HAYSTAC Axion Dark Matter Experiment: Phase III

There are three astronomical observations that, to be understood, require the presence of so-called Dark Matter, which is invisible matter that almost exclusively interacts with itself and other matter through gravity. First, the velocities of stars bound in galaxies (including the Milky Way) are too fast to be accounted for by the total mass of the visible stars.

Second, the expansion of the Universe requires the introduction of more matter than we can observe as ordinary atoms and particles. Third, the filamentary structure of the distribution of galaxies cannot be established based on gravitational interactions between galaxies, based on their observed masses. The fundamental nature of dark matter is one of the leading unsolved problems of modern science.

Much work has been done to detect new types of massive dark matter particles that are predicted based on an extended theory of particle physics called supersymmetry. So far, no such particles have been seen in accelerator or dark matter detectors. Another possible theoretical dark matter particle is the axion, which was introduced to make the strong, or nuclear, force independent of the direction of time, as has been experimentally determined to very high precision.

Although the axion has never been observed, its properties can be calculated, and it is perfect for a dark matter candidate. Such axions would form halos around galaxies, including our own. To search for axions, a microwave resonator is placed in a very strong magnetic field.

The axions in the galactic halo can convert to photons, or radio waves, in the presence of the magnetic field. A sensitive amplifier is used to search for the signal arising from this conversion, the frequency of which is unknown. This system, called a haloscope, must be tuned slowly in frequency to search for the conversion signal.

The group has operated the detector in the 4-7 GHz range several years and is continuing the search with a planned increase in sensitivity. The collaboration-wide efforts in support of HAYSTAC are leading to many technical innovations that will be useful in many research and technology fields, with the principal result being that it is possible to have a delicate quantum state measurement subsystem operate in a real-world system that is subject to vibration and very large magnetic fields, while operating with nearly 100% reliability, over times scales approaching a year.

HAYSTAC also serves as a fantastic training ground for students and postdocs in particle physics, axion dark matter, and quantum sensing techniques.

HAYSTAC (Haloscope at Yale Sensitive to Axion Cold Dark Matter) has been in continuous operation, with interruptions for system upgrades, since the initial commissioning in the Summer of 2014. The first version of the detector was based on a Josephson parametric amplifier that was able to reach the quantum noise limit of the phase-preserving amplifier.

In 2019, the project successfully incorporated quantum-enhanced detection based on a squeeze-state receiver system that is not subject to the standard quantum limit. This increased the frequency scan speed by a factor of two with a modest increase in sensitivity, due to the approximately 4 dB of squeezing that was obtained, being limited by losses in the microwave circulators.

This system is currently taking data in the 4.5 – 5 GHz range and will continue to do so through Summer 2023, with sensitivity at the KSVZ model level. The expertise that was developed from the beginning of HAYSTAC includes vibration suppression, accurate control of the conversion cavity frequency and coupling, and highly effective magnetic shielding of the quantum amplifiers that need to operate near the 8 Tesla superconducting magnet used for conversion.

The project has developed operational techniques that allow continuous operation of the cryogenic system for months. During the first year of the proposed activity, the experiment will install a new multi-rod cavity that will allow operation at higher frequencies (favored by cosmogenic axion models) with a good volume form factor that is being developed at U.C.

Berkeley, and test its cryogenic properties. Next, a new quantum enable receiver system being developed at the University of Colorado will be integrated into HAYSTAC. This system eliminates the lossy circulators and will allow a substantial increase in sensitivity.

The group anticipates that the full commissioning of the upgraded system will be complete at the beginning in mid-2025, and operate in the 7-10 GHz range or above, with sensitivity well into the KSVZ parameter range.

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