Is the neutrino its own antiparticle?

Almost every particle has an antimatter counterpart: a particle with the same mass but opposite charge, among other qualities.

This seems to be true of neutrinos, tiny particles that are constantly streaming through us. Judging by the particles released when a neutrino interacts with other matter, scientists can tell when they’ve caught a neutrino versus an antineutrino.

But certain characteristics of neutrinos and antineutrinos make scientists wonder: Are they one and the same? Are neutrinos their own antiparticles?

This isn’t unheard of. Gluons and even Higgs bosons are thought to be their own antiparticles. But if scientists discover neutrinos are their own antiparticles, it could be a clue as to where they get their tiny masses—and whether they played a part in the existence of our matter-dominated universe.
Dirac versus Majorana

The idea of the antiparticle came about in 1928 when British physicist Paul Dirac developed what became known as the Dirac equation. His work sought to explain what happened when electrons moved at close to the speed of light. But his calculations resulted in a strange requirement: that electrons sometimes have negative energy.

“When Dirac wrote down his equation, that’s when he learned antiparticles exist,” says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “Antiparticles are a consequence of his equation.”

Physicist Carl Anderson discovered the antimatter partner of the electron that Dirac foresaw in 1932. He called it the positron—a particle like an electron but with a positive charge.

Dirac predicted that, in addition to having opposite charges, antimatter partners should have opposite handedness as well.

A particle is considered right-handed if its spin is in the same direction as its motion. It is considered left-handed if its spin is in the opposite direction.

Dirac’s equation allowed for neutrinos and anti-neutrinos to be different particles, and, as a result, four types of neutrino were possible: left- and right-handed neutrinos and left- and right-handed antineutrinos. But if the neutrinos had no mass, as scientists thought at the time, only left-handed neutrinos and right-handed antineutrinos needed to exist.

In 1937, Italian physicist Ettore Majorana debuted another theory: Neutrinos and antineutrinos are actually the same thing. The Majorana equation described neutrinos that, if they happened to have mass after all, could turn into antineutrinos and then back into neutrinos again. 

The matter-antimatter imbalance

 

Whether neutrino masses were zero remained a mystery until 1998, when the Super-Kamiokande and SNO experiments found they do indeed have very small masses—an achievement recognized with the 2015 Nobel Prize for Physics. Since then, experiments have cropped up across Asia, Europe and North America searching for hints that the neutrino is its own antiparticle.

The key to finding this evidence is something called lepton number conservation. Scientists consider it a fundamental law of nature that lepton number is conserved, meaning that the number of leptons and anti-leptons involved in an interaction should remain the same before and after the interaction occurs.

Scientists think that, just after the big bang, the universe should have contained equal amounts of matter and antimatter. The two types of particles should have interacted, gradually canceling one another until nothing but energy was left behind. Somehow, that’s not what happened.

Finding out that lepton number is not conserved would open up a loophole that would allow for the current imbalance between matter and antimatter. And neutrino interactions could be the place to find that loophole.

Neutrinoless double-beta decay

 

Scientists are looking for lepton number violation in a process called double beta decay, says SLAC theorist Alexander Friedland, who specializes in the study of neutrinos.

In its common form, double beta decay is a process in which a nucleus decays into a different nucleus and emits two electrons and two antineutrinos. This balances leptonic matter and antimatter both before and after the decay process, so it conserves lepton number.

If neutrinos are their own antiparticles, it’s possible that the antineutrinos emitted during double beta decay could annihilate one another and disappear, violating lepton number conservation. This is called neutrinoless double beta decay.

Such a process would favor matter over antimatter, creating an imbalance.

“Theoretically it would cause a profound revolution in our understanding of where particles get their mass,” Friedland says. “It would also tell us there has to be some new physics at very, very high energy scales—that there is something new in addition to the Standard Model we know and love.”

It’s possible that neutrinos and antineutrinos are different, and that there are two neutrino and anti-neutrino states, as called for in Dirac’s equation. The two missing states could be so elusive that physicists have yet to spot them.

But spotting evidence of neutrinoless double beta decay would be a sign that Majorana had the right idea instead—neutrinos and antineutrinos are the same.

“These are very difficult experiments,” de Gouvêa says. “They’re similar to dark matter experiments in the sense they have to be done in very quiet environments with very clean detectors and no radioactivity from anything except the nucleus you're trying to study."

