All astrophysical and cosmological data now point convincingly to a large component of Cold Dark Matter (CDM) in the Universe, for which a light axion is a well-motivated candidate. It has long been known that if axions are a primary constituent of the dark matter of our own Milky Way halo, they may be detected through their conversion into a narrow-band microwave signal by an apparatus consisting of a microwave-cavity resonator permeated by a magnetic field.
Since 1996, the Axion Dark Matter eXperiment (ADMX) has been searching for axions, achieving sensitivities well within the band of axion models, excluding masses in the few micro-eV range. With recent technological advances, it is now possible to extend these measurements to higher frequencies. This award will enable the Yale group to join the ADMX collaboration and to build and operate a second microwave cavity experiment, ADMX at High Frequencies (ADMX-HF), to probe the 2-20 GHz (~8 to 80 micro-eV) range. This brackets the mass of the axion which corresponds to saturation of the matter density of the universe (~ 20 micro-eV). ADMX as "one experiment, two sites" will thus be able to pursue a two-prong strategy, simultaneously pushing downwards in the axion-photon coupling and upwards in mass.
In Broader Impacts, ADMX has already proven to be a driver of leading-edge technology; NSF supported the development of the near-quantum limited SQUID amplifiers for ADMX more than a decade ago, which quickly spun off to become a critical piece of IARPA's (Intelligence Advanced Research Projects Activity) roadmap in quantum computing. ADMX-HF can similarly be expected to draw from the deep quantum electronics competency at Yale to provide new innovations benefiting basic science and security. The PI has a long history of employing undergraduates to work on his research.
Two of the great mysteries of modern science are the natures of dark matter and dark energy. That the Universe is permeated with both is based on solid astrophysical measurements. Indeed, more than 95% of the mass of the Universe has been identified as being a combination of both dark matter and dark energy, leaving less than 5% as visible matter that can be directly observed. Dark matter and dark energy are "invisible" and, at present, can only be detected through their gravitational effects on the structure of the Universe, and the dynamics of galaxies. 27% of the matter in the Universe has been identified as invisible dark matter; and its detection is the focus of our project. Over the last two decades, many hypotheses have been put forward on what dark matter might be, and very elaborate and sensitive detectors have been built and operated, and have achieved a level of sensitivity that has very nearly ruled out most hypotheses. One possible dark matter candidate that has not been ruled out is the axion, a hypothetical particle that was introduced to make the forces between atomic nuclei symmetric when the direction of time is reversed. Because axions do not interact with ordinary matter, they easily penetrate the Earth, its atmosphere, laboratory walls, and the experiment container. The idea that dark matter might comprise axions goes back to the late 1980's but did not received full attention due to previous technical limitations. The basic idea is that axions, in the presence of a strong magnetic field, will be converted to radio waves that can be detected with a sensitive amplifier. The frequency of the radio waves is determined by the axion mass, through the famous relation E=mc2, and the frequency is given by f=E/h, where h is Planck's constant. Unfortunately, the axion mass is not know precisely, but models suggest a range of values that lead to radio frequencies from about 900 MHz (the frequency of cell phones and wifi networks) to up to 50 GHz, with a preferred range between 5 and 20 GHz. Because the axion mass is not known, the detector, which is basically a sensitive radio receiver, must be tuned slowly over a frequency range. An axion would be evidence by an increase in the background noise. The basic axion dark matter detector is a microwave cavity resonator which is placed in a very strong magnetic field; for our experiment, this field is 9 Telsa (stronger than a typical MRI medical magnet). The cavity resonator builds up radio frequency energy that is then sent to a very sensitive amplifier, so sensitive that quantum mechanical zero point energy is its dominant noise source. The cavity has an internal structure that allows is resonance frequency to be precisely adjusted. The difficulty w arises from the need that the cavity and amplifier must be kept near absolute zero, with everything remotely controlled from room temperature. We have now completed the construction of a detector and have successfully completed the first commissioning stage. With this commissioning, we have shown that a quantum limited amplifier, (Josephson Parametric Amplifier or JPA) can achieve the quantum limit, and that we can tune the cavity frequency and its coupling to the JPA. We have shown that the cavity's mechanical motion, and operation of the JPA, are not degraded by the magnetic field. The frequency range of our experiment is higher than any previous axion search; success rests on the JPA technology that is applied, in our work, to an axion search for the first time ever. We refer to our project as ADMX-HF, for Axion Dark Matter eXperiment at High Frequency. The broader impact of our work are numerous. First, we have shown that JPAs can be coupled to large volume cavities and operated stably. With our cavity development, we have designed a mechanical control system based on Kevlar strings that allows 10 part per million accuracy of a cryogenic system. We are developing superconducting coating for the cavity that will increase the sensitivity, and this technology will have application in other fields. Currently, one graduate student and one postdoctoral researcher are being supported on this project, and one undergraduate is working for academic credit. In the collaborating institutions, there are two additional postdoctoral researchers, two graduate students, and about three undergraduates. ADMX-HF presents an excellent educational opportunity because everyone involved with the experiment needs to learn all aspects, from cryogenic techniques to the manipulation of large data sets. The public interest in this work has been great. We have conducted tours on request, for adults, college students, and for high school students. Now that the construction phase of ADMX-HF is nearly completion, we hope to develop a dark matter and dark energy exhibit at, and in collaboration with, a local natural history museum.
