Neutrinos Hint Of Matter-Antimatter Rift
Neutrinos Hint of Matter-Antimatter Rift
the same underground observatory in Japan where, 18 years ago, neutrinos were first seen oscillating from one “flavor” to another — a landmark discovery that earned two physicists the 2015 Nobel Prize — a tiny anomaly has begun to surface in the neutrinos’ oscillations that could herald an answer to one of the biggest mysteries in physics: why matter dominates over antimatter in the universe.
The anomaly, detected by the T2K experiment, is not yet pronounced enough to be sure of, but it and the findings of two related experiments “are all pointing in the same direction,” said Hirohisa Tanaka of the University of Toronto, a member of the T2K team who presented the result to a packed audience in London earlier this month.
“A full proof will take more time,” said Werner Rodejohann, a neutrino specialist at the Max Planck Institute for Nuclear Physics in Heidelberg who was not involved in the experiments, “but my and many others’ feeling is that there is something real here.”
The long-standing puzzle to be solved is why we and everything we see is matter-made. More to the point, why does anything — matter or antimatter — exist at all? The reigning laws of particle physics, known as the Standard Model, treat matter and antimatter nearly equivalently, respecting (with one known exception) so-called charge-parity, or “CP,” symmetry: For every particle decay that produces, say, a negatively charged electron, the mirror-image decay yielding a positively charged antielectron occurs at the same rate. But this cannot be the whole story. If equal amounts of matter and antimatter were produced during the Big Bang, equal amounts should have existed shortly thereafter. And since matter and antimatter annihilate upon contact, such a situation would have led to the wholesale destruction of both, resulting in an empty cosmos.
Somehow, significantly more matter than antimatter must have been created, such that a matter surplus survived the annihilation and now holds sway. The question is, what CP-violating process beyond the Standard Model favored the production of matter over antimatter?
Many physicists suspect that the answer lies with neutrinos — ultra-elusive, omnipresent particles that pass unfelt through your body by the trillions each second.
To that end, starting in 2010, scientists with the T2K experiment generated beams of neutrinos or antineutrinos in Tokai, Japan, and aimed them toward the Super-Kamiokande neutrino observatory, a sensor-lined tank of 50,000 tons of pure water located nearly 200 miles away in Kamioka. Occasionally, these ghostly particles interacted with atoms inside the water tank, generating detectable flashes of radiation. Detecting a difference in the behavior of the neutrinos and antineutrinos would provide an important clue about the preponderance of matter over antimatter, perhaps opening up a route beyond the Standard Model to a more complete theory of nature. Already, the strange properties of neutrinos provide a possible outline of that fuller story.
Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo At the Super-Kamiokande observatory in Kamioka, Japan — shown here when it was being filled with water in 2006 — neutrinos interact with atoms inside the water, generating flashes of radiation that are picked up by the surrounding sensors.
Primordial Neutrinos
The 1998 discovery that neutrinos switch flavors on the fly “may change our most fundamental theories,” President Bill Clinton said at the time, “from the nature of the smallest subatomic particles to how the universe itself works.”
Neutrino oscillations defied the Standard Model’s prediction that the particles are massless, like photons. In order for neutrinos to oscillate, each of their three possible flavors (electron, muon and tau) must be a quantum-mechanical mixture, or “superposition,” of three possible masses. Quantum superpositions evolve over time. So a neutrino might start out with its three mass components giving it pure muon flavor, but as the components evolve at different rates, electron flavor gradually enters the mixture, and the neutrino will have some probability of being measured as an electron neutrino.
There’s no mechanism within the Standard Model by which neutrinos might acquire their tiny, nonzero masses. Also unknown is why all neutrinos are observed to be “left-handed,” spinning clockwise with respect to their direction of motion, while all antineutrinos are right-handed, spinning counterclockwise.
