One of the great scientific searches of our time is the hunt for dark matter. Physicists believe this stuff fills the universe and think they can see evidence of it in the way galaxies rotate. Indeed, galaxies spin so quickly that they ought to fly apart unless some hidden mass is generating enough gravitational force to hold them together.
That evidence has set physicists scrabbling to find dark matter on Earth. They’ve constructed dozens of observatories, most of them in underground caverns deep beneath the surface, where background noise is low. At stake is scientific fame and fortune, with the group that finds dark matter likely to be richly rewarded.
But so far physicists have found precisely nothing. If it is out there, dark matter is very well hidden. Or physicists have been looking in the wrong place. One possibility is that dark matter particles are too small for current experiments to see. So physicists desperately want better, more sensitive ways to detect these things.
Enter Yonit Hochberg at Hebrew University of Jerusalem in Israel and a few colleagues, who have developed a promising new sensor based on tiny superconducting wires. The team’s prototype already shows the potential of this approach.
The principle behind the new device is straightforward. Cool certain metals below a critical temperature and they conduct with no resistance. But as soon as their temperature rises above this threshold, the superconducting behavior disappears.
Physicists know that dark matter particles cannot interact strongly with visible matter; otherwise they would have already seen them. But dark matter particles can collide head-on with ordinary particles.
These collisions are rare because ordinary matter is mostly empty space, so dark matter particles can pass straight through. But when they do collide with an atomic nucleus or electron in a lattice, for example, the collision causes the lattice to vibrate, thereby raising its temperature.
It is this rise in temperature that superconducting nanowires are good at revealing. The heating causes a small portion of the wire to stop superconducting, and this in turn creates a voltage pulse that is easy to measure. What’s more, such a device produces few, if any, false positives.
Hochberg and co have put their idea through its paces by building a prototype. This device consists of set of tungsten silicide nanowires just 140 nanometers wide (a human hair is about 100,000 nanometers wide) and 400 micrometers long. The entire apparatus sits just a few millidegrees above absolute zero, so that the tungsten silicide wires become superconductors.
The team then looked for the voltage pulses that might reveal a dark matter collision. With appropriate shielding in place, they found no pulses during the 10,000-second duration of their measurements.
That places important constraints on the type of dark matter that could be present and its density. It also places constraints on other types of particles that physicists speculate might exist.
One of these is the “dark photon”—essentially the dark matter equivalent of the ordinary photon. If they exist, then the new sensor did not detect a single one. “The results from this device already place meaningful bounds on dark matter-electron interactions, including the strongest terrestrial bounds on sub-eV dark photon absorption to date,” say Hochberg and co.
That’s impressive work, given that the mass of the nanowires is just a few nanograms. The next stage is to fabricate them on a larger scale. Hochberg and co say that the technology is relatively mature, so this should be possible on a short time scale. Indeed, they estimate that an academic lab could churn out a thousand 200-nanometer detectors with a total mass of 1.3 grams in just a year. “An industrial effort could realize many times that number,” they point out.
So a kilogram-scale detector could be feasible in the not too distant future. Such a machine would rival those already in operation in the search for dark matter, but it would look at different energies in a different way.
So it may be that one day, superconducting nanowires will discover dark matter—if it exists at all.
Ref: arxiv.org/abs/1903.05101 : Detecting Dark Matter with Superconducting Nanowires