Physicists are still evaluating their understanding of the elusive particles.

“There have been so many surprises coming out of neutrino physics,” says Reina Maruyama, a professor at Yale University associated with the CUORE neutrinoless double beta decay experiment. “I think it’s really exciting to think about what we don’t know.”

Fermilab/SLAC

Supercomputing the Strange Difference Between Matter and Antimatter

Supercomputers such as Brookhaven Lab's Blue Gene/Q were essential for completing the complex calculation of direct CP symmetry violation. The same calculation would have required two thousand years using a laptop.

An international team of physicists including theorists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory has published the first calculation of direct "CP" symmetry violation—how the behavior of subatomic particles (in this case, the decay of kaons) differs when matter is swapped out for antimatter. Should the prediction represented by this calculation not match experimental results, it would be conclusive evidence of new, unknown phenomena that lie outside of the Standard Model—physicists' present understanding of the fundamental particles and the forces between them.

The current result—reported in the November 20 issue of Physical Review Letters—does not yet indicate such a difference between experiment and theory, but scientists expect the precision of the calculation to improve dramatically now that they've proven they can tackle the task. With increasing precision, such a difference—and new physics—might still emerge.

"This so called 'direct' symmetry violation is a tiny effect, showing up in just a few particle decays in a million," said Brookhaven physicist Taku Izubuchi, a member of the team performing the calculation. Results from the first, less difficult part of this calculation were reported by the same group in 2012.  However, it is only now, with completion of the second part of this calculation—which was hundreds of times more difficult than the first—that a comparison with the measured size of direct CP violation can be made.  This final part of the calculation required more than 200 million core processing hours on supercomputers, "and would have required two thousand years using a laptop," Izubuchi said.

The calculation determines the size of the symmetry violating effect as predicted by the Standard Model, and was compared with experimental results that were firmly established in 2000 at the European Center for Nuclear Research (CERN) and Fermi National Accelerator Laboratory.

"This is an especially important place to compare with the Standard Model because the small size of this effect increases the chance that other, new phenomena may become visible," said Robert Mawhinney of Columbia University.

"Although the result from this direct CP violation calculation is consistent with the experimental measurement, revealing no inconsistency with the Standard Model, the calculation is on-going with an accuracy that is expected to increase two-fold within two years," said Peter Boyle of the University of Edinburgh. "This leaves open the possibility that evidence for new phenomena, not described by the Standard Model, may yet be uncovered."

Matter-antimatter asymmetry

Physicists' present understanding of the universe requires that particles and their antiparticles (which have the same mass but opposite charge) behave differently. Only with matter-antimatter asymmetry can they hope to explain why the universe, which was created with equal parts of matter and antimatter, is filled mostly with matter today. Without this asymmetry, matter and antimatter would have annihilated one another leaving a cold, dim glow of light with no material particles at all.

The first experimental evidence for the matter-antimatter asymmetry known as CP violation was discovered in 1964 at Brookhaven Lab. This Nobel-Prize-winning experiment also involved the decays of kaons, but demonstrated what is now referred to as "indirect" CP violation. This violation arises from a subtle imperfection in the two distinct types of neutral kaons.

The target of the present calculation is a phenomenon that is even more elusive: a one-part-in-a-million difference between the matter and antimatter decay probabilities. The small size of this "direct" CP violation made its experimental discovery very difficult, requiring 36 years of intense experimental effort following the 1964 discovery of "indirect" CP violation.

This calculation required more than 200 million core processing hours on supercomputers and would have required two thousand years using a laptop.

While these two examples of matter-antimatter asymmetry are of very different size, they are related by a remarkable theory for which physicists Makoto Kobayashi and Toshihide Maskawa were awarded the 2008 Nobel Prize in physics. The theory provides an elegant and simple explanation of CP violation that manages to explain both the 1964 experiment and later CP-violation measurements in experiments at the KEK laboratory in Japan and the SLAC National Accelerator Laboratory in California.

"This new calculation provides another test of this theory—a test that the Standard Model passes, at least at the present level of accuracy," said Christoph Lehner, a Brookhaven Lab member of the team.

Although the Standard Model does successfully relate the matter-antimatter asymmetries seen in the 1964 and later experiments, this Standard-Model asymmetry is insufficient to explain the preponderance of matter over antimatter in the universe today.

