Norman Cao

Norman is currently working to complete a double major in Aerospace Engineering and Physics in 2015. He likes math and physics and likes to apply these concepts to his engineering practice. He hopes to make space travel cheap and available for everyone. See you in space!

Two years ago physicists discovered the Higgs Boson, an elusive particle that gives rise to the mass of fundamental particles. The discovery was a monumental achievement in validating the Standard Model, which physicists use to describe how subatomic particles interact to produce the wide variety of phenomena we observe in the world, and prompted worldwide media frenzy. The Higgs became an instant celebrity. But there's another particle grabbing particle physics headlines these days. The humble neutrino has recently been poking holes in the Standard Model and making us rethink our understanding of the origin of mass.

First proposed in 1930 as a theoretical fix in explaining certain radioactive processes, the neutrino has long been overshadowed by other particles. Even the neutrino's original name, the neutron, was taken by another particle discovered in 1932. Since their discovery, physicists hypothesized that neutrinos would be massless like photons, the particles of light. Numerous experiments hinted that neutrinos had a number of properties that only massless particles had, but never quite provided conclusive proof one way or the other. For example, neutrinos travel at speeds near the speed of light, which requires that they're either extremely light or have no mass at all—a rather ambiguous observation on whether they're massless. However, a result in 1998 from a neutrino detector in Japan, the Super-Kamiokande, shook the particle physics world: it proved once and for all that neutrinos did indeed have mass.

Still, physicists couldn't explain why. Unlike the other types of fundamental particles that we know about, which gain mass by interacting with the Higgs Field, neutrinos might gain mass by some other mechanism. Scientists don't know how else to explain why neutrinos are so amazingly lightweight. They're at least a million times lighter than the electron. For many years, physicists were only able to infer possible values for neutrino mass indirectly. Most neutrino experiments cannot measure the absolute mass of neutrinos. Experiments like Super-Kamiokande measure neutrino oscillation, a process where neutrinos start out as a certain type, or "flavor," and then transform into other flavors after traveling a distance. This oscillation process depends on the differences in mass between different neutrino states. These experiments can and do tell us that neutrinos must have mass because they must have a sense of time in order to oscillate, and according to Einstein's theory of special relativity, massless particles have no sense of time. Still, this process doesn't tell us how much mass the neutrino has.

In recent years, scientists have developed a new class of experiment that promises to give a direct measurement of the mass of the neutrino. According to MIT neutrino physicist Joe Formaggio, "people have been trying to measure neutrino mass since 1930, so this would be a major scientific achievement." Joe is a member of the Karlsruhe Tritium Neutrino (KATRIN) Experiment in Germany. Currently under construction, KATRIN will be able to probe the mass of the neutrino at scales far smaller than anything physicists have ever looked at before.

Neutrinos are extremely difficult to observe directly because they interact primarily through something called the weak force. True to its name, the weak force is extremely weak, making neutrino collisions very difficult to detect. Billions of neutrinos stream through this page and across the entire diameter of the earth every second, yet hardly any trace of them is left at all. To work around this, KATRIN and other new experiments will measure neutrino mass by taking a fresh look at the radioactive process the neutrino was originally theorized to explain: beta decay.

First pioneered at Mainz, Germany, these experiments are based on the fundamental law of conservation of energy and the equivalence of mass and energy, popularly known as E = mc². Rather than try to catch the elusive neutrino itself, physicists instead look at how the neutrino affects the byproducts of beta decay. In beta decay, a neutron decays into a proton, releasing an electron and an antineutrino in the process. This decay releases a well-known, fixed amount of energy that gets distributed between the masses and kinetic energies of the resultant particles. In a rare but measurable number of cases, the decay will deposit all of its available energy into the electron, leaving only enough energy for the mass of the neutrino. By filtering all but the highest energy electrons using what is called a MAC-E-Filter, physicists can look at the amount of energy that must be put into creating the neutrino, and thus measure neutrino mass.

To date only two other experiments of this type using a MAC-E filter have been performed, the first at Mainz in Germany and the second at Troitsk in Russia. Neither experiment was sensitive enough to detect the miniscule neutrino mass, but they did place an upper bound on how heavy it could be. KATRIN promises to improve neutrino mass sensitivity by a factor of ten from 2 eV (electron-volts) to 0.2 eV over these experiments. For comparison, the mass of an electron has been measured to be 510,999 eV. At such small scales, improving sensitivity is an enormous technical challenge. While KATRIN's predecessors are only a couple of feet taller than the physicists operating them, KATRIN's experimental setup is almost four stories tall, 70 meters (230 feet) long, and many times bigger by volume. The main spectrometer, the largest part of the experiment, weighs approximately 200 tons and had to be transported primarily by boat since it was too large for transport by road.

The drive to measure smaller and smaller neutrino masses has involved larger and larger machines. Yet as particle physicists look closer at one of the lightest particles in existence, cosmologists have begun to suggest that neutrinos might have played a role in the distribution of galaxies in the universe, including our own Milky Way. The number of neutrinos from the birth of the universe outnumbers atoms by a huge amount (possibly as high as a billion to one!), so even very small neutrino masses are very important on a cosmological scale. With a precise measurement of neutrino mass, physicists can explore how neutrino gravity has affected the development of Galactic Superclusters and the Cosmic Microwave Background, the biggest things we have observed in the universe. By peering closer at one of the lightest and most elusive particles in the universe, we may reveal secrets about the largest structures that dominate the night sky. Not bad for a tiny particle.