While there is clear evidence that the combination of the C and P symmetries can be broken, the combination of C, P and T (CPT) seems to be preserved. In fact, it is difficult to write down a quantum theory that does not preserve CPT. CPT conservation has several important consequences. One is that matter and antimatter particles have the same mass. Another is that the energy levels of atoms and anti-atoms are identical, such that any difference would be a clear proof of CPT violation.
The ALPHA experiment studies the spectral features associated to the energy levels of antihydrogen by either shining a laser onto it to discover if it will absorb the same frequencies of light as hydrogen or by exposing it to microwave radiation.
Watch the following animation to understand how the properties of the trapped antihydrogen are measured.
Because antimatter is so scarce very little is known about some of its fundamental properties. For example, does antimatter feel the same gravitational attraction as regular matter?
The ALPHA experiment is trying to understand, for the first time, if the ratio of the gravitational mass (the gravitational force experienced by an object) and the inertial mass (the resistance of the object to acceleration by an external force) is the same for hydrogen and antihydrogen. For hydrogen this ratio is measured to be very close to one. Any discrepency in the result will indicate violation of CPT symmetry. The ALPHA experiment is testing this for the first time with antihydrogen.