"This suggests that a new mechanism must be responsible for the preponderance of matter of which we are made," said Christopher Kelly, a member of the team from the RIKEN BNL Research Center (RBRC). "This one-part-per-million, direct CP violation may be a good place to first see it. The approximate agreement between this new calculation and the 2000 experimental results suggests that we need to look harder, which is exactly what the team performing this calculation plans to do."

This calculation was carried out on the Blue Gene/Q supercomputers at the RIKEN BNL Research Center (RBRC), at Brookhaven National Laboratory, at the Argonne Leadership Class Computing Facility (ALCF) at Argonne National Laboratory, and at the DiRAC facility at the University of Edinburgh. The research was carried out by Ziyuan Bai, Norman Christ, Robert Mawhinney, and Daiqian Zhang of Columbia University; Thomas Blum of the University of Connecticut; Peter Boyle and Julien Frison of the University of Edinburgh; Nicolas Garron of Plymouth University; Chulwoo Jung, Christoph Lehner, and Amarjit Soni of Brookhaven Lab; Christopher Kelly, and Taku Izubuchi of the RBRC and Brookhaven Lab; and Christopher Sachrajda of the University of Southampton. The work was funded by the U.S. Department of Energy's Office of Science, by the RIKEN Laboratory of Japan, and the U.K. Science and Technology Facilities Council.  The ALCF is a DOE Office of Science User Facility.

Nanotechnology World Association

The Antimatter puzzle: Searching for clues with a highly integrated particle sensor

Researchers in Munich have presented a highly sensitive sensor for precise measurement of particle tracks. This is the first module for the Vertex Detector of the Belle II experiment at the Japanese accelerator center KEK. The detector is expected to start operation in 2017, recording collisions between electrons and their antiparticles, positrons.

With this experiment, scientists are pursuing the question of why there is no antimatter to speak of in today’s universe.

The sensor was developed by the MPG Halbleiterlabor (HLL), the semiconductor laboratory of the Max Planck Society (MPG). The Belle II Vertex Detector is being created in an international collaboration led by the Max Planck Institute for Physics.

In the experiment, scientists bring matter and antimatter particles into collision and analyze the decay patterns of the mesons and corresponding antiparticles produced. “We are searching for infinitesimal differences. For that, precisely spotting the decay location – also known as the vertex – is crucial,” explains Prof. Christian Kiesling, a researcher at the Max Planck Institute for Physics.

"These measurements are performed by the recently finished sensor, with characteristics that make it unrivaled worldwide.” "Made from silicon a thousand times more pure than conventional transistors or memory chips, the module integrates 200,000 DEPFET pixel cells on a surface area of eight square centimeters. (DEPFET stands for “depleted p-channel field-effect transistor.”)

It was invented at the MPG HLL and is fabricated there exclusively.The DEPFET component enables the detection of photons – or, as in this case, of highly energetic particles – with the utmost efficiency and precision. “The fundamental process is very similar to what goes on in a conventional photo or video camera,” explains Dr. Jelena Ninkovic, head of the HLL.

“However, the primary signal upon detection of individual photons or particles is very much smaller.”Self-amplifying sensorThis is where the major advantage of the DEPFET comes into play: The tiny primary signal is amplified within the sensor itself. Thus the DEPFET is the sensor material and the first stage of amplification rolled into one. Arranging many DEPFETs in a matrix produces an image sensor with which a particle’s point of origin can be precisely determined. “In our case,” Ninkovic continues, “this can be done with an accuracy of around one-hundredth of a millimeter.

”Control of the pixels in a matrix and rapid processing of the DEPFET signals require additional electronics, which have been produced in collaboration with German universities.

These electronics, in the form of application-specific integrated circuits (ASICs), are placed directly on the sensor substrate. The ASICs allow digitization of signals from the pixel matrix, as well as lossless data compression, to transmit them at the highest speed (50,000 images per second).

Complex electronics on a hair-thin filmThus the DEPFET matrix becomes a very complex module with maximal integration density, which despite all its complexity is extremely thin and light, so that the measurement of particle tracks cannot be corrupted by the sensor material itself.

The HLL has developed a unique technology for this purpose that makes it possible to fabricate extremely thin and highly integrated sensor modules. In the process, the sensitive part of the module, the DEPFET matrix, is thinned by a customized etching technique to 75 micrometers, roughly the thickness of a human hair.

