Is there a right-handed version of our left-handed universe?

To solve a long-standing puzzle about how long a neutron can "live" outside an atomic nucleus, physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed universe. They designed a mind-bending experiment at the Department of Energy's Oak Ridge National Laboratory to try to detect a particle that has been speculated but not spotted. If found, the theorized "mirror neutron" -- a dark-matter twin to the neutron -- could explain a discrepancy between answers from two types of neutron lifetime experiments and provide the first observation of dark matter.

"Dark matter remains one of the most important and puzzling questions in science -- clear evidence we don't understand all matter in nature," said ORNL's Leah Broussard, who led the study published in Physical Review Letters.

Neutrons and protons make up an atom's nucleus. However, they also can exist outside nuclei. Last year, using the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, led the most precise measurement ever of how long free neutrons live before they decay, or turn into protons, electrons and anti-neutrinos. The answer -- 877.8 seconds, give or take 0.3 seconds, or a little under 15 minutes -- hinted at a crack in the Standard Model of particle physics. That model describes the behavior of subatomic particles, such as the three quarks that make up a neutron. The flipping of quarks initiates neutron decay into protons.

"The neutron lifetime is an important parameter in the Standard Model because it is used as an input for calculating the quark mixing matrix, which describes quark decay rates," said Gonzalez, who calculated probabilities of neutrons oscillating for the ORNL study. "If the quarks don't mix as we expect them to, that hints at new physics beyond the Standard Model."

To measure the lifetime of a free neutron, scientists take two approaches that should arrive at the same answer. One traps neutrons in a magnetic bottle and counts their disappearance. The other counts protons appearing in a beam as neutrons decay. It turns out neutrons appear to live nine seconds longer in a beam than in a bottle.

Over the years, perplexed physicists have considered many reasons for the discrepancy. One theory is that the neutron transforms from one state to another and back again. "Oscillation is a quantum mechanical phenomenon," Broussard said. "If a neutron can exist as either a regular or a mirror neutron, then you can get this sort of oscillation, a rocking back and forth between the two states, as long as that transition isn't forbidden."

The ORNL-led team performed the first search for neutrons oscillating into dark-matter mirror neutrons using a novel disappearance and regeneration technique. The neutrons were made at the Spallation Neutron Source, a DOE Office of Science user facility. A beam of neutrons was guided to SNS's magnetism reflectometer. Michael Fitzsimmons, a physicist with a joint appointment at ORNL and the University of Tennessee, Knoxville, used the instrument to apply a strong magnetic field to enhance oscillations between neutron states. Then the beam impinged on a "wall" made of boron carbide, which is a strong neutron absorber.

If the neutron does in fact oscillate between regular and mirror states, when the neutron state hits the wall, it will interact with atomic nuclei and get absorbed into the wall. If it is in its theorized mirror neutron state, however, it is dark matter that will not interact.

So only mirror neutrons would make it through the wall to the other side. It would be as if the neutrons had gone through a "portal" to some dark sector -- a figurative concept used in the physics community. Yet, the press reporting on past related work had fun taking liberties with the concept, comparing the theorized mirror universe Broussard's team is exploring to the "Upside Down" alternate reality in the TV series "Stranger Things." The team's experiments were not exploring a literal portal to a parallel universe.

"The dynamics are the same on the other side of the wall, where we try to induce what are presumably mirror neutrons -- the dark-matter twin state -- to turn back into regular neutrons," said co-author Yuri Kamyshkov, a UT physicist who with colleagues has long pursued the ideas of neutron oscillations and mirror neutrons. "If we see any regenerated neutrons, that could be a signal that we've seen something really exotic. The discovery of the particle nature of dark matter would have tremendous implications."

Matthew Frost of ORNL, who received his doctorate from UT working with Kamyshkov, performed the experiment with Broussard and assisted with data extraction, reduction and analysis. Frost and Broussard performed preliminary tests with help from Lisa DeBeer-Schmitt, a neutron scattering scientist at ORNL.

Lawrence Heilbronn, a nuclear engineer at UT, characterized backgrounds, whereas Erik Iverson, a physicist at ORNL, characterized neutron signals. Through the DOE Office of Science Scientific Undergraduate Laboratory Internships Program, Michael Kline of The Ohio State University figured out how to calculate oscillations using graphics processing units -- accelerators of specific types of calculations in application codes -- and performed independent analyses of neutron beam intensity and statistics, and Taylor Dennis of East Tennessee State University helped set up the experiment and analyzed background data, becoming a finalist in a competition for this work. UT graduate students Josh Barrow, James Ternullo and Shaun Vavra with undergraduates Adam Johnston, Peter Lewiz and Christopher Matteson contributed at various stages of experiment preparation and analysis. University of Chicago graduate student Louis Varriano, a former UT Torchbearer, helped with conceptual quantum-mechanical calculations of mirror-neutron regeneration.

