An introduction to the quark model

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Few-charge systems History of the quark model Mesons Baryons Multiquarks and other exotics Outlook Four-body molecules Hydrogen molecule and variants best known, (p, p, e + , e − ) In the Born–Oppenheimer–Heitler–London approach, effective pp potential, which gives the ground-state and the first excitations. This is a very good approximation, This corresponds to the two electrons in the lowest state for given pp separation, Excited electrons → second set of levels, Positronium molecule proposed by Wheeler in 1945, In 1946, Ore publish it does not believe this is the case, In 1947, Hylleraas and (the very same) Ore have an elegant proof of the stability In 2007, indirect experimental evidence JMR Quark Model
Few-charge systems History of the quark model Mesons Baryons Multiquarks and other exotics Outlook Systematics of (m + 1 , m+ 2 , m− 3 , m− 4 ) Any state with m3 = m4 is stable. Why? Two degenerate thresholds. In particular, (m + , m + , m − , m − ) is stable (positronium molecule and variants) What about two masses? (M + , M + , m − , m − ) improves stability. (M + , m + , M − , m − ) spoils stability. It becomes unstable for M/m 2.2 (or, of course, M/m 1/2.2) So, starting from the doubly-symmetric (m + , m + , m − , m − ), and breaking Charge conjugation, or Particle identity does not produce the same result. Why? Both ways of breaking symmetry lower the ground state. But in the second case, the threshold benefits more of symmetry breaking. Hence, one gets less stability. See the section on multiquarks. JMR Quark Model
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