These bendable silicon films are supported by a monolithically integrated framework, on which the readout and control electronics are mounted. The power supply and data lines run through a flexible ribbon cable, which is attached to the end of the module.The HLL technology makes it possible to arrange the thin DEPFET matrices in a cylindrical form, without any further support, around the interaction point of the experiment. With that, the highly precise measurement of particle tracks is becoming reality.

Max Planck Institute for Physics

Physicists Measure Force that Makes Antimatter Stick Together

Peering at the debris from particle collisions that recreate the conditions of the very early universe, scientists have for the first time measured the force of interaction between pairs of antiprotons. Like the force that holds ordinary protons together within the nuclei of atoms, the force between antiprotons is attractive and strong. 

The experiments were conducted at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science User Facility for nuclear physics research at DOE's Brookhaven National Laboratory. The findings, published in the journal Nature, could offer insight into larger chunks of antimatter,including antimatter nuclei previously detected at RHIC, and may also help scientists explore one of science's biggest questions: why the universe today consists mainly of ordinary matter with virtually no antimatter to be found.

"The Big Bang—the beginning of the universe—produced matter and antimatter in equal amounts. But that's not the world we see today. Antimatter is extremely rare. It's a huge mystery!" said Aihong Tang, a Brookhaven physicist involved in the analysis, which used data collected by RHIC's STAR detector. "Although this puzzle has been known for decades and little clues have emerged, it remains one of the big challenges of science. Anything we learn about the nature of antimatter can potentially contribute to solving this puzzle."

RHIC is the perfect place to study antimatter because it's one of the few places on Earth that is able to create the elusive stuff in abundant quantities.

RHIC is the perfect place to study antimatter because it's one of the few places on Earth that is able to create the elusive stuff in abundant quantities. It does this by slamming the nuclei of heavy atoms such as gold into one another at nearly the speed of light. These collisions produce conditions very similar to those that filled the universe microseconds after the Big Bang—with temperatures 250,000 times hotter than the center of the sun in a speck the size of a single atomic nucleus. All that energy packed into such a tiny space creates a plasma of matter's fundamental building blocks, quarks and gluons, and thousands of new particles—matter and antimatter in equal amounts.

"We are taking advantage of the ability to produce ample amounts of antimatter so we can conduct this study," said Tang.

The STAR collaboration has previous experience detecting and studying rare forms of antimatter—including anti-alpha particles, the largest antimatter nuclei ever created in a laboratory, each made of two antiprotons and two antineutrons. Those experiments gave them some insight into how the antiprotons interact within these larger composite objects. But in that case, "the force between the antiprotons is a convolution of the interactions with all the other particles," Tang said. "We wanted to study the simple interaction of unbound antiprotons to get a 'cleaner' view of this force."

To do that, they searched the STAR data from gold-gold collisions for pairs of antiprotons that were close enough to interact as they emerged from the fireball of the original collision. 

"We see lots of protons, the basic building blocks of conventional atoms, coming out, and we see almost equal numbers of antiprotons," said Zhengqiao Zhang, a graduate student in Professor Yu-Gang Ma's group from the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences, who works under the guidance of Tang when at Brookhaven. "The antiprotons look just like familiar protons, but because they are antimatter, they have a negative charge instead of positive, so they curve the opposite way in the magnetic field of the detector."

"By looking at those that strike near one another on the detector, we can measure correlations in certain properties that give us insight into the force between pairs of antiprotons, including its strength and the range over which it acts," he added.

The scientists found that the force between antiproton pairs is attractive, just like the strong nuclear force that holds ordinary atoms together. Considering they'd already discovered bound states of antiprotons and antineutrons—those antimatter nuclei—this wasn't all that surprising. When the antiprotons are close together, the strong force interaction overcomes the tendency of the like (negatively) charged particles to repel one another in the same way it allows positively charged protons to bind to one another within the nuclei of ordinary atoms.

In fact, the measurements show no difference between matter and antimatter in the way the strong force behaves. That is, within the accuracy of these measurements, matter and antimatter appear to be perfectly symmetric. That means, at least with the precision the scientists were able to achieve, there doesn't appear to be some asymmetric quirk of the strong force that can account for the continuing existence of matter in the universe and the scarcity of antimatter today.