The conclusion: No evidence of neutron regeneration was seen. "One hundred percent of the neutrons stopped; zero percent passed through the wall," Broussard said. Regardless, the result is still important to the advancement of knowledge in this field.

With one particular mirror-matter theory debunked, the scientists turn to others to try to solve the neutron lifetime puzzle. "We're going to keep looking for the reason for the discrepancy," Broussard said. She and colleagues will use the High Flux Isotope Reactor, a DOE Office of Science user facility at ORNL, for that. Ongoing upgrades at HFIR will make more sensitive searches possible because the reactor will produce a much higher flux of neutrons, and the shielded detector at its small-angle neutron scattering diffractometer has a lower background.

Because the rigorous experiment did not find evidence of mirror neutrons, the physicists were able to rule out a far-fetched theory. And that takes them closer to solving the puzzle.

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Why is there any matter in the universe at all? New study sheds light

Scientists at the University of Sussex have measured a property of the neutron -- a fundamental particle in the universe -- more precisely than ever before. Their research is part of an investigation into why there is matter left over in the universe, that is, why all the antimatter created in the Big Bang didn't just cancel out the matter.

The team -- which included the Science and Technology Facilities Council's (STFC) Rutherford Appleton Laboratory in the UK, the Paul Scherrer Institute (PSI) in Switzerland, and a number of other institutions -- was looking into whether or not the neutron acts like an "electric compass." Neutrons are believed to be slightly asymmetrical in shape, being slightly positive at one end and slightly negative at the other -- a bit like the electrical equivalent of a bar magnet. This is the so-called "electric dipole moment" (EDM), and is what the team was looking for.

This is an important piece of the puzzle in the mystery of why matter remains in the Universe, because scientific theories about why there is matter left over also predict that neutrons have the "electric compass" property, to a greater or lesser extent. Measuring it then it helps scientists to get closer to the truth about why matter remains.

The team of physicists found that the neutron has a significantly smaller EDM than predicted by various theories about why matter remains in the universe; this makes these theories less likely to be correct, so they have to be altered, or new theories found. In fact it's been said in the literature that over the years, these EDM measurements, considered as a set, have probably disproved more theories than any other experiment in the history of physics. The results are reported today, Friday 28 February 2020, in the journal Physical Review Letters.

Professor Philip Harris, Head of the School of Mathematical and Physical Sciences and leader of the EDM group at the University of Sussex, said:

"After more than two decades of work by researchers at the University of Sussex and elsewhere, a final result has emerged from an experiment designed to address one of the most profound problems in cosmology for the last fifty years: namely, the question of why the Universe contains so much more matter than antimatter, and, indeed, why it now contains any matter at all. Why didn't the antimatter cancel out all the matter? Why is there any matter left?

"The answer relates to a structural asymmetry that should appear in fundamental particles like neutrons. This is what we've been looking for. We've found that the "electric dipole moment" is smaller than previously believed. This helps us to rule out theories about why there is matter left over -- because the theories governing the two things are linked.

"We have set a new international standard for the sensitivity of this experiment. What we're searching for in the neutron -- the asymmetry which shows that it is positive at one end and negative at the other -- is incredibly tiny. Our experiment was able to measure this in such detail that if the asymmetry could be scaled up to the size of a football, then a football scaled up by the same amount would fill the visible Universe."

The experiment is an upgraded version of apparatus originally designed by researchers at the University of Sussex and the Rutherford Appleton Laboratory (RAL), and which has held the world sensitivity record continuously from 1999 until now.

Dr Maurits van der Grinten, from the neutron EDM group at the Rutherford Appleton Laboratory (RAL), said:

"The experiment combines various state of the art technologies that all need to perform simultaneously. We're pleased that the equipment, technology and expertise developed by scientists from RAL has contributed to the work to push the limit on this important parameter"

Dr Clark Griffith, Lecturer in Physics from the School of Mathematical and Physical Sciences at the University of Sussex, said:

"This experiment brings together techniques from atomic and low energy nuclear physics, including laser-based optical magnetometry and quantum-spin manipulation. By using these multi-disciplinary tools to measure the properties of the neutron extremely precisely, we are able to probe questions relevant to high-energy particle physics and the fundamental nature of the symmetries underlying the universe. "

50,000 measurements

Any electric dipole moment that a neutron may have is tiny, and so is extremely difficult to measure. Previous measurements by other researchers have borne this out. In particular, the team had to go to great lengths to keep the local magnetic field very constant during their latest measurement. For example, every truck that drove by on the road next to the institute disturbed the magnetic field on a scale that would have been significant for the experiment, so this effect had to be compensated for during the measurement.

Also, the number of neutrons observed needed to be large enough to provide a chance to measure the electric dipole moment. The measurements ran over a period of two years. So-called ultracold neutrons, that is, neutrons with a comparatively slow speed, were measured. Every 300 seconds, a bunch of more than 10,000 neutrons was directed to the experiment and examined in detail. The researchers measured a total of 50,000 such bunches.

