Stretching the limits of dark matter searches with springy detectors

Title: “The Piezoaxionic Effect”

Authors: Asimina Arvanitaki, Amalia Madden, Ken Van Tilburg


We can’t find the missing five-sixths of the universe called dark matter because it doesn’t collide in detectors — but what if it shakes them? Today’s paper theorizes a new kind of stretchy detector that rapidly shrinks and expands in dark matter’s presence, creating a tiny vibration that can be measured.

Old hypothesis, new effect

Many physicists think dark matter might be made up of axions. These hypothetical particles would simultaneously explain dark matter and solve the “strong CP problem”, another gap in our understanding of particle physics.

Axions are so light that they behave more like a wave than a particle, so most attempts to find them rely on some sort of oscillatory signal they would cause in a detector. Under the right conditions, the omnipresent axion field can cause neutrons to gyrate or create electromagnetic waves, so physicists build experiments that resonate at just the right frequency to pick out these axion-induced oscillations.

Detectors called haloscopes pick up electromagnetic waves of a particular frequency. Looking for axions with a haloscope is like tuning an FM radio and trying to find a particular, very faint song, but without knowing which station it’s on. If physicists can pick out the song from the loud sea of static, the frequency they find it at will tell them the axion’s mass.

Fig. 1: Animation of the piezoelectric effect. Strains in the object create a voltage difference between its two sides. Inversely, applying a voltage across the sides causes the material to stretch or shrink. © User:Tizeff / Wikimedia Commons / CC-BY-SA-3.0

If the axion is too light to resonate a haloscope, a different type of resonator will be needed to find it. Today’s paper finds that as axions pass through certain special materials, they exert a minuscule oscillatory tug on the atoms in the material. The authors coined the term “piezoaxionic effect” for this phenomenon, an analogy to the piezoelectric effect in which EM waves pull and stretch out certain crystals, as shown in Fig. 1. In the same way, they write, axion waves should cause crystals to repeatedly stretch and shrink, like a slinky suspended from one end. Again, the frequency of these oscillatory changes in length depends on the axion’s mass. In most cases, they are too small to notice, but if the axion matches the crystal’s resonant frequency, the vibrations might get amplified enough to detect.

Stretchy detectors

Fig. 2: The strength of axion signal to which various experimental probes are sensitive, as a function of the axion’s mass. We expect axions to appear somewhere in the green band, so experiments aim for sensitivities that dip below it. The blue and red bands represent one larger and one smaller set of the proposed stretchy detectors, respectively. The broader gold band highlights what might be achievable if scientists figure out how to vary the detectors’ resonant frequency to match a given axion mass.

The paper proposes a detector made of these crystals that tries to measure their vibrations. Since every piezoaxionic crystal is also piezoelectric, its stretching and shrinking will create an oscillating electrical voltage between its edges. The authors calculated the size of this voltage signal, and compared it with the noise levels they expect if they use fancy quantum sensors for the measurement. This gave them an estimate of the crystals’ sensitivity to axions.

But a given detector is only really sensitive if the axion mass is near its resonant frequency, so it would take lots of detectors to test for a range of masses. Fig. 2 shows the sensitivity of a proposed experiment, which would operate for ten years using two sets of detectors, one with arrays of millimeter-scale crystals (red), and one with arrays of centimeter-scale crystals (blue). The green area shows where we expect axions to live within this two-dimensional space. To discover axions, you need an experiment whose sensitivity dips fully below the green band at their actual mass (which, remember, we don’t know). The gold curve is meant to highlight the technology’s future potential if physicists can figure out a way to tune the resonant frequency of a crystal detector, like tuning a haloscope.

Are axions having a moment?

Axions have been receiving heightened attention recently, since giant detectors buried underground have failed to prove the other leading dark matter theory, that of the weakly interacting massive particle. As that long-favored hypothesis becomes tenuous, many physicists are looking in new directions, and the axion is the readiest alternative.

The vibrational detectors would also probe a different range of axion masses than existing experimental efforts like haloscopes. Since the axion mass is such a huge unknown, a number of different technologies will be required to cover the full range of possibilities. Especially if the tuning of a detector’s resonant frequency becomes possible, this might become a critical new tool for dark matter hunters trying to excavate this parameter space. Perhaps in the coming years, it will be the buzz of one of these vibrating detectors that finally alerts us to dark matter’s true nature.

