I always tell my students that LIGO (laser interferometer gravitational wave observatory) is one of humanity’s most stunning achievements. Basically, a group of people set out to measure the unmeasurable. They methodically overcame every source of noise that swamped the signal until gravitational waves were found. Along the way, LIGO scientists have created the most sensitive instrument in existence.

And once you’ve got a nice sensor, everyone wants to use it for their experiments. The latest bunch in the queue are the particle physicists, who think that the exquisite sensitivity of LIGO might make its equipment a good detector for particles.

LIGO knows when you rock out

Before we get to how to turn LIGO into a particle physics detector, let’s quickly look at how LIGO works.

LIGO is a giant interferometer: a laser beam is split in two by a partially reflective mirror. The two light beams are sent to distant mirrors. The light beams reflect off the mirrors and return from whence they came. At the partially reflective mirror, the two light beams are recombined. Light is a wave, which means that it has an amplitude that oscillates between a positive and negative value. When the two waves are recombined, the outcome depends on their amplitudes at the moment they are recombined (this is called interference—hence "interferometry").

If the two waves combine when they are in phase, then the waves always have the same amplitude value at the partially reflective mirror. The light wave that exits the interferometer in this case will be bright. We call this "constructive interference." But if the two waves combine so that they are anti-phased, then the amplitudes of both waves are equal but opposite in sign. In this case, the result will be darkness: no light exits the interferometer.

The factor that determines how the waves add together is the difference in the distance they travelled. If the interferometer is set up so that no light exits the interferometer under normal conditions, a small movement of one mirror will result in a sudden light in the darkness.

This allows LIGO to be sensitive to mirror displacements that are on the order of 10-19 meters (for comparison, the diameter of a proton is 10-15 meters). Even small test interferometers that were built to test LIGO technologies have similar displacement sensitivities.

Turning LIGO into a cheap and cheerful ATLAS

So interferometers are spectacular displacement sensors. How do we turn that into a particle detector?

The basic idea is very simple. A beam of high energy particles is shot into the back of one of the mirrors. The force imparted by the particles colliding with the mirror material displaces the mirror, providing a signal. In fact, what you really want is a particle beam that switches on and off at about a thousand cycles per second, or 1kHz. The result is that the mirror starts to vibrate at 1kHz, with the amplitude of the kHz proportional to the energy imparted by the particle collisions going on in the mirror.

The researchers show that a small and comparatively cheap interferometer should have useful sensitivity to particle collisions. A proof-of-principle experiment should not be out of reach.

The big question is what we get out of this that we do not get out of existing particle physics experiments, and that's where things gets a bit vague. One of the more concrete suggestions is that these measurements provide more accurate stopping power measurements—an important parameter when choosing radiation therapy doses for cancer treatment.

Beyond that, the main idea seems to be that this provides a new window on particle physics. In particular, there are some types of scattering that are hard to detect in traditional particle detectors. The paper isn’t really explicit about this, but I think they mean that they have a more direct view on what are called "missing mass calculations."

We basically figure out what's happened during some collisions via the absence of energy—if we know the energy of the collision, and account for all the energy and particles that come out of it, then any missing energy suggests that the collision also produced some particles we didn't detect. However, by directly detecting changes in the mirror's behavior, we measure the energy of the collisions directly. Maybe we don’t need to work so hard to look for missing energy if we get a more direct picture of these collisions.

There may yet be a number of baby-LIGOs installed in small particle accelerators around the world. Their measurements will feed into calculations for the big colliders and help guide particle searches.

This is why everyone should have at least one interferometer.

European Physics Letters, 2018, DOI: 10.1209/0295-5075/123/41001 (About DOIs)