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------------------------- 2619 - NEUTRINOS - and other mysterious particles?
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- Three times since 2016, ultra-high-energy particles have blasted up through the ice of Antarctica, setting off detectors in the Antarctic Impulsive Transient Antenna (ANITA) experiment, a machine dangling from a NASA balloon far above the frozen surface. The balloon is sent circling high above he South Pole.
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- Several particles detected later at the buried Antarctic neutrino observatory, IceCube, don't match the expected behavior of any Standard Model particles either. The particles look like ultra high-energy neutrinos. But ultra high-energy neutrinos shouldn't be able to pass through the Earth. That suggests that some other kind of particle. One that's never been seen before is colliding with our Antarctic detectors.
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- A collection of hyperactive neutrino sources somewhere in our northern sky could have blasted enough neutrinos into Earth that we'd detect particles shooting out of the southern tip of our planet. But the IceCube researchers didn't find any evidence of that collection out there, which suggests new physics must be needed to explain these mysterious particles.
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- Why are these mystery particles so unsettling for the Standard Model or particle Physics? Neutrinos are the faintest particles we know about; they're difficult to detect and nearly massless. They pass through our planet all the time, mostly coming from the sun and rarely, if ever, colliding with the protons, neutrons and electrons that make up our bodies and the earth beneath our feet.
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- Ultra-high-energy neutrinos from deep space are different from their low-energy ones we typically measure. Much rarer than these low-energy neutrinos, they have wider "cross sections," meaning they're more likely to collide with other particles as they pass through them.
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- The odds of an ultra-high-energy neutrino making it all the way through Earth intact are so low that you'd never expect to detect it happening. That's why the ANITA detections were so surprising.
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- Many ultra-high-energy cosmic neutrinos come from the interactions of cosmic rays with the cosmic microwave background (CMB), the faint afterglow of the Big Bang. Every once in a while, those cosmic rays interact with the CMB in just the right way to fire high-energy particles at Earth.
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- This is called the "flux," and it's the same all over the sky. Both ANITA and IceCube have already measured what the cosmic neutrino flux looks like to each of their sensors, and it just doesn't produce enough high-energy neutrinos that you'd expect to detect a neutrino flying out of Earth at either detector even once.
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- If the events detected by ANITA belong to this diffuse neutrino component, ANITA should have measured many other events at other elevation angles. In theory, there could have been ultra-high-energy neutrino sources beyond the sky-wide flux.
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- If it is not a matter of neutrinos produced by the interaction of ultra-high-energy cosmic rays with the CMB, then the observed events can be either neutrinos produced by individual cosmic accelerators in a given time interval or some unknown Earthly source.
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- Blazars, active galactic nuclei, gamma-ray bursts, starburst galaxies, galaxy mergers, and magnetized and fast-spinning neutron stars are all good candidates for those sorts of accelerators.
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- We know that cosmic neutrino accelerators do exist in space; in 2018, IceCube tracked a high-energy neutrino back to a blazar, an intense jet of particles coming from an active black hole at the center of a distant galaxy.
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- ANITA picks up only the most extreme high-energy neutrinos and if the upward-flying particles were cosmic-accelerator-boosted neutrinos from the Standard Model, most likely tau neutrinos, then the beam should have come with a shower of lower-energy particles that would have tripped IceCube's lower-energy detectors. (See references at the end of this review to learn more about neutrinos.)
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- IceCube data results don't completely eliminate the possibility of an accelerator source out there. But they do "severely constrain" the range of possibilities, eliminating all of the most plausible scenarios involving cosmic accelerators and even many less-plausible ones.
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- Researchers don't know what's next. Neither ANITA nor IceCube is an ideal detector for the needed follow-up searches, leaving the researchers with very little data on which to base their assumptions about these mysterious particles. It's a bit like trying to figure out the picture on a giant jigsaw puzzle from just a handful of pieces.
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- Right now, many possibilities seem to fit the limited data, including a fourth species of "sterile" neutrino outside the Standard Model and a range of theorized types of dark matter. Any of these explanations would be revolutionary. But none is strongly favored at this point in the research..
