Billions of them are passing through our bodies right now, and yet they are almost impossible to catch. For this year's Dvořák Lecture, we hosted one of the world's leading experts on neutrinos, Professor Stefan Söldner-Rembold, who heads the Department of Physics at Imperial College London.
Söldner-Rembold was not visiting us for the first time. His ties to the institute go back to the late 1990s. "It feels a bit like a second scientific home," the physicist joked at the very start. His career has taken him through the field's largest laboratories, including DESY, CERN and Fermilab. He then opened his lecture with the subject he has devoted much of his career to: neutrinos.
All around us, yet invisible
Neutrinos are the most abundant matter particles in the universe. For every proton, neutron and electron, there are roughly a billion neutrinos. Even so, they are almost impossible to detect. They carry no electric charge, their mass is tiny, and they pass through ordinary matter, and therefore through us, almost without a trace.
It is precisely this elusiveness that makes them interesting. Neutrinos hold a special place in the Standard Model, the theory that describes the known particles and forces. Söldner-Rembold illustrated this with the well-known "particle zoo," where particles are arranged by mass like animals. The heaviest elementary particle, the top quark, corresponds to a blue whale, while neutrinos belong among the smallest creatures at the far end of the scale.
Physicists suspect that these tiny particles may help answer a far bigger question. Why is the universe made of matter at all, instead of dissolving into pure energy shortly after the Big Bang?
A particle that breaks the mould
Söldner-Rembold then pointed out that the long-used particle zoo is out of date. According to today's measurements, neutrino masses are far lower than the image suggests. "If we stayed on that scale, a neutrino would be the size of a fruit fly," he noted.
That is exactly what makes the neutrino unique. Its mass is incomparably smaller than that of any other particle in the Standard Model, the theory that describes all known elementary particles and the forces between them. And yet it is not zero. Why? That is one of the great unsolved questions of modern physics.
At the same time, he warned against an oversimplified picture. "We intuitively think of elementary particles as little balls, but neutrinos are purely quantum objects and in many ways are closer to photons." A ball would have a definite position and a definite path along which it moves, whereas a particle is not so sharply defined.
From the Sun to a detector deep underground
Most of the neutrinos we can observe arise in nuclear processes. They are born in the Sun, where thermonuclear fusion creates them, and in supernova explosions, where protons and electrons merge into neutrons and release a stream of neutrinos and antineutrinos. But neutrinos also arise here on Earth, for instance, in nuclear reactors, and even right beneath our feet, in the decay of uranium and thorium inside the planet.
The typical energy of these neutrinos ranges roughly from a few to twenty megaelectronvolts. But there is a catch. "Neutrinos barely interact with anything, which is a problem," the physicist explained. A neutrino with an energy of about ten megaelectronvolts has a mean free path in lead of roughly half a light-year. To catch any at all, scientists have to build enormous detectors, have an extremely intense source of neutrinos, or both.
Both are combined in the DUNE experiment, which Söldner-Rembold led for four years. The intense source is provided by an accelerator at Fermilab near Chicago, from where a beam of neutrinos travels through the Earth to South Dakota, more than a thousand kilometres away. There, deep underground, a giant detector filled with liquid argon awaits it.
Along the way, the neutrinos change their flavour, as the state is called. "We start with a muon neutrino and measure how its flavour changes with energy over a long distance," Söldner-Rembold explained. In doing so, scientists watch whether neutrinos behave differently from their antimatter twins, and thus whether they break the symmetry between matter and antimatter.
About the Dvořák Lecture
The Dvořák Lecture is one of FZU's prestigious events, delivered each year by an internationally renowned scientist. It bears the name of Vladimír Dvořák (1934–2007), a solid-state physicist who directed the institute from 1993 to 2001 and led its reforms after 1989. Dvořák was known for his work on ferroelectricity and structural phase transitions, but he is also remembered as an outstanding teacher who could explain even difficult physics clearly.