A new international standard is set

The researchers' latest results supported and enhanced those of their predecessors: a new international standard has been set. The size of the EDM is still too small to measure with the instruments that have been used up until now, so some theories that attempted to explain the excess of matter have become less likely. The mystery therefore remains, for the time being.

The next, more precise, measurement is already being constructed at PSI. The PSI collaboration expects to start their next series of measurements by 2021.

Search for "new physics"

The new result was determined by a group of researchers at 18 institutes and universities in Europe and the USA on the basis of data collected at PSI's ultracold neutron source. The researchers collected measurement data there over a period of two years, evaluated it very carefully in two separate teams, and were then able to obtain a more accurate result than ever before.

The research project is part of the search for "new physics" that would go beyond the so-called Standard Model of Physics, which sets out the properties of all known particles. This is also a major goal of experiments at larger facilities such as the Large Hadron Collider (LHC) at CERN.

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Tracking down the mystery of matter

Researchers at the Paul Scherrer Institute PSI have measured a property of the neutron more precisely than ever before. In the process they found out that the elementary particle has a significantly smaller electric dipole moment than was previously assumed. With that, it has also become less likely that this dipole moment can help to explain the origin of all matter in the universe. The researchers achieved this result using the ultracold neutron source at PSI. They report their results today in the journal Physical Review Letters.

The Big Bang created both the matter in the universe and the antimatter -- at least according to the established theory. Since the two mutually annihilate each other, however, there must have been a surplus of matter, which has remained to this day. The cause of this excess of matter is one of the great mysteries of physics and astronomy. Researchers hope to find a clue to the underlying phenomenon with the help of neutrons, the electrically uncharged elementary building blocks of atoms. The assumption: If the neutron had a so-called electric dipole moment (abbreviated nEDM) with a measurable non-zero value, this could be due to the same physical principle that would also explain the excess of matter after the Big Bang.

50,000 measurements

The search for the nEDM can be expressed in everyday language as the question of whether or not the neutron is an electric compass. It has long been clear that the neutron is a magnetic compass and reacts to a magnetic field, or, in technical jargon: has a magnetic dipole moment. If in addition the neutron also had an electric dipole moment, its value would be very much less -- and thus much more difficult to measure. Previous measurements by other researchers have borne this out. Therefore, the researchers at PSI had to go to great lengths to keep the local magnetic field very constant during their latest measurement. Every truck that drove by on the road next to PSI disturbed the magnetic field on a scale that was relevant for the experiment, so this effect had to be calculated and removed from the experimental data.

Also, the number of neutrons observed needed to be large enough to provide a chance to measure the nEDM. The measurements at PSI therefore ran over a period of two years. So-called ultracold neutrons, that is, neutrons with a comparatively slow speed, were measured. Every 300 seconds, an 8 second long bundle with over 10,000 neutrons was directed to the experiment and examined. The researchers measured a total of 50,000 such bundles.

"Even for PSI with its large research facilities, this was a fairly extensive study," says Philipp Schmidt-Wellenburg, a researcher on the nEDM project on the part of PSI. "But that is exactly what is needed these days if we are looking for physics beyond the Standard Model."

Search for "new physics"

The new result was determined by a group of researchers at 18 institutes and universities in Europe and the USA, amongst them the ETH Zurich, the University of Bern and the University of Fribourg. The data had been gathered at PSI's ultracold neutron source. The researchers had collected measurement data there over two years, evaluated it very carefully in two teams, and through that obtained a more accurate result than ever before.

The nEDM research project is part of the search for "new physics" that would go beyond the so-called Standard Model. This is also being sought at even larger facilities such as the Large Hadron Collider LHC at CERN. "The research at CERN is broad and generally searches for new particles and their properties," explains Schmidt-Wellenburg. "We on the other hand are going deep, because we are only looking at the properties of one particle, the neutron. In exchange, however, we achieve an accuracy in this detail that the LHC might only reach in 100 years."

"Ultimately," says Georg Bison, who like Schmidt-Wellenburg is a researcher in the Laboratory for Particle Physics at PSI, "various measurements on the cosmological scale show deviations from the Standard Model. In contrast, no one has yet been able to reproduce these results in the laboratory. This is one of the very big questions in modern physics, and that's what makes our work so exciting."

Even more precise measurements are planned

With their latest experiment, the researchers have confirmed previous laboratory results. "Our current result too yielded a value for nEDM that is too small to measure with the instruments that have been used up to now -- the value is too close to zero," says Schmidt-Wellenburg. "So it has become less likely that the neutron will help explain the excess of matter. But it still can't be completely ruled out. And in any case, science is interested in the exact value of the nEDM in order to find out if it can be used to discover new physics."

Therefore, the next, more precise measurement is already being planned. "When we started up the current source for ultracold neutrons here at PSI in 2010, we already knew that the rest of the experiment wouldn't quite do it justice. So we are currently building an appropriately larger experiment," explains Bison. The PSI researchers expect to start the next series of measurements of the nEDM by 2021 and, in turn, to surpass the current one in terms of accuracy.

