The Dawn of the LHC ERA
A Confrontation with Fundamental
Questions
Michael Dine
UCSC 2007
Aerial view of LHC
Size of LHC
In a magnetic field B, a particle of charge q and momentum
momentum p travels in a circle of radius R given by
p
R
qB
At the LHC, the desired beam energy is 7 TeV and the
state of the art dipole magnets have a field of 8 Tesla.
Plugging in and converting units gives a radius of 3 km
and a circumference of 18 km.
Addition of quadrupoles, RF cavities, etc., increases
the circumference of LHC to 27 km.
2 in 1 superconducting
dipole magnet being
installed in the CERN tunnel
Magnet Pictures
LHC dipoles waiting to be installed.
Detecting Particle Collisions
When high energy particles collide, they produce many particles.
gg  H  Z0 Z0         
Simulation of an event
in ATLAS detector.
White lines are the four
muons. The other tracks
are due to particles
from quarks in the
protons.
ATLAS Detector
Tracker Pictures
Tracker
Inserting silicon detector into tracker
Inserting solenoid into calorimeter
Calorimeter Installation
Muon Toroids
Muon superconducting
toroids.
Endcap Muon Sectors
Endcap muon sector
SCALE OF THE PROJECT
• The stored energy in the beams is equivalent roughly to
the kinetic energy of an aircraft carrier at 10 knots
(stored in magnets about 16 times larger)
• There will be about a billion collisions per second in
each detector.
• The detectors will record and stores “only” around 100
collisions per second.
• The total amount of data to be stored will be 15
petabytes (15 million gigabytes) a year.
It would take a stack of CDs 20Km tall per year.
Fundamental Questions of Another
Era
If one asked a physicist in 1935, what are the great
questions of “fundamental" physics, might have
answered:
• How does one quantize electrodynamics?
• What accounts for the strong interactions?
• Does the Fermi theory provide a description of the
weak interactions.
The focus was to provide an understanding of how
protons, neutrons, electrons and neutrinos interact to
describe the world around us.
Evolving horizons
By 1960, QED understood, well tested; four Fermi theory well tested,
basic structure understood. But a proliferation of strongly
interacting particles, as well as the discovery of the muon and its
neutrino.
Now might have asked:
• What is the nature of the strong interactions? Is it described by a
quantum field theory, like QED?
• The four fermi theory of the weak interactions provides a
successful phenomenology, but can't describe physics at very high
energies. What takes its place? E.g. does it arise from exchange of
massive vector fields (short range?).
• Given the success of field theory for some interactions, tempting to
look at the quantum mechanics of general relativity. But hard to
make sense of it (e.g. Feynman)
The Standard Model
Quarks and leptons, interacting through
exchange of gauge particles (photon, W§, Zo,
gluons).
The Standard Model (I)
quantum field theory, describing interactions between
pointlike spin-1/2 particles (quarks and leptons)
via exchange of spin-1 vector bosons (photon, W and Z, gluon)
fundamental particles (fermions)
2 (particle pair) *
3 (generations)*
2 (anti-particles)
1995
2000
By 1995, the strong and weak interactions were understood at the sort
of precision level of QED in 1960. the Standard Model was
triumphant; no interesting discrepancies. All questions in our list
answered (except general relativity)!
If past fundamental questions were truly fundamental,
we would quit. But intoxicated by our success, we are
now far more ambitious. We are also deeply dissatisfied;
the very success of the Standard Model is a source of
frustration.
Candidate Fundamental Questions
• The Standard Model possesses many parameters. Some
are extremely peculiar; e.g. me/mt = 3 x 10-6.
• The electric charges of the quarks and leptons are exact
rational multiples of one another (e.g. Qe=Qp). Why?
• General relativity cannot be combined sensibly with the
Standard Model, without some significant modification.
• The Standard Model cannot account for most of the
energy density of the universe. About 25% dark
matter; about 70% dark energy; only 5% baryons.
• The Standard Model cannot explain why there are
baryons at all (baryogenesis).
