The Dawn of the LHC ERA
A Confrontation with Fundamental
Questions
Michael Dine
Quarknet, UCSC, 2008
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.
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 store “only” approx. 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 this
much data.
Today: A Theorist’s View of the
LHC
• Why is this machine, perhaps the largest
scientific instrument ever built,
interesting?
• What do we expect to learn? What
questions might we hope to answer?
The Standard Model
By 1980, the Standard Model of particle
physics offered a nearly complete picture of
the elementary particles and their
interactions.
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)!
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
Puzzles of the Standard Model
• 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 20% dark
matter; about 75% dark energy; only 5% baryons.
• The Standard Model cannot explain why there are
baryons at all (baryogenesis).
The Hierarchy Problem (or the
failure of dimensional analysis)
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. Problem is
one of dimensional analysis. We know there
are large energy scales in nature. Biggest is the
“Planck mass”, Mpg = GN1/2= 1019 GeV
Why isn’t MH = C Mp, where C » 1?
H
e pi
e pi
A possible solution: Supersymmetry
Symmetry between
Fermions ↔ Bosons
(matter)
(force carrier)
... doubled particle spectrum ... ☹
Solves hierarchy problem
• Now dimensional analysis requires greater
care. It turns out that because of the
symmetry,
MH = C Ms
• New physics at TeV (LHC!) scales
• Explains dark matter
• Gives prediction of strong interaction strength
Interaction Strength in Supersymmetry
without SUSY
... BUT some of our puzzles
solved ...
Successful unification of
forces
Lightest susy particle stable, and
oproduced in abundance to be dark matter
Readily explains baryon asymmetry
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.
I am a fan of the supersymmetry hypothesis; I'm not
alone. About 12,000 papers in the SPIRES data base (also
a good fraction of your faculty). 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
•Hierarchy
•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, an extension of the ideas of grand
unification, 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 3/4 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 known 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.
Popular Treatments of String
Theory
Rhapsodic about string theory
Denounces string theory
Should the public care?
• Green: too focused on the mathematics of
string theory, too little on what we actually see,
observe in nature. Given string theory’s
limited successes, seems to me this should be
``end of the book” material.
• Smolin: some valid criticisms, but promoting
his own agenda; no more interest in physical
phenomena than Green.
Dine rant
See handout. Not yet prepared to put on my
website.