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A first 'snapshot' of the complete spectrum of neutrinos emitted by the sun

The Borexino instrument located deep beneath Italy's Appenine Mountains detects neutrinos as they interact with the electrons of an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 1,000 tons of water.

About 99 percent of the Sun's energy emitted as neutrinos is produced through nuclear reaction sequences initiated by proton-proton (pp) fusion in which hydrogen is converted into helium, say scientists including physicist Andrea Pocar at the University of Massachusetts Amherst. Today they report new results from Borexino, one of the most sensitive neutrino detectors on the planet, located deep beneath Italy's Apennine Mountains.

"Neutrinos emitted by this chain represent a unique tool for solar and neutrino physics," they explain. Their new paper in Nature reports on "the first complete study of all the components of the pp-chain performed by Borexino." These components include not only the pp neutrinos, but others called Beryllium-7 (7Be), pep and Boron-8 (8B) neutrinos. The pp fusion reaction of two protons to produce deuteron, nuclei of deuterium, is the first step of a reaction sequence responsible for about 99 percent of the Sun's energy output, Pocar says.

He adds, "What's new today is incremental, it's not a leap, but it is the crowning of more than 10 years of data-taking with the experiment to show the full energy spectrum of the Sun at once. Our results reduce uncertainty, which is perhaps not flashy but it's type of advance that is often not recognized enough in science. The value is that measurements get more precise because with more data and thanks to the work of dedicated young physicists, we have a better understanding of the experimental apparatus."

"Borexino offers the best measurement ever made for the pp, 7Be and pep neutrinos," he adds. "Other experiments measure the 8B neutrinos more precisely, but our measurement, with a lower threshold, is consistent with them."

Further, "Once you have more precise data, you can feed it back into the model of how the Sun is behaving, then the model can be refined even more. It all leads to understanding the Sun better. Neutrinos have told us how the Sun is burning and, in turn, the Sun has provided us with a unique source to study how neutrinos behave. Borexino, scheduled to run for another two to three years, has strengthened our understanding of the Sun very profoundly."

For earlier studies of pp, 7B, pep and 8B neutrinos, the team had focused on each one separately in targeted analyses of the collected data in restricted windows of energy, "like trying to characterize a forest by taking one picture each of many individual types of trees," Pocar notes. "Multiple pictures give you an idea of a forest, but it's not the same as the photo of the entire forest."

"What we have done now is take a single photo that reflects the whole forest, the whole spectrum of all the different neutrinos in one. Instead of zooming in to look at little pieces, we see it all at once. We understand our detector so well now, we are comfortable and confident that our one shot is valid for the whole spectrum of neutrino energies."

Solar neutrinos stream out of the star at the center of our system at nearly the speed of light, as many as 420 billion hitting every square inch of Earth's surface per second. But because they only interact through the nuclear weak force, they pass through matter virtually unaffected, which makes them very difficult to detect and distinguish from trace nuclear decays of ordinary materials, Pocar says.

The Borexino instrument detects neutrinos as they interact with the electrons of an ultra-pure organic liquid scintillator at the center of a large sphere surrounded by 1,000 tons of water. Its great depth and many onion-like protective layers maintain the core as the most radiation-free medium on the planet. It is the only detector on Earth capable of observing the entire spectrum of solar neutrino simultaneously, which has now been accomplished, he notes.

The UMass Amherst physicist, one principal investigator on a team of more than 100 scientists, is particularly interested in now turning his focus to measure yet another type of solar neutrino known as CNO neutrinos, which he hopes will be useful in addressing an important open question in stellar physics, that is the metallicity, or metal content, of the Sun.

"There are two models that predict different levels of elements heavier than helium, which for astronomers is a metal, in the Sun; a lighter metallicity and a heavier model," he notes. CNO neutrinos are emitted in a cyclic fusion reaction sequence different from the pp chain and subdominant in the Sun, but thought to be the main source of power for heavier stars. The CNO solar neutrino flux is greatly affected by the solar metallicity.

Pocar says, "Our data is possibly showing some slight preference for heavy metallicity, so we'll be looking into that because neutrinos from the Sun, especially CNO, can help us disentangle this."

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Primordial black holes may have helped to forge heavy elements

Artist’s depiction of a neutron star.

Astronomers like to say we are the byproducts of stars, stellar furnaces that long ago fused hydrogen and helium into the elements needed for life through the process of stellar nucleosynthesis.

As the late Carl Sagan once put it: "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff."

But what about the heavier elements in the periodic chart, elements such as gold, platinum and uranium?

Astronomers believe most of these "r-process elements" -- elements much heavier than iron -- were created, either in the aftermath of the collapse of massive stars and the associated supernova explosions, or in the merging of binary neutron star systems.

