The Muonic Hydrogen Collaboration is a group of research physicists from Asia, Europe, the USA and Canada, who study muon interactions in hydrogen at TRIUMF. The goal of the research is to understand the atomic, molecular, and nuclear reactions involving muons and different isotopes of hydrogen, particularly those which are important for muon-catalyzed fusion.

The muon is an elementary particle which can have a negative or positive charge and has approximately 207 times the mass of an electron. The negatively charged variety carries the same charge as an electron; like the electron, it can be part of an atomic system or bind atoms together in a molecule. Intense beams of muons are produced at TRIUMF from the decay of pions, which are in turn produced when hydrogen ions (protons) moving at 3/4 the speed of light strike a stationary target. The muon decays with an average lifetime of only 2.2 microseconds (millionths of a second), but it participates in a rich variety of processes in that short period.

There are three isotopes (varieties) of hydrogen atoms. First there is protium (H), by far the most common of the three, which has one proton (a massive positively charged particle) as its nucleus. The single positive charge of the nucleus is balanced by one electron to form the neutral protium atom. Then there is deuterium (D), whose nucleus is made up of one proton and one neutron (slightly more massive than a proton, and with no electric charge) bound together as a deuteron (d). About 0.015% of all hydrogen atoms in nature are deuterium. Finally there is tritium (T), with a proton and two neutrons forming a triton (t) to make up the nucleus. Tritium is radioactive and its safe handling and containment require special experimental procedures. Two atoms from any combination of these isotopes can be bound together by orbiting electrons to form a hydrogen molecule (the most common of which is 2 protium atoms making normal hydrogen gas).

A negative muon can, like an electron, also bind any two hydrogen atoms into a molecular ion. A molecular ion is just a molecule in which the charges of the nuclei are not exactly neutralized by the surrounding electrons, or,in this case, the negative muons. Because the muon is so much heavier than the electron, its normal orbit is much closer to the two nuclei, so the muonic molecular system is much smaller and more tightly bound than its electronic version. When at least one of the isotopes is deuterium or tritium, the hydrogen nuclei can fuse together, forming a heavier nucleus, and releasing energy. The muon effectively shields the repulsive electrical force between the two positive hydrogen nuclei, allowing them to come together closely enough to bind via the "strong" nuclear force. The muon in most cases survives so that it can cause further muon molecular ion formation and fusion. Because the muon acts as a catalyst to enable the process without being consumed, this is known as muon-catalyzed fusion. If the same muon could go on to catalyze enough reactions, we could use the energy created as a source of clean and inexpensive power. However, sometimes the muon sticks to a charged fusion product such as an alpha particle, and is lost to the cycle. To date, over 100 fusions per muon have been recorded in experiments at other laboratories, but it is estimated that it would take somewhat more than this in order to "break even" energy-wise.

Unlike other fusion processes, muon-catalyzed fusion can occur at or below room temperature. In fact, the TRIUMF group uses a target of solid hydrogen at about 3 degrees Kelvin (-270 degrees Celsius). To create muon-catalyzed fusion, a beam of negative muons is stopped in a frozen layer formed from a mixture of hydrogen isotopes. The unique method allows us to study the formation of muonic molecular systems, to determine the parameters which control the fusion processes, and to learn more about the sticking of the muon to a fusion product. The solid target experiments can show features of the reactions which are not accessible via other methods. Even though the production of clean, inexpensive energy from muon-catalyzed fusion is beyond our present capability, we are able to learn more about the fascinating behaviour of negative muons in hydrogen.