Experts overwhelmingly favor a double-duty explanation of neutrino mass and single-handedness called the “seesaw mechanism,” whereby the known, lightweight, left-handed neutrinos have much heavier right-handed counterparts, and the known antineutrinos likewise have superheavy left-handed counterparts (the light and heavy masses are inversely related, like two sides of a seesaw). For this seesaw explanation to work, the neutrinos and antineutrinos on each side of the seesaw must actually be the same particle, except for their opposite handedness. Numerous experiments are now hunting for an extremely rare radioactive decay that would confirm this “Majorana” nature of neutrinos, thereby shoring up the logic of the seesaw mechanism.
If the theory is correct, then the heavy neutrinos and antineutrinos would have populated the hot young universe, when there was enough energy to beget beastly particles. They would have since decayed. Physicists wonder: Might their decays have produced the matter-antimatter asymmetry? This is the question to which an answer may be emerging — and much sooner than expected.
Tilted Seesaw
There’s good reason to think that neutrinos violate CP symmetry. The one established instance of CP violation in the laws of physics arises among the quarks — the building blocks of protons and neutrons — whose flavor mixing is described by a mathematical matrix similar to the one for neutrino mixing. In the quark case, though, the value of a numerical factor in the matrix that creates a disparity between quarks and antiquarks is very small. Quarks and antiquarks behave far too symmetrically to account for the universe’s matter-antimatter imbalance.
Lucy Reading-Ikkanda for Quanta Magazine
But the neutrino mixing matrix comes equipped with its own factor by which neutrinos and antineutrinos can violate CP symmetry. (Paradoxically, they can behave differently from one another even if they are Majorana particles, identical except for their opposite handedness.) If the lightweight neutrinos and antineutrinos violate CP symmetry, then the hypothetical heavy primordial neutrinos and antineutrinos must as well, and their asymmetric decays could easily have produced the universe’s glut of matter. Discovering CP violation among the lightweight neutrinos “will boost that general framework,” said Neal Weiner, a theoretical physicist at New York University.
The question is, how large will the CP-violation factor be? “The fear was that it would be small,” said Patricia Vahle, a physicist at the College of William & Mary — so small that the current generation of experiments wouldn’t detect any difference between neutrinos’ and antineutrinos’ behavior. “But it is starting to look like maybe we will be lucky,” she said.
To search for CP violation, the T2K scientists looked for evidence that neutrinos and antineutrinos oscillated between muon and electron flavors with unequal probabilities as they traveled between Tokai and Kamioka. The amount of CP violation once again works like a seesaw, with the rate of muon-to-electron neutrino conversions on one side, and corresponding antineutrino conversions on the other. The larger the value of the factor in the matrix, the greater the seesaw’s tilt.
If the seesaw is balanced, signifying perfect CP symmetry, then (accounting for differences in the production and detection rates of neutrinos and antineutrinos) the T2K scientists would have expected to detect roughly 23 electron neutrino candidates and seven electron antineutrino candidates in Kamioka, Tanaka said. Meanwhile, if CP symmetry is “maximally” violated — the seesaw tilted fully toward more neutrino oscillations and fewer antineutrino oscillations — then 27 electron neutrinos and six electron antineutrinos should have been detected. The actual numbers were even more skewed. “What we observed are 32 electron neutrino candidates and four electron antineutrino candidates,” Tanaka said.
With so few total events, it’s too soon to know whether the apparent tilt of the seesaw, signifying a large amount of CP violation, is real or a statistical aberration. Two other new hints of CP violation, however, strengthen the case. First, the newly running NOvA experiment, which generates a beam of muon neutrinos in Illinois and measures electron neutrinos in Minnesota, found a large number of these oscillations, again suggesting that the seesaw may be tilted in favor of neutrino oscillations and away from antineutrino oscillations. Second, researchers at the Super-Kamiokande observatory detected a similar enhancement of electron neutrinos coming from Earth’s atmosphere. (T2K and NOvA both plan to submit their findings for publication later this year.)
Vahle, who presented NOvA’s new results this month in London, urged caution; even when the T2K and NOvA results are combined, their statistical significance remains at a low level known as “2 sigma,” where there’s still a 5 percent chance the apparent deviation from CP symmetry is a random fluke. The results “do give me hope that finding CP violation in neutrino oscillations won’t be as hard as many feared it would be,” she said, “but we aren’t there yet.”
