To further our pursuit of knowledge, the world’s most sensitive scale has been built to weigh the universe’s most elusive particle.
Neutrinos are the stuff that holds the universe together. The problem: They are so small that until recently they were practically impossible to examine. But a team of international researchers has developed a scale that is sensitive enough to weigh even these tiny “ghost” particles.
Neutrinos pose a puzzling quandary. They are the tiniest and possibly even the most important particles in all the universe. They are everywhere—but no one knows why. Of all the known particles, they are outnumbered only by photons, and neutrinos are the only ones whose mass had been a mystery. Unfortunately for scientists, they are so small that for a long time there was no way to even conduct research. But that has changed now. Particle astrophysicist Guido Drexlin has spent the last 20 years working with a team of 150 scientists to build the world’s most sensitive scale—the Karlsruhe Tritium Neutrino Experiment (KATRIN) in Karlsruhe, Germany. The plan is to utilize KATRIN to determine the properties of neutrinos, the ghost particles that are believed to form the building blocks of the entire universe. Until quite recently, they’d long eluded measurement. After all, how do you weigh something that until the turn of the last century was believed to have no mass at all? First you spend $70 million to construct KATRIN, a 220-ton spectrometer. Its 230-footlong line of superconducting magnets and cold traps can track the decay of a radioactive isotope. But why go to all this bother? What are neutrinos?
“KATRIN is a shining beacon of fundamental research and an outstandingly reliable high-tech instrument.”
Guido Drexlin, professor of particle astrophysics at the Karlsruhe Institute of Technology
And how could you possibly measure them if they have no mass? The very existence of neutrinos was not proved until 1956, several years after two American physicists, Clyde Cowan and Frederick Reines, first set out to demonstrate it. But British physicist James Chadwick had suspected the existence of neutrinos in 1914 while he was studying radioactive decay and noticed that there was apparently energy missing. In 1930 Austrian-born physicist Wolfgang Pauli predicted the existence of neutrinos after he’d observed that a portion of the energy and angular momentum appeared to vanish when a neutron transformed into a proton and electron. Inspired by his prediction, Cowan and Reines began their work at a nuclear reactor in Washington State in the early 1950s before moving on to a more powerful one in South Carolina. There they had conducted what they dubbed “the Poltergeist Project.” Fission reactors were their first choice as an intense source of neutrinos (the alternative would’ve been a nuclear bomb), and in a 1956 test at South Carolina’s Savannah River Plant they detected neutrinos for the first time. In 1995 Reines was awarded the Nobel Prize in Physics for their work after Cowan’s death. Neutrinos have always been with us. It is believed that they were created 13.8 billion years ago, just a split-second after the Big Bang, and that unimaginably large amounts of neutrinos from that event are still
present today. Neutrinos are one of the most abundant particles in the universe and a billion times more plentiful than the particles that make up the stars, planets, and everything we see around us. Measuring their mass is important because it will help researchers solve a number of other mysteries. “Knowing the mass of a neutrino will allow scientists to answer fundamental questions in cosmology, astrophysics, and particle physics, including how the universe evolved and what physics exists beyond the Standard Model,” explains Hamish Robertson, an experimental physicist and professor emeritus of physics at the University of Washington as well as a KATRIN scientist. The Standard Model theory describes fundamental particles and how they interact.
500,000 TIMES LIGHTER THAN AN ELECTRON
Thanks to KATRIN, we are now aware that neutrinos weigh no more than 1.1 electronvolt (ev). That equates to approximately one-billionth of the mass of the lightest atom, hydrogen.
Other research groups had already established a lower limit for neutrino mass at 0.02 ev. It is not possible to weigh neutrinos directly. KATRIN’S trick is to monitor the decay of tritium, a heavy isotope of hydrogen. During decay, a neutron becomes a proton, emitting an electron and a neutrino in the process. While KATRIN cannot detect neutrinos, it can measure the energy released by the electrons as they shoot around the chamber of the spectrometer, the largest ultrahigh-vacuum system in the world. And that measurement can reveal the energy of the neutrinos and thus their mass. Dr. Guido Drexlin regards the success of the experiment as a vindication of almost 20 years of work since KATRIN was first designed. And he believes the results are only a foretaste of what the spectrometer can achieve. “This is just showing the community that KATRIN is up and running,” he says. Over the next seven years he hopes to improve the technology, making KATRIN so sensitive that it actually determines the absolute mass of a neutrino. Observations thus far have indicated it could be 0.1 ev or even lighter. But so much about neutrinos remains to be discovered. Scientists believe they exist in three “flavors”: electron, muon, and tau. The electron neutrinos may be the lightest, but that distinction could also be held by tau neutrinos. A better understanding of them could help scientists decipher what holds the universe together and why it’s expanding and perhaps even clear up the mystery of dark matter. Peter Doe, a University of Washington research professor of physics and a scientist at KATRIN, says: “Neutrinos are strange little particles. They’re so ubiquitous, and there is so much we can learn from confirming their mass. Solving the mass of the neutrino leads us into a brave new world of creating a new Standard Model.”