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- Symmetries in nature power our fundamental understanding of the cosmos, from the universality of gravity to the unification of the forces of nature at high energies.
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- In the 1970s, physicists uncovered a potential symmetry that united all the kinds of particles in our universe, from the electrons to the photons and everything in between. This connection, known as “super symmetry“, relies on the strange quantum property of “spin“, and potentially holds the key to unlocking a new understanding of physics.
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- For centuries, symmetries have allowed physicists to find underlying connections and fundamental relationships throughout the universe. When Isaac Newton first came up with the idea that the gravity that pulls an apple from a tree is the exact same force that keeps the moon in orbit around the sun, he discovered a symmetry: the laws of gravity are truly universal. This insight allowed him to make a tremendous leap in understanding how nature works.
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- Throughout the 1800s, physicists around the world puzzled over the strange properties of electricity, magnetism and radiation. What caused electric current to flow down a wire? How could a spinning magnet push that same current around? Was light a wave or a particle?
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- Decades of difficult pondering culminated in a clean mathematical breakthrough by James Clerk Maxwell, who unified all these distinct branches of inquiry under a single set of simple equations: electromagnetism.
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- Albert Einstein made his mark too by taking Newton's insights one step further. Taking as a maxim that all physical laws should be the same regardless of your position or velocity, he came up with “special relativity“; the notions of time and space had to be rewritten to preserve this symmetry of nature. And adding gravity to that mix led him to general relativity, our modern understanding of that force.
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- Even our conservation laws, the conservation of energy, the conservation of momentum and so on, depend on symmetry. The fact that you can run an experiment day after day and get the same result reveals a symmetry through time, which through the mathematical genius of Emmy Noether lead to the law of conversation of energy. (See other Reviews about Emmy Noether and her contributions to science).
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- If you pick up your experiment and move it across the room and still get the same result, you just uncovered a symmetry through space, and the corresponding conservation of momentum.
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- In the macroscopic world, that just about sums up all the symmetries that we've encountered in nature. But the subatomic world is a different story. The fundamental particles of our universe have an interesting property known as "spin." It was first discovered in experiments that shot atoms through a varied magnetic field, causing their paths to deflect in the exact same way that a spinning, electrically charged metal ball would.
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- However subatomic particles are not spinning, electrically charged metal balls; they just act like them in certain experiments. And unlike their regular-world analogs, subatomic particles can't have any amount of rotation they wish. Instead, each kind of particle gets its own unique amount and direction of spin.
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- For various obscure mathematical reasons, some particles like the electron get to have a spin of ½, while other particles like the photon get a spin of 1. If you're wondering how a photon could possibly behave like a spinning charged metal ball, then don't sweat it too much; you are free to just think of "spin" as yet another property of subatomic particles that we have to keep track of, like their mass and charge. And some particles have more of this property, and some have less.
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- In general, there are two great "families" of particles: those with half-integer (1/2, 3/2, 5/2, etc.) spin, and those with whole-integer (0, 1, 2, etc.) spin. The half integers are called "fermions" and are made up of the building blocks of our world: electrons, quarks, neutrinos and so on. The whole integers are called "bosons" and are the “carriers” of the forces of nature: photons, gluons, and the rest of the bosons.
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- In the 1970s, string theorists began to look critically at this property of spin and started to wonder if there might be a symmetry of nature there. The idea quickly expanded outside the string community and became an active area of research across particle physics. If true, this "supersymmetry" would unite these two seemingly disparate families of particles. But what would this supersymmetry look like?
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- The basic is that, in supersymmetry, every fermion would have a "superpartner particle" (or "sparticle" for short, and, the names are only going to get worse) in the boson world, and vice versa, with the exact same mass and charge but a different spin.
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- But if we go looking for the sparticles, we don't find any. For example, the sparticle of the electron (the "selectron") should have the same mass and charge as the electron, but a spin of 1. That particle doesn't exist that we can find.
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- Somehow, this symmetry must be broken in our universe, driving up the masses of the sparticles outside the range of our particle colliders. There are many different ways of achieving supersymmetry, all predicting different masses for the selectrons, the stop quarks, the sneutrinos and everybody else.
