Posted on 02/08/2017
Dark matter, making up more than 25% of the matter in the Universe is still well unknown and mysterious; an important step towards the detection of these mysterious particles was made last week, when the DEAP-3600 experiment presented its first result. Researchers from the Dark Matter group of Royal Holloway are part of an international collaboration among 10 institutions from Canada, Mexico and the UK, which has developed the DEAP–3600 detector.
3600 kg of liquid argon, installed 2 km underground, aiming to solve one of the biggest scientific mysteries and detect the most favourable candidates for dark matter: Weakly-Interacting Massive Particles (otherwise known as WIMPs). The experiment is designed to detect the very small amount of light that would be produced if a WIMP was to interact with a liquid argon atom in the detector.
This may sound simple, but observing a WIMP interaction is rather challenging. As their name suggests, WIMPs are weakly interacting, meaning that we expect them to interact in a detector extremely rarely. There are two main ways experiments can give themselves the best chance of observing a WIMP interaction: the experiment should have a large enough target mass as possible, increasing the number of atoms and hence enhancing the probability of an interaction, and the experiment should have as little background as possible, which could potentially ‘mimic’ a WIMP signal. To address this second point, DEAP-3600 is installed deep underground; this way it is shielded it from background events, originating from cosmic rays travelling towards Earth from space.
On top of its underground shield, the use of liquid argon as the target mass provides extremely good discrimination between the various interactions. WIMPs are expected to scatter off the nucleus of a liquid argon atom, causing a nuclear recoil that results in the production of scintillation light. Beta or gamma particles however, two of the main sources of background events present in DEAP-3600, will scatter off of the electrons of the atom rather than the nucleus, causing an electronic recoil. Thanks to the use of liquid argon as the target mass, the detector can very precisely distinguish between these two types of interaction; a very powerful tool for WIMP searches.
The DEAP-3600 experiment has been under construction for the last 6 years, and exploits new particle detector technologies to conduct its highly sensitive dark matter searches. As mentioned, the experiment uses liquid argon as the target mass, which is held in a spherical acrylic vessel (AV) capable of storing a maximum target mass of 3600kg. The AV is viewed by 255 photomultiplier tubes, which measure the scintillation light arising from interactions in the liquid argon target mass. The AV is housed in a steel shell, which is then submerged in an 8-metre diameter water tank, all held in a cavity 2km underground at SNOLAB, in Canada.
“On the 26th July 2017, the DEAP-3600 experiment presented its first scientific result of its dark matter search at the TAUP Conference held at SNOLAB, Canada” says Ashlea Kemp, PhD student at Royal Holloway and member of the dark matter group. “DEAP-3600 is special, as it is the first experiment of its kind”.
This first result was based on 4.44 days of data, with a nearly full detector (3322kg of liquid argon). “The result demonstrates a very stable detector performance, the best electronic recoil rejection in argon, and most importantly, the leading limit on WIMP-nucleon spin-independent cross section measured in argon, for a WIMP mass of 100 GeV” Ashlea concludes, letting us know that DEAP-3600 continues to take data DEAP-3600 continues to take data, and with a full dataset aims to reach an order of magnitude better sensitivity than the current leading limits.
Find out more about our work in direct detection of dark matter here