“Physics is in a unique situation, like never before,” remarks CERN scientist Marzio Nessi. “For sure there is a new physics somewhere, but it will either take more work or more statistics for it to be seen or else it is not appearing at the energies we can currently reach [at CERN’s giant particle accelerator, the LHC].” Nessi refers here to the fact that the landmark discovery of the Higgs boson did not usher in new experimental results able to answer the big open questions. He predicts that one of the next major stories will revolve around neutrinos, nature’s “ghost particles” as they have come to be known, which, he believes “will open a window onto the new physics.” The ProtoDUNE detectors his team built at CERN are the prototype of what will be the largest ever neutrino detector (DUNE). Nessi talked about the process and its implications at his 8 October lecture in the BBVA Foundation, Madrid, as part of the CERN-BBVA Foundation particle physics series.

Marzio Nessi is Project Leader of the CERN Neutrino Platform, the organization’s main contribution to the global neutrino research effort. Previously a head of the LHC’s ATLAS experiment which played a lead role in the Higgs discovery of 2012, Nessi retains a vivid memory of those moments: “I have to say that not everybody, at the beginning, was convinced that the Higgs boson really existed, that it was not an artefact of a given school of thought. So for many of us its existence was really a surprise, even though the detector was constructed and tuned to allow testing for just that hypothesis.”

Since this milestone event, physicists have been sifting through the LHC data for any sign of supersymmetric particles that could, for example, reveal the nature of dark matter. But so far, there have been no sightings. “We found the Higgs. And the next expectation drawn from the theory is to find a new level of symmetry (supersymmetry) which could solve many open problems (including dark matter) and insert new fundamental particles into the picture. (…) The community is therefore in a very new situation. We speak about unknown unknowns, meaning unknowns in what we can expect from the theory, unknowns in what we might find. For sure there is a new physics somewhere. The Standard Model has a lot of problems and cannot explain what we see or think we see in nature. We might need new ways of thinking and a new type of experiments to decide in which direction to go.”

One of these new experiments could be the future large neutrino detectors. This at least is the belief of the more than 1,000 scientists and engineers from 32 countries taking part in DUNE, the biggest neutrino experiment currently under construction, with giant underground detectors in South Dakota (United States) set to come on stream in 2026. To test that the DUNE technology will work in practice, CERN, near Geneva, has spent two years building two prototype detectors known as ProtoDUNE and a few weeks ago announced that one of them had detected the first neutrinos. “Our first [ProtoDUNE] detector is already gathering beautiful data,” Nessi confirms, “and the second, using a different technology, will be online in a few months’ time.”

Beyond the Standard Model

The ProtoDUNE detectors occupy the equivalent of a three-story building, but the definitive DUNE will be 20 times bigger; dimensions, in this case, directly commensurate with the importance of its potential findings. “Neutrinos,” Nessi explains, “are among the most abundant particles in the universe, but we understand very little about them.” What we do know, however, gives us every incentive to learn a whole lot more.

Neutrinos have been described as nature’s “ghost particles” because they barely interact with matter. This means that, every second, trillions of neutrinos pass through the Earth, and our bodies, leaving almost no trace. It is on the back of this “almost” that the neutrino-hunting experiments have been mounted. For very sporadically a neutrino slams into an atom, producing particles that detectors are able to observe. Hence the 2000 discovery that, counter to the predictions of theory, neutrinos do have mass. This finding, which merited the Nobel Prize in Physics in 2015, posed a raft of new questions, and new possibilities.

Among the questions: What exact mass do neutrinos have? Might neutrinos make up at least part of dark matter, which we are sure exists although its nature is unknown? And among the possibilities: that of finding a theory to explain the neutrinos’ mass that might, in turn, explain why the universe has more matter than antimatter.

The Standard Model, which is consistent with all the elementary particles detected to date – the Higgs included, predicts that the universe was born with equal amounts of matter and antimatter. If the amounts of each had remained the same, today’s universe would comprise only energy, since matter and antimatter annihilate each other. But there is now far more matter than there is antimatter. What process might be behind this? One hypothesis is that the primeval universe was home to far more massive neutrinos than today’s, and it was their gradual conversion into the particles we are now detecting that disrupted the matter/antimatter balance.

“We know that neutrinos oscillate between at least three different states,” says Nessi, “which implies that they have a mass, even if a very small one. This runs against the predictions of the Standard Model. For the moment we are not capable of measuring it. In recent years we have managed to parametrize this oscillating behavior, establishing a set of parameters with good precision. Some more parameters need to be measured, some of them containing new information on the fundamental behavior of nature; for example, on the evident asymmetry between matter and antimatter. All this is our new window on new physics, beyond the Standard Model.”

A huge technological challenge

Neutrinos could also prove to be the missing piece in other astrophysical puzzles, like the functioning of stars. “Many processes in the universe are driven by neutrinos,” Nessi continues. “Most of the energy in the explosion of a supernova, for example, goes on the production of a large quantity of neutrinos. For this reason, understanding the behavior of these particles will open up new horizons in our understanding of the dynamics of stars.”

But to get deeper insight into neutrinos and what they can teach us we need to trap far more than we have so far been able to measure. This is the first technological challenge facing the ProtoDUNE detectors at CERN: “Neutrino detectors are very peculiar,” Nessi points out. “They need very large volumes and a lot of details. These days, for instance, we are working with detectors based on a cryogenic liquid (argon) for the future DUNE experiment. This requires handling unprecedentedly large volumes of cryogenic liquid of very high purity and detectors that operate fully in a cryogenic environment, including all intelligent electronics. This is all new and needs to be demonstrated on a big enough scale to be credible. Today our prototypes are at a level of 1,000 tons of liquid. This should be sufficient to give us confidence in detectors that in the near future will be 20 times bigger. A lot of details, a lot of new technologies to test. But it works!”

It has taken two years to build the first ProtoDUNE detectors and eight weeks to fill them with liquid argon, which must be handled at temperatures of below -184º C. Detectors can pick out traces in this argon of particles from both astronomical sources and the beams generated in the CERN accelerator complex. The way it works is this: some passing neutrinos smash into the argon nuclei and create charged particles, which leave ionization tracks in the liquid; these tracks are visible to the detectors, and it is even possible to create 3D images of the process. Now that the first tracks have been seen, scientists will run the detector in coming months to put the technology through its paces.

In 2013, the European Strategy for Particle Physics tasked CERN with helping European neutrino physics groups to get involved in the American and Japanese programs. This motivated the setup of the CERN Neutrino Platform, which “should be seen as a tool for all neutrino activities to explore new ways to build detectors and make an impact,” in the global collaboration network, Nessi concludes.

CERN and the BBVA Foundation

The collaboration between CERN and the BBVA Foundation dates from 2014, when the supranational organization opted to celebrate its 60th anniversary in Spain in partnership with the Foundation. The result was the lecture series “The Secrets of Particles. Fundamental Physics in Everyday Life” whose closing speaker was CERN’s outgoing Director-General, Rolf Heuer. This was followed by a second edition where speakers included Heuer’s successor, Fabiola Gianotti. All lectures in the three series are available in full on the website www.fbbva.es and the playlists of our Youtube channel.