Strange Effects: The Mystifying History of Neutrino Experiments

The news spread at a barely slower pace, fascinating the public. One thing everyone knows is that a very famous physicist named Albert Einstein once said that nothing should travel faster than light speed.

In February, the OPERA researchers found a couple small problems with their experimental set-up, calling into question the original faster-than-light neutrino result. The event highlighted the difficulty of science at the edge of the unknown -- and neutrinos are especially tricky.

More often than not, neutrino experiments throughout history have turned up perplexing results. While most of these experiments didn’t get the high-profile attention that disputing Einstein provides, they've challenged scientists and helped them learn ever more about the natural world.

In this gallery, we take a look at some of the strangest historical neutrino results and the findings that still have scientists scratching their heads.

Researchers need to know how neutrinos work because they're relevant to many areas of physics. These ubiquitous specks came into existence milliseconds after the Big Bang, and new neutrinos are created during the radioactive decay of elements, nuclear reactions within stars and the explosive collapse of supernovas.

“They’re one of the dominant particles in the universe but we still know very little about them,” said physicist Bill Louis of Los Alamos National Lab, co-spokesperson for the MiniBooNE neutrino experiment.

Neutrinos are so hard to study because they barely interact with other matter. Unlike the more familiar electron, they have no electromagnetic charge. They pass as easily through lead walls as through mist, and are so light that scientists long thought they had no mass at all. Detecting them requires closely watching a large tank of material, such a water, on the off chance that a neutrino will hit another particle and produce an observable change.

The tiny particles first came to scientists’ attention in beta decay, a radioactive process discovered at the end of the 19th century in which an atom's nucleus emits an electron and transforms itself into a different atom.

In the 1910s, researchers noticed something strange about beta decay. If the only particle emitted was an electron, then the process seemed to violate two physical laws: conservation of energy and conservation of momentum. No one at the time understood how this could happen, but evidence for the violations grew stronger and stronger with each new experiment.

In 1930, physicist Wolfgang Pauli wondered if the nuclear process was more complex than previously thought. If an atom also emitted something else during beta decay, the apparent contradictions to physical law could be solved. That something else turned out to be the neutrino.

But in order for neutrinos to exist, they would have to be very light and not very interactive. No one had ever seen particles that could fit this description, and no one could think of a way to find them. For a long time, scientists thought neutrinos would prove impossible to detect.

Many researchers realized that these reactors, which emitted 300 trillion neutrinos per square inch every second, could be harnessed to detect neutrinos. Though they hardly ever interact with matter, there's a tiny probability that, given enough material, a neutrino will collide with something. In a process that's basically the reverse of beta decay, this direct hit will generate gamma radiation.

That year, physicists Clyde Cowan and Frederick Reines built a detector and placed it near theSavannah River Plant in South Carolina. With the reactor on, their experiment unambiguously detected neutrinos for the first time.

Though Cowan died in 1974, Reines would go on to win the 1995 Nobel Prize in Physics for the discovery.

Astronomers want to detect those neutrinos because they contain important information about processes going on in the sun’s center. In 1964, physicist Ray Davis and astronomer John Bacall built an experiment in the Homestake mine in South Dakota to find these neutrinos. The detector needed to be placed deep underground because cosmic rays hitting the Earth’s atmosphere would interfere with the results.

After the Homestake experiment was calibrated and run, the researchers noticed an anomaly. According to their calculations, the sun should have been producing three times as many neutrinos as they actually detected. So they went back to the drawing board, looking for mistakes and making more refined estimates. But they still couldn’t figure out where they went wrong.

The Homestake experiment ran for more than 30 years, always showing the same result: three times fewer neutrinos than expected. Astronomers feared that their models of the sun might be totally incorrect. The problem persisted into the mid-'90s.

By this point, researchers had discovered that neutrinos come in three different types. The neutrino produced during beta decay or in the sun's center is an electron neutrino, but other processes will create particles known as muon or tau neutrinos.

You might guess why the three-type finding was important to a puzzle in which researchers found one-third the number of neutrinos they expected. Researchers realized that during their flight between the sun and Earth, electron neutrinos -- the type detected at Homestake -- were transforming into the other types. As a result, the experiment missed two-thirds of the neutrinos. When new detectors were built that could catch all three types of neutrinos, the discrepancy vanished.

The finding had profound implications. While some scientists had previously considered the neutrino to be massless, oscillating between different types required the particles to have mass.

But if protons regularly decayed, it would be a problem to life on Earth. The atoms in our bodies could start chaotically changing into other elements. No one wants carbon in their DNA spontaneously turning into boron.

So theorists said that protons might decay, but over timescales 20 orders of magnitude longer than the age of the universe. To test this, scientists watched huge numbers of protons – all those contained in, for example, a giant tank of water.

Watching for the decay of one proton out of 10³², or 100 nonillion protons, is the ultimate needle-in-a-haystack challenge. These experiments had to be placed deep underground to prevent cosmic rays from entering and fooling scientists into thinking a proton had decayed.

