Neutrinos Suggest Solution to Mystery of Universe’s Existence

Updated results from a Japanese neutrino experiment continue to reveal an inconsistency in the way that matter and antimatter behave.

A neutrino passing through the Super-Kamiokande experiment creates a telltale light pattern on the detector walls.

A neutrino passing through the Super-Kamiokande experiment creates a telltale light pattern on the detector walls.

From above, you might mistake the hole in the ground for a gigantic elevator shaft. Instead, it leads to an experiment that might reveal why matter didn’t disappear in a puff of radiation shortly after the Big Bang.

I’m at the Japan Proton Accelerator Research Complex, or J-PARC — a remote and well-guarded government facility in Tokai, about an hour’s train ride north of Tokyo. The experiment here, called T2K (for Tokai-to-Kamioka) produces a beam of the subatomic particles called neutrinos. The beam travels through 295 kilometers of rock to the Super-Kamiokande (Super-K) detector, a gigantic pit buried 1 kilometer underground and filled with 50,000 tons (about 13 million gallons) of ultrapure water. During the journey, some of the neutrinos will morph from one “flavor” into another.

In this ongoing experiment, the first results of which were reported last year, scientists at T2K are studying the way these neutrinos flip in an effort to explain the predominance of matter over antimatter in the universe. During my visit, physicists explained to me that an additional year’s worth of data was in, and that the results are encouraging.

Researchers don’t know why. “There must be some particle reactions that happen differently for matter and antimatter,” said Morgan Wascko, a physicist at Imperial College London. Antimatter might decay in a way that differs from how matter decays, for example. If so, it would violate an idea called charge-parity (CP) symmetry, which states that the laws of physics shouldn’t change if matter particles swap places with their antiparticles (charge) while viewed in a mirror (parity). The symmetry holds for most particles, though not all. (The subatomic particles known as quarks violate CP symmetry, but the deviations are so small that they can’t explain why matter so dramatically outnumbers antimatter in the universe.)

Last year, the T2K collaboration announced the first evidence that neutrinos might break CP symmetry, thus potentially explaining why the universe is filled with matter. “If there is CP violation in the neutrino sector, then this could easily account for the matter-antimatter difference,” said Adrian Bevan, a particle physicist at Queen Mary University of London.

Researchers check for CP violations by studying differences between the behavior of matter and antimatter. In the case of neutrinos, the T2K scientists explore how neutrinos and antineutrinos oscillate, or change, as the particles make their way to the Super-K detector. In 2016, 32 muon neutrinos changed to electron neutrinos on their way to Super-K. When the researchers sent muon antineutrinos, only four became electron antineutrinos.

That result got the community excited — although most physicists were quick to point out that with such a small sample size, there was still a 10 percent chance that the difference was merely a random fluctuation. (By comparison, the 2012 Higgs boson discovery had less than a 1-in-1 million probability that the signal was due to chance.)

This year, researchers collected nearly twice the amount of neutrino data as last year. Super-K captured 89 electron neutrinos, significantly more than the 67 it should have found if there was no CP violation. And the experiment spotted only seven electron antineutrinos, two fewer than expected.


Researchers aren’t claiming a discovery just yet. Because there are still so few data points, “there’s still a 1-in-20 chance it’s just a statistical fluke and there isn’t even any violation of CP symmetry,” said Phillip Litchfield, a physicist at Imperial College London. For the results to become truly significant, he added, the experiment needs to get down to about a 3-in-1000 chance, which researchers hope to reach by the mid-2020s.

But the improvement on last year’s data, while modest, is “in a very interesting direction,” said Tom Browder, a physicist at the University of Hawaii. The hints of new physics haven’t yet gone away, as we might expect them to do if the initial results were due to chance. Results are also trickling in from another experiment, the 810-kilometer-long NOvA at the Fermi National Accelerator Laboratory outside Chicago. Last year it released its first set of neutrino data, with antineutrino results expected next summer. And although these first CP-violation results will also not be statistically significant, if the NOvA and T2K experiments agree, “the consistency of all these early hints” will be intriguing, said Mark Messier, a physicist at Indiana University

A planned upgrade of the Super-K detector might give the researchers a boost. Next summer, the detector will be drained for the first time in over a decade, then filled again with ultrapure water. This water will be mixed with gadolinium sulfate, a type of salt that should make the instrument much more sensitive to electron antineutrinos. “The gadolinium doping will make the electron antineutrino interaction easily detectable,” said Browder. That is, the salt will help the researchers to separate antineutrino interactions from neutrino interactions, improving their ability to search for CP violations.

