Liquid Argon TPCs for neutrino physics

Event display from ArgoNeuT

Introduction

Water Cherenkov neutrino detectors are a proven technology capable of being scaled to very large sizes.  However, because of their comparatively high threshold, they do not show details of the interaction, particularly any low-momentum recoil protons or soft pions.  They are also not good at reconstructing high-multiplicity events, as the multiple overlapping rings are difficult to sort out.  Liquid argon TPCs are a new type of neutrino detector which aims to address these issues, providing high-resolution 3D track reconstruction which can in principle provide unprecedented detail in neutrino event reconstruction.

How it works

As a charged particle passes through a medium, it will create ionisation.  In a TPC, an electric field causes the produced electrons to drift to anode wires at the end of the vessel.  The drift time and the position of the hit wire are combined to provide a 3D reconstruction of the event, as shown in the animation.

Animation showing the operation of a liquid argon TPC

Gas TPCs were developed by Dave Nygren from an idea by the late Georges Charpak, and the first large-scale implementation was Nygren's PEP-4 detector at SLAC, which operated from 1980.  The applicability of the technique to noble liquids, specifically liquid argon (LAr), was first proposed as early as 1977, by Carlo Rubbia, but the first large-scale implementation of the idea was the ICARUS experiment in 2010.  Liquid argon TPCs are ideal for neutrino experiments because they provide a relatively large target mass (unlike gas TPCs, which by their nature have relatively low densities). 

Advantages and Disadvantages

The advantage of LAr TPCs is of course their high precision and low energy threshold, which together should allow highly detailed reconstruction of neutrino events.  The principal disadvantage is that they cannot be made anywhere near as large as water Cherenkov detectors: the DUNE far detector will be only about one-tenth the mass of Hyper-Kamiokande, when both are complete.  To some extent, the higher precision can be traded off against a smaller sample—for example, it should be possible to select cleaner samples of particular event topologies, and if you have much lower background you do not need as large a signal to achieve the same significance.  There are also analyses, particularly those involving detailed reconstruction of complex multi-particle final states, which simply cannot be done in water Cherenkovs.  But there are some situations in which there is no substitute for high statistics, such as trying to track fast oscillations in the neutrino signal from a Galactic supernova.

In fact, LAr and water Cherenkov technologies are highly complementary.  One gives a higher target mass, the other greater precision.  At low energies, for solar and supernova neutrinos, water Cherenkovs see mostly electron antineutrinos (from inverse beta decay, ν̅e + p → e+ + n) whereas LAr detectors see mostly electron neutrinos (from quasi-elastic scattering, νe + Ar → e + K).  The detector systematic errors on oscillation measurements will also be different.  This is why Sheffield has chosen to be involved in programmes based on both technologies.