|A highly sensitive experiment now underway to find the axion,
a mysterious species of subatomic particle that may or may not exist, could be made even
more sensitive in the next generation of the experiment. The development of a radio
frequency amplifier based on a new type of high-gain, ultra-low noise Superconducting
QUantum Interference Device (SQUID) should make it possible for the first time ever to
detect even the most elusive axions.
The new amplifier uses a direct current SQUID made from superconducting niobium and configured so that its input coil is used as a microstrip resonator. With this unique configuration, the input signal is coupled between one end of the input coil and the SQUID loop which acts as a groundplane for the microstrip resonator. The result is a radio frequency amplifier operating at frequencies approaching 1.0 gigahertz the highest at which a SQUID amplifier has ever been successfully operated.
This amplifier was developed by the research group of John Clarke, a physicist who holds a joint appointment with the U.S. Department of Energy's Lawrence Berkeley National Laboratory and the University of California at Berkeley. Key contributions were made by group members Marc-Olivier André and Michael Mück.
One of the first applications of the new amplifier will be to enhance what is already the most sensitive search for axions in science. An axion is a hypothetical elementary particle that has been proposed to explain the absence of an electrical dipole moment for the neutron. If it exists, an axion would have neither electric charge nor spin and a mass that might be a trillion times less than that of an electron.
Despite its slight presence, the axion is widely considered to be an excellent candidate for explaining the nature of "dark matter" in the universe. Even in light of recent new findings about the Cosmological Constant, there still seems to be an enormous amount of matter in the Universe which cannot be seen but which makes its massive gravitational presence known through the motions of the galaxies. Axions could help explain dark matter through sheer numbers; an estimated 100 trillion of them are packed into every cubic centimeter of space in our galaxy. But first, scientists have to prove they exist.
An experiment to detect axions began in 1995 at the Lawrence Livermore National Laboratory (LLNL). This ongoing experiment is led by LLNL physicist Karl van Bibber and physicist Les Rosenberg of the Massachusetts Institute of Technology. In addition to scientists from LLNL and MIT, it also involves Clarke and his group at Berkeley Lab/UC Berkeley, plus scientists from the University of Florida, the University of Chicago, and Fermi National Accelerator Laboratory.
In this experiment, the static magnetic field of a powerful (8 Tesla), 12-ton superconducting electromagnet is used to stimulate axions to decay into microwave photons via the "Primakoff effect." An excess of microwave photons above thermal and electronic noise signals the decay of an axion. This signal is extremely faint, however, and must be amplified in order to be detected.
With the addition to the experiment of the Clarke group's radio frequency amplifier and its niobium SQUID, signals from the decay of dark-matter axions should get a much needed boost. Clarke and his group report they have obtained resonant frequencies 200 MHz to 720MHz and gains of about 20 decibels. What this would mean to the axion search, according to van Bibber and Rosenberg, is that for the first time, the experiment should be able to reach weakly-coupled axions at fractional halo density. These axions are considered to be the most promising of the axion-based dark-matter models.