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.”

New Insights Into Antimatter, 20 Years In the Making.



Anti-hydrogen particles react with the walls of the experimental chamber, producing the flashes of light seen here.

Antimatter is more than a science fiction concept that allows engineers to power the Enterprise. It’s an actual — albeit small — constituent of our universe. While antimatter is rare, it can exist under the right conditions. Information about the way antimatter behaves provides a powerful tool for testing the Standard Model of particle physics we currently use to understand the forces that govern the way particles behave.

For Every Particle, An Anti-

Antimatter was first predicted by British physicist Paul Dirac in 1928. He proposed that every particle of matter should have a corresponding antiparticle. These antiparticles are identical to their particle counterparts in every way except for charge. For example, the antimatter counterpart to the negatively-charged electron is the positively-charged antielectron, also called the positron.

When matter and antimatter meet, they annihilate each other and leave only energy behind. The Big Bang should have created matter and antimatter in equal amounts, but today, our universe is dominated by matter, with very little antimatter present. Understanding why this asymmetry exists would be a significant step towards understanding the origin and evolution of our universe.

However, naturally-occurring antimatter is often immediately destroyed when it encounters the universe’s abundant matter. Today, particle physicists can routinely create antimatter for study at the CERN Antiproton Decelerator facility, which has led to several new breakthroughs in the characterization of antimatter.

Identical Lines

In a recently-published Nature article, CERN’s ALPHA collaboration has announced the very first measurement of a spectral line in an antihydrogen atom. This result, which was over 20 years in the making, was achieved using a laser to observe the 1S-2S transition in antihydrogen. To within experimental limits, the ALPHA collaboration’s results show that this transition is identical in both hydrogen and antihydrogen atoms — a condition required by the Standard Model. If these transitions were different, it would essentially break our current understanding of physics.

The 1S-2S transition is one of many that contribute to hydrogen’s spectrum. A spectrum is created when electrons that have been excited by radiation “fall” from a higher energy level inside an atom to a lower one. This process releases energy at precise wavelengths. Each element produces a unique spectrum, like a fingerprint. Astronomers often use spectra to determine an object’s composition based on the light it produces.

Making Antimatter

To observe the 1S-2S transition in antihydrogen, the ALPHA collaboration first had to create antiatoms and keep them stable — no easy task. ALPHA’s recipe for antihydrogen consisted of mixing plasmas containing antiprotons and positrons together to produce antihydrogen atoms. The resulting antiatoms were then magnetically trapped to hold them for experimentation.

From an original batch of 90,000 antiprotons, researchers could create 25,000 antihydrogen atoms; of these, the ALPHA collaboration managed to trap and study an average of 14 antiatoms per trial. By illuminating the antihydrogen atoms with a laser tuned to provide exactly the energy needed to achieve the proposed transition, researchers were then able to observe the resulting emission to look for deviations from the spectrum of normal hydrogen.

The ALPHA collaboration’s result, along with the results from other antimatter experiments performed by the ASACUSA and BASE collaborations, shows just how far antimatter research has come at CERN. The ALPHA collaboration plans to further refine the precision of their results in the future for even more robust testing of the Standard Model. Such high-precision antimatter testing may also be able to shed light on the matter-antimatter asymmetry we observe in our universe.

What Is Antimatter And Why Are We Searching For It?

Scientists have been looking for antimatter since the early 1920’s. Is antimatter real?

Antimatter catches a wave: Accelerating positrons with plasma is a step toward smaller, cheaper particle colliders

Antimatter catches a wave at SLAC
Simulation of high-energy positron acceleration in an ionized gas, or plasma — a new method that could help power next-generation particle colliders. The image shows the formation of a high-density plasma (green/orange color) around a positron beam moving from the bottom right to the top left. Plasma electrons pass by the positron beam on wave-like trajectories (lines) Credit: W. An/UCLA

A study led by researchers from the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory and the University of California, Los Angeles has demonstrated a new, efficient way to accelerate positrons, the antimatter opposites of electrons. The method may help boost the energy and shrink the size of future linear particle colliders – powerful accelerators that could be used to unravel the properties of nature’s fundamental building blocks.