“Right now, we are probably willing to bet that CP is violated in the neutrino sector, but we won’t be shocked if it is not,” said André de Gouvêa, a physicist at Northwestern University. Wascko is a bit more optimistic. “The 2017 T2K result has not yet clarified our understanding of CP violation, but it shows great promise for our ability to measure it precisely in the future,” he said. “And perhaps the future is not as far away as we might have thought last year.”

The real reason that nothing goes faster than the Light.

Light is faster than anything else (Credit: Amana Images Inc/Alamy Stock Photo)

It was September 2011 and physicist Antonio Ereditato had just shocked the world.

The announcement he had made promised to overturn our understanding of the Universe. If the data gathered by 160 scientists working on the OPERA project were correct, the unthinkable had been observed.

Particles – in this case, neutrinos – had travelled faster than light.

This time the scientists got it wrong

According to Einstein’s theories of relativity, this should not have been possible. And the implications for showing it had happened were vast. Many bits of physics might have to be reconsidered.

Although Ereditato said that he and his team had “high confidence” in their result, they did not claim that they knew it was completely accurate. In fact, they were asking for other scientists to help them understand what had happened.

In the end, it turned out the OPERA result was wrong. A timing problem had been caused by a poorly connected cable that should have been transmitting accurate signals from GPS satellites.

There was an unexpected delay in the signal. As a consequence, the measurements of how long the neutrinos took to travel the given distance were off by about 73 nanoseconds, making it look as though they had whizzed along more quickly than light could have done.

Despite months of careful checks prior to the experiment, and plentiful double-checking of the data afterwards, this time the scientists got it wrong. Ereditato resigned, though many pointed out that mistakes like these happen all the time in the hugely complex machinery of particle accelerators.

Why was it such a big deal to suggest – even as a possibility – that something had travelled faster than light? And are we really sure that nothing can?

Let’s take the second of those questions first. The speed of light in a vacuum is 299,792.458 km per second – just shy of a nice round 300,000km/s figure. That is pretty nippy. The Sun is 150 million km away from Earth and light takes just eight minutes and 20 seconds to travel that far.

He needed to use ever-larger amounts of additional energy to make ever-smaller differences to the speed

Can any of our own creations compete in a race with light? One of the fastest human-made objects ever built, the New Horizons space probe, passed by Pluto and Charon in July 2015. It has reached a speed relative to the Earth of just over 16km/s, well below 300,000km/s.

However, we have made tiny particles travel much faster than that. In the early 1960s, William Bertozzi at the Massachusetts Institute of Technology experimented with accelerating electrons at greater and greater velocities.

Because electrons have a charge that is negative, it is possible to propel – or rather, repel – them by applying the same negative charge to a material. The more energy applied, the faster the electrons will be accelerated.

You might imagine that you just need to increase the energy applied in order to reach the required speed of 300,000km/s, but it turns out that it just is not possible for electrons to move that fast. Bertozzi’s experiments found that using more energy did not simply cause a directly proportional increase in electron speed.

As objects travel faster and faster, they get heavier and heavier

Instead, he needed to use ever-larger amounts of additional energy to make ever-smaller differences to the speed the electrons moved. They got closer and closer to the speed of light but never quite reached it.

Imagine travelling towards a door in a series of moves, in each of which you travel exactly half the distance between your current position and the door. Strictly speaking, you will never reach the door, because after every move you make you still have some distance still to travel. That is the kind of problem Bertozzi encountered with his electrons.

But light is made up of particles called photons. Why can these particles travel at the speed of light when particles like electrons cannot?

 “As objects travel faster and faster, they get heavier and heavier – the heavier they get, the harder it is to achieve acceleration, so you never get to the speed of light,” saysRoger Rassool, a physicist at the University of Melbourne, Australia.

“A photon actually has no mass,” he says. “If it had mass, it couldn’t travel at the speed of light.”

For the most part it is fair to say that light travels at 300,000km/s

Photons are pretty special. Not only do they have no mass, which gives them free reign when it comes to zipping about in vacuums like space, they do not have to speed up. The natural energy they possess, travelling as they do in waves, means that the moment they are created, they are already at top speed.

