- 3089 - SUPER SYMMETRY - Our best model of particle physics can not contain all the weirdness in the universe. The reigning physics model is the Standard Model. Its replacement has been predicted for decades.
------------------- 3089 - SUPER SYMMETRY
- Strange results from laboratory experiments suggest flickers of ghostly new species of neutrinos beyond the three described in the Standard Model. And the universe seems full of “dark matter” that no particle in the Standard Model can explain.
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- Recent tantalizing evidence is coming from the 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.
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- Several additional particles detected later at the buried “Antarctic neutrino observatory” , “IceCube” don't match the expected behavior of any Standard Model particles. The particles look like “ultra high-energy neutrinos“.
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- 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 flinging itself into the cold southern sky.
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- 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 dirt beneath our feet.
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- But ultra-high-energy neutrinos from deep space are different from their low-energy cousins coming from the sun. 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. Then IceCube added more detctions.
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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.
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- But in theory, there could have been ultra-high-energy neutrino sources beyond the sky-wide flux, those neutrino guns, or cosmic accelerators.
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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 being particle accelerators.
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- We know that cosmic neutrino accelerators do exist in space. IceCube has tracked a high-energy neutrino back to a blazar, an intense jet of particles coming from an active blackhole at the center of a distant galaxy.
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- ANITA picks up only the most extreme high-energy neutrinos. 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.
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- 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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- 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.
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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.
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- When Isaac Newton first clicked on 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“. (See another Review, the equations are not so simple)
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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 revealed 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 leads to the law of conversation of energy. There is a symmetry through space, and the corresponding symmetry through the 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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- But 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 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 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 halfsies are called "fermions" and are made up of the building blocks of our world: electrons, quarks, neutrinos and so on. The wholsies are called "bosons" and are the carriers of the forces of nature: photons, gluons, and the rest.
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- At first glance, these two families of particles couldn't possibly be any different.
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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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- 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.
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- 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 the rest.
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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.
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- If supersymmetry isn't really found in nature, what should we think of next if we can't find anything.
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March 12, 2021 SUPERSYMMETRY 3089
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