Download ibm_seminar - Stony Brook University

Paul Grannis
Stony Brook University
IBM Seminar Oct. 12, 2007
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Particle Physics
at the Crossroads
 Developing the paradigm ‘Standard Model’
 Where we are now
 Future directions at the Terascale (1 TeV = 1012 eV)
time
The scene in the ’60’s
4 microscopic fundamental forces between matter
particles, transmitted by boson. EM interaction
(QED) is the prototype. Spin-1 photon couples to
the electric charge of the matter particles.
EM – described
by QED. Long
range via
massless photon
exchange.
Strong force –
attractive, short
range; binds nuclei
and hadrons.
Exchanges of p
mesons etc.etc.
f2
f1
boson exchange
coupling
constant
f3
f4
Q2 = momentum transfer2
carried by exchange quantum
Weak force –
radioactivity; solar
reactions; particle
decays. Very
short range. Heavy
W bosons?
Gravity – very
weak, long range;
no quantum
theory. Massless
gravitons?
No connection or unification among these forces.
Particle classification
Hadrons = particles that feel the Strong int’n: Half integer spin
baryons (p, n, L etc.) and integer spin mesons like p+, r0, K- … )
Leptons = particles that feel the Weak interaction but not
Strong. Both charged (e-, m-) and neutral (neutrinos, ne , nm )
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The scene in the ’60’s
Starting from a few ‘elementary’ hadrons –
proton, neutron, p meson etc., hundreds
more burst on the scene in the ’60’s.
The bewildering array of quantum numbers
(charge, parity, spin etc.) and complex
interactions defied easy explanation.
(p,n, p, K, h, D, N*, w, r, f, A1, A2, B, L, S, X, W … we ran
out of Roman and Greek letters and still they came!)
Often particles come in multiplets with
similar mass & properties except for charge
 ‘isotopic spin’ internal symmetry (I) with
same SU(2) group properties as spin. Family
with isospin I has (2I+1) members. e.g. I=1/2
nucleon: p has IZ = +½ , n has IZ = -½. Similarly, I= 3/2 D: (D++, D+ , D0 , D-)
and I=1 pions (p+, p0, p-)
Some particles were ‘strange’ – produced strongly in pairs but slow to
decay (via weak int’n). Must be a new quantum number: strangeness, S,
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conserved in strong but not weak interactions.
Building the Standard Model – quarks
In early 60’s, Gell Mann and Zweig noted that observed
hadrons could be built from smaller entities – quarks – to
build the known hadrons. Three ‘flavors’ of quarks fill the
fundamental representation of SU(3). Three antiquarks in
conjugate representation.
S
n
S-
S0
X-
p
L
IZ
S+
X0
S
IZ=-1/2, S=0
d
IZ=+1/2, S=0
u
IZ
IZ=0, S=-1
s
Mesons made of quark and antiquark: eg p- = (du) .
Baryons from 3 quarks: eg n = (ddu)
Quark model explained known (and missing) hadrons,
but quarks not observed, so seemed like bookkeeping
artiface. Some states [e.g. W- (sss)] are fermions but
with totally symmetric wavefns = statistics problem.
baryon octet
In 70’s SLAC expts on e- p scattering at
large Q2, found evidence for point-like
objects within the proton, with charges e
1/3e and 2/3e , just like the constituent
p
quarks. These expts were a direct
analogy with Rutherford a-Au scattering.
S
S
S
S
g
e
q
q
q
So the quark
hypothesis seemed
to have physical
validity after all.
But why were there
no free quarks
observed?
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Building the SM – QCD
1973: Gross, Politzer, Wilczek
Puzzles resolved by development of QCD, a local gauge theory like QED
but based upon a new SU(3) ‘color’ symmetry. Analog of g is a set of 8
massless ‘gluons’ which couple to ‘color charge’ carried by quarks.
Unlike QED, the gluons have color themselves, so couple also to other
gluons. Each quark flavor (u,d,s) comes in 3 distinct colors (R, G, B).
Observed mesons and baryons are color-less combinations with
antisymmetric color wavefns (solves statistics problem).
