Science with Antiatoms: The Quest To Study Antihydrogen

Initial attempts to trap useful numbers of antihydrogen atoms have revealed the need to produce colder atoms.  The deepest magnetic traps that can be produced with superconducting technology can only confine atoms whose energy (in temperature units) is below 0.5 K.    So far the antiprotons and positron plasmas from which antihydrogen is formed, and hence the antihydrogen itself, have temperatures far enough above 0.5 K that there are not enough atoms in the low energy tail of the Boltzmann distribution.  In pursuit of lower temperature plasmas the centrifugal separation of antiprotons from the electrons that cool them have been observed.  Adiabatic cooling is shown to be a simple and effective method for cooling for cooling charged particles in a trap.  No particle loss is required, making adiabatic cooling an attractive alternative to cooling by the evaporative loss of particles.  The temperature following the cooling of more than two million antiprotons is shown to have the expected dependence upon harmonic well depths down to 6 K -- the lowest directly measured temperature for antiprotons.  Likely the temperature is much lower than can currently be directly measured, and evaporative cooling after adiabatic cooling should produce even lower temperatures (with particle loss).