Read More

“Axion dark matter: What is it and why now?” – Review of axions in Science Advances

“New Results from HAYSTAC’s Phase II Operation with a Squeezed State Receiver” – Recent preprint from a leading haloscope experiment

“Experimental Searches for the Axion and Axion-Like Particles” – Review of experiments trying to discover axions

A hint of CEvNS heaven at a nuclear reactor

Title : “Suggestive evidence for Coherent Elastic Neutrino-Nucleus Scattering from reactor antineutrinos”

Authors : J. Colaresi et al.

Link :

Neutrinos are the ghosts of particle physics, passing right through matter as if it isn’t there. Their head-on collisions with atoms are so rare that it takes a many-ton detector to see them. Far more often though, a neutrino gives a tiny push to an atom’s nucleus, like a golf ball glancing off a bowling ball. Even a small detector can catch these frequent scrapes, but only if it can pick up the bowling ball’s tiny budge. Today’s paper may mark the opening of a new window into these events, called “coherent neutrino-nucleus scattering” or CEvNS (pronounced “sevens”), which can teach us about neutrinos, their astrophysical origins, and the even more elusive dark matter.

A scrape with a ghost in a sea of noise

CEvNS was first measured in 2017 by COHERENT at a neutron beam facility, but much more data is needed to fully understand it. Nuclear reactors produce far more neutrinos than other sources, but they are even less energetic and thus harder to detect. To find these abundant but evasive events, the authors used a detector called “NCC-1701” that can count the electrons knocked off a germanium atom when a neutrino from the reactor collides with its nucleus.

Unfortunately, a nuclear reactor produces lots of neutrons as well, which glance off atoms just like neutrinos, and the detector was further swamped with electronic noise due to its hot, buzzing surroundings. To pick out CEvNS from this mess, the researchers found creative ways to reduce these effects: shielding the detector from as many neutrons as possible, cooling its vicinity, and controlling for environmental variables.

An intriguing bump with a promising future

After all this work, a clear bump was visible in the data when the reactor was running, and disappeared when it was turned off. You can see this difference in the top and bottom of Fig. 1, which shows the number of events observed after subtracting the backgrounds, as a function of the energy they deposited (number of electrons released from germanium atoms).

Fig. 1: The number of events observed minus the expected background, as a function of the energy the events deposited. In the top panel, when the nuclear reactor was running, a clear bump is visible at low energy. The bump is moderately to very strongly suggestive of CEvNS, depending on which germanium model is used (solid vs. dashed line). When the reactor’s operation was interrupted (bottom), the bump disappeared – an encouraging sign.

But measuring CEvNS is such a new enterprise that it isn’t clear exactly what to look for – the number of electrons a neutrino tends to knock off a germanium atom is still uncertain. This can be seen in the top of Fig. 1, where the model used for this number changes the amount of CEvNS expected (solid vs dashed line).

Still, for a range of these models, statistical tests “moderately” to “very strongly” confirmed CEvNS as the likely explanation of the excess events. When more data accumulates and the bump becomes clearer, NCC-1701 can determine which model is correct. CEvNS may then become the easiest way to measure neutrinos, since detectors only need to be a couple feet in size.

Understanding CEvNS is also critical for finding dark matter. With dark matter detectors coming up empty, it now seems that dark matter hits atoms even less often than neutrinos, making CEvNS an important background for dark matter hunters. If experiments like NCC-1701 can determine CEvNS models, then dark matter searches can stop worrying about this rain of neutrinos from the sky and instead start looking for them. These “astrophysical” neutrinos are cosmic messengers carrying information about their origins, from our sun’s core to supernovae.

This suggestive bump in the data of a tiny detector near the roiling furnace of a nuclear reactor shows just how far neutrino physics has come – the sneakiest ghosts in the Standard Model can now be captured with a germanium crystal that could fit in your palm. Who knows what this new window will reveal?

Read More

Ever-Elusive Neutrinos Spotted Bouncing Off Nuclei for the First Time” – Scientific American article from the first COHERENT detection in 2017

Hitting the neutrino floor” – Symmetry Magazine article on the importance of CEvNS to dark matter searches

Local nuclear reactor helps scientists catch and study neutrinos” – story about these results