When we have observed distant galaxies we noticed something peculiar about the way they move. Instead of moving like we’d expect them to, with things further away from the centre moving slower than those closer, everything past a certain point moves at about the same speed. This is contrary to how other, non-galaxy sized systems like our solar system move and so we’ve long looked for an explanation as to how this could occur. The commonly accepted theory is that there’s extra matter present throughout the universe that we can’t see but interacts through gravity, called dark matter. Direct detection of dark matter has so far eluded us however due to its incredibly elusive nature. However there’s one experiment, called XENON1T, that could potentially shine some light on this elusive substance.
XENON1T is an evolution of previous experiments XENON10 and XENON100 which all shared the same goal: direct detection of a dark matter particle. Even though dark matter is theorized to be abundant everywhere in the universe (millions of dark matter particles would be passing through you every second) since they rarely interact detection is incredibly difficult. However just like the neutrinos before them there are ways of making detection possible and that’s what the XENON series of experiments aimed to do. At the heart of the experiment is a cylinder of liquid xenon, a chilly -95°C, bounded at each end by an array of photomultiplier tubes which are essentially extremely sensitive cameras. The thinking goes that, should a dark matter particle interact with the liquid xenon, a flash of light will occur which can then be analysed to see if it was a dark matter interaction.
The process by which this is determined is pretty interesting relying on 2 different types of interactions in order to determine what kind of particle interacted with the liquid xenon. The first signal, dubbed S1, is the flash of light that occurs at a very specific frequency of 178nm (ultraviolet). The photomultipliers are sensitive enough to be able to detect single photon events so even the smallest interaction will be captured. This photon is then brought upwards through the liquid xenon by an electric field that’s present in the gaseous section of the XENON1T device and when it leaves the liquid xenon it rockets upwards. The photon then leaves a trail of ionization in the gas behind it, dubbed the S2 signal, which allows for the exact position of the interaction to be determined. This is critical as the researchers only want events from the centre of the device as that has a greatly reduced rate of background noise.
The methodology was thoroughly validated by the two previous experiments even though both of them failed to directly detect any dark matter. They did put bounds on the properties of dark matter however, notably their size and potential electron spin. XENON1T is going to be around 100 times more sensitive than its previous brethren so even if it fails to see anything it will put even more stringent constraints on how a dark matter particle could be constructed. This is critical in validating or eliminating certain particle physics models and could give credence to other non-standard physics models like Modified Newtonian Dynamics (MOND).
Research such as this is incredibly important in developing an accurate understanding of how our universe operates. No matter the outcome of the experiment we’ll learn something valuable and it could potentially drive research towards a whole new world of physics. It won’t change our daily lives but ensuring our understanding of the world is as close to its knowable truth is the heart of science and that’s why this research is so important.