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Muon g-2 breaks the Laws of Physics
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- First time: When the muon was first discovered, Isidor Isaac Rabi reacted saying “Who ordered that?” Rabi, a Nobel laureate who helped America develop the atom bomb, was reflecting physicists’ general surprise that muons, which are just heavy and unstable versions of electrons, actually exist. To an orderly physicist’s mind they somehow seemed superfluous to Nature’s requirements.
- Details: Establishing the muon’s nature was an important part of the creation of what is known as the Standard Model of particle physics. This, along with Einstein’s general theory of relativity (actually a theory of gravity), is one of the two foundation stones on which modern physics is built. Yet the Standard Model is known to be incomplete for several reasons, one of which is precisely the fact that it does not yet embrace gravity. So it seems fitting that an answer to Rabi’s question, and with it a path to an explanation of physics beyond the Standard Model, may now have been opened by a measurement made on muons.
- Latest study: The study in question, called Muon g-2, used a superconducting storage device to look at the magnetic behaviour of muons. Experiments conducted with this machine at Brookhaven National Laboratory, in New York state, in the 1990s, had suggested an anomaly in such behaviour—a deviation of about 0.1% from theoretical predictions about the way that muons should spin in magnetic fields—but without sufficient statistical power to be sure. If this anomaly were real, it would suggest that an unknown force was tugging on the muons in the experiment.
- To have another go at finding out, the storage device was shipped to Fermilab, outside Chicago, in 2013. There, it was linked to equipment which gave it more oomph.
- This boost has, indeed, confirmed the previous result—though irritatingly not quite unambiguously enough for physics’ finicky requirements.
- These demand “five sigma” of significance (five standard deviations from the mean, for the mathematically inclined).
- The new data, added to the old, and announced on April 7th, give only 4.2 sigma. That, nevertheless, suggests there is only one chance in 40,000 that the result is a fluke.
- LHC: This is the second time in a month that a group of physicists has published a result which might lead beyond the Standard Model, for on March 23rd researchers on a project being conducted at cern, home of the Large Hadron Collider, the world’s largest particle accelerator, pulled a similar surprise. Their work involved the decay of particles called b-mesons into electrons, muons and their antimatter equivalents. Again, the details are not yet quite as statistically robust as might be desired. But two such findings in short order give hope that the hunt for physics beyond the Standard Model may soon run its quarry down.
- Knowledge centre:
- Standard model of Physics - The theories and discoveries of thousands of physicists since the 1930s have resulted in a remarkable insight into the fundamental structure of matter: everything in the universe is found to be made from a few basic building blocks called fundamental particles, governed by four fundamental forces. Our best understanding of how these particles and three of the forces are related to each other is encapsulated in the Standard Model of particle physics. Developed in the early 1970s, it has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments, the Standard Model has become established as a well-tested physics theory.
- Four fundamental forces - There are four fundamental forces at work in the universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force is the strongest of all four fundamental interactions.Three of the fundamental forces result from the exchange of force-carrier particles, which belong to a broader group called “bosons”.
- Subatomic world - Normal matter is made of molecules, which are themselves made of atoms. Inside the atoms, there are electrons spinning around the nucleus. The nucleus itself is generally made of protons and neutrons but even these are composite objects. Inside the protons and neutrons, we find the quarks, but these appear to be indivisible, just like the electrons.Quarks and electrons are some of the elementary particles we study at CERN and in other laboratories. But physicists have found more of these elementary particles in various experiments, so many in fact that researchers needed to organize them, just like Mendeleev did with his periodic table.This is summarised in a concise theoretical model called the Standard Model.
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