Particle Physics and Particle Astrophysics Research

DUNE: the Deep Underground Neutrino Experiment at Fermilab

Introduction

The current generation of neutrino oscillation experiments, including T2K, has made great progress in understanding the physics of neutrino oscillations. However, a new generation of neutrino experiments is still required to understand CP violation in the neutrino sector – a possible explanation of the matter-antimatter asymmetry of the universe – and to address questions such as the mass hierarchy and the possibility of sterile neutrinos. The two main future long-baseline neutrino experiments are Hyper-Kamiokande in Japan and DUNE in the USA. These are complementary designs, Hyper-K opting for an extremely large water Cherenkov detector with a very strong non-accelerator research programme in addition to the long-baseline experiment, while DUNE has chosen to build a ground-breaking liquid argon far detector, much smaller than Hyper-K but with prospects for much higher precision in event reconstruction. For several years Sheffield has been pioneering development of LAr in the UK, focusing on new ideas to make more feasible a massive LAr detector for neutrinos with exceptional particle identification sensitivity.

Liquid Argon development

Recently there has been exceptional progress with our LAr programme including publication of a world-leading breakthrough in readout technology [1]. Recent highlight R&D at Sheffield is as follows:

New 10 kg LAr test rig built and fully operational

10 kg LAr test detector

A complete 10 kg LAr test detector with purification and DAQ has been designed, built and routinely operated at Sheffield for analysis of optical and charge readout device performance, to assess LAr purity, measure pulse shape discrimination, optimize light collection and wavelength shifter techniques and evaluate PMTs.

New LAr purification chemistry for multi-tonne use developed

We have successfully developed a new concept of molecular sieves, anhydrous complexes, and oxygen scavengers to remove electronegative species in LAr (which limit charge transfer) and UV quenchers (which attenuate scintillation light). The effects on LAr performance parameters have been extensively assessed. The result is a new reduced-cost technique with improved purity levels - a key objective for multi-tonne large volume detectors. A test of this in ArDM at CERN is now being studied.

10 kg LAr test detector Scintillation from test detector

New LAr recirculation concept

We have also developed, with Technodyne Ltd. (a world-leading UK company in the LPG industry), a new cryogenic LAr recirculation pump design with no moving parts, reduced wear and contamination. This is also vital R&D required to reduce purification costs in a large-scale device.

New LAr wavelength shifter technique

A new methodical study of the chemistry of wavelength shifter production and application on reflective surfaces has successfully produced improved light collection technology for LAr using TPB [2]. This has been adopted in ArDM dark matter experiment.

Wavelength shifter Wavelength shifter

New pulse shape discrimination (PSD) data for LAr

The new wavelength shifter technique allowed us to confirm the known excellent dE/dx particle ID of LAr, study new ways to make use of this in purification studies.

Wavelength shifter scintillation plot Wavelength shifter discrimination plot

New cryogenic PMTs built (PIPSS)

A PIPSS with Electron Tubes Ltd., on cryogenic PMTs for LAr and other noble gases was completed in 2008. This resulted in successful development of a new PMT with integral metal vacuum flange. The PMT window can be exposed directly to cryogenic liquid (for improved light collection) but with dynode chain outside at RTP.

World-first characterisation of new silicon photomultipliers (SiPMs) with LAr

SiPMs provide a potential better route to efficient light collection via in-liquid photon-readout. Via strong links with partners SensL, Hamamatsu and Warwick, we have extensively evaluated new pixel SiPMs at LAr temperature and in LAr with the Sheffield detector. We have produced and published world-first data that successfully demonstrates operation with robust gain, achieving thresholds of 1-3 keV [3].

SiPM board TGEM SiPM scintillation plot

First demonstration of charge gain using thick gas electron multipliers (TGEMs) in GAr

This has been our greatest R&D achievement (publised in [1]), acknowledged as a pioneering result. It opens prospect for achieving higher gain a LAr without the need for costly dual-phase operation. Although charge gain within LAr is unrealistic, we have found that (unlike in LXe) secondary scintillation light (and hence gain) can be produced in a stable way within the holes of our TGEMs. We observed this using our SiPM devices mounted directly on the TGEMs, for instance recording a 55Fe spectrum from the light. Although more work is needed this result does look to be a key step in development of large volume modules for use in multi-tonne targets at lower cost.

Secondary scintillation rig Secondary scintillation spectrum Secondary scintillation vs drift field

LAr particle physics and simulations

Finally, a LAr simulation has been developed using a combination of the Geant 4 toolkit and the GENIE event generator to model neutrino interactions in a large liquid argon TPC. Fully simulated event tracking has been applied including finite readout resolution and diffusion of drifted electrons. Events have been simulated in an electric, and with and without a magnetic field. Based on the simulated events created by this model, an event viewer with track reconstruction has been developed which can equally be used for experimental data.

Reconstructed Simulated Event from a 15 GeV νμ interaction

FJNE 15 GeV ν<sub>μ</sub> Simulated Event

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The FJNE project, 3 ton test detector at CERN, 1 kon at JPARC

Work has started for the new FJNE programme at Sheffield in the following areas:

Light tracking and purification hardware for the 3 tom LAr detector being constructed at CERN

Despite the known tracking potential of LAr there is a recognised lack of detailed beam data and associated track reconstruction know-how to validate the technique. Beam tests are planned to resolve this using a basic 3 ton module now being constructed at CERN (figs below thanks to collaboration with ETHZ).

Tracking and purification hardware model Tracking and purification hardware photo

We plan to design, construct and operate the light tracking and LAr purification hardware for this detector. This is well matched to our key work for the 1 ton ArDM detector at CERN. We will design and build the PMT array and DAQ; develop the wavelength shifter planes and contribute hardware and chemistry expertise to the purification system, making use of our close relationship with ETHZ.

Large volume LAr particle tracking simulations and analysis of 3 ton detector beam data

The test beam work above is vital to validating and training the automated track reconstruction software we are developing for charged particle ID and for our detector simulations (including of the electronics). CERN will provide the necessary controlled fluxes of GeV protons, electrons and pions. Here we will aim, in particular, to examine electron interactions, the signature (νe appearance) for CP violation.

Readout R&D for a Kton-scale experiment

The simulation and 3 ton beam data physics work above has a vital underlying aim to guide generic scale-up design issues such as choice of spatial resolution. However, the challenge remains to demonstrate a practical implementation that is cost effective and scalable to the kton level. Designs for this have already started as below.

3kton detector model

Our pioneering work with a single SiPM showing that gain can be achieved in single phase LAr via electroluminescence readout of light with TGEMs, thereby avoiding the need for complex two-phase operation such as in GLACIER, puts us in an excellent position to make a major contribution here. Our aim now with Warwick and other partners, based on experience constructing our 10 kg detector, will be to build and operate at Sheffield a 100 kg single phase LAr demonstrator module and show that full 3D tracking can be achieved with sufficient sensitivity in single-phase.

Model of cryogenics system

References

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