Sympathetic cooling of protons and antiprotons

Antiproton cooling methods

Cooling methods are essential for precision measurements in atomic physics, since the thermal motion of the sample causes limitations or systematic uncertainties in the measurement precision.

Antiproton precision measurements require in particular the cooling of antiprotons from their production energy in CERN's antimatter facility of about 26 GeV to a few keV for initial trapping and subsequent cooling to energies of  0.1 K (~8.6 µeV) or below for the precision measurements of the antiproton magnetic moment.

Protons can be directly produced in the trap from hydrogen which is frozen out on the cold trap surfaces. Common methods are electron impact ionization using a field emission point or laser ablation, and require cooling from ~1 000 K to 0.1 K.

Presently, we reach the coldest temperatures for proton and antiproton by resistive cooling in a Penning trap. This allows cooling the proton/antiproton to the environment temperature, which is 4.2 K in our cryogenic trap system. So far, we have cooled the cyclotron mode of the proton/antiproton to about 0.1 K by selecting a cold thermal state after thermalizing a single particle with our image-current detection system, which also provides the resistance for cooling. This is a time consuming process, since the probability to obtain a cold particle is low.

Currently, we are implementing sympathetic cooling methods for single protons and antiprotons with laser-cooled ions in our apparatus. To this end, we have build a double-trap system with a common end cap electrode following an early proposal by Heinzen and Wineland. Here, the proton can interact via image currents with a cloud of laser-cooled beryllium ions, so that these particles form a coupled harmonic oscillator. Consquently, the proton can exchange energy with the laser-cooled ions and cool down to the Doppler limit of laser-cooled beryllium ions in our novel trap system. We expect to reach temperatures of a few mK in cooling cycles which are at least 100 times faster compared to the resistive cooling approach.

This new cooling method will be essential for the next generation of CPT invariance tests and searches for new physics in proton and antiproton precision experiments.

The common end-cap double-trap system for sympathetic cooling of protons and antiprotons.