A massive experiment to determine the mass of one of the Universe’s most mystifying particles has finally worked out how big the neutrino might actually be. Initially considered massless, the particle’s mass is now understood to weigh a maximum of a single electronvolt. Although not a precise answer, this does bring us a step closer to a substantial solution for one of modern physics’ biggest mysteries.
Neutrinos are eccentric. Despite being one of the most abundant particles in the Universe, they are hard to detect. Owing to their unique properties, they interact very little with normal matter as they flow through the Universe at near light-speed. Billions of neutrinos are in fact zipping through your body right now giving them the name of ‘ghost particles’.
After years of calibrations and facility assessments, the Karlsruhe Tritium Neutrino (KATRIN) experiment commenced its series of tests last spring to calculate the resting mass of the neutrino. Earlier this month, representatives from the collaboration presented their first set of results at a conference in Japan.
There’s still a way to go, and their findings are yet to be published, but the team’s efforts have already halved the prevalent mass estimates down from the previous upper limit of around 2 electronvolts to just 1. Unlike units of pounds and kilograms, this measurement isn’t an easy one to picture. MIT physicist Joseph Formaggio and leading member of the KATRIN experimental group recommends starting tiny and then going smaller.
Formaggio explained to MIT News writer, Jennifer Chu, “Each virus is made up of roughly 10 million protons. Each proton weighs about 2,000 times more than each electron inside that virus. And what our results showed is that the neutrino has a mass less than 1/500,000 of a single electron!”
But no one is surprised that the base mass of a neutrino might be so inconceivably low. In fact, when the particles were first proposed as part of the Standard Model of particle physics, it was assumed that they didn’t have any mass at all. This supposition was empirically challenged in the late 1990s by the results of a milestone experiment demonstrating neutrinos streaming from the Sun changed form, or flavour, in a way that indicated that their mass couldn’t be zero. Subsequently, for more than two decades, numerous experiments have attempted to define the limits on just how big – or small – it might be.
The biggest challenge is posed by the fact that neutrinos mind their own business in a pretty effective way. The sole interaction they have with the particles constituting our measuring tools is through the weak nuclear force, which difficult to detect.
University of Washington physicist, Hamish Robertson elaborates, “If you filled the Solar System with lead out to fifty times beyond the orbit of Pluto, about half of the neutrinos emitted by the Sun would still leave the Solar System without interacting with that lead”.
Therefore, physicists have had to improvise with less direct methods to carry out observations on these ghostly particles. Astronomical observations have suggested the particles have a mass of at least 0.02 electronvolts. Other experiments grounded on the vast shower of electrons released by crumbling atoms of tritium have proposed that it can’t be more than 2.2 electronvolts.
KATRIN picked up where the last upper-limit estimates left off, upscaling the pursuit for an answer in tritium’s decay by keeping a vigilant watch on the radioactive gas inside a 70-meter-long piece of equipment. When the hydrogen isotope decays, it can release a pretty energetic pair of particles in the form of an electron and an antineutrino. As physicists are certain that the mass of a neutrino is the same as its antiparticle, this breaking down process provides a prime opportunity for making a precise measurement.
Typically, the 18,560 electron volts of energy that sends the electron flying is shared somewhat equally with the antineutrino. According to the classic E=mc2 formula, mass and energy are two sides of the same coin. An accelerating antineutrino has a boost of kinetic energy that counts towards its mass.
But physicists are interested in its non-accelerating energy. For this, they need to sift through a multitude of decay events to identify the few that give the electron most of the energy. Theoretically, remains of that shared energy should establish the limit on how heavy a resting neutrino could be. Fortunately, their source of tritium produces around 25 billion pairs of the two particles every second guaranteeing that at least some will have the greedy electrons and undernourished antineutrinos they need.
Narrowing down this number to a precise figure would aid updating a wide variety of physics that still eludes us- from the nature of dark matter to an explanation of why ‘stuff’ exists at all.
Physicist Peter Doe from the University of Washington says,” Neutrinos are strange little particles. They’re so ubiquitous, and there’s so much we can learn once we determine this value.”