Most experiments at the Antiproton Decelerator (AD) at CERN involve laser or microwave studies of atoms such as antiprotonic helium (pbarαe) and antihydrogen (pbare+). These may throw light on outstanding questions concerning, for example, the apparent absence of cosmic antimatter and possible limits to the validity of the charge–parity time-reversal (CPT) theorem. In this research, antiprotons are brought to rest in a container – a helium-gas target chamber in the first case, and a high-vacuum electromagnetic trap containing positrons in the second. In either case, interpretation of the results requires a full understanding of how the atoms are created, what their quantum states are and how they subsequently behave. However, it is rather like performing chemistry in a test tube where residues of impurity gases might also be present. Though unwanted these could have important effects and the studies at the AD have indeed led to some unexpected, serendipitous discoveries.

The ATHENA collaboration, whose primary is to study antihydrogen spectroscopically, has reported evidence that metastable protonium atoms (i.e. antiprotonic hydrogen, pbarp) can be created in binary antiproton reactions with H2+ ions. These ions were produced when the positrons in the trap collided with H2 molecules, inevitably present as "dirt in the test tube". This serendipitous method of making protonium turns out to be interesting because it seems to produce it in states with principal quantum number (n) near 68 and angular momentum quantum number l < 10.

Ground-state n = 1, l = 0 protonium can be produced easily and has been known for many years. However, it annihilates almost instantaneously owing to the marked overlap of the p and pbar wave functions. In high-n protonium, however, there is little overlap, since the Bohr-model orbit radius is proportional to n2. The p and pbar can then come into contact only by de-exciting radiatively to l ∼ 0 via a chain of transitions that the ATHENA team estimates to take about 1 ms. This extreme longevity should enable detailed laser-spectroscopy experiments on the protonium atom, leading to values of the antiproton's properties relative to those of the proton, and so to a new class of CPT-invariance tests (N Zurlo et al. 2006). Two-body atoms are especially valuable in this respect since their transition frequencies can be calculated analytically

Another experiment at the AD, ASACUSA, has been exploiting longevity against annihilation for some years with the (neutral) antiprotonic helium atom, pbarHe+. Although this is a three-body atom, its high-n, high-l, pbarHe states have microsecond annihilation lifetimes and are easily produced when antiprotons with electron-volt energies collide with ordinary helium atoms. As in the antihydrogen experiment, H2 impurities are always present in the "test tube" at some level and have long been known to reduce, or quench, the pbarHe+ lifetime, even at very low molecular concentrations, via binary collisions between H2 and pbarHe+.

To understand this fully, the ASACUSA team introduced H2 and D2 molecules into the helium target at various temperatures and concentrations and then deduced the quenching cross-section from the annihilation lifetime of the antiproton in the (n,l) = (37,34) and (n,l) = (39,35) states, as a function of these variables (B Juhász et al. 2006). Below 30 K the cross-section levelled off in the first case, revealing a tunnelling effect with a small activation barrier, while the (39,35) state had a 1/v "Wigner"-type dependence. Such results can perhaps serendipitously fill some gaps in our understanding of astrophysics, since the measured cross-sections should be similar to those for binary reactions of hydrogen and deuterium, which play an important role in cold interstellar and protostellar clouds, but have not been well studied at low temperatures.

A final unsought discovery has resulted from ASACUSA's quest for ever lower systematic errors in the laser-spectroscopy experiments on antiprotonic helium. This forced the team to go to extremely low helium target pressures. At helium densities less than 3 × 1016 cm-3 they noticed a lengthening of the tail of the spectrum of time intervals between the formation of the pbarHe+ atom and the subsequent annihilation of the antiproton. This could only be explained by longevity of the pbarHe++ two-body, doubly charged ion, which in higher-pressure gas is a short-lived intermediate stage between the formation of the neutral pbarHe+ atom and the "contact" ppbar annihilation (Hori et al. 2005). Once again, a two-body atom promises to become serendipitously available as a test bench for CPT tests. Following up this possibility is an important part of the ASACUSA experimental programme.

Further reading

M Hori et al. 2005, Phys. Rev. Letts. 94 063401.
B Juhász et al. 2006 Chem. Phys. Letts. 427 246.
N Zurlo et al. 2006 Phys. Rev. Letts. 97 153401.