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A very simple - Introduction to the Standard Model of Physics If I cut a slice out of butter, what I get is still butter. Isn't it? Well then another slice out of this smaller slice, and another out of that and so on.. till where can I go and still that what I've cut out is still butter?? The answer is till I reach a molecule of butter, it is still butter. Beyond the molecule of butter, it no longer is butter :-) Then we get at the atomic level, nucleus protons and so on. So what's the smallest level to which we can get down here and still say, well this is still a particle!! This is the question that the current standard model of physics tries to answer. We have matter in this universe and the matter interact via the four known fundamental forces i.e., electromagnetic force, strong nuclear force, weak nuclear force and gravitational force. As far the last one is concerned, there are still debates as to whether gravity is a force similar to others?? At least I don't think so. Rather than being a force, it basically is a geometric structure of space time. Coming back to the standard model, so now we have particles that make up the matter and the particles that carry the above mentioned forces between the matter particles. The particles that make up the matter are called fermions. The particles that carry the forces are called bosons. Fermions that are at the fundamental level are quarks and leptons. When we say fundamental level, we mean those particles which are not made up of other particles, in other words those particles that do not have an internal structure which can be further simplified. Electrons, tau particle and muons are leptons. Each of these particles also have a corresponding neutrino associated with them. So we also have electron neutrino, tau neutrino and muon neutrinos as leptons. quarks are the fermions that make up protons and neutrons. So protons and neutrons are composite fermions. Composite fermions are called baryons. We also have composite bosons and they are called mesons. Mesons and baryons together are called hadrons. So the overall model looks somewhat like this. Fermions - particles that make up the matter Fermions = fundamental fermions + composite
fermions Now, is the standard model complete?? As of today (15 Oct 2006), the standard model does not explain gravity, so it is incomplete. Also, the existence of all particles described in the standard model has been confirmed experimentally except for the Higg's boson which is yet to be observed. - by Gurudev
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