Transcript Contemporary Physics in your classroom Gordon J. Aubrecht

Everything you wanted
to know about the LHC
but were afraid to ask
Gordon J. Aubrecht, II
Physics Education Research Group
Department of Physics, Ohio State University,
former chair of CPEP
Abstract:
The Large Hadron Collider (LHC) is a particle accelerator based at
CERN on the Swiss-French border. The LHC was turned on last
September to fears of the “end of the world,” but the experience
turned almost into the end of the LHC. An accident took the
machine out of service until September 2009. What is the LHC for?
Why is it important? What caused the accident? When will it return
to service? I hope to address some or all of these issues in this
presentation.
I have long been associated with the
Contemporary Physics Education Project, is
known as CPEP for short.
CPEP began as a way to bring particle physics
into high school (and college) classrooms. At that
time, twenty years ago, the Standard Model of
particles had jelled into something respectable.
We at CPEP thought that presentation of
cutting-edge physics and the knowledge that
there were still many open questions could lead
students to consider future careers as scientists.
This was how we built on the idea. Notice
Gordon had a lot less gray in his hair!
This is the original version of the published
chart.
I am passing out the CPEP particles and
interaction chart for you to look at and keep.
This is the newest version of the particles chart,
however, I do not have copies of this one with me.
There are materials available to help students and
teachers as well. CPEP thought that we needed to
assist serious study as well as providing visual
beauty and provoking curiosity through charts.
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Amazon.com: The Charm of Strange Quarks:
Mysteries and Revolutions of Particle Physics: R
Michael Barnett, Henry Muehry, Helen R. Quinn,
G. J. Aubrecht, ...
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What do we mean by the “hadron” in the Large
Hadron Collider?
There are two sorts of particles shown on the
chart I gave you—leptons and hadrons. They are
completely different in their properties from one
another, but all leptons behave in certain ways
and all hadrons behave in certain other ways.
Leptons interact gravitationally,
electromagnetically, and via the weak interaction.
Hadrons are the only ones that interact via the
strong interaction. Quarks are hadrons.
The hadrons are the
strongly interacting
particles.
This is important: the hadrons act over really
short distances—
distances of a femtometer (10-15 m).
The Standard Model (see the chart) has been the
most successful model ever in describing the
actions of particles.
The Standard Model explains all the particle
physics of the past 30 years.
Explorations of the Standard Model have been
responsible for 32 Nobel Prizes over the last 30
years.
However, there are some little problems …
The Standard Model uses as input 24 parameters:
12 quark and lepton masses
12 coupling constants
Where should these parameters come from?
All fundamental particles start with zero mass, but
I’m sure you’re aware that you have mass, as does
everything we see around us.
Why just quarks and leptons?
Why not, say, leptoquarks?
In 1964, Peter Higgs proposed a particle to
explain the Standard Model (before it really
existed in concept).
Wikipedia says: “broken symmetry in electroweak theory,
explaining the origin of mass of elementary particles in
general and of the W and Z bosons in particular. This socalled Higgs mechanism, which had several inventors besides
Higgs, predicts the existence of a new particle, the Higgs
boson (often described as “the most sought-after particle in
modern physics”). Although this particle has not turned up in
accelerator experiments so far, the Higgs mechanism is
generally accepted as an important ingredient in the Standard
Model of particle physics, without which particles would
have no mass.”
The outstanding problems are there as we saw … and the
more complete model, if any, should be able to answer the
questions:
Why is there an accelerating universe?
Why is there so little antimatter in the universe?
What is the origin of mass?
Where could dark matter come from?
Why is there a huge range of masses?
The forces of nature are introduced as interactions:
gravitational interaction, electroweak interaction, strong
interaction.
The strong interaction involves hadrons.
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The Large Hadron Collider (LHC) is a place
where interactions can occur through particle
collisions.
According to Wikipedia,
“The Large Hadron Collider (LHC) is the world’s largest and
highest-energy particle accelerator, intended to collide
opposing particle beams, protons at an energy of 7
TeV/particle or lead nuclei at 574 TeV/particle.”