If CP violation among neutrinos is real and as large as it currently seems, then the evidence will slowly strengthen in the coming years. T2K’s signal could reach 3-sigma significance by the mid-2020s. “It’s a very exciting time as we look forward to a lot more data from both experiments,” said Peter Shanahan, a NOvA co-spokesperson.
It isn’t yet known exactly how CP violation in the light neutrino oscillations would translate into CP-violating decays of the heavy set. But discovering the former would point physicists in the latter’s general direction. “If we are starting to see [CP violation] in the neutrino sector, it is certainly a critical result,” Weiner said.
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Pacific Lamprey (Entosphenus tridentatus)
Lampreys in their mature stage are parasites, latching onto other fish with their suction cup like mouths. Keratinised teeth rasp away their host’s skin, as the lamprey releases anticoagulants to ensure a steady supply of blood. Many victims die of infections or blood loss.
Dave Herasimtschuk and Jeremy Monroe
When We Were Very Young A A Milne decorations by Ernest H Shepherd London Methuen & Co Ltd. Thirty Second Edition 1941 (Cheap Form) [First Published 1924]
The 38th poem in the book, “Teddy Bear”, that originally appeared in Punch magazine in February 1924, was the first appearance of the famous character Winnie-the-Pooh.
These travel booklets from throughout the UK were collected by Barbara Denison over the course of three decades, part of a larger collection consisting of dozens of volumes. Text-dense and diagram-heavy guides like these were meant to both give guidance while visiting and act as inexpensive momentos afterwards. Most of the booklets in the collection concern cathedrals, abbeys, and ruined castles that Denison visited over the course of her travels.
(Image caption: This type of electrocorticography (ECoG) grid, which is implanted in patients about to undergo epilepsy surgery, enables researchers to record and transmit electrical signals to and from the surface of the brain. Credit: Mark Stone/University of Washington)
For the first time in humans, researchers use brain surface stimulation to provide ‘touch’ feedback to direct movement
In the quest to restore movement to people with spinal cord injuries, researchers have focused on getting brain signals to disconnected nerves and muscles that no longer receive messages that would spur them to move.
But grasping a cup or brushing hair or cooking a meal requires other feedback that has been lost in amputees and individuals with paralysis — a sense of touch. The brain needs information from a fingertip or limb or external device to understand how firmly a person is gripping or how much pressure is needed to perform everyday tasks.
Now, University of Washington researchers at the National Science Foundation Center for Sensorimotor Neural Engineering (CSNE) have used direct stimulation of the human brain surface to provide basic sensory feedback through artificial electrical signals, enabling a patient to control movement while performing a simple task: opening and closing his hand.
It’s a first step towards developing “closed loop,” bi-directional brain-computer interfaces (BBCIs) that enable two-way communication between parts of the nervous system. They would also allow the brain to directly control external prosthetics or other devices that can enhance movement — or even reanimate a paralyzed limb — while getting sensory feedback.
The results of this research will be published in the Oct.-Dec. 2016 issue of IEEE Transactions on Haptics. An early-access version of the paper is available online.
“We were able to provide a baseline degree of sensory feedback by direct cortical stimulation of the brain,” said lead author and UW bioengineering doctoral student Jeneva Cronin. “To our knowledge this is the first time it’s been done in a human patient who was awake and performing a motor task that depended on that feedback.”
The team of bioengineers, computer scientists and medical researchers from the CSNE and UW’s GRIDLab used electrical signals of different current intensities, dictated by the position of the patient’s hand measured by a glove he wore, to stimulate the patient’s brain that had been implanted with electrocorticographic (ECoG) electrodes. The patient then used those artificial signals delivered to the brain to “sense” how the researchers wanted him to move his hand.
“The question is: Can humans use novel electrical sensations that they’ve never felt before, perceive them at different levels and use this to do a task? And the answer seems to be yes,” said co-author and UW bioengineering doctoral student James Wu. “Whether this type of sensation can be as diverse as the textures and feelings that we can sense tactilely is an open question.”
They would also allow the brain to directly control external prosthetics or other devices that can enhance movement — or even reanimate a paralyzed limb — while getting sensory feedback.
