in next few years!
• Other new projects (HEPAP whitepaper timelines)
• m Storage Ring might be ready for decision
  2010 nature of physics questions fairly clear
  (n matrix/CP) Next generation expts will probably
  teach us much.
• m Collider or multi-TeV ee collider 2010
• VLHC after 2010
• Physics case for the multi-TeV colliders is not
  yet clear to me Higher energy than LHC/LC may
  not be highest priority if there is rich TeV
  scale physics.
• Very large underground laboratories (proton
  decay, solar neutrinos, neutrino oscillations,
  supernova watch)
• CERN is evaluating its future beyond LHC

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How do we proceed?

• 2. Should the LC be the next world HE machine?
• Inevitable that the LC decision is the next that
  will be taken by the worldwide community. Real
  proposals exist potential alternatives much
  further off. Not all regions may propose a LC in
  their region, but we will make decisions soon.
• Worldwide support for the LC (somewhere) will be
  essential if it is to succeed. Arguing against
  the LC will not enhance the prospect for other
  projects. LC should not be the last frontier
  accelerator.
• Particularly in the US community, we must engage
  the LC question, and consequence of opting for
  other paths. Snowmass 2001 and HEPAP subpanel
  affords the chance to confront these issues as a
  community.

• 3. Is the Linear Collider too expensive?
• One hears, particularly in the US, that the
  likely cost of the LC is too large to sell to the
  government. But ANY future collider discussed is
  at least comparable cost. An endemic problem!
  We need to be optimistic enough to expect to
  succeed in arguing for a next project if it has a
  clear scientific justification!
• Cost of the LC seen by some as the primary driver
  toward the initial stage at 500 GeV. Will such
  a stage address the crucial next questions?
  Physics arguments above show clear role for 500
  GeV program. But expect that upgrades will be
  needed.
• Cost is a factor, and we must press all ways to
  control it. But we must not lose sight of the
  probable need for future evolution in the design.

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How do we proceed?
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• Steps toward Internationalization
• Need a collective decision on the right sequence
  of next steps (for the US, Snowmass 2001 and
  HEPAP subpanel are critical activities).
  Proceed to a world view on the preferred next
  step.
• Site is likely chosen by funding which region
  will put up 2/3(??) of the cost?
• For LC (or other projects), allocate spheres of
  responsibility empower all regions to take
  primary responsibility for major systems from
  leadership of design, RD, construction,
  commissioning, operations. e.g. for LC, might
  assign injector/damping rings rf/linacs
  final focus/beam delivery/monitors to separate
  regions.
• Need international advocacy for the project
  ICFA, Physical Societies, Global Science Forum
  involvement. International cooperation in
  presenting proposals to national governments
  stressing the joint ownership of project, shared
  access and continuity of world wide effort.
• Connect governmental science policy officials
  through forums on how to forge international
  science project structures.
• International review comparative cost,
  performance upgradability, technical risks
  assessment (being done for LC). International
  oversight of accelerator, detectors, scientific
  program.

• The LC (or any other frontier project) should be
  fundamentally international. Each region needs
  strong involvement at the frontier to retain
  health of HEP and accelerator physics in that
  region. With LHC, Europe takes the energy
  frontier Asia and North America will need
  frontier facilities to remain healthy.

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Conclusion
The physics case for the LC with a 1st stage at
500 GeV is very strong. We need a linear
collider to understand EWSB in any scenario. We
know enough to make the choice now. Community
engagement through Snowmass, HEPAP is essential
if we are to reach a consensus. With present
lack of understanding of how EWSB is manifested,
flexibility of Linear Collider design (energy, L
, beam particles) is essential. The LC will be
an evolving facility. The cost will be high.
Unless we internationalize so as to satisfy the
needs of all regions and allow productive
collaboration, we jeopardize the prospect of the
LC, or any other new frontier facility anywhere.