"A different kind of furnace was needed to forge gold, platinum, uranium and most other elements heavier than iron," explained George Fuller, a theoretical astrophysicist and professor of physics who directs UC San Diego's Center for Astrophysics and Space Sciences. "These elements most likely formed in an environment rich with neutrons."

In a paper published August 7 in the journal Physical Review Letters, he and two other theoretical astrophysicists at UCLA -- Alex Kusenko and Volodymyr Takhistov -- offer another means by which stars could have produced these heavy elements: tiny black holes that came into contact with and are captured by neutron stars, and then destroy them.

Neutron stars are the smallest and densest stars known to exist, so dense that a spoonful of their surface has an equivalent mass of three billion tons.

Tiny black holes are more speculative, but many astronomers believe they could be a byproduct of the Big Bang and that they could now make up some fraction of the "dark matter" -- the unseen, nearly non-interacting stuff that observations reveal exists in the universe.

If these tiny black holes follow the distribution of dark matter in space and co-exist with neutron stars, Fuller and his colleagues contend in their paper that some interesting physics would occur.

They calculate that, in rare instances, a neutron star will capture such a black hole and then devoured from the inside out by it. This violent process can lead to the ejection of some of the dense neutron star matter into space.

"Small black holes produced in the Big Bang can invade a neutron star and eat it from the inside," Fuller explained. "In the last milliseconds of the neutron star's demise, the amount of ejected neutron-rich material is sufficient to explain the observed abundances of heavy elements."

"As the neutron stars are devoured," he added, "they spin up and eject cold neutron matter, which decompresses, heats up and make these elements."

This process of creating the periodic table's heaviest elements would also provide explanations for a number of other unresolved puzzles in the universe and within our own Milky Way galaxy.

"Since these events happen rarely, one can understand why only one in ten dwarf galaxies is enriched with heavy elements," said Fuller. "The systematic destruction of neutron stars by primordial black holes is consistent with the paucity of neutron stars in the galactic center and in dwarf galaxies, where the density of black holes should be very high."

In addition, the scientists calculated that ejection of nuclear matter from the tiny black holes devouring neutron stars would produce three other unexplained phenomenon observed by astronomers.

"They are a distinctive display of infrared light (sometimes termed a "kilonova"), a radio emission that may explain the mysterious Fast Radio Bursts from unknown sources deep in the cosmos, and the positrons detected in the galactic center by X-ray observations," said Fuller. "Each of these represent long-standing mysteries. It is indeed surprising that the solutions of these seemingly unrelated phenomena may be connected with the violent end of neutron stars at the hands of tiny black holes."

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New simulations could help in hunt for massive mergers of neutron stars, black holes

This image, from a computerized simulation, shows the formation of an inner disk of matter and a wide, hot disk of matter 5.5 milliseconds after the merger of a neutron star and a black hole.

Now that scientists can detect the wiggly distortions in space-time created by the merger of massive black holes, they are setting their sights on the dynamics and aftermath of other cosmic duos that unify in catastrophic collisions.

Working with an international team, scientists at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed new computer models to explore what happens when a black hole joins with a neutron star -- the superdense remnant of an exploded star.

Using supercomputers to rip open neutron stars

The simulations, carried out in part at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC), are intended to help detectors home in on the gravitational-wave signals. Telescopes, too, can search for the brilliant bursts of gamma-rays and the glow of the radioactive matter that these exotic events can spew into surrounding space.

In separate papers published in a special edition of the scientific journal Classical and Quantum Gravity, Berkeley Lab and other researchers present the results of detailed simulations.

One of the studies models the first milliseconds (thousandths of a second) in the merger of a black hole and neutron star, and the other details separate simulations that model the formation of a disk of material formed within seconds of the merger, and of the evolution of matter that is ejected in the merger.

That ejected matter likely includes gold and platinum and a range of radioactive elements that are heavier than iron.

Any new information scientists can gather about how neutron stars rip apart in these mergers can help to unlock their secrets, as their inner structure and their likely role in seeding the universe with heavy elements are still shrouded in mystery.

"We are steadily adding more realistic physics to the simulations," said -- Foucart, who served as a lead author for one of the studies as a postdoctoral researcher in Berkeley Lab's Nuclear Science Division.

"But we still don't know what's happening inside neutron stars. The complicated physics that we need to model make the simulations very computationally intensive."

Finding signs of a black hole-neutron star merger

Foucart, who will soon be an assistant professor at the University of New Hampshire, added, "We are trying to move more toward actually making models of the gravitational-wave signals produced by these mergers," which create a rippling in space-time that researchers hope can be detected with improvements in the sensitivity of experiments including Advanced LIGO, the Laser Interferometer Gravitational-Wave Observatory.

In February 2016, LIGO scientists confirmed the first detection of a gravitational wave, believed to be generated by the merger of two black holes, each with masses about 30 times larger than the sun.

The signals of a neutron star merging with black holes or another neutron star are expected to generate gravitational waves that are slightly weaker but similar to those of black hole-black hole mergers, Foucart said.

