Aim

The magnetic moment of the antiproton can be determined from the microwave spectroscopy of the antiprotonic helium hyperfine spitting by comparison of the measured transition frequencies with three-body Quantum Electrodynamic (QED) calculations. Such a matter-antimatter comparison yields a test of CPT invariance. Simultaneously these measurements provide a vigorous test of three-body QED calculations.  Further details on this work can be found here:

Pask_Master.pdf

3.9 M

Antiprotonic helium

Antiprotonic helium is an exotic metastable three-body system consisting of a helium nucleus, an electron and an antiproton.   When an antiproton approaches a helium atom, at an energy less than the helium ionisation energy, it can simultaneously eject one of the two ground state electrons and become captured, see Fig. 1.  97% of the captured antiprotons annihilate within nanoseconds with one of the nucleons in the nucleus but ~ 3% occupy metastable states with lifetimes of the order of ~ 1.5 µs.  A hyperfine structure (HFS) arises from the interactions of the angular momentum and spin of its constituents.

Method

The HFS of antiprotonic helium is investigated by a three step, laser-microwave-laser spectroscopy, technique.   The first laser pulse depopulates one of the HF doublet states (e.g. F⁺) shown in Fig. 2, by transferring it to a short lived daughter state.  A microwave pulse is fired, which can transfer the population from F^-, refilling F⁺. Finally a second laser pulse, with the same frequency as the first, transfers the population to the daughter state again.   The experiment is repeated, scanning the microwave frequency over the expected range.  Because the daughter state has a relatively short lifetime, each laser pulse induces an annihilation peak on the detector which is proportional to the population transferred see Fig. 3.  Therefore when the microwave frequency is resonant with a transition a lager peak occurs on the detector than when the frequency is off resonance.  By plotting the peak height as a function of microwave frequency the resonances can be observed.

Apparatus

The experiment is carried out at CERN’s Antiproton Decelerator (AD) which provides a pulsed beam of antiprotons. The target helium gas is contained in a cylindrical microwave cavity surrounded by a cryostatic gas chamber.  The annihilation products are detected by Cherenkov counters.  A continuous wave (cw) laser beam is split into two seed beams which are pumped to produce amplified pulses. The microwave signal is produced by a vector network analyser (VNA), amplified with a pulsed travelling wave tube amplifier (TWTA) and transported to the target through a rectangular waveguide.  A schematic of the target region is show in Fig. 4.

Results

In 1996, at LEAR, a precise laser spectroscopy scan at 726.10 nm, resolved the two hyperfine transitions from the parent (37, 35) to the daughter (38, 34) state.  In 2001, the first laser-microwave-laser measurement was performed, resolving the hyperfine structure for the first time. [read more]

During the following years, developments in laser technology allowed improvements to be made to the laser system achieving (1) a narrow line width, so that one HF line could be depopulated without affecting the other; (2) single mode and high shot to shot stability, which reduced the quantity of statistics required for each measurement; (3) a long pulse length, which increased the laser depopulation efficiency because it was of the order of the daughter lifetime; and (4) an unlimited time difference between laser pulses.  In 2006-7 it was demonstrated that, with these improvements, a the signal to noise ratio and microwave resonance line width could be improved.  See Fig. 5. [read more]

In 2008 a three year high statistic and systematic study was completed. A comparison, between the experimentally determined HF splitting and three-body QED predictions, resulted in a value for the antiproton magnetic moment of −2.7862(83) µ_N, which agrees in magnitude with that of the proton to 0.3%.  This constitutes an independent measurement, on a three body system, of the antiproton magnetic moment with the same precision as the value quoted by the particle data group. [read more]

Future

Antiprotonic helium 3 is a more complex system.  The additional nucleon spin produces a total of eight, instead of four, states, see Fig. 6.  Planned measurements will address a small deviation from theory that was observed in laser spectroscopy experiments. In principle the experiment is identical to the above description but with different laser and microwave frequencies.  For the microwave this means a new resonant cavity.  This experiment is expected to take three years.

A new 11 GHz cavity is under construction at SMI. Simulations were carried in out to determine the cavity dimensions and optimise the construction materials. Incorporated in the new design is a new compressor cooled cryostat which removes the dependence on liquid helium and allows the experiment to run without interruption.  A prototype cavity is shown in Fig. 7.