Parameters of the Standard Model
• Lepton masses
me = 0.511 MeV m = 113 MeV mt = 1.777~ GeV
• Quark masses
mu ¼ 1.5-4~ MeV md ¼ 4-8 MeV
ms ¼ 80-130 MeV mc ¼ 1.15-1.35 GeV
mb ¼ 4.1-4.4~ GeV mt ¼ 174.3 § 5 GeV
• Quark mixing angles
.
MZ= 91.1876(21) GeV
a(MZ) ¼ 1/128
as(MZ) = 0.1187(20)
sin2(qw)= 0.23120(15)
MH > 115 (more later)
Cosmological Parameters (WMAP Satellite)
A theory parable: Grand Unification
(a little math, but bear with me -- spectacular results)
Standard Model: Interactions with three gauge groups,
SU(3) x SU(2) x U(1).
SU(2): very familiar. E.g. isospin
is a good symmetry of strong interactions.
In the Standard Model, a similar symmetry:
Why not enlarge the symmetry (Georgi and Glashow)?
Group quarks and leptons into larger objects:
SU(3) acts on the d's; SU(2) on the leptons. G is an SU(5)
matrix. The SU(5) must be a broken symmetry, but this is
already true of SU(2). And right away we explain
one of our big fundamental questions, and make some
progress on another:
Charge quantization
Electric charge must be one of the generators of the group,
like t3= s3 for SU(2). Like the Pauli matrices, it must be
traceless. In this case:
So electric charge is quantized,
automatically!
Coupling unification:
This theory has one gauge coupling, rather than three. So
we should be able to compute two of the couplings in terms
of the third. This isn't quite right; it turns out we need one
more parameter, the energy scale at which the symmetry
breaks, Mgut. It is easy to compute these.
Coupling constants
EM
Weak
Strong
A more exciting prediction – and
spectacular failure
Quarks carry B= 1/3; in this theory they can turn into leptons.
E.g.
X; MX=1014 GeV
u
d
Leads to p ! e+ po, t ¼ 1026 years
dc
e+
Super Kamiokande
t(p -> e+ po)>1033 years
And one even more stupendous failure:
Dirac: existence of magnetic monopoles ) charge quantization.
The arrow seems to go the other way; any time theorists invent a
framework for charge quantization, they find monopoles. The
SU(5) theory has monopoles as solitons (static solutions of the
non-linear field equations).
Beautiful idea. But very heavy (almost a microgram), produce
lots of them in early universe.
(about 10-5/cc)
Certainly not there!
One success: Baryogenesis.
Because baryon number and CP not conserved, these theories produce
some net amount of matter (baryon number =.
The same heavy particles which mediate proton decay are
produced in the hot big bang; their decays need not produce
same number of protons and antiprotons.
Can barely account for the observed
matter/antimatter asymmetry
X
q
e
_
X
_
q
_
e
Rates for process and charge conjugate
process not exactly the same (need loop
corrections), due to CP violation, departure
from equilibrium when T ¼ MX.
So we have a prototype for understanding quantization of
electric charge and for explaining the value of a coupling.
Also for connections between theoretical ideas and cosmology.
But overall a failure.
The Standard Model Higgs Boson
time [year]
Last missing particle in SM
(EW symmetry breaking – mass)
Light SM Higgs preferred
MH = 126 +73 -48 GeV
< 280 GeV (95% CL)
Higgs Search at LEP:
mass limits:
obs. mh >
exp. m >
h
114.4 GeV
115.3 GeV
Hierarchy Problem and the LHC
But, apart from our failure to discover it up to
now, the Higgs field presents a deeper puzzle.
It may be too heavy to see without an LHC but
the real puzzle is that it is so light.
g
H
e pi
e pi
If you are not an expert at relativistic Feynman diagrams, you
can see the problem from some simple quantum mechanics.
The first two factors represent matrix elements and
density of states; last is the energy denominator.
This expression is badly divergent for large p. Still
the theory, mathematically, makes sense. The magic
of renormalization hides the real difficulty. If you cut
off the integral at p = L, you have a correction to the
mass:
m 2 = mo2 + L 2.
So unless mo is very finely tuned, one would expect
m2 ¼ L 2.
The need to cut off the integral reflects our
ignorance of phenomena at scales L and
higher. So this argument strongly suggests
that there must be some new phenomena at
an energy scale less than the TeV scale,
which cuts off the divergence.