In fact, in some ways it makes more sense to think of light as energy rather than as a flow of particles, though truthfully it is – a little confusingly – both.

Still, light sometimes appears to travel more slowly than we might expect. Although internet technicians like to talk about communications travelling at “the speed of light” through optical fibres, light actually travels around 40% slower through the glass of those fibres than it would through a vacuum.

In reality, the photons are still travelling at 300,000km/s, but they are encountering a kind of interference caused by other photons being released from the glass atoms as the main light wave travels past. It is a tricky concept to get your head around, but it is worth noting.

Similarly, special experiments with individual photons have managed to slow them down by altering their shape.

 Still, for the most part it is fair to say that light travels at 300,000km/s. We really have not observed or created anything that can go quite that quickly, or indeed more quickly. There are a few special cases, mentioned below, but before those, let’s tackle that other question. Why is it so important that this speed of light rule be so strict?

Even though the distance has increased, Einstein’s theories insist that the light is still travelling at the same speed

The answer lies, as so often in physics, with a man named Albert Einstein. His theory of special relativity explores many of the consequences of these universal speed limits.

One of the important elements in the theory is the idea that the speed of light is a constant. No matter where you are or how fast you are travelling, light always travels at the same speed.

But that creates some conceptual problems.

Imagine shining light from a torch up to a mirror on the ceiling of a stationary spacecraft. The light will shine upwards, reflect off the mirror, and come down to hit the floor of the spacecraft. Let’s say the distance travelled is 10m.

Now let’s imagine that the spacecraft begins travelling at a hair-raising speed, many thousands of kilometres per second.

Time travels slower for people travelling in fast-moving vehicles

When you shine the torch again, the light will still seem to behave as before: it will shine upwards, hit the mirror, and bounce back to hit the floor. But in order to do so the light will have to travel diagonally rather than just vertically. After all, the mirror is now moving quickly along with the spacecraft.

The distance the light travels therefore increases. Let’s imagine it has increased overall by 5m. That is 15m in total, instead of 10m.

And yet, even though the distance has increased, Einstein’s theories insist that the light is still travelling at the same speed. Since speed is distance divided by time, for the speed to be the same but the distance to have increased, time must also have increased.

Yes, time itself must have got stretched. That sounds wacky, but it has been proved experimentally.

 It is a phenomenon known as time dilation. It means time travels slower for people travelling in fast-moving vehicles, relative to those who are stationary.

For example, time runs 0.007 seconds slower for astronauts on the International Space Station, which is moving at 7.66 km/s relative to Earth, compared to people on the planet.

The muons are generated with so much energy that they’re moving at velocities very near the speed of light

Things get interesting for particles, like the electrons mentioned above, that can travel close to the speed of light. For these particles, the degree of time dilation can be great.

Steven Kolthammer, an experimental physicist at the University of Oxford in the UK, points to an example involving particles called muons.

Muons are unstable: they quickly fall apart into simpler particles. So quickly, in fact, that most muons leaving the Sun should have decayed away by the time they reach the Earth. But in reality muons arrive at Earth from the Sun in great numbers. This was something scientists long found difficult to understand.

“The answer to this puzzle is that the muons are generated with so much energy that they’re moving at velocities very near the speed of light,” says Kolthammer. “So their sense of time, if you will, their internal clock, actually runs slow.”

The muons were “kept alive” longer than expected, relative to us, thanks to a real, natural bending of time.

 When objects move quickly relative to other objects, their length contracts as well. These consequences, time dilation and length contraction, are both examples of how space-time changes based on the motion of things – like you, me or a spacecraft – that have mass.

There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light

Crucially, as Einstein said, light does not get affected in the same way – because it has no mass. That is why it is so important that all of these principles go hand-in-hand. If things could travel faster than light, they would disobey these fundamental laws that describe how the Universe works.

That sums up the key principles. At this point, we can consider a few exceptions and caveats.

For one thing, while nothing has ever been observed travelling faster than light, that does not mean it is not theoretically possible to break this speed limit in very special circumstances.

Take, for instance, the expansion of the Universe itself. There are galaxies in the Universe moving away from one another at a velocity greater than the speed of light.

There is yet another possible way in which faster-than-light travel is technically possible

Another interesting situation concerns particles that seem to be expressing the same properties at the same time, no matter how far apart they are.

