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Neutrinos, elementary particles with nearly no mass and no electrical charge, are notoriously elusive. They silently traverse through most matter without leaving a trace, rendering them virtually undetectable. This trait compelled Wolfgang Pauli, the physicist who postulated their existence in 1930, to fear that these particles might never be detected. However, in 1956, this seemingly intangible particle was finally confirmed, and the subsequent years even revealed a second variety – the muon neutrino.
Despite their elusive nature, neutrinos aren’t entirely invisible. On occasion, these unassuming particles engage with ordinary matter, albeit through a notably weak nuclear force. The weak force is aptly named as a neutrino could journey through a lightyear of lead before encountering an interaction. But with vast amounts of neutrinos and sufficiently large detectors, these rare interactions can indeed be observed.
The neutrino’s interaction with regular matter is the key to its detection. During these scarce occurrences, neutrinos can transform into more familiar particles, such as electrons, muons, and taus. They also possess the ability to dislodge protons and neutrons from atomic nuclei and produce charged particles. These charged entities shed electrons from atoms while moving, creating visible paths. A magnetic field can then differentiate the particles by direction based on their charge.
Investigating these elusive neutrinos is akin to a detective meticulously solving a mystery. The task involves identifying the particle type, its energy, and direction based on the visible trails left in the detector. It’s as if deducing the type, speed, and direction of cars from the intermingling tire tracks in the mud. With the acquired knowledge of the particles resulting from the interaction, physicists can estimate the original neutrino’s type and energy.
However, the reconstruction of the neutrino interaction is not always flawless, and crucial details can be overlooked. The detection process might miss some particles, such as neutrons. While not as elusive as neutrinos, neutrons don’t provide a detailed trajectory, making them difficult to track. Nevertheless, using conservation laws of energy and momentum, physicists can identify when a particle goes undetected, indirectly leading to the detection of the elusive neutrinos.
Contributing to these intricate studies of neutrinos is Professor Andrew Furmanski, an Assistant Professor at the University of Minnesota. Having gained his education and early research experience in the UK, Furmanski shifted his focus to the US, spending considerable time at Fermilab before relocating to Minnesota.
Primarily, Furmanski employs detectors using liquid argon to observe neutrinos, focusing on the unique behavior of neutrinos known as ‘neutrino oscillations’—the phenomenon where neutrinos change their type or ‘flavour’ as they travel. A key challenge in these studies involves discerning neutrino interactions with large atomic nuclei such as argon due to their infrequency. Invisible particles produced during these interactions can hamper the understanding of neutrino behavior. However, Furmanski and his team are pioneering methods to estimate this ‘missing energy’, enhancing our ability to infer the presence of additional undetected particles.