The study demonstrates a new method to search for meV dark photons

The study demonstrates a new method to search for meV dark photons

Dilution refrigerator (left) used to achieve a background-free single-electron quantum cyclotron. (top right) silver Penning trap inside which a single electron will be suspended. (Bottom right) demonstration of resolution of quantum cyclotron states. A microwave training is applied to induce the transition. Credit: Xing Fan.

About 85% of the mass of our galaxy consists of dark matter, matter that does not emit, absorb or reflect light and therefore cannot be observed directly. While several studies have suggested or theorized about its composition, it remains one of the greatest unsolved problems in physics.

Physicists around the world have been searching for dark matter or trying to come up with new methods to directly observe different dark matter candidates. A hypothetical form of dark matter that has so far eluded detection is dark photon dark matter.

An intriguing possibility is that dark matter consists of dark photons, which look like photons (ie, the particles that make up visible light) but interact with charges of weak strength. These dark photons could theoretically have masses in the milli-electrovolt range, about a million times lighter than those of electrons, and thus notoriously difficult to detect.

Researchers at Northwestern University, Stanford University, and Fermilab recently introduced a new method that could be used to search for meV dark photons. The validity of the method, outlined in a paper published in Physical Review Letterswas demonstrated in a short proof-of-principle process, which also helped establish new constraints on dark matter with dark photons.

“The idea for our study arose from discussions between experimentalists and theorists facilitated by the DOE SQMS Center,” Gabriel Gabrielse, one of the researchers who conducted the study, told “At Northwestern we were looking for BSM applications for an unusual one-electron detector that had no background. We developed a new detector to measure the electric dipole moment of electrons and test the most accurate prediction of the standard model.”

The study demonstrates a new method to search for meV dark photons

A dark photon that enters and excites an electron to a higher excited state. Credit: Harikrishnan Ramani.

After learning about the work being done by Gabrielse and his colleagues at Northeastern, a team of theoretical physicists at Stanford reached out and pointed out the potential of their detector to search for meV dark photons. This sparked a series of interactions and collaborations between the two research groups, including Roni Harnik, a theorist at Fermilab.

The new method introduced by the researchers is based on the use of trapped electrons as high-Q resonators for the detection of dark matter by meV dark photons. Its basic assumption is that when the rest energy of a dark photon matches the energy splitting of the two lowest cyclotron levels, the first electron cyclotron state will be excited.

“If a meV dark photon enters the trap in which a single electron is suspended, then the electron can be excited from the ground state to the first excited state of the cyclotron motion,” explained Gabrielse. “There is no background and the single excitation of a single trapped electron can be unambiguously detected. The failure to see such excitations for several days allowed us to set a limit on the strength of the dark photon field that passed, based on theor. calculations of the efficiency with which a dark photon could produce such an excitation.”

To demonstrate the practicality of the proposed method, Gabrielse and his colleagues used it to collect an initial measurement using a single electron. This attempt showed that their strategy was backgroundless for a search that lasted just over 7 days.

The researchers were also able to set a new limit for dark photon dark matter, specifically at 148 GHz (0.6 meV). In the future, their work could pave the way for new studies to evaluate and use the proposed strategy to search for meV dark photons.

“The most notable achievement of our work is the concrete demonstration of an entirely new method of searching for meV dark matter,” added Gabrielse. “We now plan to do a broad search for meV dark photons in an apparatus that is designed for this. The apparatus in which the demonstration measurement took place was optimized for measuring the magnetic moment of electrons in a narrow band, while the new apparatus and the new ideas we are developing will allow extensive searches.”

More information:
Xing Fan et al., One-Electron Quantum Cyclotron as a Milli-eV Dark Photon Detector, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.261801

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