This is called “quantum entanglement”. In essence, a photon will flip back and forth between two possible states at random – but the flips will exactly mirror the flipping of another photon somewhere else, if the two are entangled.

Two scientists each studying their own photon will therefore get the same results at the same time, faster than the speed of light.

However, in both these examples it is crucial to note that no information is travelling faster than the speed of light between two entities. We can calculate the Universe’s expansion, but we cannot observe any faster-than-light objects in it: they have disappeared from view.

 As for the two scientists with their photons, while they might achieve the same result simultaneously, they could not confirm the fact with each other any more quickly than light could travel between them.

“This gets us out of any problems, because if you are able to send signals faster than light you can construct bizarre paradoxes, under which information can somehow go backwards in time,” says Kolthammer.

What if instead you actively distorted space-time in a controlled way?

There is yet another possible way in which faster-than-light travel is technically possible: rifts in space-time itself that allow a voyager to escape the rules of normal travel.

Gerald Cleaver at Baylor University in Texas has considered the possibility that we might one day build a faster-than-light spacecraft. One of the ways to do this might be to travel through a wormhole. These are loops in space-time, perfectly consistent with Einstein’s theories, which could allow an astronaut to hop from one bit of the Universe to another via an anomaly in space-time, a sort of cosmic shortcut.

The object travelling through the wormhole would not exceed the speed of light, but it could theoretically reach a certain destination faster than light could if it took a “normal” route.

But wormholes might not be available for space travel. What if instead you actively distorted space-time in a controlled way, to travel faster than 300,000km/s relative to someone else?

 Cleaver has investigated an idea known as an “Alcubierre drive”, proposed by theoretical physicist Miguel Alcubierre in 1994. Essentially, it describes a situation in which space-time is squashed in front of a spacecraft, pulling it forward, while space-time behind the craft is expanded, creating a pushing effect.

“But then,” says Cleaver, “there’s the issues of how to do that, and how much energy it’s going to take.”

Faster-than-light travel remains a fantasy at the moment

In 2008, he and graduate student Richard Obousy calculated some of the energies involved.

“We worked out that, if you assume a ship that’s about 10m x 10m x 10m – you’re talking 1,000 cubic metres – that the amount of energy it would take to start the process would need to be on the order of the entire mass of Jupiter.”

After that, the energy would have to continue being provided constantly in order to ensure the process did not fail. No-one knows how that would ever be possible, or what the technology to do it would look like.

“I don’t want to be misquoted centuries from now for predicting it would never come about,” says Cleaver, “but right now I don’t see solutions.”

Faster-than-light travel, then, remains a fantasy at the moment.

But while that may sound disappointing, light is anything but. In fact, for most of this article we have been thinking in terms of visible light. But really light is much, much more than that.

 Everything from radio waves to microwaves to visible light, ultraviolet radiation, X-rays and the gamma rays emitted by decaying atoms – all of these fantastic rays are made of the same stuff: photons.

The difference is the energy, and therefore their wavelength. Collectively these rays make up the electromagnetic spectrum. The fact that radio waves, for instance, travel at the speed of light is enormously useful for communications.

Space-time is malleable and that allows for everyone to experience the same laws of physics

In his research, Kolthammer builds circuitry that uses photons to send signals from one part of the circuit to another, so he is well placed to comment on the usefulness of light’s awesome speed.

“The idea that we’ve built the infrastructure of the internet for example and even before that, radio, based on light, certainly has to do with the ease with which we can transmit it,” he points out.

He adds that light acts as a communicating force for the Universe. When electrons in a mobile phone mast jiggle, photons fly out and make other electrons in your mobile phone jiggle too. It is this process that lets you make a phone call.

The jiggling of electrons in the Sun also emits photons – at fantastic rates – which, of course, produces the light that nourishes life on Earth.

Light is the Universe’s broadcast. That speed – 299,792.458 km/s – remains reassuringly constant. Meanwhile, space-time is malleable and that allows for everyone to experience the same laws of physics no matter their position or motion.