Radioactive 'waste' in space

Daniel Kasen, a scientist in the Nuclear Science Division at Berkeley Lab and associate professor of physics and astronomy at UC Berkeley who participated in the research, said that inside neutron stars "there may be exotic states of matter unlike anything realized anywhere else in the universe."

In some computer simulations the neutron stars were swallowed whole by the black hole, while in others there was a fraction of matter coughed up into space. This ejected matter is estimated to range up to about one-tenth of the mass of the sun.

While much of the matter gets sucked into the larger black hole that forms from the merger, "the material that gets flung out eventually turns into a kind of radioactive 'waste,'" he said. "You can see the radioactive glow of that material for a period of days or weeks, from more than a hundred million light years away." Scientists refer to this observable radioactive glow as a "kilonova."

The simulations use different sets of calculations to help scientists visualize how matter escapes from these mergers. By modeling the speed, trajectory, amount and type of matter, and even the color of the light it gives off, astrophysicists can learn how to track down actual events.

The weird world of neutron stars

The size range of neutron stars is set by the ultimate limit on how densely matter can be compacted, and neutron stars are among the most superdense objects we know about in the universe.

Neutron stars have been observed to have masses up to at least two times that of our sun but measure only about 12 miles in diameter, on average, while our own sun has a diameter of about 865,000 miles. At large enough masses, perhaps about three times the mass of the sun, scientists expect that neutron stars must collapse to form black holes.

A cubic inch of matter from a neutron star is estimated to weigh up to 10 billion tons. As their name suggests, neutron stars are thought to be composed largely of the neutrally charged subatomic particles called neutrons, and some models expect them to contain long strands of matter -- known as "nuclear pasta" -- formed by atomic nuclei that bind together.

Neutron stars are also expected to be almost perfectly spherical, with a rigid and incredibly smooth crust and an ultrapowerful magnetic field. They can spin at a rate of about 43,000 revolutions per minute (RPMs), or about five times faster than a NASCAR race car engine's RPMs.

The aftermath of neutron star mergers

The researchers' simulations showed that the radioactive matter that first escapes the black hole mergers may be traveling at speeds of about 20,000 to 60,000 miles per second, or up to about one-third the speed of light, as it is swung away in a long "tidal tail."

"This would be strange material that is loaded with neutrons," Kasen said. "As that expanding material cools and decompresses, the particles may be able to combine to build up into the heaviest elements." This latest research shows how scientists might find these bright bundles of heavy elements.

"If we can follow up LIGO detections with telescopes and catch a radioactive glow, we may finally witness the birthplace of the heaviest elements in the universe," he said. "That would answer one of the longest-standing questions in astrophysics."

Most of the matter in a black hole-neutron star merger is expected to be sucked up by the black hole within a millisecond of the merger, and other matter that is not flung away in the merger is likely to form an extremely dense, thin, donut-shaped halo of matter.

The thin, hot disk of matter that is bound by the black hole is expected to form within about 10 milliseconds of the merger, and to be concentrated within about 15 to 70 miles of it, the simulations showed. This first 10 milliseconds appears to be key in the long-term evolution of these disks.

Over timescales ranging from tens of milliseconds to several seconds, the hot disk spreads out and launches more matter into space. "A number of physical processes -- from magnetic fields to particle interactions and nuclear reactions -- combine in complex ways to drive the evolution of the disk," said Rodrigo Fernández, an assistant professor of physics at the University of Alberta in Canada who led one of the studies.

Simulations carried out on NERSC's Edison supercomputer were crucial in understanding how the disk ejects matter and in providing clues for how to observe this matter, said Fernández, a former UC Berkeley postdoctoral researcher.

What's next?

Eventually, it may be possible for astronomers scanning the night sky to find the "needle in a haystack" of radioactive kilonovae from neutron star mergers that had been missed in the LIGO data, Kasen said.

"With improved models, we are better able to tell the observers exactly which flashes of light are the signals they are looking for," he said. Kasen is also working to build increasingly sophisticated models of neutron star mergers and supernovae through his involvement in the DOE Exascale Computing Project.

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Gravitational Waves to Crack Neutron Star Mystery

We are now in an incredible new era of astronomy where the faint ripples in spacetime caused by distant black hole collisions are being detected and studied. These are the most energetic events in the cosmos and, by "listening in" to their gravitational wave signals, these black hole mergers have finally been directly observed.

Although the first detections of gravitational waves by the Laser Interferometer Gravitational-wave Observatory (LIGO) have, so far, been exclusive to the collisions of black holes, astrophysicists have far loftier goals. The next step will be to detect the collisions of neutron stars and, hopefully, use their gravitational wave signals to crack open the super-dense husks of stellar matter, revealing what they are really made of.

Gravitational waves traveling through spacetime can be imagined as ripples traveling across the surface of a pond. And, like those water ripples, gravitational waves carry energy away from massive objects that accelerate, collide or explode. The historic Feb. 11 announcement of the detection of gravitational waves that were recorded by LIGO on Sept. 14, 2015, revealed the first direct observation of two black holes, one 29 solar masses and another 36 solar masses, colliding and merging as one.