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- To date, no evidence for supersymmetry has been found, and experiments at the Large Hadron Collider have ruled out the simplest supersymmetric models. While it's not quite the last nail in the coffin, theorists are scratching their heads, wondering if supersymmetry isn't really found in nature, and what we should think of next if we can't find anything.
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- Maybe the mysteries of neutrinos will lead us to new symmetries that open new science for our confused ones.
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-------------------------------------- Other reviews available:
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- 2223 - Astronomers have learned much more about the Universe with their “microwave telescopes”. They have determined the Universe to be 13,800,000,000 years old. They have determined that only 5% or the Universe is visible or ordinary matter. The rest is Dark Matter (25%) and Dark Energy (70%) . To learn more astronomers want to be able to use “neutrino telescopes” and “gravity wave telescopes”.
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- 2131 - Neutrinos - The Little Neutral Ones. The neutrino is a tiny elementary particle that is a billion times more abundant than protons and electrons that make up our normal atoms. Neutrinos are produced in the fusion reactions of our Sun and in the natural radioactive decay of elements in the Earth’s crust. Potassium 40 in your body is emitting 340,000,000 neutrinos every day. This review contains the history of discoveries of neutrinos.
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- 2093 - Neutrinos - What have we learned? - Neutrinos are the smallest atomic particles. If we could see neutrinos they would be exceptional probes into our environment. Neutrinos are produced in fusions reactions in the Sun and stars, and in radioactive decay in the earth's crust. The ICECUBE neutrino detector at the South Pole has over 5,000 light sensors to detect neutrinos interacting with atoms in the ice.
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- 2026 - Many more discoveries are needed to explain neutrinos. A detector in the ice at the South Pole may make these new neutrino discoveries. Another experiment is sending neutrinos from Illinois to South Dakota. Neutrinos are a billion times more abundant than electrons.
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- 1978 - Neutrinos are sub-atomic particles that reside with electrons and protons in the center of atoms. They release in mass from the fusion reactions that go on in the center of our Sun. They are nearly massless with no charge and trillions have passed through your body as you have read this sentence.
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- 1840. - Are there sterile neutrinos? They would be right handed. That would be a neutrino with right handed spin. It is predicted in the math in physics. But, it is yet to be discovered. It took 50 years later after its prediction for the Higgs Boson to be discovered.
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- 1814. - Defines what are neutrinos are and provides an index of seven more reviews on the subject.
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- 1631. - Because neutrinos are neutral particles they travel through space in a straight line unaffected by magnetic fields. They arrive hours ahead of the light coming from a supernovae explosions. Astronomers hope that neutrino detectors can be used to study supernovae. They can get their telescopes looking in the right places before the explosion happens.
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- 1608. - While you are reading this sentence 5,000,000 neutrinos passed through your thumbnail. They are generated in the Earths crust, the Sun, the Big Bang, supernovae, and even from inside your own body. Neutrinos come in 3 flavors. Only the left handed neutrinos interact with the weak nuclear force.
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- 1589.- Neutrinos are unaffected by the strong nuclear force. Neutrinos are created in radioactive decay. These small particles were proposed to exist in 1930. They were not discovered until 1942. Neutrinos may be their own anti-particle. Neutrinos are so small they comprise only 0.3% percent of the mass-energy of the universe.
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- 1511. - Particles that the Standard Model of Particle physics predicts, but, that we have yet to find. Sterile neutrinos may explain the structure of galaxies. It may explain the distance between galaxies.
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- 1219. - ICECUBE is a neutrino telescope located deep in the ice at the South Pole. It will not see photons. It will see neutrinos. New discoveries are bound to occur.
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- 1139 - Neutrino telescopes could look further back in time then light telescopes can detect. The universe will “light up” when we can see with neutrino eyes.
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- 732. - Neutrinos are leptons. An electron neutrino mass is less than 0.1 electron volts. A supernova releases 99% of its energy in the first 10 second burst of neutrinos. This review contains the thermal history of the universe. After 13.7 billion years we have cooled down to 2.728 Kelvin degrees.
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- February 14, 2020 2619
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