Six reasons to get excited about neutrinos

The headlines from the recent International Conference on High Energy Physics (ICHEP) in Chicago trended sad, focused on the dearth of discoveries from the Large Hadron Collider. (See, for example, “Prospective particle disappears in new LHC data.”) Yet there was some optimism to be found in the Windy City, particularly among neutrino physicists. Here are six reasons to believe that neutrinos might provide the window into new physics that the LHC has not:

Neutrinos are proof that the standard model is wrong. Sure, we know that dark matter and dark energy are missing from the standard model. But neutrinos are standard-model members, and the theoretical predictions are wrong. Prevailing theory says that neutrinos are massless; the Nobel-winning experiments at the Sudbury Neutrino Observatory and Super-Kamiokande demonstrated definitively that neutrinos oscillate between three flavors (electron, muon, and tau) and thus have mass. André de Gouvêa, a theoretical physicist at Northwestern University, deems neutrinos the “only palpable evidence of physics beyond the standard model.” Everything we learn about neutrinos in the coming years is new physics.

This signal from May 2014 denotes the detection of an electron neutrino by Fermilab’s NOvA experiment. Credit: NOvA Neutrino Experiment

This signal from May 2014 denotes the detection of an electron neutrino by Fermilab’s NOvA experiment.

Neutrinos’ ability to morph from one flavor to another is only now starting to be understood. Each of neutrinos’ three flavors is actually a quantum superposition of three different mass states. By understanding the interplay of the three mass states, characterized by parameters called mixing angles, physicists can pin down how neutrinos transform between flavors. Fresh data from the NOvA experiment at Fermilab near Chicago suggest that neutrino mixing may not be as simple as most theories predict.

Neutrinos may exhibit charge conjugation–parity (CP) violation. All known examples of CP violation, in which particle decays proceed differently with matter than with antimatter, take place in processes involving quark-containing particles like kaons and B mesons. But at the Neutrino 2016 meeting in London and at ICHEP, the T2K experiment offered fresh data hinting at matter–antimatter asymmetry for neutrinos. After firing beams of muon neutrinos and antineutrinos at the Super-Kamiokande detector in Japan, scientists expected to detect 23 electron neutrinos and 7 electron antineutrinos; instead they have spotted 32 and 4, respectively. T2K isn’t anywhere close to achieving a 5 σ result, but the evidence for CP violation seems to be growing as the experiment acquires more data.

Neutrinos may be the first fundamental particles that are Majorana fermions. Because the neutrino is the only fermion that is electrically neutral, it is also the only one that could be a Majorana fermion, a particle that is identical to its antiparticle. Learning whether neutrinos are Majorana particles or typical Dirac fermions would provide invaluable insight as to how neutrinos acquired mass at the dawn of the universe, de Gouvêa says. To determine the nature of neutrinos, physicists are hunting for a process called neutrinoless double beta decay. In typical double beta decay, two neutrons transform into protons and emit a pair of antineutrinos. If those antineutrinos are Majorana particles, they could annihilate each other. A 16 August paper from the KamLAND-Zen experiment in Japan reports the most stringent limits for the rate of neutrinoless double beta decay, further constraining the possibility that neutrinos are Majorana particles.

Another neutrino flavor may be waiting to be discovered. The discovery of a fourth neutrino flavor, the sterile neutrino, would make every particle physicist forget about the LHC’s particle drought. Such a neutrino could enable physicists to explain dark matter or the absence of antimatter in the universe. The Antarctic detector IceCube just reported a negative result in the hunt for a sterile neutrino, but results from prior experiments still leave some wiggle room for the particle’s existence.

Multiple powerful neutrino experiments are on the horizon. The NOvA experiment is up and running and delivering data that, at least so far, seem to complement T2K’s hints of CP violation. Fermilab scientists are already excited about the Deep Underground Neutrino Experiment, which should come on line around 2025.Hyper-Kamiokande, a megadetector in Japan with a million-ton tank of water for neutrino detection, should start operations around the same time.



From where do these neutrinos come?

The IceCube Neutrino Observatory near the South Pole of the Earth has begun to detect nearly invisible particles of very high energy. Although these rarely-interacting neutrinos pass through much of the Earth just before being detected, where they started remains a mystery. 

Pictured here is IceCube’s Antarctic lab accompanied by a cartoon depicting long strands of detectors frozen into the crystal clear ice below.

Candidate origins for these cosmic neutrinos include the violent surroundings of supermassive black holes at the centers of distant galaxies, and tremendous stellar explosions culminating in supernovas and gamma ray bursts far across the universe. As IceCube detects increasingly more high energy neutrinos, correlations with known objects may resolve this cosmic conundrum — or we may never know.

