Introduction to Elementary Particles

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Lecture 6 35 Oct 30, 2002 5 The Quanta of the Standard Model A long series of experiments and theoretical studies and advances has established the so-called Stan- dard Model of elementary particle physics. This theory encompasses the electromagnetic theory (uni- fied by Maxwell), the weak interaction (responsible for radioactive decays of for instance the neutron and the muon, and for the burning of hydrogen into helium and heavier elements in the Sun), and the strong interaction (which binds protons and neutrons inside the nucleus). The SM is the (incomplete) theory (in the sense of our current best approximation to the truth) of the fundamental matter particles and their interactions. We know that the Standard Model is incomplete: it does not truly unify the electroweak and the strong interaction, just combines them. Further, it does not predict the model’s parameters (of which there are 21) and only experimentation lets us determine the values of these. It does not include the much weaker gravitational interaction in the same quantum me- chanical framework, because a the quantization of gravity, unlike that for electromagnetism, is very poorly understood. Finally, SM calculations break down – i.e. something new must happen – at ener- gies of a few TeV, energies that will become accessible by 2006 at the Large Hadron Collider now un- der construction at CERN near Geneva. However, all present data are in excellent agreement with pre- dictions made within the framework of the Standard Model, and it is the best theory we have at pre- sent. Many famous and less famous physicists, both experimenters and theorists, have contributed to the present picture of the elementary particles, which developed from the early 1900s until now. The Stan- dard Model is thus the result of a long and arduous search for the correct description of nature at its most elemental level. In that search, experimental study and theoretical insight always go hand-in- hand: theory tends to diverge if not checked by experimental feedback, and experimentation becomes meaningless without theoretical analysis of its findings. We expect that the next generation of experi- ments, at the Fermilab Tevatron and at the CERN LHC will result in exploration of the physics beyond the Standard Model. The Standard Model divides the world of elementary particles in fermionic matter: leptons (6 electron- like particles, and 6 anti-leptons), and quarks (6 quarks of different flavors, and 6 anti-quarks), see Table 4. The fermions are the sources of a variety of fields:
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