Compilation of many
Key QCD property: coupling ‘constant’ aS  ∞ at small
aS experiments
Q2 or long distance (infrared slavery) and
logarithmically  0 at large Q2 or small distance
(asymptotic freedom). So, quarks do not emerge
freely, but fragment to ‘jet’ of colorless hadrons.
√Q2
As Q2 increases (a more powerful
microscope), see more resolved
substructure within the proton.
This causes a variation of the
coupling as a function of Q2 and
deviation from simple ‘Rutherford’
e- p scattering – observed in expt.
(Q=momentum of g)
e
S
S g
S
S
p
e
q
g
q
q
q
q
Quark and gluon dist’s in the
proton have been mapped to
Q2 ~106 GeV2 (down to ~1am).
q
g
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Building the SM – weak bosons
JH
n
The 4 fermion ‘charged current’ interaction of
Fermi with (V-A) currents qualitatively explained
observed weak interactions.
p
e
n
JL
neutron b decay
n
n
p
But the 4 fermion interaction violates unitarity for
energies above about 600 GeV. In analogy with
W+
QED, postulate spin 1 boson carrier, W+. The W+
e
n
must be heavy to give the short range observed for
the Weak Int’n, thus theory non-renormalizable.
s-wave unitarity violations,
Weinberg and Salam (1973) predicted weak
neutral currents. These imply massive
p
0
neutral Z boson. Such events seen.
n
though delayed, still occur (e.g.
in nn → W+W- at high energy).
p
Z0
n
The W± and Z0 were discovered
in 1983 (Rubbia, van der Meer)
with about 100x proton mass
(81 and 91 GeV/c2 respectively)
n
n
CC event
NC event
m
n
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Building the SM – Electroweak Int’n
Glashow, Salam, t’Hooft,
Weinberg, Veltmann,
b
(isospin 0)
Propose unification of Weak and EM interactions.
(w-, w0, w+) (isospin 1)
Postulate four fundamental massless spin 1 gauge
bosons within a U(1)xSU(2) (both EM and Weak) group symmetry.
0
Unifies the EM and Weak int’n into Electroweak Force and fixes unitarity
and renormalizability – but alas does not give massive W and Z bosons!
So, introduce (ad hoc) a complex doublet of spin 0 Higgs fields
– one pair (f, f*) is neutral and one (f+, f- ) is charged.
The symmetric theory is spontaneously broken – the b0 and w0 states mix.
In the process, the w± and w0 acquire mass by absorbing 3 of Higgs fields
to become W± and Z, whereas one combination (g) remains massless. One
Higgs field is left, giving a physical Higgs boson, not yet observed but
constrained: 115< mH <150 GeV. Quarks and leptons
g = cosqW b0 + sinqW w0
Z = -sinqW b0 + cosqW w0
also acquire their mass from Higgs coupling.
B
A familiar example: an external B field breaks
the spatial symmetry in a ferromagnet.
The Higgs mechanism is a Rube Goldberg device? You
bet – but many experiments agree with predictions!
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Completing the quark & lepton lineup
By 1973, 3 quarks (u,d,s) and 4 leptons (e,ne,m,nm )
had been seen. But theory diverges if Nq ≠ Nl
e+e-→
hadrons
J/y
1974: BNL & SLAC experiments see narrow resonance
at 3.1 GeV, bound state of charm quark-antiquark pair.
4 q, 4 l
1976: FNAL expt sees dimuon resonance at 9.5 GeV
interpreted as bottom (b) quark - antibottom state.
pp→ m+m-
1976: SLAC finds t lepton (1.8 GeV), partner to e,m and
infers the related nt (not seen directly until 2004)
U
5 q, 4 l
5 q, 6 l
1995: FNAL experiments discover top quark at mass
175 GeV (~Au nucleus - but no substructure!).
m/e (obs/exp)
no oscillation
downgoing
upgoing
pp→
6 q, 6 l
1998 : Japanese exp’t shows
that neutrinos have non-zero
mass (nm oscillates into ne or nt ).
tt
t
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The Standard Model
Fermion (‘charge’ carrying) matter particles
3 quark ‘flavor’ isospin doublets (generations).