Strange Neutrinos from the Sun Detected for the First Time .

An underground neutrino detector has found particles produced by the fusion of two protons in the sun’s core
The Borexino neutrino detector

The Borexino neutrino detector uses a sphere filled with liquid scintillator that emits light when excited. This inner vessel is surrounded by layers of shielding and by about 2,000 photomultiplier tubes to detect the light flashes.

Deep inside the sun pairs of protons fuse to form heavier atoms, releasing mysterious particles called neutrinos in the process. These reactions are thought to be the first step in the chain responsible for 99 percent of the energy the sun radiates, but scientists have never found proof until now. For the first time, physicists have captured the elusive neutrinos produced by the sun’s basic proton fusion reactions.

Earth should be teeming with such neutrinos—calculations suggest about 420 billion of them stream from the sun onto every square inch of our planet’s surface each second—yet they are incredibly hard to find. Neutrinos almost never interact with regular particles and usually fly straight through the empty spaces between the atoms in our bodies and all other normal matter. But occasionally they will collide with an atom and knock an electron loose, creating a quick flash of light visible to extremely sensitive detectors. That is how the Borexino experiment at Italy’s Gran Sasso National Laboratory found them. Its detection of so-called pp neutrinos—neutrinos created by the fusion of two protons in the sun—was a feat far from guaranteed. “Their existence was not in question, but whether some group was capable of building such an exquisitely pristine detector to see these low-energy neutrinos in real time, event by event, was,” says Wick Haxton, a physicist at the University of California, Berkeley, who was not involved in the experiment. “Borexino accomplished this through a long campaign to reduce and understand background events.”

[Slide Show: Giant Experiments Seek Out Tiny Neutrinos]

Borexino uses a vat of liquid scintillator—a material designed to emit light when excited—contained in a large sphere surrounded by 1,000 tons of water, cocooned in layers upon layers of shielding and buried 1.4 kilometers underground. These defenses are meant to keep out everything but neutrinos, thereby excluding all other background radiation that could mimic the signal. “Unfortunately for the pp neutrinos all this is not enough,” says Andrea Pocar of the University of Massachusetts Amherst who is also a member of the Borexino collaboration and lead author of a paper reporting the results in the August 28 Nature (Scientific American is part of Nature Publishing Group).

Some background contamination cannot be shielded because it originates inside the experiment. “The main background is the presence of carbon 14 in the scintillator itself,” Pocar says. Carbon 14 is a radioactive isotope common on Earth. Its predictable decay schedule allows archaeologists to date ancient specimens. When it decays, however, carbon 14 releases an electron that creates a flash of light very similar to that of a pp neutrino. The physicists had to look in a narrow sliver of energies where pp neutrinos can be distinguished from errant carbon 14 decays. Even then, once in a while two carbon 14 atoms in the scintillator will decay simultaneously, and the energies of the electrons they release can “pile up” on top of one another to exactly mimic the pp neutrino flash. “We had to understand these pileup events very precisely and subtract them out,” Pocar explains. The team invented a new way to count the events, and gathered data over multiple years before the researchers were convinced they had isolated a true signal. “This was a very difficult measurement to make,” says Mark Chen of Queen’s University in Ontario, who was not involved in the project. “The campaign by Borexino to purify the liquid scintillator in their detector paid off.”

Borexino’s discovery of pp solar neutrinos is a reassuring confirmation of physicists’ main theoretical models describing the sun. Previous experiments have found higher-energy solar neutrinos created by later stages of the fusion process involving the decay of boron atoms. But the lower-energy pp neutrinos were harder to find; their detection completes the picture of the sun’s fusion chain as well as bolsters plans for next-generation Earthbound neutrino experiments.

A strange quirk of these elementary particles is that they come in three flavors—called electron, muon and tau—and they have the bizarre ability to swap flavors, or “oscillate.” Because of the complex particularities of proton fusion reactions, all of the sun’s neutrinos happen to be born as electron neutrinos. By the time they reach Earth, however, some portion of them have morphed into muon and tau neutrinos.

Each neutrino flavor has a slightly different mass, although physicists do not yet know exactly what those masses are. Determining the masses and how they are ordered among the three flavors is one of the most important goals of current neutrino experiments. The mass differences between flavors are the main factor affecting how neutrinos oscillate.

