![]() ![]() Though picturesque, the island is not easy to reach. The meeting was being held on the picturesque Italian island of Elba on the Tyrrhenian Sea. I got involved in the business of lepton flavor universality violation about 10 years ago, as a postdoc in Bern, Switzerland, when I was invited to a meeting about the proposed SuperB collider to be built in Tor Vergata near Rome. The E821 experiment and the discovery of mysterious muon behavior happened before my time in particle physics. A measured violation of the Standard Model would be a flashlight pointing the way toward this higher theory we seek. Physicists have proposed a huge number of such extensions, but at most one of these theories can be correct, and so far none of them has received any direct confirmation. Therefore, the Standard Model must be merely an approximate description that we will need to supplement by adding new particles and interactions. The theory doesn’t explain why neutrinos have mass, nor what makes up the invisible dark matter that seems to dominate the cosmos, nor why matter won out over antimatter in the early universe. If particles really were breaking this rule, that would be exciting in its own right and also because physicists believe that the Standard Model can’t be the ultimate theory of nature. If they don’t, then they violate lepton flavor universality-and the unexpected g-factor measurement suggested that’s just what was happening. ![]() The rule of lepton flavor universality says that because electrons and muons are charged leptons, they should all interact with other particles in the same way (barring small differences related to the Higgs particle). Muons and electrons are both part of a class of particles called leptons (along with a third particle, the tau, as well as the three generations of neutrinos). The measurement wasn’t what the Standard Model predicted. Things started to change in 2004, when the E821 experiment at Brookhaven National Laboratory on Long Island announced its measurement of a property of the muon-a heavy version of the electron-known as its g-factor. For several decades after the invention of the Standard Model, particles seemed to obey this rule. Here the rule I’m thinking of is called “lepton flavor universality,” and it is one of the predictions of our Standard Model of particle physics, which describes all the known fundamental particles and their interactions (except for gravity). This is true not just in life but also in particle physics. The explanation on the video (at the bottom of the page) sets out to show some of the most important properties of these particles and how they relate to one another.Breaking the rules is exciting, especially if they have held for a long time. Showing the range of particles we call fermions and the connections between them To us, the most important baryons are protons and neutrons. Hadrons are in turn divided into two sub groups, mesons and baryons. Included as fermions are combinations of quarks, these are hadrons. The main properties of these are shown in the table. Showing some of the main properties of leptons and quarks However, confusingly, there are six different quarks and six leptons. The fundamental/ elementary fermions, recognised by the Standard Model are quarks and leptons. These are called fermions.įermions are the building blocks of our universe, held together by gauge bosons. However there are many more particles which do or can exist, often very briefly, which make up the full set of matter particles. These protons and neutrons are in turn made up from quarks. All matter, as we experience it, is made up from three main particles – protons, neutrons and electrons. ![]()
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