Many proposals: technicolor, large extra
dimensions, supersymmetry. Each makes
distinctive predicdtions for phenomena at
the LHC.
An attractive Extension: Supersymmetry
Symmetry between
Fermions ↔ Bosons
(matter)
(force carrier)
... doubled particle spectrum ... ☹
Supersymmetry
Symmetry between
Fermions ↔ Bosons
(matter)
(force carrier)
... doubled particle spectrum ... ☹
Interaction Strength in Supersymmetry
without SUSY
... BUT some Standard Model
Problems solved ...
... extension in string theory is
candidate for Grand Unified Theory ...
... lightest SUSY particle stable
⇨candidate for dark matter ...
... unification of forces ...
with SUSY
1 TeV
Interaction energy in GeV
l
c
l
c
l
c
q
q
g
q
l
l
c
l
q
Production and decay of superparticles at the LHC. Here, jets,
Leptons, missing energy.
In a broad class of supersymmetric models, the
lightest new particle is stable (R-parity); typically
the partner of the Higgs or Z boson or photon.
Produced in early universe.
N
W+
N
W-
Range of supersymmetry parameters consistent with dark matter density;
here partner of photon is essentially the dark matter.
I am a fan of the supersymmetry hypothesis; I'm not
alone. About 12,000 papers in the SPIRES data base. If
true, quite exciting: a new symmetry of physics, closely
tied to the very nature of space and time. Dramatic
experimental signatures. A whole new phenomenology,
new questions. But neither the limited evidence nor these
sorts of arguments make it true; there is good
experimental as well as theoretical reason for skepticism.
This is not the only explanation offered for the hierarchy,
and all predict dramatic phenomena in this energy range.
• Large extra dimensions
• Warped extra dimensions
• Technicolor
• It’s just that way (anthropic?)
Hypothetical answers to our fundamental questions:
• Too many parameters
• Charge quantization
• Quantum general relativity
• Dark Matter
• Dark energy
• Baryogenesis
Other proposals have some success with each
of the starred items; perhaps fair to see that
supersymmetry does best.
STRING THEORY
String theory has pretensions to attack the remaining
problems on this list:
• A consistent theory of quantum gravity
• Incorporates gauge interactions, quarks and leptons,
and other features of the Standard Model.
• Parameters of the model can be calculated, in principle.
• Low energy supersymmetry emerges naturally – all of
this proliferation, which seemed artificial, almost
automatic.
Has string theory delivered?
• String theory is hard. We don’t have a wellunderstood set of principles. Some problems of
quantum gravity are resolved, but many of the
challenges remain.
• String theory seems able to describe a vast number
of possible universes, only a small fraction of
which are like ours.
• Until recently, no progress on one of the most
difficult challenges to particle physics: the dark
energy.
Dark Energy/Cosmological Constant
• About 2/3 of energy of universe. Satisfies
p = -r
– an energy density of the vacuum.
• Dimensional analysis: L » M4.
Mp4? MW4? (1076,108)
Measured: 10-47!
Progress and Controversy
• Many states of string theory now know with
properties close to those of the Standard Model.
Possibly 10500 or more!
• Among these, a uniform distribution of L. So
many consistent with observation.
• Banks, Weinberg: in such a circumstance, only
form galaxies in those states with L close to
observation. Perhaps universe, in its history,
samples all? (This argument actually predicted the
observed value of the dark energy).
Can string theorists make other
predictions?
• Supersymmetry at LHC, or not?
• If yes, spectrum of superpartners?
• If no, alternatives (“just” a Higgs, large
extra dimensions, “warping”?)
• Cosmology?
We are at the dawn of a very exciting era. We
may resolve some of our fundamental questions.
Atlas Detector
Beam pipe
Muon System
Tracker
Hadron
Calorimeter
Electromagnetic
Calorimeter
Tracker
Tracks (e, , p, p, p, K)
Layers of
Silicon pixels
Silicon strips
Drift tubes
2 Tesla
Magnetic
Field

B

Measure q and p
2.5 m diameter
Measure each
point to about
20 to 100 
Measure
momentum of
100 GeV/c
particle to 4%.