The Top Quark has been one of the most elusive particles in the history of particle accelerators, with only the graviton being more evasive of physicists’ inquiring clutches. Scientists have been looking for the top quark ever since the bottom quark was discovered at Fermilab in 1977, but only in 1994, April 22, was any physical evidence found to support the top’s existence.

In the Standard Model of Fundamental Particles and Interactions , all hadrons are made of smaller particles; quarks. There are three "flavors" of quarks -- u, d, and s -- and each flavor has three possible "colors" -- labeled red, green, or blue. For every type of quark exists an antiquark with a corresponding anticolor (cyan, magenta, and yellow). Hadrons come in two varieties: baryons, made of three quarks, and mesons, made of a quark and an antiquark. So far, quarks have come in pairs with corresponding properties like isospin. The up and down pair and the charm and strange pair are called isospin doublets, and unless the bottom quark is part of a new, unexplained isospin singlet, then the top quark must exist. According to the Standard Model, the bottom quark will behave differently depending on whether it is part of an isospin singlet or doublet. Experiments thus far show that the bottom quark should be part of a doublet, and therefore there should be a top quark.

Top quarks are produced in pairs when a light quark in the proton and a ligh antiquark in the antiproton annihilate to form a top-antitop pair. Since the Standard Model predicts that the top should almost always decay into a particle and a bottom quark, we expect a top-antitop event to produce a W+W- pair and bottom and antibottom quarks. This assumption must be tested. The top is so massive, it might decay in ways not included in the Standard Model.

If there is evidence for the top, why is there still question of its existence? There is no solid evidence so far, no stationary top quark at which we can point and say "see? It exists!" Instead, we have to rely on statistical evidence that says a certain type of other particle (theoretically produced by top quark decay) is occurring too often to be a coincidence.

The detectors involved at Fermilab, the CDF (Collider Detector at Fermilab) and DZero , weigh thousands of tons and have hundreds of thousands of electronic channels to transmit information about the collisions of the proton streams. There must be so many channels and layers of detector-material because all the particles produced by the accelerator collisions have to react with the detector in order to make the most of the collisions, and give a complete picture of what happened. The detector must indirectly determine, the path, energy, and type (charge, mass, etc.) of each particle spawned by each collision. The detector must function over the largest possible angular range, and also provide a wide range of cross checks to protect against measurement errors. Supercomputers with millions of lines of code sift through the information given by the detectors; they must extract interesting classes of events from beackgrounds of hundreds of millions of times more frequent uninteresting ones. The computers only record abnormal particle signatures, such as extra-frequent production of bottom quarks. The detectors use several different methods for determining particle characteristics:

• Charged particles leave a trail of ionized atoms as they pass through matter (gas, liquid, or solid). Some layers of the detector measure the position and magnitude of ionization trails. The amount of energy loss from ionization per unit length can be used to determine the particle velocities.
• Particle velocities may also be determinded from the time intereval between the particle passing two points. Unfortunately, the interval is only practically measurable for relatively low-energy particles.
• Charged particles are deflected into curved orbits in the presence of a magnetic field. The radius of this curvature can be used to calculate the momentum of the particle. Either its mass or velocity can be calculated from the momentum if the other is known.
• Cerenkov radiation is characteristic, but weak, radiation coherently emitted by particles passing through material. When they pass between different materials, they emit transition radiation. Passing through magnetic fields, they emit synchrotron radiation. The intensity and characteristics of these three can be used for a velocity measurement.
• Elenctrons or photons passing through matter produce electromagnetic cascades of radiation, which leave ionized atoms. The energy of the original particle is eventually converted completely into ionization. A detector that uses a thick block of instrumented material can measure the incident energy of a particle.
• Hadrons interact strongly as they pass through matter, producing secondary hadrons, and again leaving a core of ionized matter. The measurement of a hadron's incident energy through this property is called hadron calorimentry. A hadron calorimeter in a detector may require a thickness of 3 to 5 feet of steel, weighing hundreds of tons.
• The initial layers of the detector characterize the charge of the particles and are designed to alter their trajectories as little as possible. Outer layers use large amounts of material to convert particles' energy into detectable ionization. The outer layer of a detector is frequently used to detect muons, usually the only particles to travel through the other layers

As accelerators have gotten more and more powerful, they are able to produce more massive particles (since mass and energy are equivalent). When the bottom quark was first discovered, physicists theorized that the top quark had a mass of about 15 GeV, but accelerators were soon powerful enough to produce particles with masses of over 100 GeV, and still no top quark was discovered. Because of this, the theoretical mass of the top quark has grown along with the power of accelerators, up until evidence began appearing that the actual mass is in the range of 174 GeV, +/- 17 . Only the Tevatron at Fermilab has enough energy to produce such a massive particle, which is incredibly huge for a "subatomic" particle; more massive than a total gold atom. The enormous mass leads scientists to believe the top has a special relationship with gravity, and the yet unidentified gravity particle, the massless graviton.

There are four types of interactions between particles: Strong, Electromagnetic, Weak, and Gravitational. In Strong, Electromagnetic, and Weak Interactions, there are known particles that are exchanged to give rise to the force, but no gravitational particle has been found. A large-mass particle could possibly have more interactions with gravitons, or otherwise give a hint at the true nature of gravity. When we understand how gravity works, we can develop a unifying theory of all forces, vastly simplifying all physicists' lives and reducing tremendously the amount of math required to do physics.