In new studies, researchers explore new ways to hunt dark matter

For decades, astronomers and physicists have tried to solve one of the deepest mysteries of the cosmos: around 85% of its mass is missing. Many astronomical observations indicate that the visible mass in the universe is nowhere near enough to hold galaxies together and explain how matter clumps together. Some sort of invisible, unknown subatomic particle called dark matter must provide the extra gravitational glue.

In underground laboratories and in particle accelerators, scientists have been searching for this dark matter without success for more than 30 years. NIST researchers are now exploring new ways to search for the invisible particles. In one study, a prototype for a much larger experiment, researchers used state-of-the-art superconducting detectors to hunt for dark matter.

The study has already placed new limits on the possible mass of a hypothetical type of dark matter. Another NIST team has proposed that trapped electrons, commonly used to measure the properties of ordinary particles, could also serve as highly sensitive detectors of hypothetical dark matter particles if they carry a charge.

In the study of superconducting detectors, NIST scientists Jeff Chiles and Sae Woo Nam and their collaborators used superconducting tungsten silicide nanowires only one-thousandth the width of a human hair as dark matter detectors.

“Superconductive” refers to a property that certain materials, such as tungsten silicide, have at ultra-low temperatures: no resistance to the passage of electric current. Systems of such wires, officially known as superconducting nanowire single photon detectors (SNSPDs), are extremely sensitive to very small amounts of energy transmitted by photons (particles of light) and possibly matter particles. black when they collide with the detectors.

The researchers operate the SNSPDs at a temperature just below the threshold required for the nanowires to become superconducting. This way, even a tiny amount of energy deposited by an incoming particle will produce enough heat to develop electrical resistance in the wire.

With current flow through the nanowire now obstructed, current travels along a second path connected to an electrical amplifier. The current generates a brief but measurable voltage – a signal that part of the nanowire has heated up by interacting with a photon or, perhaps, a dark matter particle.

The SNSPD experiment consisted of a small square array of nanowires, each 140 nanometers (nm or billionths of a meter) in diameter and spaced 200 nm apart, confined in a light-tight box. The researchers added a stack of two types of insulating material, designed to make it more likely that the system could seek out a type of hypothetical dark matter particle known as a dark photon.

According to theoretical predictions, a dark photon colliding with the battery would be likely to annihilate and generate an ordinary infrared photon in its place. A lens would then focus the photon onto the SNSPD circuit, where it could interact with the nanowires and be detected as a voltage signal.

The small 180-hour experiment found no evidence of dark photons in the low-mass range of 0.7 to 0.8 electron volts/c² (eV/c²⁾, less than half a millionth the mass of the electron, the lightest known stable particle. (Because the masses of subatomic particles are far too small to be conveniently expressed in terms of fractional kilograms, physicists use Einstein’s definition of mass in E=mc² In place.)

Although the experiment needs to be scaled up with many more detectors to provide an expanded data set, it is still the most sensitive dark photon search performed to date in this mass range, a declared Nam. The researchers, including collaborators from Massachusetts Institute of Technology, Stanford University, University of Washington, New York University and the Flatiron Institute, reported their findings in a paper by Physical examination letters.

In a second report, some of the same NIST researchers and their collaborators analyzed the data from the first study in a different way. Scientists ignored the potential effects of the stack of insulating materials and focused only on the ability of any type of dark matter particle to interact with individual electrons in the nanowire detector itself, either by scattering an electron or by being absorbed by it.

Although small, this study placed the strongest limits of any experiment to date – excluding astrophysical research and studies of the sun – on the strength of interactions between electrons and dark matter in the mass range less than one million eV. This makes it likely that a large-scale version of the SNSPD configuration could make a significant contribution to the search for dark matter, Chiles said.

He and his colleagues at the Hebrew University of Jerusalem, the University of California, Santa Cruz, the Santa Cruz Institute for Particle Physics at the University of California; and MIT reported this analysis in an article in the December 8 edition of Physical examination D.

In a third study, a NIST physicist and colleagues proposed that single electrons, electromagnetically confined to a small region of space, could be sensitive detectors of charged dark matter particles. For more than three decades, scientists have used a much heavier population of positively charged beryllium ions to probe the electrical and magnetic properties of ordinary (non-dark) charged particles.

Electrons, however, would make ideal detectors for detecting dark matter particles if those particles have the slightest electrical charge. This is because electrons have the lowest mass of any known charged particle and are therefore easily pushed or pulled by the slightest electrical disturbance, such as a particle with a small electrical charge passing nearby.

Only a few trapped electrons would be needed to detect charged dark matter particles with only one-hundredth the charge of an electron, said NIST physicist Jake Taylor, fellow at the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science. , research partnerships between NIST and the University of Maryland.

Electromagnetically trapped electrons would be cooled to a fraction of a degree above absolute zero to limit the jitter inherent in the particle. Taylor, along with Daniel Carney of Lawrence Berkeley National Laboratory in California, Hartmut Haffner of the University of California, Berkeley, and David C. Moore of Yale University, described their experimental design in a Physical examination letters.

By configuring the trap so that the strength of the electron’s confinement is different along each dimension – length, width and height – the trap could also provide information about the direction from which the dark matter particle arrived.

However, scientists face a technological challenge before they can use electron trapping to search for dark matter. Photons are used to cool, manipulate and detect motion of trapped ions and electrons. For beryllium ions, these photons, generated by a laser, are in the visible light range.

The technology that allows visible light photons to manipulate trapped beryllium ions is well established. In contrast, the photons needed to detect the motion of single electrons have microwave energies, and the necessary detection technology has not yet been perfected. However, if interest in the project is strong enough, scientists could develop an electron trap capable of detecting dark matter in less than five years, Carney estimated.

In another study, a NIST researcher and an international group of colleagues are looking beyond Earth to hunt for dark matter. A team that includes Marianna Safronova of the University of Delaware and the Joint Quantum Institute has proposed that a new generation of atomic clocks, installed on a spacecraft that flies closer to the sun than Mercury’s orbit, could search for signs of ultra-light dark matter.

This hypothetical type of dark matter, bound to a halo surrounding the sun, would cause minute variations in the fundamental constants of nature, including the mass of the electron and the fine structure constant.

Changes in these constants would alter the frequency at which atomic clocks vibrate – the rate at which they “tick”. Among the wide variety of atomic clocks, researchers would carefully choose two that have different sensitivities to changes in fundamental constants driven by ultralight dark matter. By measuring the ratio of the two varying frequencies, scientists could reveal the presence of dark matter, the researchers calculated.

They describe their analysis in an article posted in natural astronomy.

This story is republished with the kind permission of NIST. Read the original story here.