## 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.

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

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.

“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 world with no weak forces

Gravity, electromagnetism, strong, and weak — these are the beating hearts of the universe, the four fundamental forces. But do we really need the last one for us to exist?

Harnik, Kribs and Perez went about building a world without weak interactions and showed that, indeed, life as we know it could emerge there. This was a counter-proof by example to a famous anthropic argument by Agrawal, Barr, Donoghue and Seckel for the puzzling tininess of the weak scale, i.e. the electroweak hierarchy problem.

Let’s ask first: would the Sun be there in a weakless universe? Sunshine is the product of proton fusion, and that’s the strong force. However, the reaction chain is ignited by the weak force!

So would no stars shine in a weakless world? Amazingly, there’s another route to trigger stellar burning: deuteron-proton fusion via the strong force! In our world, gas clouds collapsing into stars do not take this option because deuterons are very rare, with protons outnumbering them by 50,000. But we need not carry this, er, weakness into our gedanken universe. We can tune the baryon-to-photon ratio — whose origin is unknown — so that we end up with roughly as many deuterons as protons from the primordial synthesis of nuclei. Harnik et al. go on to show that, as in our universe, elements up to iron can be cooked in weakless stars, that they live for billions of years, and may explode in supernovae that disperse heavy elements into the interstellar medium.

A “weakless” universe is arranged by elevating the electroweak scale or the Higgs vacuum expectation value ($\approx$ 246 GeV) to, say, the Planck scale ($\approx 10^{19}$ GeV). To get the desired nucleosynthesis, care must be taken to keep the u, d, s quarks and the electron at their usual mass by tuning the Yukawa couplings, which are technically natural.

And let’s not forget dark matter. To make stars, one needs galaxy-like structures. And to make those, density perturbations must be gravitationally condensed by a large population of matter. In the weakless world of Harnik et al., hyperons make up some of the dark matter, but you would also need much other dark stuff such as your favourite non-WIMP.

If you believe in the string landscape, a weakless world isn’t just a hypothetical. Someone somewhere might be speculating about a habitable universe with a fourth fundamental force, explaining to their bemused colleagues: “It’s kinda like the strong force, only weak…”

Bibliography

Viable range of the mass scale of the standard model
V. Agrawal, S. M. Barr, J. F. Donoghue, D. Seckel, Phys.Rev.D 57 (1998) 5480-5492.

A Universe without weak interactions
R. Harnik, G. D. Kribs, G. Perez, Phys.Rev.D 74 (2006) 035006