If neutrinos are traveling through matter, their interactions with it will also alter their oscillation rates. The oscillations of higher-energy neutrinos, it turns out, are more altered by matter, leading to a larger chance they will oscillate—and therefore to fewer of them surviving as electron neutrinos by the time they reach Earth.

The Sudbury Neutrino Observatory in Ontario and Japan’s Super-Kamiokandeexperiment measured this phenomenon decades ago when they detected the higher-energy solar neutrinos from boron decays. Now, Borexino’s findings confirm the effect: more of the lower-energy neutrinos seen by Borexino persisted as electron flavor than the higher-energy neutrinos measured by those previous experiments. “This is important because matter effects have so far only been seen in the sun, yet we want to use this effect on Earth in future ‘long-baseline neutrino experiments’ to fully determine the pattern of neutrino masses,” Haxton says.

These experiments, such as the Fermi National Accelerator Laboratory’s Long-Baseline Neutrino Experiment (LBNE) planned to open in 2022, will probe how neutrinos traveling though matter oscillate. Rather than using solar neutrinos, these projects will create powerful beams of neutrinos in particle accelerators and fine-tune their pathways to make precision measurements. Fermilab’s experiment will generate a stream of neutrinos from its base laboratory near Chicago to the Sanford Underground Research Facility in South Dakota. As the neutrinos fly through about 1,285 kilometers of Earth’s mantle on their journey (the so-called “long baseline”), many will oscillate. By studying how the mantle matter interacts with the different flavors to affect their oscillation rates, the researchers hope to reveal which neutrino flavors are lighter and which are heavier.

Solving the neutrino mass puzzle, in turn, could point to a deeper theory of particle physics than the current Standard Model, which does not account for neutrino masses. Borexino’s latest feat of precision neutrino measurement suggests that experiments are finally becoming powerful enough to pry such secrets from the evasive particles.

Borexino Collaboration succeeds in spotting pep neutrinos emitted from the sun.

 Now, researchers deep beneath the ground in a mountain in Italy, working together in a group known as the Borexino Collaboration, have spotted the more elusive proton to electron to proton neutrino, a pep reaction that results in the formation of deuterium, a heavy form of hydrogen. To detect them, the team, as they describe in their paper published in Physical Review Letters, the team had to develop a method of filtering out virtually all other neutrinos, including those from outer space.

To detect the presence of neutrinos, researchers build underground facilities to use the Earth’s natural filtering abilities to remove particle clutter. Then, they fill a big vat with a special kind of liquid that reacts with the type of neutrino they are looking for. When one of the neutrinos strikes the liquid, a tiny flash or sparkle occurs. By measuring the number of sparkles that occur over a period of time the researchers can describe the amount of such neutrinos that are emitted by the sun, which helps to more fully understand the nuclear reactions that occur inside of it.

Pp neutrinos have been easy to count, they are plentiful and high energy, which makes it easy to detect them when hitting the liquid. Pep, neutrinos on the other hand are low energy and more elusive and up till now have been mostly a theoretical concept. To detect their presence the team had to devise a means of filtering out virtually all other cosmic particles and then use a liquid that causes a sparkle when struck by a particle that has just 1.44 mega-electron-volts of energy, the distinctive signature of the pep neutrino. The team succeeded on both counts and were able to detect 3.1 pepneutrino strikes per day, per 100 tons of liquid.

The new technique for cleaning and filtering out unwanted particles is ground breaking work and likely will be used by other scientists looking to measure other particles in other research efforts.

We observed, for the first time, solar neutrinos in the 1.0–1.5 MeV energy range. We determined the rate of pep solar neutrino interactions in Borexino to be 3.1±0.6stat±0.3syst  counts/(day·100  ton). Assuming the pep neutrino flux predicted by the standard solar model, we obtained a constraint on the CNO solar neutrino interaction rate of <7.9  counts/(day·100  ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. The necessary sensitivity was achieved by adopting data analysis techniques for the rejection of cosmogenic 11C, the dominant background in the 1–2 MeV region. Assuming the Mikheyev-Smirnov-Wolfenstein large mixing angle solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6±0.3)×108  cm-2 s-1 and <7.7×108  cm-2 s-1 (95% C.L.), respectively, in agreement with both the high and low metallicity standard solar models. These results represent the first direct evidence of the pep neutrino signal and the strongest constraint of the CNO solar neutrino flux to date.