What are neutrinos and how can we measure their mass?

Of all the elementary particles in the universe, neutrinos may be the strangest. These mysterious little energy packets, also known as “ghost particles,” are uncharged, have little mass, and come in at least three different varieties. With new research, science is closer than ever to understanding the properties of neutrinos, from their size to their basic properties.

Neutrinos are surprisingly small. Kathrin Valerius, a cosmic particle researcher at the Karlsruhe Institute of Technology in Germany, states that each weighs less than 0.8 electron volts and is “hundreds of thousands of times lighter than the next lightest particle, the electron.”

They are also everywhere. Dozens of trillions of neutrinos pass through your body every second, mainly from the sun. However, due to its small size and lack of charge, it rarely interacts with tissues and other things. “If one neutrino interacts with you throughout your life, you’re lucky,” says Sowjanya Gollapinni, an experimental particle physicist at the Los Alamos National Laboratory.

Despite being aware of the existence of neutrinos for nearly a century, theoretical physicists still know little about neutrinos. In 1930, the famous physicist Wolfgang Pauli was puzzled by a seemingly impossible challenge. Over several experiments, Pauli’s contemporaries noticed an accounting error when observing beta decay, the process by which certain radioactive atoms decay. Instead of being emitted as electrons, a small portion of the energy of the decaying atom was clearly gone.

This observation broke the first law of thermodynamics that energy cannot be produced or destroyed. Pauli then proposed what he described as a “desperate remedy.” It is a new type of small, uncharged elementary particle that is emitted with electrons and causes lost energy. The idea of ​​neutrinos was born.

Pauli’s neutral particles were only confirmed in 1956 in experiments that proved their existence, but their size was not confirmed. Theory predicted that neutrinos would be completely massless.

However, in 2015, Takaaki Kajita of the University of Tokyo and Arthur McDonald of Queen’s University in Ontario won the Nobel Prize in Physics for their work demonstrating that particles actually have mass. In the mid-2000s, the Mainz neutrino mass experiment in Germany set the upper limit of neutrino mass to 2.3 electron volts. And data from the Karlsruhe tritium neutrino experiment (KATRIN) in Germany in early 2022.

Such accurate measurements require very sensitive and very large equipment. KATRIN’s 200-meter ton spectrometer and 70-meter ultra-high vacuum tube can reach temperatures from -270.15 degrees Celsius to 250 degrees Celsius, allowing researchers to detect billions of particles. .. Extremely low temperatures keep the heat-sensitive supermagnetic material cold enough to generate a strong magnetic field that allows the detector to capture individual particles. Experiments switch to high temperatures when cleaning is required. Valerius, who works on the project, describes it as a “big pizza oven.”

However, even with this setting, it is not possible to directly detect elusive ghost particles. Instead, the spectrometer measures the energy of the electrons emitted by the radioactive hydrogen along with the neutrinos as they decay. The maximum energy of these electrons is well documented. If the scientist records the total energy from this experiment, it’s just a matter of subtracting the electron’s energy: everything that remains belongs to the neutrino.

Researchers are currently developing new experiments to better understand neutrinos. One of them, called the Deep Underground Neutrino Experiment (DUNE), aims to understand another mysterious property of neutrinos: how neutrinos oscillate or change types. It is said that.

Neutrinos have three “flavors”: electronic, muon, and tau. However, these identities have not been modified. “If a neutrino was born as a particular flavor, it can transform into other flavors as it moves,” explains Gollapinni, part of the DUNE collaboration. “It’s like changing your identity.” For example, some electron neutrinos from the Sun turn into muons and tau neutrinos by the time they reach Earth. To understand why and how this change occurs, DUNE is a neutrino beam traveling about 800 miles underground from the experimental headquarters of the Fermi National Accelerator Laboratory in Batavia, Illinois to the Sanford Underground Laboratory in South Dakota. Observe. ..

Researchers have found that experiments like these are the nature of dark matter (maybe the fourth, yet undetected flavor of neutrinos called “sterile neutrinos”), other major cosms such as black holes. Hope to help get rid of neutrinos, even the form, or even the origin of the substance itself. “KATRIN’s collaboration has done a great job,” says Anthony Ezeribe, a particle physicist at the University of Sheffield in the United Kingdom, also part of DUNE. “There’s still work to be done.”

Valerius agrees. And, like many neutrino scientists, she is excited about the enormous research potential of this tiny particle. “Our understanding of neutrinos, or lack of understanding, is not perfect,” she says. “We don’t even know what we don’t know yet.”


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