The scientists had previously shown that boosting the energy of charged particles by having them “surf” a wave of , or plasma, works well for . While this method by itself could lead to smaller accelerators, electrons are only half the equation for future colliders. Now the researchers have hit another milestone by applying the technique to positrons at SLAC’s Facility for Advanced Accelerator Experimental Tests (FACET), a DOE Office of Science User Facility.

“Together with our previous achievement, the new study is a very important step toward making smaller, less expensive next-generation electron-positron colliders,” said SLAC’s Mark Hogan, co-author of the study published today in Nature. “FACET is the only place in the world where we can accelerate positrons and electrons with this method.”

SLAC Director Chi-Chang Kao said, “Our researchers have played an instrumental role in advancing the field of plasma-based accelerators since the 1990s. The recent results are a major accomplishment for the lab, which continues to take accelerator science and technology to the next level.”

Shrinking Particle Colliders

Researchers study matter’s fundamental components and the forces between them by smashing highly energetic particle beams into one another. Collisions between electrons and positrons are especially appealing, because unlike the protons being collided at CERN’s Large Hadron Collider – where the Higgs boson was discovered in 2012 – these particles aren’t made of smaller constituent parts.

“These collisions are simpler and easier to study,” said SLAC’s Michael Peskin, a theoretical physicist not involved in the study. “Also, new, exotic particles would be produced at roughly the same rate as known particles; at the LHC they are a billion times more rare.”

Antimatter catches a wave at SLAC
Future particle colliders will require highly efficient acceleration methods for both electrons and positrons. Plasma wakefield acceleration of both particle types, as shown in this simulation, could lead to smaller and more powerful colliders than today’s machines. Credit: F. Tsung/W. An/UCLA/SLAC National Accelerator Laboratory

However, current technology to build electron-positron colliders for next-generation experiments would require accelerators that are tens of kilometers long. Plasma wakefield acceleration is one way researchers hope to build shorter, more economical accelerators.

Previous work showed that the method works efficiently for electrons: When one of FACET’s tightly focused bundles of electrons enters an ionized gas, it creates a plasma “wake” that researchers use to accelerate a trailing second electron bunch.

Creating a Plasma Wake for Antimatter

For positrons – the other required particle ingredient for electron-positron colliders – plasma wakefield acceleration is much more challenging. In fact, many scientists believed that no matter where a trailing positron bunch was placed in a wake, it would lose its compact, focused shape or even slow down.

“Our key breakthrough was to find a new regime that lets us accelerate positrons in plasmas efficiently,” said study co-author Chandrashekhar Joshi from UCLA.

Instead of using two separate particle bunches – one to create a wake and the other to surf it – the team discovered that a single positron bunch can interact with the plasma in such a way that the front of it generates a wake that both accelerates and focuses its trailing end. This occurs after the positrons have traveled about four inches through the plasma.

Antimatter catches a wave at SLAC
Computer simulations of the interaction of electrons (left, red areas) and positrons (right, red areas) with a plasma. The approximate locations of tightly packed bundles of particles, or bunches, are within the dashed lines. Left: For electrons, a drive bunch (on the right) generates a plasma wake (white area) on which a trailing electron bunch (on the left) gains energy. Right: For positrons, a single bunch can interact with the plasma in such a way that the front of the bunch generates a wake that accelerates the bunch tail. Credit: W. An/UCLA

“In this stable state, about 1 billion positrons gained 5 billion electronvolts of energy over a short distance of only 1.3 meters,” said former SLAC researcher Sébastien Corde, the study’s first author, who is now at the Ecole Polytechnique in France. “They also did so very efficiently and uniformly, resulting in an accelerated bunch with a well-defined energy.”

Looking into the Future

All of these properties are important qualities for particle beams in accelerators. In the next step, the team will look to further improve their experiment.

“We performed simulations to understand how the stable state was created,” said co-author Warren Mori of UCLA. “Based on this understanding, we can now use simulations to look for ways of exciting suitable wakes in an improved, more controlled way. This will lead to ideas for future experiments.”

Although plasma-based particle colliders will not be built in the near future, the method could be used to upgrade existing accelerators much sooner.

“It’s conceivable to boost the performance of linear by adding a very short plasma accelerator at the end,” Corde said. “This would multiply the accelerator’s energy without making the entire structure significantly longer.”