Then, on June 14, a second detection was announced, revealing yet another black hole merger. Only this time the black holes were smaller, "weighing in" at 14 solar masses and 8 solar masses.

Both observations revealed an astonishing amount of information about the pairs of black holes before and after their powerful collisions, including the fact they were spinning and their precise masses. What's more, the gravitational wave signals were very much in line with our theoretical predictions of what gravitational waves generated by black hole mergers should look like. These first detections are a testament to our theoretical understanding of black holes, considering that, up until Sept. 15, 2015, we'd never actually observed any kind of direct signal from a black hole (though we already had an abundance of indirect evidence that black holes and gravitational waves were physically out there).

So, we have direct observational proof of the existence of gravitational waves (and, indeed, black holes themselves) and we now know we can build laser interferometers sensitive enough to detect the most minuscule fluctuations in spacetime. Now astrophysicists want to probe even deeper in the hope of detecting the collisions of neutron stars, which are a few orders of magnitude less massive than black holes and their gravitational waves will be much fainter.

Neutron stars are created after massive stars run out of fuel and explode as supernovae. After the stellar fireworks, a tiny spinning mass of ultradense matter may be left behind. The intense gravity of this neutron star crushes normal matter into an extreme state governed by quantum dynamics. Thought to be a massive ball of neutrons -- subatomic particles found in the nuclei of normal atoms -- a neutron star can be thought of as a giant atom unto itself; a massive nucleus composed of only neutrons trapped in a crushing gravitational field, spinning fast and often possessing a powerful stellar magnetic field.

Some of these neutron stars can turn into pulsars, blasting intense radiation from their magnetic poles, appearing as flashing lighthouses in the universe.

However, there are many unanswered questions in the field of neutron star physics. One key question is whether neutron stars fit the "textbook" explanation of what a neutron star is -- basically a ball of neutrons -- or have the neutrons themselves been crushed into oblivion and other quantum rules are being applied? Could neutron stars actually be an ultra-dense ball of quarks, the even smaller subatomic particles that make up the neutrons themselves?

Before Feb. 11, these questions would have remained a theoretical curiosity, but now we have a natural probe into some of the most massive objects in the universe and gravitational waves could be used to reveal the hearts of colliding neutron stars, possibly cracking them open and revealing the physics that lie within.

"Ultimately, (gravitational waves) may answer the question, whether neutron stars are composed solely of ordinary atomic nuclei, or if they contain more exotic matter in the form of dense deconfined quark matter," said physicist Aleksi Vuorinen, of the University of Helsinki, Finland, in a statement.

The key problem with trying to understand neutron star physics is that there is currently no way we can simulate the extreme conditions neutron star matter experiences on Earth. The closest we can come is to collide particles inside powerful accelerators like the Large Hadron Collider (LHC) near Geneva, Switzerland, to produce a very short-lived, hot quark-gluon plasma. Though particle collisions may reveal some qualities of this exotic state of matter, governed by Quantum Chromodynamics (QCD), it comes woefully short of simulating a neutron star's dense environment.

"The quark-gluon plasma that is produced in heavy ion collisions can be thought of as a hot but not very dense soup of quarks and gluons, while quark matter is a very dense and cold, essentially solid state, of matter," said Aleksi Kurkela (from CERN and the University of Stavanger in Norway) in an interview with Phys.org. "Our work in fact bridges the gap between these two systems, as our result is applicable at all temperatures, unlike any of the previous results."

So it seems likely that the only way we can "see" what neutron stars are made of would be to try to detect the gravitational wave signals generated after two neutron stars collide. But once we do detect the first gravitational waves generated by neutron star mergers, we need to decipher what the signal actually means, and this is why Vuorinen and Kurkela are working on some pretty exciting simulations of neutron star smashups.

Read more at Discovery News

Scientists reveal single-neuron gene landscape of the human brain

A team of scientists at The Scripps Research Institute (TSRI), University of California, San Diego (UC San Diego) and Illumina, Inc., has completed the first large-scale assessment of single neuronal "transcriptomes." Their research reveals a surprising diversity in the molecules that human brain cells use in transcribing genetic information from DNA to RNA and producing proteins.

The researchers accomplished this feat by isolating and analyzing single-neuronal nuclei from the human brain, allowing classification of 16 neuronal subtypes in the brain's cerebral cortex, the "gray matter" involved in thought, cognition and many other functions.

"Through a wonderful scientific collaboration, we found an enormous amount of transcriptomic diversity from cell to cell that will be relevant to understanding the normal brain and its diseases such as Alzheimer's, Parkinson's, ALS and depression," said TSRI Professor and neuroscientist Jerold Chun, who co-led the study with bioengineers Kun Zhang and Wei Wang of UC San Diego and Jian-Bing Fan of Illumina.

The study was published on June 24 in the journal Science.

