Experiments at CERN’s low-energy antiproton ring (LEAR), closed in 1996, brought many very-high-precision and sometimes surprising antiproton results. Some continue to appear, the latest being the apparent independence of the size of the target of the antiproton-nucleus annihilation rate at very low energy. Clearly antiproton annihilation is a mysterious business.
Antiproton-nucleus annihilation was measured at LEAR by the OBELIX experiment at very low antiproton momenta, down to 40 MeV/c. This momentum seems quite large with respect to the characteristic momentum in particle-antiparticle systems bound by electromagnetic attraction (Coulomb force). For proton-antiproton, this is of the order of 4 MeV/c. Nevertheless, this attraction appears to be important and can even affect the annihilation rate.
In fact, in this energy range, Bethe’s usual 1/v law is replaced by a 1/v2 one, where v is the relative velocity of the interacting particles.
This 1/v2 regime was predicted in 1948 by Wigner and is well known in atomic physics. In nuclear physics, in contrast, one usually encounters electromagnetic repulsion between protons, which gives rise to an exponential decrease of the reaction rate at low energies, a phenomenon that is particularly important in nuclear astrophysics.
The OBELIX experiment, for the first time, investigated with very high precision the behaviour of the reaction rate in a system with Coulomb attraction. In the figure, the measured antiproton-proton, antiproton-deuteron and antiproton-helium annihilation cross-sections are presented as a function of antiproton momentum.
These cross-sections are multiplied by the square of the relative velocity. For the proton-antiproton system, the situation is very clear: one can see that the product tends to a constant value with decreasing antiproton momentum. For a 1/v behaviour, this product should tend to zero. For the deuteron and helium cases, the analysis is more complicated.
This change of regime is instructive but not really unexpected. The most interesting observation comes from the comparison of the values of these three cross-sections. At high energies they are quite different – the antiproton nucleus annihilation cross-sections are several times that for antiproton-proton. Surprisingly, at low antiproton momentum, the antiproton-deuteron and antiproton-helium annihilation cross-sections drop to the proton antiproton level or even below it.
An accurate analysis of these annihilations shows that this is not a kinematic effect; it is a direct result of the dynamics of the antiproton-nucleus interaction.
This was confirmed independently by another LEAR experiment – PS207 – which measured, for the first time, the shift and the broadening of the antiproton-deuteron atomic ground state. This extremely difficult experiment showed that the width of this level, entirely determined by the annihilation process, is approximately the same for antiproton-proton and antiproton-deuteron atoms.
A geometrical picture of annihilation would suggest that the probability of this process should increase with the number of possible annihilating partners – the number of nucleons in nuclei. However, these experiments demonstrate clearly that this is not the case.
To understand the mystery, these experiments should be continued at lower energies and with heavier nuclei, not only to understand the dynamics of the annihilation process but also to measure the cross-sections.
This knowledge would be important, in particular for astrophysicists, who search for antimatter in the universe and need to know about the properties of low-energy matter-antimatter interaction. CERN’s antiproton decelerator (AD), currently starting operations, will be a powerful tool in obtaining this precious antimatter information.
