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Particle accelerators
• Charged particles can be accelerated by an electric field.
• Colliders produce head-on collisions which are much more
energetic than hitting a fixed target. The center of mass
energy is 2E in a collider but only m2E for a fixed target
(E = energy, m = mass of the particles, E»m, c=1).
• The LHC collides protons with m 1GeV, E=7TeV. It produces the same center of mass energy as a proton with
E=105 TeV hitting a proton at rest.
• Cosmic rays enter the atmosphere with energies well beyond that achievable by accelerators (up to E=108 TeV).
But they are scarce and are detected with a fixed target.
• 1 TeV = 1000 GeV = 1012 eV
Electrostatic accelerators
• An electrostatic van de Graaf accelerator uses high voltage
for acceleration, which is obtained by mechanical transfer
of electrons from one material to another.
• Energies of 10 MeV can be reached, which are typical for
nuclear physics.
Linear accelerator
• Particles gain energy by surfing an electromagnetic wave.
• This happens in a microwave cavity. High energy is reached
by having a long series of cavities. The 3 km long Stanford
Linear Accelerator (SLAC) accelerates electrons to 50 GeV.
Linear accelerators have been abandoned in favor of
circular accelerators (synchrotrons). These are more
compact and use only a few microwave cavities.
Circular accelerators (synchrotrons)
•
Use again a microwave cavity for acceleration, except
that the particles keep coming around in a big circle.
They are accelerated each time they pass the cavity.
•
LEP at CERN (Geneva): 115 GeV electrons vs. positrons.
Discovered the W+,W -,Z bosons of the weak interaction.
•
Tevatron at Fermilab (Chicago): 1000 GeV = 1 TeV
protons vs. antiprotons.
Discovered the top quark, the last missing quark.
•
LHC at CERN (Geneva): 7 TeV protons vs. protons.
Discovered the Higgs boson, the last missing particle
of the Standard Model.
Fermilab
CERN
LHC (large hadron collider)
Highest energy worldwide.
Found the Higgs boson.
Still looking for supersymmetric particles and
candidates for dark matter.
27 km
Particle detectors
• As the energy of the incident particles increases, there
is more and more energy available for producing other
particles. Feynman compared this to shooting two Swiss
precision watches against each other and trying to find
out from the debris how a watch is built.
• Detectors have become larger, and the number of particles produced at high energy is enormous. There is so
much information that most of the data have to be preselected automatically. A “trigger” committee decides
on the algorithm for that. The number of scientists in a
collaboration is reaching 3000 at the LHC.
LHC Detector
One high energy
event
Cosmic rays
Cosmic rays (mainly protons) produce a shower of particles when
they strike a nucleus in the upper
atmosphere. The shower spreads
out over miles.
We don’t know where cosmic rays
are accelerated. Galaxies with a
huge black hole at the center can
emit particle jets over distances
as large as a galaxy. Such jets may
act as huge linear accelerators.
The Auger cosmic ray detector in Argentina
Cosmic rays are observed with energies of more than 1020 eV,
100 million times greater than the energy reached with our
accelerators. This is the energy of a 90 mph tennis ball compressed into a single proton !
A particle shower (red line) is
observed in a group of ground
detectors (orange) and in four
light detectors, which look up
into the sky (blue and green).
One of 1600 ground detectors
Neutrino detectors
• Neutrinos are very difficult to detect, because they don’t
possess electric or strong charge. They can only interact
via the weak interaction.
• The weak interaction is transmitted by the W, Z bosons.
Both have very high masses (approaching 100 GeV), while
neutrinos from the Sun and from radioactivity have only
energies in the MeV range. They have to emit short-lived
‘virtual’ W or Z bosons, and that happens rarely.
• Therefore, a neutrino detector needs to have a very large
detection volume, such as the Kamiokande detector in a
mine in Japan or the Ice Cube detector at the South Pole,
where a cubic kilometer of clear ice serves as detector.
The South Pole
Ice Cube detector at the South Pole
Strings of photon detectors are lowered into the ice along cables.
Particles are tracked by the
emitted light and by timing.
A photon
detector
Particle accelerators are microscopes
The uncertainty relation requires a large momentum range
p to focus onto a small spot x.
Large momentum implies large kinetic energy -- therefore
the need for high energy accelerators.
To get down to the Planck length (the smallest length scale)
one would need the Planck energy (largest particle energy):
Planck energy: 1028 eV
Cosmic rays: 31020 eV
The LHC:
71012 eV
It has been estimated that one would need an accelerator
the size of the universe to reach the Planck energy.
Particle accelerators are time machines
Make the particle energy equal to the thermal energy soon after the Big Bang.
Atoms
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Fri. Dec 3
Phy107 Lecture 34
400 000
Years