The weak int’n quark states ≠ strong states:
Mixing matrix VQ connects.
3 lepton flavor isospin doublets (generations).
Mass eigenstates are rotated from flavor
states: transformation matrix VW.
+ the other
2 color sets
u
c
t
d
s
b
ne nm n t
e
m
t
(3 generations is minimum needed for CP violation as seen in nature)
Interactions/force carriers:
Strong (QCD)
Electroweak
SU(3) x SU(2) x U(1)
26 arbitrary parameters:
12 fermion masses
8 mixing matrix parameters
3 force couplings
2 EW boson masses
+ 1 strong CP phase
g
g
+ the other color
gluon states
W±
Z
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The Standard Model has withstood 1000’s of tests
etc. etc.
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So whats wrong with the Standard Model?
1. Why those 26 ad hoc parameters? (Arbitrary, but they matter.)
If m(u) > m(d), proton (uud) is heavier than the neutron (ddu) and
thus proton decays : stars, cosmic microwave background,
H atoms, CM physics and people don’t exist.
 Why do fermion masses vary by 10 orders of magnitude?


Lepton and Quark mixing matrices are very different.
2. SM permits CP violation, but not enough to explain why there is the
huge asymmetry between number of baryons and antibaryons in the
universe. A new source of CP violation is needed.
3. The Strong and EW interactions are just pasted together in SM. If
extrapolate the three coupling constants to high energy, they come
close to a common value at the grand unification scale ~ 1017 GeV
close but no cigar
g3
g2
g1
No unification
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So whats wrong?
4. Higher order quantum corrections (loop diagrams) would cause the
Higgs, W, Z boson masses to diverge to Planck or grand unification
scale unless there is some fantastic accidental tuning of couplings
to keep these at TeV scale. (fine tuning / hierarchy problem)
W, Z, H
masses are here
But would
tend to here
5. Galaxies show substantial dark matter, also evident in early galaxy
formation. DM seems to be massive particles, left from the early
universe. SM provides no candidate.
6. Dark energy, pushing the universe apart in the present epoch, has no
explanation in the SM.
7. The SM would give WL (energy density due to cosmological constant)
be O(10120). One might understand some new symmetry causing it to
be zero, but WL ~ 1. The biggest fine tuning problem of them all !
8. Gravity is not included in SM
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So, despite the SM successes we strongly
believe it must be superceded
First we need to find what plays the role of the Higgs boson to break
EW symmetry. Moreover, to solve the SM defects (fine tuning of
Higgs mass, provide dark matter particle, unify the forces … ) there
needs to be new physics at few 100 to 1000 GeV – the Terascale.
The new theory must reproduce the successes of the SM while adding
new ingredients – much as Quantum Mechanics gives Classical
Mechanics in the correspondence limit.
There are several classes of theoretical models suggested for the
new paradigm:
 New symmetries of nature
 New forces and particles
 New kinds of space dimensions
Each model class has many variants
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An experimentalist’s dream !
We know there is a new playing field at the Terascale, but
have no idea who the players are, or what the rules of the
game might be.
Go there and find out!
And there are two demonstrated new accelerator colliding beam
facilities that will give a complementary view of the new terrain:
 The Large Hadron Collider (LHC), will start in 2008 at CERN:
proton-proton collisions at ECM = 14 TeV
 The International Linear Collider (ILC) being designed by global
international collaboration: e+e- collisions at ECM = 0.5 – 1 TeV.
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Colliders for the energy frontier
 High energy reach
LHC
 Broad range of quark/gluon
energies simultaneously (ECM not fixed)
 Large event rate
 Large QCD backgrounds
 Don’t know initial state quantum #s
 Event pileup – spectator quarks &
proton
proton
other pp collisions
 Radiation damage issues
 Known initial quantum state
 Well-defined ECM and pol’zn
 low bkgd → allows ambitious
ILC
experimental techniques
 Event rates low; need
+
e
e
sequential runs at different
ECM and polarization
 Complex machine detector
LHC & ILC collider characteristics
interface; need exquisite
are highly complementary
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control of beam optics
The LHC
Mt. Blanc
The 14 TeV (ECM), 27 km circumference
Large Hadron Collider (proton-proton) at
CERN on the Swiss-French border –
complete in 2008. The LHC will be the
highest energy accelerator for many years.
