Situated on 27 square kilometers of prairie 45 miles West of Chicago, Fermilab is the largest high energy particle physics research lab in the United States, second only to CERN worldwide. It is home to the highest energy particle accelerator in the world, a 4 mile long ring capable of accelerating particles to almost one terra-electron volts (one trillion eV), named the Tevatron.
The Tevatron initially began operation as a fixed target accelerator in 1983, firing 800 GeV protons into stationary targets such as Copper, Beryllium and Lead. Later, the Tevatron was converted into a collider, smashing together beams of protons and antiprotons each with energies of 900 GeV. Beam energies have gradually been increased to today’s peak operating energy of 980 GeV.
When protons and antiprotons collide they annihilate in a burst of energy. This energy then produces new particles. The more energy that is available for particle production, the heavier the particles that can be created. Colliding-beam experiments can produce heavier particles than fixed target accelerators because both particles involved in the collision have significant kinetic energy and after annihilation all of this energy along with the particles’ rest mast energy goes into the production of new particles. Colliding beams of 900 GeV protons and antiprotons produce approximately 1.8 TeV of energy.
The Accelerator Chain
The Tevatron accelerates protons and antiprotons to such impressively high energies by sending them through a number of different accelerators connected in a chain. Initially, Hydrogen gas is ionised in the Cockroft-Walton pre-accelerator and the resulting H- ions accelerated via a positive voltage up to 750 keV. Next these ions pass through a linear accelerator which uses traveling electromagnetic waves to speed the ions to 400 MeV as they surf the wavefronts. The negative ions then pass through a carbon foil which strips them of their electrons, leaving only positively charged protons. These protons then pass through a circular accelerator known as the Booster which uses powerful magnets to curve the protons round its loop approximately 20,000 times, each time slightly increasing their kinetic energy, up to a maximum of 8 GeV.
Next the accelerated protons enter the Main Injector. Some of these protons are accelerated from 8 GeV to 150 GeV and injected into the Tevatron. The rest are accelerated to 120 GeV and sent to the Antiproton Source. There the protons collide with a Nickel target, producing (among other things) 120 GeV antiprotons which are sent back to the Main Injector via the Accumulator Ring. Back in the Main Injector, the antiprotons are accelerated to 150 GeV and injected into the Tevatron along with the 150 GeV protons.
The Tevatron itself is the largest accelerator of all, a circular loop that, like the Booster, uses electromagnetic fields to accelerate charged particles. In this last stage the particles reach their maximum kinetic energies of almost 1 TeV. Due to their opposite charges the protons and antiprotons travel in opposite directions around the ring. The two beams are designed to cross at the sites of two massive detectors, the Collider Detector at Fermilab (CDF) and DZero, which contain a plethora of detectors to identify the particles that are produced in the huge burst of annihilation energy. The solenoidally shaped CDF is a magnetic detector with a high-resolution silicon vetrtex detector. DZero, in comparison, has no central magnetic field, instead making heavy use of calorimetry to detect and track particle energies.
Probing the Standard Model
In 1995 the Tevatron was used to produce firm experimental evidence for the existence of the top quark. The top quark is the heaviest quark predicted by the Standard Model, the unified theory of the electroweak interaction and quantum chromodynamics. At the time it was the last remaining undetected particle required to complete electroweak theory. It was signaled experimentally by the way bottom quarks disintegrate and the way they interact via Z boson exchange with other particles. In their experiments, both the CDF and the DZero teams found evidence for the existence of top quarks in the decay of W bosons (mediators of the weak force) into electrons or muons along with neutrinos.
One of the current jobs of the Tevatron is to further pinpoint the mass of the top quark. This work is very relevant to the current search for the Higgs boson, the particle theoretically postulated to explain why particles have mass. The mass of the Higgs boson is strongly related to the mass of the top quark, hence pinpointing the mass of the top quark gives us a better indication of the likely mass of the Higgs boson and where to look for it.
The predicted mass of the Higgs boson is of the order of several hundred GeV, which would seem to indicate that finding it would be easy for the 2 TeV annihilation energy producing Tevatron. However the task is incredibly complicated, primarily because a multitude of other unwanted particles are produced in the energetic annihilation. Any Higgs signals must be separated clearly from the cachophany of other signals. In practice the Tevatron is only capable of finding a Higgs boson of mass 100 GeV to 200 GeV. Unfortunately for the Tevatron, last year’s best estimate of the mass of the top quark (found using the DZero detector) puts an upper limit on the mass of the Higgs boson of 219 to 251 GeV, just out of the Tevatron’s reach.
This means that the Tevatron will probably miss out on discovering the Higgs boson, if it really exists at all. That honour will most likely go to the $2.6 billion dollar Large Hadron Collider (LHC) set to begin operation at CERN in 2007. There is, however, a very interesting possibility that the Tevatron will overturn the Standard Model altogether by uncovering the lightest of the not one but FIVE Higgs bosons predicted by Supersymmetry, an alternative theory which predicts that every particle we know about has a heavy partner just waiting to be discovered.
Only time will tell.
1.“Inquiring Minds”, Fermilab Official Website, http://www.fnal.gov/pub/inquiring/index.html, accessed 20/08/2005.
2.“Discovery of the Top Quark”, Chris Quigg, APS News-Online/Physics News in 1995, May 1996, http://lutece.fnal.gov/Papers/PhysNews95.html, accessed 20/08/2005.
3.“Mass Hysteria”, Alexander Hellemans & Valerie Jamieson, New Scientist, Issue 2456, 17 July 2004, p38.
4.“Particles and Nuclei: An Introduction to the Physicsal Concepts”, 4th Ed, Povh et al, Springer, 2003.