Before shutting down on Sept. 29, 2011, the Tevatron was the world’s largest proton-antiproton collider. Residing at Fermilab, the Tevatron accelerated and stored beams of protons and antiprotons traveling in opposite directions around an underground ring four miles in circumference at almost the speed of light before colliding them at the center of two detectors.
The detectors, called CDF, for Collider Detector at Fermilab, and DZero, named for its location on the accelerator ring, contain many detection subsystems that identified the different types of particles emerging from the collisions. Scientists explored the structure of matter, space and time by analyzing the showers of particles created.
The Tevatron guided beams of protons and antiprotons through a vacuum pipe around the accelerator ring using more than 1,000 superconducting magnets.
When cooled with liquid helium to negative 450 degrees Fahrenheit, the cable inside the superconducting magnets could conduct electric current without resistance. The lack of resistance permited the magnets to operate at higher currents, which gave operators the ability to steer particles accelerated to higher energies.
Because protons and antiprotons have opposite charges, beams of each traveled through the beam pipe in opposite directions. An acceleration section gave additional energy to the particles as they circled around the tunnel about 48,000 times per second.
Once they reached their maximum energy, the beams of protons and antiprotons collided at each of two four-story detectors, CDF and DZero, which are positioned at two different points along the Tevatron. The beams crossed paths an average of 1.7 million times per second at each detector. Each time presented an opportunity for one or more collisions between circulating protons and antiprotons.
Collisions created showers of new particles. Physicists took interest in collisions that stood out due to the force of their impact or for the types of particles they produced. For interesting events, about 400 of which occured each second, CDF and DZero recorded each particle’s flight path, energy, momentum and electric charge. Physicists used this information to identify the types of particles created by the collisions and to determine if they discovered something new.
CDF and DZero, two detectors positioned on opposite sides of the Tevatron’s beam pipe, used different technologies to capture and analyze similar information about the particles proton-antiproton collisions produce.
CDF and DZero are built differently, but the particles that sprayed out from collisions traveled through three similar basic layers in both. The inner layer of each detector is comprised of tracking detectors, which recorded the flight paths of passing electrically charged particles.
Because the layers of tracking detector are located within magnetic fields, charged particles such as electrons, muons and charged hadrons followed curved paths through them. The slower or less massive the particles, the greater was the magnet’s effect on them, and the more they curved. Scientists therefore used the amount which a particle’s track curved to determine its momentum. This information helped them determine what kinds of particles were produced immediately after the proton and antiproton collided.
The next layers were made up of calorimeters, which measured the energy of particles.
These are massive detectors that absorbed the energy of charged particles. When a high-energy particle collided with a layer of lead, steel, or uranium in the calorimeter, it showered, creating a cascade of lower-energy, charged particles. These in turn hit other metal atoms and created their own showers of even lower-energy particles. In this way, one 100-GeV electron could become 100 1-GeV electrons by the end of the ride.
The CDF and DZero calorimeters are sampling calorimeters, which means that they absorbed most of the energy of particles in dense layers of metal, but “sampled” the energy of the showering particles using materials such as liquid argon or plastic scintillator interspersed between the metal layers. Every time the remaining particles passed through a layer of scintillator or argon, they deposited energy, which was converted to light or electrical current, which was measured by fast electronics. The amount of light or current determined the energy of the particle very precisely.
Only muons, which are like electrons, but much heavier, could pass through the inner layers of the detectors and still leave their traces in the outer layers of the detector. Thick walls of steel shielding separated the outermost muon chambers from the rest of the detector. Normally only muons and neutrinos passed all the way through to the muon chambers. But neutrinos hardly interact with matter at all; practically all of them coursed through the detector without leaving any signal. By recording how many particles the outermost detector found, physicists could determine which particles that came out of the collision were high-energy muons.
Physicists also gathered information about particles that slipped through detectors undetected. Physicists know that according to the law of conservation of energy, what they put into the system should equal what they get out of it. Missing energy meant the collision created some particles that the detector could not see. These could be particles such as neutrinos that don’t interact with matter very often. Or they could be particles that have not been discovered yet. Many searches for new physics postulate the existence of particles that the detector would not have been able to see directly. Looking for missing energy was the only way to validate the existence of these particles using the detectors.
The two collaborations verified each other’s results, which ensured that they do not misinterpret data anomalies as discoveries. Working in tandem, the detector teams could search mass and energy ranges more efficiently than they could alone.
Physicists observed the first proton-antiproton collisions produced by the Tevatron on Oct. 13, 1985. Researchers at the CDF experiment and at DZero, which began operating later in 1992, have used the Tevatron to study matter at ever smaller scales.
On March 2, 1995, physicists at CDF and DZero announced the discovery of the top quark. Researchers in both collaborations had statistically proven observation of the top quark in collisions at their detectors.
The top quark, which is as heavy as a gold atom but much smaller than a proton, was the last undiscovered quark of the six predicted to exist by current scientific theory. Scientists worldwide had sought the top quark since the discovery of the bottom quark at Fermilab through fixed-target experiments in 1977.
Both collaborations were subsequently able to measure the mass of the top quark to high precision. Particle physicists measure particle masses to verify their particle models. Knowing the value of the top quark mass has allowed physicists to zero in on the mass of the undiscovered Higgs boson, a crucial component of the theoretical framework of particle physics.