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PPT : Physics 7802.01 Introduction
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Over the years inquiring minds have asked: “Can we describe the known physics with just a few building blocks ?” Þ Historically the answer has been yes. Elements of Mendeleev’s Periodic Table (chemistry) nucleus of atom made of protons, neutrons proton and neutron really same “particle” (different isotopic spin)
By 1950’s there was evidence for many new particles beyond g, e, p, n It was realized that even these new particles fit certain patterns: pions: p+(140 MeV) p-(140 MeV) po(135 MeV) kaons: k+(496 MeV) k-(496 MeV) ko(498 MeV)
Some sort of pattern was emerging, but ……….. lots of questions Þ If mass difference between proton neutrons, pions, and kaons is due to electromagnetism then how come: Mn > Mp and Mko > Mk+ but Mp+ > Mpo
Lots of models concocted to try to explain why these particles exist: Þ Model of Fermi and Yang (late 1940’s-early 50’s): pion is composed of nucleons and anti-nucleons (used SU(2) symmetry)
note this model was proposed before discovery of anti-proton !
With the discovery of new unstable particles (L, k) a new quantum number was invented: Þ strangeness P780.02 Spring 2003 L8
Gell-Mann, Nakano, Nishijima realized that electric charge (Q) of all particles could be related to isospin (3rd component), Baryon number (B) and Strangeness (S): Q = I3 +(S + B)/2= I3 +Y/2 Coin the name hypercharge (Y) for (S+B)
Interesting patterns started to emerge when I3 was plotted vs. Y:
Particle Model of Sakata (mid 50’s): used Q = I3 +(S + B)/2 assumed that all particles could be made from a combination of p,n, L tried to use SU(3) symmetry In this model:
This model obeys Fermi statistics and explains why: Mn > Mp and Mko > Mk+ and Mp+ > Mpo
Unfortunately, the model had major problems…. P780.02 Spring 2003 L8
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Tags: nucleus of atom, particle model, protons neutrons