Lake Geneva
airport
But …
The protons are bags of many
quarks and gluons (partons) which
share the proton beam momentum.
Parton collisions have a wide range
of energies – up to ~2000 GeV.
Initial quantum state is not fixed.
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The International Linear Collider (in planning)
Linear to beat synchrotron radiation (~E4/R). Just one pass of beams
(but electrons are cheap).
Collide beams with energy tuneable up to Ecm = 500 GeV (upgrade to Ecm
= 1000 GeV). Two identical linear 11 (20) km long accelerators, bringing
beams to head-on collision in 6 nm high spot.
Damping rings (wigglers) cool transverse phase space; bunch compressor
squeezes longitudinal bunches.
Superconducting rf acceleration (35 MV/m) at f=1.3 GHz in main linacs.
pre-accelerator
few GeV
source
Layout of electron arm
KeV
damping
ring
few GeV
few GeV
bunch
compressor
250-500 GeV
main linac
extraction
& dump
final focus
IP
collimation
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The physics program for the LHC and ILC
The LHC should discover the Higgs if
it exists up to >1 TeV (10 times
higher than SM-indicated value).
Fermilab still has a shot!
e+ g,Z
Z
e-
interaction rate
Curves denote different Higgs
boson spins; ILC data cleanly
discriminate.
collision energy
significance
1. Find the agent for Electroweak symmetry breaking –
in the SM, the Higgs boson.
H
Higgs mass →
The ILC will tell us if what LHC sees is the
SM Higgs or some surrogate. It can detect
the ‘Higgs’ even if it decays into invisible
particles. It can tell us the Higgs quantum
numbers, and its couplings to different
particles.
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LHC/ILC Physics
2. Mapping the Higgs boson
Yukawa coupling
In the SM, Higgs couplings to fermions,
W/Z are directly proportional to mass;
they differ in other models. Measuring
these couplings to few % level is a
sensitive test of whether we have the SM
or some new physics. LHC can only get to
few 10’s%, for only some couplings. ILC
can do to few %.
ILC
precision
Particle mass →
The Higgs couplings to other particles distinguishes different models.
Extra dimension model
Points for different H decay
modes, relative to SM.
New symmetries model
SM
value
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LHC/ILC Physics
3. New Symmetries
Supersymmetry (SUSY) introduces new fermionic space-time
coordinates, resulting in a new boson for every existing SM fermion
and vice versa. (Partner of the spin ½ electron is a spin 0 selectron,
etc.). In unbroken SUSY, selectron mass = electron mass etc. We
know this is not true, so SUSY is a broken symmetry. All the other
properties of the selectron are like the electron (charge, couplings).
There are many model variants, and many parameters, so it will be
difficult to unscramble.
 SUSY boson and fermion contributions to Higgs
mass cancel those from SM particles, so the
hierarchy / fine tuning problem is solved.
 SUSY has a natural dark matter candidate.
 SUSY could provide the CP violation needed.
 SUSY modifications to SM predictions are small,
so not in conflict with data.
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LHC/ILC Physics
4. Learning about Supersymmetry
The LHC and ILC have complementary strengths in mapping the SUSY
spectrum – LHC sees quark and gluon partners; ILC sees lepton and
W/Z/Higgs partners. Together they can extrapolate to the scale
where SUSY is broken and tell us how that happens.
Mass and coupling unification
pattern deduced from ILC &
LHC reveals how SUSY broken.
(Plots show two model
possibilities).
LHC
ILC
DM cosmic density 
energy →
DM mass 
SUSY provides a good candidate for DM
(lightest SUSY particle). LHC and
particularly ILC can determine its mass
and density.
