Have you heard of neutrinos? They’re a set of subatomic particles that are likely to cause quite some scientific uproar in the next years. Catch up on what makes these shy little fellows so special and how Swiss particle physicists are involved in solving one of the Universe’s most tenacious mysteries….
A neutrino walks into a bar. The bartender asks: “Can I get you something?” “No thanks,” says the neutrino, “just passing through…”
Why is that funny? Well, because it refers to one of the most prominent features of the neutrino, one of the most abundant elementary particles: it passes through everything all the time and hardly ever interacts. That makes it both fascinating to physicists – and pretty hard to study. After all, how do you study something that you cannot see, feel or catch; something that doesn’t show up in conventional particle detectors (which have a habit of making invisible things visible) but that holds the key to a treasure chest of new knowledge about the Universe? There are ways to do it and people who know these ways. Let us take you on a journey into the world of the neutrino and the people who are on its case – including many Swiss researchers.
But first things first: why is this tiny thing such a big deal? “There are many open questions concerning neutrinos,” says Prof. Michele Weber from the University of Bern. For example, they are contenders for an answer to the question why the Universe is made of matter and not antimatter, even though both must have been produced in equal amounts at the Big Bang. A tiny difference between particles and their antiparticles – which according to theory should be identical in every aspect except their charge – account for the imbalance, and scientists think that neutrinos could at least contribute to finding an explanation why matter dominates over antimatter.
Neutrinos are also messengers from outer space. They are the first particles that fly out of exploding stars like supernovae, and catching them means scientists catch the earliest signs of a major stellar event and are able to observe it in full. Watching a black hole come into existence and new stars being born in nearly real time will send us miles ahead of our current knowledge of these universal processes.
“But, like all elementary particles, neutrinos are also fascinating in themselves,” says Weber, who, like his physics colleagues, likes a challenge. “Neutrinos are especially so because they are so hard to measure. We don’t know one of their most fundamental parameters – their mass! Might there be a fourth neutrino, or new effects that go beyond the Standard Model of particle physics?” His colleague from the University of Geneva, Prof. Federico Sánchez, agrees. “There are many reasons why we should study neutrinos. We also don’t know how they get their mass and why this process is apparently so different from the way other particles acquire mass,” he explains. Sánchez and Weber are on a quest to find out, along with some 5000 physicists all over the world including some key players in several Swiss universities and institutes.
So let’s take stock of what we do know. Neutrinos, first postulated in 1930 and discovered in the 50s, are uncharged particles. They are very light and come in three kinds, or “flavours”, that are directly linked to the family of electrons, whose heavier cousins are called muon and tau. That means that there are electron neutrinos, muon neutrinos and, you guessed it, tau neutrinos. For a long time, scientists had suspected neutrinos might not have any mass at all, and that is how they still are implemented in the Standard Model of particle physics, the theory that describes all elementary particles and the forces that act between them. But some twenty years ago, scientists found proof that neutrinos can change from one flavour to the other – a process called oscillation. This in turn meant that cannot be massless – but what their exact masses are and which of the flavours is the heaviest one remains a mystery.
Remember the joke from the beginning? The fact that they just pass through without interacting is not much of a joke when you try to study neutrinos, because they are so hard to catch. Just to give you an idea: trillions of them pass through your little toe every second, and not only through your toe, but through the earth underneath your feet as well. In fact, most travel through the entire planet without ever being stopped by matter. The only way to heighten your chance of seeing one is to send lots of them on a journey and to have giant and highly sensitive detectors with plenty of material to cause them to interact and catch all traces of what happens when one neutrino interacts.
And that’s exactly how here on Earth, the hunt is on. Actually, most of the hunt is not on *on Earth* but underneath it – in huge water-filled tanks under mountains and in old mines equipped with detectors. Physicists have devised these experiments with neutrino beams deep underground because they want to block out outside influences like cosmic rays for a clearer signal.
One of these is experiments is the “T2K” project in Japan. Beams of neutrinos or antineutrinos are sent from a research facility on Japan’s east coast 295 km through the ground to the Kamioka observatory in a disused mine on the west coast, into a water-filled tank as big as 20 olympic-sized swimming pools 1000 metres underground. The neutrino beam comes from a pulsed beam of protons onto a target and contains a trillion very high-energy neutrinos (or antineutrinos) per pulse. Most of these are never seen again – but those that are will be scrutinised by researchers like Federico Sánchez. He is currently the International Spokesperson of the collaboration of some 500 scientists and works on a crucial piece of equipment: the near detector. In order to understand what happened to the beam along the way, you have to know what it looks like at the beginning – so there is another detector 280 metres away from the point where the neutrinos are produced, called ‘near detector’.