The Higgs particle may soon be discovered at the
LHC. The Higgs particle rescues something my
colleague Richard Kass at OSU called a physical
Ponzi scheme (speaking of the Standard Model).
Another colleague once called this the “broom
and rug approach” to physics—
use the broom to sweep the dirt together, pull up
the rug and sweep the dirt under it, then put the
rug back down.
We hid a lot of dirt in the Standard Model. …
We can’t predict the Higgs mass, for example.
We don’t know if there’s one Higgs or many.
There are those 24 free parameters.
Why are the electric charges of e- (a basic lepton)
and p (a composite hadron) exactly the same
size?
There are problematic infinities in the model.
Let’s think a bit.
The resolution of objects depends on the
wavelength of the probing object. A
wave of wavelength  bends around
objects of size d. Waves and particles are
not more than different evocations of
some underlying reality. Particles have
momentum p that is related to the
wavelength : p = h/.
Because
p = h/,
 is comparable in size to the object (d),
and the energy of a particle is given by
E = (p2c2 + m2c4)1/2 = mc2,
we see that to “see” a small object (d very
small), p must be very large, and so in
turn E must be very large.
This means that particle physicists are
always searching to increase the energy
of collisions. They do this by
accelerating the particles in an
accelerator.
The first accelerators were designed in
the 1920s—Cockroft and Walton
designed a linear accelerator (linac), and
E. O. Lawrence designed a circular
accelerator (cyclotron).
Lawrence’s machine was called a
cyclotron (not prefix), and today particle
physicists use both linacs and
synchrocyclotrons to study particle
physics.
The synchronization is necessary due to
the effects of special relativity.
This is an experimental sketch from a
1950s paper. Note that the mass energy of
the pi particle is ~ 140 MeV, or 0.14 GeV
(in the 1950s, this was denoted “Bev”).
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Here’s another
experimental
result. See the
“resonance” (the
particle) in these
data?
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Experiments led to this “particle zoo.”
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Then, in the 1960s, Murray Gell-Mann,
George Zweig, and others invented ways to
categorize these many particles and the
result is called the quark model.
You saw the quarks from the chart earlier.
A proton is uud, a neutron is udd, etc.
The model also produces mesons—
hadrons that are made of quark-antiquark
pairs.
CERN’s LHC will allow us to glimpse
interactions at really high energy. This shows
ATLAS, which is one of the detectors at the
LHC.
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The LHC is a circular accelerator ring 27 km
around. Particles are steered in both directions
using superconducting magnets and made to
collide in several regions loaded with detectors
like the Atlas detector.
Because the ring is so big, the particles’
energies are immense—~10 TeV—and the
particles are traveling at essentially the speed of
light: E =  mc2 =  1 GeV, so
 10 TeV/(1 GeV) = 10,000, giving
v = c - 1.5 m/s.
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LHC preaccelerators
p and Pb: Linear accelerators for protons (Linac 2) and Lead (Linac 3)
(not marked) Proton Synchrotron Booster
PS: Proton Synchrotron
SPS: Super Proton Synchrotron
LHC experiments
ATLAS
A Toroidal LHC Apparatus
CMS
Compact Muon Solenoid
LHCb
LHC-beauty
ALICE
A Large Ion Collider Experiment
TOTEM
Total Cross Section, Elastic Scattering and Diffraction Dissociation
LHCf
LHC-forward
ATLAS is about 45 meters long,
more than 25 meters high, and
has a mass of about 7,000
tonnes.
More than 1700
physicists work on
this collaboration.
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ALICE is about 26
meters long, and 12
meters high and wide,
and has a mass of
about 10,000 tonnes.
This
experiment is
a
collaboration
of over 1000
physicists.
The Compact Muon Solenoid (CMS)
is 21 meters long and 15 meters wide
and high. It has a mass of 12,500
tonnes.
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LHCb (Large Hadron Collider beauty) is
21 meters long, 10 meters high, and
13 meters wide, with a mass of 5600
tonnes.
