Measure of neutrinos, nature's most elusive particles

-February 27, 2015

Of all the tiny, point-like particles of matter that we know of, quarks and gluons are the weirdest, but neutrinos are the most elusive. There are plenty of them out there; if you combined all the neutrinos zipping around the universe, they’d weigh more than all the stars combined. Still, they’re damn near impossible to detect.

Neutrinos were postulated by Wolfgang Pauli in 1930 to explain how decays of certain radioactive elements appeared to violate Newton's laws of motion. In beta decay, an atomic nucleus spits off an electron or a positron (the antimatter equivalent of an electron), but the sum of the momentum of that beta particle and the remaining nucleus doesn't account for the atom's momentum prior to the decay.

Picture a game of pool played with an invisible 8-ball. When the cue ball hits that 8-ball, it veers off and you can reconstruct the 8-ball's direction and speed from the initial and final directions and speeds of the cue ball. Pauli did the same thing by proposing that an invisible neutral particle, the neutrino, must carry away the missing momentum in beta decay.

Atomic nuclei are built of neutrons and protons. Tremendous force is required to hold the positively charged protons within the few femtometer (1E-15 m) nuclear diameter. The force goes up fast with atomic number, so heavy nuclei have many more neutrons than protons. The neutrons dilute the electric repulsion between protons. In many cases, nuclei can drop to a lower energy state by kicking out a unit of positive charge through proton decay:

The positron, e+, is the beta particle, and the neutrino is indicated by the Greek letter nu. To understand why neutrinos are so difficult to detect, let's review the known forces of nature. I listed them in my article about quarks and gluons quarks and gluons. In order of increasing strength: gravity, the weak nuclear force, magnetism, electricity, and the strong nuclear force. Reducing the number of these forces has been a goal of physics since Aristotle's air, water, earth, and fire were debunked.

In the 19th century, Faraday, Maxwell, Poynting, and cohorts discovered that electricity and magnetism are different facets of the same force, thus becoming electromagnetism.

In the 20th century, Sheldon Glashow (big cigar smoker), Steven Weinberg (famous atheist), Abdus Salam (devout Muslim), Carlo Rubbia (once banned from flying on a certain airline), and everyone who worked on CERN's UA1 and UA2 experiments showed that the electromagnetic force and the weak nuclear force are different facets of an electroweak force.

Grand Unified Theories (GUTs) attempt to find the underlying link that will unite the strong and electroweak forces and, someday, gravity—so that when we say, "May the force be with you," the wisecracking physicist can't respond, "Which force?"

Neutrons and neutrinos are both electrically neutral, but neutrons are easy to see because they are made up of quarks which carry both electric charge and the strong nuclear force’s color charge. The reason that neutrinos—and probably whatever makes up dark matter—are so hard to detect is that they only interact through the gravitational and weak forces. If they absorbed or emitted light, like stuff that carries electric charge, they’d be easy to see.

The detectors at the big particle colliders, like ATLAS and CMS at CERN's LHC, account for neutrinos through their absence, like the invisible 8-ball.

Because neutrinos only interact through the weak force, detecting them requires extremely low-noise, precision instrumentation. Neutrino detectors are usually placed deep underground, safe from stray particles like cosmic rays. Since weak interactions are so weak, we can only expect a few neutrinos to leave behind any sign of their passage. To up the likelihood of seeing them, the detectors are big and filled with large volumes of absorbers, like ultra-purified water.

As a neutrino passes through the detector it has a small chance of interacting with another particle. Electrons and the quarks that make up neutrons and protons all carry weak charge so neutrinos can tap an electron or the nucleus of an atom in the absorber. By "tap" I mean the reverse of beta decay, such as:

Super-Kamiokande is a neutrino experiment that's in a mine 1000 feet underground in Japan. It holds 50,000 tons of pure water surrounded by detectors. If a neutrino taps a nucleus or an electron in the water hard enough, then the electron and/or a chunk of nuclear matter from the collision, flies through the water. If this electrically charged cue ball flies faster than the speed of light in the water, then it will generate a cone of blue to ultraviolet light called Cherenkov radiation. By detecting the energy and direction of that light, experimentalists can reconstruct the neutrino’s momentum.

The Super-Kamiokande detector, nearly full of water. Source: Kamioka Observatory, University of Tokyo.

Let me back up a bit because that last paragraph included some pretty cool physics.

Einstein demands that nothing can travel faster than the speed of light in vacuum. Within media, like water, glass, or cables, electromagnetic radiation (a.k.a., light) travels slower than it does in a vacuum. A particle with enough energy can travel in a medium faster than light travels in that medium but still slower than the speed light travels in a vacuum—so relativity is secure. If that particle carries electric charge, then this totally cool, weird thing happens.

Picture the charged particle as a boat cruising along in a harbor. As long as the boat travels slower than the speed of waves in that water, it doesn’t produce a wake. But as soon as it leaves the harbor and accelerates above the speed of water waves, it produces a wake, that is, a shock wave in the shape of a cone propagating in the direction of the boat. The same phenomenon applies when jets accelerate above the speed of sound and create the shock waves that cause a sonic boom. For the charged particle going faster than the speed of light in a medium, the shock wave/wake forms a cone of blue to ultraviolet light that’s called Cherenkov radiation.

The Cherenkov light cones are easy to detect with low noise photon counters, Super-Kamiokande uses a vast array of photo-multiplier tubes. The conic geometry of the radiation conveys the momenta of the particles, the cue ball, from which the momentum of the neutrino, the 8-ball, can be reconstructed.

Dusting off the Super-Kamiokande photo-multiplier tubes Source: Kamioka Observatory, University of Tokyo.

Neutrinos are the darkest type of matter that we've so far discovered, but they aren't massive enough to account for the missing mass of the universe, called dark matter. We’ll investigate dark matter next time.