But the scientists point out that we wouldn't know that if they hadn't done these experiments. 

"There are many ways to test for matter/antimatter asymmetry, and there are more precise tests, but in addition to precision, it's important to test it in qualitatively different ways. This experiment was a qualitatively new test," said Richard Lednický, a STAR scientist from the Joint Institute for Nuclear Research, Dubna, and the Institute of Physics, Czech Academy of Sciences, Prague.

"The successful implementation of the technique used in this analysis opens an exciting possibility for exploring details of the strong interaction between other abundantly produced particle species," he said, noting that RHIC and the Large Hadron Collider (LHC) are ideally suited for these measurements, which are difficult to assess by other means.

Brookhaven National Laboratory

Physicists create antimatter in record density

Positrons are plentiful in ultra-intense laser blasts 

Physicists from Rice University and the University of Texas at Austin have found a new recipe for using intense lasers to create positrons — the antiparticle of electrons — in record numbers and density.

In a series of experiments described recently in the online journal Scientific Reports published by Nature, the researchers used UT’s Texas Petawatt Laser to make large number of positrons by blasting tiny gold and platinum targets.

Although the positrons were annihilated in a fraction of a microsecond, the experiments have implications for new realms of physics and astrophysics research, medical therapy and perhaps even space travel, said Rice physicist Edison Liang, lead author of the study.

“There are many futuristic technologies related to antimatter that people have been dreaming about for the last 50 years,” said Liang, the Andrew Hays Buchanan Professor of Astrophysics. “One is that antimatter is the most efficient form of energy storage. When antimatter annihilates with matter, it becomes pure energy. Nothing is left behind, unlike in fusion or fission or chemical-based reactions.”

With laser pulses as short as 130 femtoseconds (one femtosecond equals one-quadrillionth of a second), and peak intensity reaching almost 2 billion-trillion watts per square centimeter, the Texas Petawatt Laser is one of the world’s most intense short-pulse, high-energy lasers. In experiments that began in 2012, the Rice-UT team led by Liang completed more than 130 successful laser shots using gold and platinum targets.

“The physical conditions created in our experiments are more extreme than in previous experiments,” Liang said. “Antimatter has been created in accelerators for many decades, but the key difference here is that the laser pulses are very short and ultra intense. The density of antimatter created is high because it’s all concentrated in a tiny amount of space.”

In some shots, the team found they produced jets of particles that included about half as many positrons as electrons. Liang said a high ratio of positrons to electrons is important because physicists would like to produce a mixture that approaches a neutral “pair plasma” with equal numbers of positrons and electrons.

“Pair-dominated plasmas of positrons and electrons are fixtures in the universe,” he said. “They are believed to exist in the winds of pulsars, jets of quasars and gamma-ray bursts, and they are also thought to dominate the universe in the milliseconds following the Big Bang. We hope to use this laboratory platform to simulate some of those phenomena.”

Liang said the group would ultimately like to both create a dense pair-dominated plasma and find a way to capture it in a magnetic bottle for further study.

“Once you trap pure antimatter, the antiparticles can live a long time as long as they don’t touch any matter,” he said. “Since the 1950s, the Air Force, NASA and others have been talking about using antimatter for space travel. In space, weight is the key factor in determining how much fuel you can take with you.

Under the right conditions, positrons and electrons can pair up and orbit one another without touching or annihilating themselves. This paired arrangement, which behaves like a hydrogen atom with almost no mass, is known as positronium.

“But my main interest, which I’ve been concerned with for about 25 years, is to create a very high concentration of antimatter in a very small volume. We want to create a new form of quantum matter, a Bose-Einstein condensate (BEC), of antimatter. That is a really new domain of quantum physics that we could explore.”

A BEC is a collection of atoms or particles that are cooled to such low temperatures that their behavior is dictated by the laws of quantum mechanics. In a BEC, thousands of individual particles act collectively, marching in lock step as if they were a single entity.

“If we have a very large concentration of electrons and positrons, we could imagine making a BEC,” he said. “The beauty of positronium is that it is so light, it can actually become a BEC at relatively mild temperatures — about the temperature at which you would store liquid helium (minus 452 degrees Fahrenheit).”

The research is supported by the Department of Energy and the Rice University Faculty Initiative Fund.

Rice University