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650 physicists
belong to this
experimental
collaboration.
TOTEM is 440 meters long, 5 meters
high and 5 meters wide. It has a mass
of 20 tonnes. Fifty physicists work on
this experiment.
<-- This is CMS.
The long red thing is TOTEM.
View of one quarter of the CMS detector with the TOTEM forward trackers T1 and T2. The CMS calorimeters, the
solenoid and the muon chambers are visible. Note also the forward calorimeter CASTOR.
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LHCf (Large Hadron
Collider forward)
LHCf has two
detectors, each
measuring 30 cm long,
80 cm high, 10 cm
wide, with a mass of 40
kg each. Twenty-two
physicists work on this
experiment, which uses
the LHC to simulate
cosmic rays.
The experiments ALICE, ATLAS, LHCb, etc.,
will be looking for traces of the Higgs particle(s),
and we know that the Tevatron at Fermilab has
already constrained the Higgs mass to be above
100 GeV. We need to get to those high energies
the LHC promises to see what’s what.
We need to look for evidence of what lies beyond
the Standard Model …
… such as supersymmetry (colloquially known
as SUSY) or some more exotic things (whatever
they might be). SUSY might be able to explain
dark matter, the mysterious extra mass that helps
hold galaxies together. As in the Pauli joke
explanation, the lowest-mass object is stable; it
doesn’t decay. The lowest-mass SUSY particle
could be the source of this dark matter.
SUSY might tell us that grand unification is
correct (the couplings are the same at high
enough temperature [energy]).
For a circular accelerator, the magnets that bend
the particles are situated along the path.
The LHC has more than 1600 superconducting
magnets (most of which mass over 27 tonnes).
Around 96 tonnes of liquid 4He is needed to keep the
magnets at their operating temperature of 1.9 K.
There are 1232 dipole magnets that keep the beams
on their circular path, with an additional 392
quadrupole magnets that steer the beams.
To get the high magnetic fields needed, the
superconducting magnets carry huge currents
(millions of amperes) losslessly.
On 19 September 2008 came an unanticipated
disaster. A fault occurred in the electrical bus
connection in the region between a dipole and a
quadrupole. This led to an electric arc, which
punctured a helium enclosure. All of a sudden the
huge current heated everything up, so the
temperature went up, vaporizing the helium and
sending it blasting through the tunnel. It spread to
other helium enclosures and damage occurred over
half a kilometer.
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A faulty electrical connection between
two magnets (in red) was the cause of
the incident in sector 3-4 of the LHC.
CERN said: “The forces on the vacuum barriers
attached to the quadrupoles at the subsector ends were
such that the cryostats housing these quadrupoles
broke their anchors in the concrete floor of the tunnel
and were moved away from their original positions,
with the electric and fluid connections pulling the
dipole cold masses in the subsector from the cold
internal supports inside their undisplaced cryostats.
The displacement of the quadrupoles cryostats
damaged ‘jumper’ connections to the cryogenic
distribution line, but without rupturing its insulation
vacuum.”
Some of the damage.
CERN also said that 5 quadrupole and 24 dipole magnets
needed to be repaired. CERN is checking out several dozen
other magnets they don’t think are damaged just in case there are
problems.
Repairs are costing over $20M, and restart will come only this
coming September, after having been anticipated for July, 2009.
According to CERN, “The cause of this delay is due to several
factors such as implementation of a new enhanced protection
system for the busbar and magnet splices; installation of new
pressure-relief valves to reduce the collateral damage in case of
a repeat incident; application of more stringent safety
constraints; and scheduling constraints associated with helium
transfer and storage.”
• http://www.youtube.com/watch?v=j50Zs
sEojtM
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So, we hope from this September
to see new, interesting physics.
What will we learn?:
•
•
•
•
•
The origin of dark matter?
Whether there is one Higgs or many Higgs?
Whether SUSY is possible?
Why the universe is mostly matter?
??????????????????????
Let the good physics roll!
I’ll be happy to take
questions!
Thank you.