But cosmic rays hitting the atmosphere also create neutrinos, and even moving deep underground doesn’t shield you from them. Since a neutrino passing through the detector would look like a decaying proton, researchers needed to know how many neutrinos they might expect to see.

When they measured the number, scientists found something very weird. Neutrinos coming from above the experiment outnumbered those arriving from below by two to one.

After about 10 years of puzzlement, scientists finally figured it out: During flight, neutrinos traveling from below (through the 8000-mile-wide Earth) had time to transform into a different neutrino type. The experiments were only sensitive to one type of neutrino, so they missed the half that changed.

The finding confirmed that neutrinos changed as a function of the distance they traveled.

“When people figured this out, they got very excited, ” said theoretical physicist Andre de Gouvêa. “It showed us that neutrinos had properties that we didn’t know existed at all.”

To this day, no one has seen proton decay.

LSND remains famous among scientists because it saw a small excess of electron antineutrinos appear seemingly from nowhere. The best explanation for this odd anomaly required completely new physics.

All known subatomic particles come in groups of three. The electron, for instance, is associated with particles called the muon and the tau, which act much like electrons but are heavier. But what the LSND experiment saw could best be explained if, instead of three neutrino types, there were four or more.

“The implications of this are potentially huge,” said physicist Bill Louis.

The existence of a fourth neutrino would throw serious doubt on the current models of particle physics. But it might also help explain certain unresolved problems, such as the details of supernova explosions.

In order to have remained hidden for this long, a fourth neutrino would have very special properties. There are four fundamental forces: the strong, electromagnetic, weak, and gravitational forces. A proton interacts with all of them, an electron with all but the strong, and neutrinos only interact with the weak and gravity. But this new fourth neutrino would only interact with gravity.

Because this type of neutrino is so unexpected, many researchers remain skeptical of the LSND findings. As well, several other experiments that could have observed the LSND phenomenon didn’t see it.

“That doesn’t prove it wrong, but it does restrict the parameters,” said theoretical physicist Andre de Gouvêa.

The LSND findings remain one of the great unsolved mysteries in neutrino physics.

Beginning in 2002, scientists began running a new experiment named MiniBooNE at Fermi National Accelerator Laboratory in Illinois. MiniBooNE’s aim was to confirm or deny the controversial LSND results. Their initial results seemed to disprove the LSND anomaly, but further data changed that picture.

“Now it looks like MiniBooNE is consistent with LSND,” said physicist Bill Louis, co-spokesperson for the experiment.

If this is true, theorists may need to come up with new physical models for how neutrinos behave. These results could potentially impact many fields outside of the neutrino physics community, including cosmology and astrophysics.

“On top of everything, MiniBooNE might have run into another anomaly,” said theoretical physicist Andre de Gouvêa.

During its preliminary run, MiniBooNE spotted a strange excess of low-energy neutrinos. While many scientists thought these results could be simply be background noise, later data confirmed the surplus. As of today, physicists trying to explain the odd finding remain stumped.

Working in a completely different vein, researchers ran into one more abnormal result in Jan. 2011. New, more careful calculations on the number of neutrinos expected to come out of nuclear reactors showed that estimates from the last 20 years might be wrong.

“Combined with the LSND and MiniBooNE results, this reactor anomaly seems to indicate something weird is going on,” said de Gouvêa. To address this problem, researchers will need to build new detectors and rerun experiments that until now seemed to be closed cases.

Taken together, LSND, MiniBooNE, and the reactor anomalies suggest that neutrino scientists still have many puzzles left to figure out.

For this, physicists in the U.S. are hoping to get approval for the Long-Baseline Neutrino Experiment(LBNE). This dual-facility test would shoot the world’s most intense neutrino beam from Fermilab in Illinois 800 miles through solid earth to the Homestake mine in South Dakota, the same location where solar neutrinos were first detected.

At the proposed Deep Underground Science and Engineering Laboratory, researchers would catch the neutrinos, watching for their oscillations and any other interesting phenomena. Though this experiment might not be built for more than 15 years, the LBNE collaboration already has 400 eager scientists signed on.

In one future experiment, scientists hope to use LBNE to answer a profound question: why is the universe made of matter rather than antimatter? Though you, being made of matter, likely don’t give the idea much thought, it's a perplexing issue in modern physics.

Most physics equations predict that the universe would have been created with equal parts matter and antimatter, which are identical in every respect but their charge. The problem is that putting the two together causes them to annihilate into pure energy. So if the number of particles and antiparticles was originally equal after the Big Bang, every bit of matter in the cosmos would eventually find a partner and disappear in a puff of radiation.

Yet matter persists. Scientists have already found a small difference, called charge-parity (CP) violation, which affects some types of matter and antimatter and helps explain why matter dominates. But the finding still isn't enough to account for the amount of matter in the universe.