Compare with cosmic microwave bknd,
underground DM experiments to see
if the picture is consistent.
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5. New forces
LHC/ILC Physics
New forces and the particles they introduce provide a new energy
scale. This would stabilize the hierarchy problem of the SM.
The prototype candidate was a new interaction similar to QCD
(“Technicolor”) with new particles at O(few TeV). The simplest of
these models would however produce deviations from the SM that are
not seen, but many more complex variants exist.
These models give new quarks, bosons, ‘leptoquarks’, etc. that would be
seen at LHC and ILC.
An example: a new higher mass Z
boson seen at LHC
production rate
dimuon mass
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6. Hidden dimensions
LHC/ILC Physics
String theory requires at least 6 extra spatial
dimensions (beyond the 3 we already know). The
extra dimensions are curled up like spirals on a
mailing tube. If their radius is ‘large’ (~1 attometer
= billionth of an atomic diameter) or larger, they
could lower the effective Planck mass, eliminate the
hierarchy problem and unify all forces (including
gravity?) at the new Planck scale.
If a particle created in an energetic
collision goes off into the extra
dimensions, it becomes invisible in our
world and the event shows missing
energy and total momentum imbalance.
e.g.
e+ e-  g + ‘nothing’ at ILC
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LHC/ILC Physics
7. Untangling New Dimensions
Combination of data from LHC and ILC allow the determination of the
reduced Planck scale and the number of extra dimensions.
production rate
dimuon mass
Wavefunctions trapped inside a ‘box’ of extra
dimensions yields a series of resonance states
(like new heavy Z bosons). At LHC, these are
indistinguishable from other possible sources.
This?
ILC measurements of the couplings
(vector and axial vector) allow us to
distinguish what new physics is
operating. Different models have
quite different couplings.
or this?
or … ??
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Example of ILC and LHC complementarity
4 ways to produce a ‘signal’ with same final
objects: jet (quark), 2 leptons & missing energy
Observed final particles
a) & b) SUSY with different choices of
dark matter particle (lightest SUSY
particle) = partner of photon or of
neutrino.
c) & d) Extra Dimensions models with
different character of excited Z.
These all look the same at LHC.
At ILC, the cross-sections and angular distributions for specified initial
state polarizations tell us which is happening.
This information can in turn be used by LHC to deduce the heavy
particle masses.
Crudely, LHC discovers and ILC discoverss what the discovery was.
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Detectors at LHC and ILC
Conceptually “simple” – charged
particle tracking, surrounded by
calorimeters (particle energies),
surrounded by muon detectors.
CMS detector at LHC
at LHC
In detail, very large (to contain
the high energy particles), and
complex (to identify and measure
objects precisely). Detector
collaborations of >1000 people
from many nations.
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Is there a discovery guarantee?
The important things to note about all the postulated models of new
physics:
 All known models have observable new phenomena within reach at the
LHC and ILC.
 Each model class has many variants, each with a large degree of
freedom of parameters. The LHC and ILC are needed to give a
complementary, binocular view of new phenomena. Together, they will
tell us much more than either alone.
“Pardon me, I thought you
were much farther away”
But of course Nature could be more
cunning than we, so we eagerly await the
early results from LHC to help certify
the case for the ILC.
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Politics of big science: Planning the ILC
 The most important step was working out the detailed physics
case along the lines I’ve outlined. The ILC has been driven by a
science-based consensus from the field (not as for ITER, top down
from government).
 Obtain high priority from high-level advisory panels.
 2006 US National Academy Report (members from industry, academe,
other sciences, other physics disciplines):
1.
2.
Exploit LHC opportunity
Plan program to become world-leading center for R&D for ILC, and do
what is necessary to mount a compelling bid to host in US.
 2003 & 2007: top of the list of DOE Office of Science new intermediate
term initiatives
 Similar exercises in Asia and Europe
 Internationalize from the start:
Asia, Europe, Americas are equal
partners. Critical decision on
technology (SC vs. room temp. rf)
made by 2004 international panel.
Oversight Steering Committee
equally represented by all regions.
