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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

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Title: 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


1
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
2
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.
3
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.
4
This was how we built on the idea. Notice Gordon
had a lot less gray in his hair!
5
This is the original version of the published
chart.
6
I am passing out the CPEP particles and
interaction chart for you to look at and keep.
7
This is the newest version of the particles
chart, however, I do not have copies of this one
with me.
8
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.
Amazon.com The Charm of Strange Quarks
Mysteries and Revolutions of Particle Physics R
Michael Barnett, Henry Muehry, Helen R. Quinn, G.
J. Aubrecht, ... www.amazon.com/Charm-Strange-Quar
ks-Mysteries-Revolutions/dp/0387988971 - 307k -
9
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10
What do we mean by the hadron in the Large
Hadron Collider?
11
There are two sorts of particles shown on the
chart I gave youleptons 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.
12
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13
The hadrons are the strongly interacting
particles.
14
This is important the hadrons act over really
short distances distances of a femtometer (10-15
m).
15
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.
16
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 Im sure youre aware that you have
mass, as does everything we see around us. Why
just quarks and leptons? Why not, say,
leptoquarks?
17
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
so-called 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.
18
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?
19
The forces of nature are introduced as
interactions gravitational interaction,
electroweak interaction, strong interaction. The
strong interaction involves hadrons.
20
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 worlds
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.
21
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.
22
We hid a lot of dirt in the Standard Model.
We cant predict the Higgs mass, for example.
We dont know if theres 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.
23
Lets 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/?.
24
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.
25
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 1920sCockroft and Walton
designed a linear accelerator (linac), and E. O.
Lawrence designed a circular accelerator
(cyclotron).
26
Lawrences 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.
27
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).
28
Heres another experimental result. See the
resonance (the particle) in these data?
29
Experiments led to this particle zoo.
30
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.
31
CERNs LHC will allow us to glimpse interactions
at really high energy. This shows ATLAS, which is
one of the detectors at the LHC.
32
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 immense10 TeVand 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.
33
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
34
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35
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.
36
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.
37
The Compact Muon Solenoid (CMS) is 21 meters long
and 15 meters wide and high. It has a mass of
12,500 tonnes.
38
LHCb (Large Hadron Collider beauty) is 21 meters
long, 10 meters high, and 13 meters wide, with a
mass of 5600 tonnes.
650 physicists belong to this experimental
collaboration.
39
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.
lt-- 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.
40
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.
41
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 whats what. We need to look for
evidence of what lies beyond the Standard Model
42
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
doesnt 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).
43
For a circular accelerator, the magnets that bend
the particles are situated along the path.
44
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46
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.
47
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.
48
A faulty electrical connection between two
magnets (in red) was the cause of the incident in
sector 3-4 of the LHC.
49
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.
50
Some of the damage.
51
CERN also said that 5 quadrupole and 24 dipole
magnets needed to be repaired. CERN is checking
out several dozen other magnets they dont 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.
52
  • http//www.youtube.com/watch?vj50ZssEojtM

53
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!
54
Ill be happy to take questions! Thank you.
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