DarkSide-20k as a neutrino observatory

Author: Lucy Kotsiopoulou

Beyond the Dark matter search

The goal of DarkSide-20k is ultimately to try and detect particle-like dark matter. But what if it could also detect neutrinos? Neutrinos are the lightest particles with the smallest mass of all the elementary particles within the Standard Model, so light that they were believed to be massless until about twenty years ago. They come in three flavours, the electron neutrino νe, muon neutrino νμ and tau neutrino ντ, and only interact via the Weak force and Gravity. In fact, they interact so weakly with matter that every second 100 trillion neutrinos pass through your body. Nearly twenty years ago, it was experimentally observed that they oscillate between their flavours and are in actuality massive. They are very tiny indeed… but not as tiny as originally thought! As it happens, there is a high probability that they all will be released from a very special galactic source: a supernova explosion. Looking further back, nearly thirty years ago the most recent near to Earth supernova, SN 1987A in the Large Magellanic Cloud, exploded, providing the only occasion on which supernova neutrinos have been detected on Earth. After travelling for 168,000 years, and sweeping through Earth in around 13 seconds, a total of 25 supernova neutrino events were detected by three neutrino detectors at the time: IMB, Baksan and Kamiokande II. Now, in preparation for the next supernova explosion, specifically a Type II core-collapse one, as SN 1987A, the only thing one can do is wait. Or maybe not?!

The visible light when there is a supernova explosion in the sky can be as bright as the moon and visible even in daylight. Nonetheless, this light represents less than 0.01% of the total energy released from the star’s final moments. 1% becomes the kinetic energy of the debris of the star being plunged outwards, while 99% is neutrinos of all flavours launching into outer space. Astronomers gather plentiful information about the explosion by detecting the light through telescopes, and have been doing so for centuries. However, this light can last from only a few seconds up to a few days, and often it is not in the visible spectrum, leaving astronomers unprepared as to where to turn their telescopes. Neutrinos however are different; as they only interact very weakly with the matter within the star just before it explodes, they can escape before the photons of light and reach Earth earlier, alerting us to incoming photons. Therefore neutrino and dark matter detectors can form a crucial role in predicting the next supernova, mainly within the joint effort of the Supernova Neutrino Early Warning System (SNEWS). Many neutrino experiments of all kinds and dark matter experiments are part of this programme and are poised for action. DarkSide-20k is now working on becoming its newest member!

The impact of DarkSide-20k within SNEWS

Supernova neutrinos have an energy of about 15 MeV, which for a neutrino is considered low. As DarkSide-20k is a dual-phase TPC liquid argon detector, the detection method will be based on interactions between the incoming neutrino and the argon nuclei floating within the confines of the detector. There are two ways in which a supernova neutrino of the above energies could interact: either by scattering off the entire nucleus or by actually breaking it apart. The first process is called Coherent Elastic neutrino (ν) – Nucleus Scattering (CEνNS) and the signal that will be detected by DarkSide-20k’s TPC is the small recoil energy from the scattered argon nucleus, thanks to the amplified signal in the gas pocket at the top of the TPC, capable of detecting recoils as low as 0.5 keV. For a more technical description, our published paper can be found at the end of this article as well as on our Publications page. This paper focuses only on the CEνNS interactions, where our experiment can detect all three types of neutrinos. Thanks to CEνNS only,DarkSide-20k will be able to discover a supernova explosion as far out as the edge of the Milky Way galaxy for a star eleven times the size of our Sun or bigger. The other process however, in which the nucleus is broken apart by the neutrino, at this energies can only occur with electron neutrinos. This reaction, called the Charged Current Interaction (CCI) or electron neutrino absorption in argon, releases an electron and an excited potassium nucleus. By combining the information from both interactions, one can compare the total flux of neutrinos reaching Earth with only the electron neutrinos, yielding possible answers to how the neutrinos oscillated within the supernova before reaching Earth.

Thanks to such a high-statistics, multi-channel, multi-messenger observation from the next event we will be able to constrain the details of the last moments of a star’s life, while also providing an independent inference of the nature of neutrinos and their mass, yielding answers to the neutrino mass hierarchy problem: examining order of their masses, and which are the lightest.

Sources:

The DarkSide-20k Collaboration, “Sensitivity of future liquid argon dark matter search experiments to core-collapse supernova neutrinos” (2020), https://arxiv.org/pdf/2011.07819
Horiuchi S, Kneller JP, “What can be learned from a future supernova neutrino detection?” (2018), https://arxiv.org/pdf/1709.01515.
Phillips, AC, “The Physics of Stars”, 2nd Edition (1999).