All the Same

While parts of the cerebral cortex look different under a microscope--with different cell shapes and densities that form cortical layers and larger regions having functional roles called "Brodmann Areas"--most researchers treat neurons as a fairly uniform group in their studies.

"From a tiny brain sample, researchers often make assumptions that obtained information is true for the entire brain," said Chun.

But the brain isn't like other organs, Chun explained. There's a growing understanding that individual brain cells are unique, and a possibility has been that the microscopic differences among cerebral cortical areas may also reflect unique transcriptomic differences--i.e., differences in the expressed genes, or messenger RNAs (mRNAs), which carry copies of the DNA code outside the nucleus and determine which proteins the cell makes.

To better understand this diversity, the researchers in the new study analyzed more than 3,200 single human neurons--more than 10-fold greater than prior publications--in six Brodmann Areas of one human cerebral cortex.

With the help of newly developed tools to isolate and sequence individual cell nuclei (where genetic material is housed in a cell), the researchers deciphered the minute quantities of mRNA within each nucleus, revealing that various combinations of the 16 subtypes tended to cluster in cortical layers and Brodmann Areas, helping explain why these regions look and function differently.

Neurons exhibited anticipated similarities, yet also many differences in their transcriptomic profiles, revealing single neurons with shared, as well as unique, characteristics that likely lead to differences in cellular function.

"Now we can actually point to an enormous amount of molecular heterogeneity in single neurons of the brain," said Gwendolyn E. Kaeser, a UC San Diego Biomedical Sciences Graduate Program student studying in Chun's lab at TSRI. Kaeser was co-first author of the study with Blue B. Lake and Rizi Ai of UC San Diego and Neeraj S. Salathia of Illumina.

Many New Questions

Interestingly, some of these differences in gene expression have roots in very early brain development taking place before birth. The researchers found markers on some neurons showing that they originated from a specific region of fetal brain called the ganglionic eminence, which generates inhibitory neurons destined for the cerebral cortex. These neurons may have particular relevance to developmental brain disorders.

The enormous transcriptomic diversity of single neurons was predicted by earlier work from Chun's laboratory and others showing that the genomes--the DNA--of individual brain cells can be different from cell to cell. In future studies, the researchers hope to investigate how single-neuron DNA and mRNA differs in single neurons, groups and between human brains--and how these may be influenced by factors such as stress, medications or disease.

Read more at Science Daily

New state of water molecule discovered

Neutron scattering and computational modeling have revealed unique and unexpected behavior of water molecules under extreme confinement that is unmatched by any known gas, liquid or solid states.

In a paper published in Physical Review Letters, researchers at the Department of Energy's Oak Ridge National Laboratory describe a new tunneling state of water molecules confined in hexagonal ultra-small channels -- 5 angstrom across -- of the mineral beryl. An angstrom is 1/10-billionth of a meter, and individual atoms are typically about 1 angstrom in diameter.

The discovery, made possible with experiments at ORNL's Spallation Neutron Source and the Rutherford Appleton Laboratory in the United Kingdom, demonstrates features of water under ultra confinement in rocks, soil and cell walls, which scientists predict will be of interest across many disciplines.

"At low temperatures, this tunneling water exhibits quantum motion through the separating potential walls, which is forbidden in the classical world," said lead author Alexander Kolesnikov of ORNL's Chemical and Engineering Materials Division. "This means that the oxygen and hydrogen atoms of the water molecule are 'delocalized' and therefore simultaneously present in all six symmetrically equivalent positions in the channel at the same time. It's one of those phenomena that only occur in quantum mechanics and has no parallel in our everyday experience."

The existence of the tunneling state of water shown in ORNL's study should help scientists better describe the thermodynamic properties and behavior of water in highly confined environments such as water diffusion and transport in the channels of cell membranes, in carbon nanotubes and along grain boundaries and at mineral interfaces in a host of geological environments.

ORNL co-author Lawrence Anovitz noted that the discovery is apt to spark discussions among materials, biological, geological and computational scientists as they attempt to explain the mechanism behind this phenomenon and understand how it applies to their materials.

"This discovery represents a new fundamental understanding of the behavior of water and the way water utilizes energy," Anovitz said. "It's also interesting to think that those water molecules in your aquamarine or emerald ring -- blue and green varieties of beryl -- are undergoing the same quantum tunneling we've seen in our experiments."

While previous studies have observed tunneling of atomic hydrogen in other systems, the ORNL discovery that water exhibits such tunneling behavior is unprecedented. The neutron scattering and computational chemistry experiments showed that, in the tunneling state, the water molecules are delocalized around a ring so the water molecule assumes an unusual double top-like shape.

"The average kinetic energy of the water protons directly obtained from the neutron experiment is a measure of their motion at almost absolute zero temperature and is about 30 percent less than it is in bulk liquid or solid water," Kolesnikov said. "This is in complete disagreement with accepted models based on the energies of its vibrational modes."

Read more at Science Daily