It’s difficult for a person to mimic natural movements — whether using a prosthetic device or a limb that has become disconnected from the brain by neurological injury — without sensation. Though there are devices to assist patients with paralysis or who have undergone amputations with basic function, being able to feel again ranks highly on their priorities, researchers said.
Restoring this sensory feedback requires developing an “artificial” language of electrical signals that the brain can interpret as sensation and incorporate as useful feedback when performing a task.
The UW CSNE team frequently works with patients about to undergo epilepsy surgery who have recently had an ECoG electrode grid implanted on the surface of their brain. For several days or weeks, doctors constantly monitor their brain activity to pinpoint the origin of their seizures before operating.
By consenting to participate in research studies during this period when their brain is “wired,” these patients enable researchers to answer basic neurological questions. They can test which parts of the brain are activated during different behaviors, what happens when a certain region of the brain’s cortex is stimulated and even how to induce brain plasticity to promote rehabilitation and healing across damaged areas.
The potential to use ECoG electrodes implanted on the surface of the brain in future prosthetic or rehabilitative applications offers several advantages — the signals are stronger and more accurate than sensors placed on the scalp, but less invasive than ones that penetrate the brain, as in a recent study by University of Pittsburgh researchers.
In the UW study, three patients wore a glove embedded with sensors that provided data about where their fingers and joints were positioned. They were asked to stay within a target position somewhere between having their hands open and closed without being able to see what that target position was. The only feedback they received about the target hand position was artificial electrical data delivered by the research team.
When their hands opened too far, they received no electrical stimulus to the brain. When their hand was too closed – similar to squeezing something too hard – the electrical stimuli was provided at a higher intensity.
One patient was able to achieve accuracies in reaching the target position well above chance when receiving the electrical feedback. Performance dropped when the patient received random signals regardless of hand position, suggesting that the subject had been using the artificial sensory feedback to control hand movement.
Providing that artificial sensory feedback in a way that the brain can understand is key to developing prosthetics, implants or other neural devices that could restore a sense of position, touch or feeling in patients where that connection has been severed.
“Right now we’re using very primitive kinds of codes where we’re changing only frequency or intensity of the stimulation, but eventually it might be more like a symphony,” said co-author Rajesh Rao, CSNE director and UW professor of computer science & engineering.
“That’s what you’d need to do to have a very natural grip for tasks such as preparing a dish in the kitchen. When you want to pick up the salt shaker and all your ingredients, you need to exert just the right amount of pressure. Any day-to-day task like opening a cupboard or lifting a plate or breaking an egg requires this complex sensory feedback.”
How Humans Change Space Itself
It’s no surprise that humans influence the surface of our planet, but our reach can go farther than that. Humans affect space, too.
We know storms from the sun can naturally change the space environment around Earth, which can have an impact on satellites and power grids.
Scientists now know that Cold War era nuclear tests in the 1950s caused similar effects.
Particles around Earth are organized into layers known as radiation belts. These 1950s tests created a temporary extra layer of radiation closer to Earth.
The effects of this could be seen all around the world. Aurora appeared at the equator instead of the poles, utility grids in Hawaii were strained, and in some cases, satellites above test sites were affected.
Some types of communications signals can also affect Earth’s radiation belts.
Very low-frequency waves, or VLFs, are used for radio communications. They are often used to communicate with submarines, because these waves can penetrate deep into the ocean.
The waves can also travel far into the space environment around Earth. When these waves are in space, they affect how high-energy particles move, creating a barrier against natural radiation.
The outer edge of this radio-wave barrier corresponds almost exactly the inner edge of Earth’s natural radiation belts – meaning it could be human activity that at least partly shapes this natural radiation around Earth.
For more NASA sun and space research, visit www.nasa.gov/sunearth and follow us on Twitter and Facebook.
Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com
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George Michael was 23 when he sat for this interview, which has never been broadcast before.
He had just embarked on his solo career after leaving Wham!. During the conversation, George Michael talks about his sexuality and shedding his pop-ballad persona ala Careless Whisper. He reflects on the power of his fame, as he recalls the historical trip Wham! took to play in communist-China in 1985.
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