“The majority of physics is actually done in the near detector,” Sánchez says. “We use it to check how many particles there are and what energy they have before they oscillate,” he explains. The data from the near and the far detector are then compared to trace the oscillations that took place in between. This is done for two sets of data: one from neutrinos and one from antineutrinos. If there is a difference in how they behave, this is an indication for an imbalance that could be the key to the question why we are made of matter rather than antimatter. In fact, their findings made it onto the cover of Nature in 2020 because they point to a rather large difference. It’s not enough proof to claim a discovery, but Sánchez and his team from the University of Geneva and other groups around the world are currently working on an upgrade that should be installed around the end of this year. With the improved sensitivity and more data to analyse, they hope to get closer to understanding that difference.
Physicists wouldn’t be physicists if they didn’t aim for even better detectors, higher rates and energies and much more data. That’s why two experiments are currently under construction that complement each other on their quest to cross-examine the neutrino – the HyperKamiokande (HK) detector in Japan and the DUNE experiment in the United States. Both are set to start running at the end of the decade. The University of Geneva and ETHZ are involved in HK, the successor of T2K, putting their expertise in the near and the far detector – which will be 10 times larger than its predecessor and contain 500 000 tons of ultra-pure water. At the same time, Michele Weber and his team from the University of Bern are currently busy constructing a prototype for DUNE’s near detector (in collaboration with 30 other institutes from around the world).
DUNE, which stands for Deep Underground Neutrino Experiment, has the same principle as T2K: a neutrino beam produced by an accelerator, this time at Fermilab near Chicago, is sent to a detector in a mine in South Dakota, some 1300 kilometres away. Once finished, it will house the largest detector of its type ever built, using 70’000 tons of liquid argon. Both near and far detectors, like their Japanese counterparts, are chucked full of advanced technology. The current prototype consists of modules that measure one by one by three metres – some 70 % of the size of the final version. In the end, the near detector will consist of 35 of these modules. The principle is the same for all these detectors: when a passing neutrino interacts with an atom in the liquid that fills the detector, it produces particles that knock out the electrons in the liquid’s atoms. These electrons drift to a positive pole in the module and are there read out by pixel sensors. Each type of neutrino produces particles that have a characteristic signature and artificial intelligence programs help hunt them down and separate them from other signals. “Thanks to the modular design, we can measure neutrino interactions very precisely and get real 3-D images of the tracks of the charged particles passing through the module,” Weber says. “In the end, this is what science is all about: take 3-D pictures of physical phenomena to find out more about the world.”
Neutrino physics is not new in Switzerland. Some of the most relevant discoveries on how neutrinos interact with matter or the most precise determination of the number of neutrino flavours were achieved at CERN. Swiss Universities have been involved in the neutrino oscillation quest since 2002, with contributions to the K2K experiment in Japan (the predecessor of T2K). The University of Bern was also involved in the OPERA experiment, the experiment that observed for the first time a transition of a muon neutrino to a tau neutrino. There have also been contributions to experiments such as the MINERvA and MicroBooNE experiments in USA or FASER-nu at CERN now in operation to help understanding the interaction of neutrinos with matter.
For the T2K and K2K experiments, researchers from the Universities of Geneva and Bern and ETH-Zurich got the 2015 breakthrough prize in fundamental physics as members of the community that helped to establishment of neutrino oscillations.
What is more, the groups of Universities Zurich , and previously the University of Neuchatel, is involved in the quest for the rare decays that might provide inside on the mass generation mechanism for neutrinos. But this is a different story.
Still, the neutrino community’s biggest dream is to find evidence for differences between the way neutrinos and antineutrinos behave. In the meantime, they are analysing their data from current experiments and getting their new systems ready for the next big catch. Stay tuned, things are going to stay interesting over the next years!
Author: Barbara Warmbein for CHIPP
Dr Angela Benelli
Conseil Européen pour la Recherche Nucléaire (CERN)
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