The search for Axions

Exotic particles addressing major issues of fundamental physics like Dark Matter. By Dr. Lino Miceli.

PROJECT PHOTOS

INTRODUCTION

Axions are elusive, exotic particles thought to exist since the 70s, but never observed. Axion search is at the forefront of modern science, since if an axion is found, two major issues of fundamental physics may be addressed, namely the nature of Dark Matter (DM), and the so called “Strong CP Problem”, flagged by the vast inconsistency between the predicted and the observed level of Charge-Parity (CP) violation in strong interactions (nuclear forces). “C” is the symmetry by which the laws of physics do not depend on the sign of the electric charge. “P” is the symmetry by which the laws of physics remain unchanged if applied in a mirror space. Axion search is therefore at the crossroad between cosmology and particle physics.

Microwave cavities

One way of searching for Cold Dark Matter (CDM) axions, possibly composing the halo of our galaxy, is with tunable microwave cavities inserted in high magnetic fields, and operated at cryogenic temperatures. The operating principle of this axion detector is the following: the high magnetic field stimulates an axion of the galactic halo to decay into a photon whose frequency corresponds to the axion mass. It is so because the energy (E) of the converted photon is a manifestation of the axion mass, by the famous equation E=mc2, while the photon energy and frequency (E) are equivalent, since E=hv for electromagnetic fields. As a consequence, there is a higher chance that an axion is detected if the cavity is made to resonate at this frequency. To do so, the cavity must be tunable, similarly to a radio able to be tuned to a specific station. If detected, the frequency of this radio signal tells us what the axion mass is.

Center for Axion and Precision Physics Research

IBS/CAPP

A growing number of experiments around the world hunts for axions. In particular, the Institute for Basic Science Center for Axion and Precision Physics Research (IBS/CAPP), at the Korea Institute of Science and Technology (KAIST), in Daejeon, South Korea, is poised to take the world lead in axion search. While pushing fundamental physics research, CAPP contributes to advanced technology, due to the high level experimental challenges of this research field. As part of its international efforts, the CAPP leads an axion search at CERN, the CAST-CAPP project, using the large prototype LHC magnet of the CAST facility [Fig. 1].

The cooperation with JPE greatly helps this project meeting some of its technical challenges.

The CAST Magnet at CERN
Figure 1. The CAST Magnet at CERN

The experiment

In this experiment, long, tunable rectangular cavities will be inserted in the 8.8 T CAST magnet, operating at 1.8 K. One such cavity can be tuned by changing the distance between two thin sapphire plates inserted parallel to each other [Fig. 2]. This “tuner” requires high sensitivity, high stability, and high precision positioning, in the order of a few nm, since the axion frequency bandwidth is very narrow. If stability, or position resolution, is insufficient, the feeble axion signal (~ 10-23 W!) is washed out. The JPE Cryo Translation Stage (CTS) can satisfy these requirements, while operating in the harsh physical environment, i.e. very low temperatures and high magnetic fields, of this kind of experiments.

The JPE CTS connected to the tuning mechanism in a short cavity test assembly.
Figure 2. The JPE CTS connected to the tuning mechanism in a short cavity test assembly.

While typical CDM axion experiments are based on solenoid magnets, where translators and rotators of the cavity tuners can be placed at higher temperatures and much lower fields, axion search in dipole magnets represents a higher challenge to these actuators. The cooperation between JPE and IBS/CAPP is showing crucial in the CAST-CAPP experiment.