News & Analysis Physics World  January 2018
(Reidar Hahn / Fermilab)

Neutrino hunter

Nigel Lockyer, director of Fermilab in the US, talks to Michael Banks about the future of particle physics – and why neutrinos hold the key

Fermilab is currently building the Deep Underground Neutrino Experiment (DUNE). How are things progressing?

Construction began last year with a ground-breaking ceremony held in July at the Sanford Underground Research Facility, which is home to DUNE. By 2022 the first of four tanks of liquid argon, each 17,000 tonnes, will be in place detecting neutrinos from space. Then in 2026, when all four are installed, Fermilab will begin sending the first beam of neutrinos to DUNE, which is some 1300 km away.

Why neutrinos?

Neutrinos have kept throwing up surprises ever since we began studying them and we expect a lot more in the future. In many ways, the best method to study physics beyond the Standard Model is with neutrinos.

What science do you plan when DUNE comes online?

One fascinating aspect is detecting neutrinos from supernova explosions. Liquid argon is very good at picking up electron neutrinos and we would expect to see a signal if that occurred in our galaxy. We could then study how the explosion results in a neutron star or black hole. That would really be an amazing discovery.

And what about when Fermilab begins firing neutrinos towards DUNE?

One of the main goals is to investigate charge–parity (CP) violation in the lepton sector. We would be looking for the appearance of electron and antielectron neutrinos. If there is a statistical difference then this would be a sign of CP violation and could give us hints as to the reason why there is more matter than antimatter in the universe. Another aspect of the experiment is to search for proton decay.

How will Fermilab help in the effort?

To produce neutrinos, the protons smash into a graphite target that is currently the shape of a pencil. We are aiming to quadruple the proton beam power from 700 kW to 2.5 MW. Yet we can’t use graphite after the accelerator has been upgraded due to the high beam power so we need to have a rigorous R&D effort in materials physics.

What kind of materials are you looking at?

The issue we face is how to dissipate heat better. We are looking at alloys of beryllium to act as a target and potentially rotating it to cool it down better.

What are some of the challenges in building the liquid argon detectors?

So far the largest liquid argon detector is built in the US at Fermilab, which is 170 tonnes. As each full-sized tank at DUNE will be 17,000 tonnes, we face a challenge to scale up the technology. One particular issue is that the electronics are contained within the liquid argon and we need to do some more R&D in this area to make sure they can operate effectively. The other area is with the purity of the liquid argon itself. It is a noble gas and, if pure, an electron can drift forever within it. But if there are any impurities that will limit how well the detector can operate.

How will you go about developing this technology?

The amount of data you get out of liquid argon detectors is enormous, so we need to make sure we have all the technology tried and tested. We are in the process of building two 600 tonne prototype detectors, the first of which will be tested at CERN in June 2018.

The UK recently announced it will contribute £65m towards DUNE, how will that be used?

The UK is helping build components for the detector and contributing with the data-acquisition side. It is also helping to develop the new proton target, and to construct the new linear accelerator that will enable the needed beam power.

Are you worried Brexit might derail such an agreement?

I don’t think so. The agreement is between the UK and US governments and we expect the UK to maintain its support.

Japan is planning a successor to its Super Kamiokande neutrino detector – Hyper Kamiokande – that would carry out similar physics. Is it a collaborator or competitor?

Well, it’s not a collaborator. Like Super Kamiokande, Hyper Kamiokande would be a water-based detector, the technology of which is much more established than liquid argon. However, in the long run liquid argon is a much more powerful detector medium – you can get a lot more information about the neutrino from it. I think we are pursuing the right technology. We also have a longer baseline that would let us look for additional interactions between neutrinos and we will create neutrinos with a range of energies. Additionally, the DUNE detectors will be built a mile underground to shield them from cosmic interference.

In the long run liquid argon is a much more powerful detector medium – you can get a lot more information about the neutrino from it

Regarding the future at the high-energy frontier, does the US support the International Linear Collider (ILC)?

The ILC began as an international project and in recent years Japan has come forward with an interest to host it. We think that Japan now needs to take a lead on the project and give it the go-ahead. Then we can all get around the table and begin negotiations.

And what about plans by China to build its own Higgs factory?

The Chinese government is looking at the proposal carefully and trying to gauge how important it is for the research community in China. Currently, Chinese accelerator scientists are busy with two upcoming projects in the country: a free-electron laser in Shanghai and a synchrotron in Beijing. That will keep them busy for the next five years, but after that this project could really take off.