(Learn the lessons from the SSC failure.)
US
Canada
France
Germany
Italy
Poland
Russia
Spain
Suisse
UK
etc
China
India
Japan
Korea
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Planning the ILC
Or herding
cats …
 Formed international team “Global Design
Effort” in 2005 to guide R&D and design effort.
65 members equally from all regions. No central
home – a virtual lab with constant collaboration
and a well defined project management.
 Feb. 2007: Deliver Reference Design Report (detailed conceptual
design), Value Cost Estimate, physics program, detector conceptual
designs and a generally accessible brochure. http://www.linearcollider.org
 Value estimate in international units: cost of delivered materials,
services, average civil cost for 3 sample sites, manpower. No
contingency, escalation, taxes, detectors, operations costs as national
accounting practices vary widely on these.
 Ref. Design is basis for developing a full Engineering Design (2010),
guide R&D to prove principles, industrialize key technologies. GDE is
reorganizing for the Eng. Des. phase, assigning project responsibilities,
‘projectizing’ the work plan.
4.80x109 ILCU
technical systems
1.82x109 ILCU
site-specific costs
14.1K man yrs
(1 ILCU = $1 F07)
Ref. Design/Value Est formally received by ICFA (subcommittee of
IUPAP) and international funding agencies group.
 Expect Host region to pay ~1/2; other two regions ~1/4 each.
Need process for selecting site to proceed in parallel with Eng Design.
R&D expenditures ~equal in 3 regions ($60M in US in FY08)
5
Technical challenges & payoffs
 35 MV/m (Q>1010) cavities have been achieved,
but not reliably. Processing the Nb surface is a
black art, not yet reliably industrialized. Much of
R&D will be toward high yield industrial fabrication.
 Damping rings prepare very low emittance beams
(synchrotron radiation in wigglers) needed to
achieve the small collision spot (6 nm). Challenges
for controlling instabilities (e-cloud, fast ion
instability), low emittance tuning, and fast kicker magnets (3ns
rise/fall time) for extraction.
electropolished
 Polarized positron source (helical undulator, g  e+e-, remote handling
rotating target, e+ magnetic capture device)
 High power high rep rate laser for polarized e Dynamic feedback on nm scale for beam position control, ground motion
correctors
 Final focus optics, beam dumps, experiment interface, high precision
and energy, luminosity, polarization measurements bunch by bunch
4
Wider impacts - is it worth it?
The primary motivation for the LHC and ILC is the large scientific
discovery potential.
But, there are scientific benefits outside particle physics.
Accelerators are now basic tools for material science, nuclear physics,
chemistry, environmental science, structural biology, plasma physics …
Techniques developed for LHC & ILC will enable a new generation of
light sources, XFELs, ERLs, rare ion accelerators and neutron sources.
Such large projects are magnets for drawing young people into science.
The impact on the broader society are much harder to predict.
"I think there is a world market for maybe five computers." – Thomas J. Watson (1943)
Gladstone, Chancellor of the Exchequer, asking about Faraday’s discoveries of electric
induction:“But after all, what use is it?” Faraday: “I do not know sir, but soon you will
be able to tax it.”
But one can see potential wider applications – high power monochromatic
X ray sources (Compton backscatter e- from lasers), medical diagnostics,
isotope production, nano-lithography, container scanning, shipboard
defense, linac-based fusion and waste transmutation …
3
The outlook
The Standard Model and measurements in hand provide a vista of new
unity and interconnectedness within the microscopic world.
go here with LHC/ILC
sense whats happening here
The experimental tools to take us there are in hand. LHC will start next
year. The ILC prospects have improved steadily but the project depends
on what we find at LHC and has yet to be approved by world governments.
ILC is expensive and must be a fully international collaboration.
2
Summary
 Over the course of 40 years, our
understanding of the fundamental
forces and constituents of matter
has been revolutionized.
 The SM paradigm is about to be
broken in ways that we cannot
predict. The next generation of
experiments will tell us a
fascinating new story.
 A truly exciting time for
particle physics !