What are the Elementary Constituents of Matter? What are the forces that control their behaviour at the most basic level?

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Presentation transcript:

What are the Elementary Constituents of Matter? What are the forces that control their behaviour at the most basic level?

History of Constituents of Matter AD

In Nuclear Reactions momentum and mass- energy is conserved – for a closed system the total momentum and energy of the particles present after the reaction is equal to the total momentum and energy of the particles before the reaction In the case where an alpha particle is released from an unstable nucleus the momentum of the alpha particle and the new nucleus is the same as the momentum of the original unstable nucleus

Large variations in the emission velocities of the  particle seemed to indicate that both energy and momentum were not conserved. This led to the proposal by Wolfgang Pauli of another particle, the neutrino, being emitted in  decay to carry away the missing mass and momentum. The neutrino (little neutral one) was discovered in Wolfgang Pauli __

u u u 1 u = 1 J = __ kg eV

Mass difference kg u

It has been found by experiment that the emitted beta particle has less energy than MeV Neutrino accounts for the ‘missing’ energy J J eV MeV

+ +

First artificial splitting of nucleus First transmutation using artificially accelerated particles First experimental verification of E = mc 2 Irish Nobel Prize E.T.S. Walton 1951 Cockroft and Walton Ernest WaltonJohn Cockcroft

Proton + Lithium Two alpha particles + Energy 1 MeV17.3 MeV Experimental verification of E = mc 2

Ancient Greeks: Earth, Air, Fire, Water By 1900, nearly 100 elements By 1936, back to three particles: proton, neutron, electron

CERN LEP APPLET

The Four Fundamental Forces

Particle zoo

Thomson (1897): Discovers electron

Q = -1e almost all trapped in atoms Q= 0 all freely moving through universe _

Just as the equation x 2 =4 can have two possible solutions (x=2 OR x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. Dirac interpreted this to mean that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron, for instance, there should be an "antielectron" called the positron identical in every way but with a positive electric charge.

1928 Dirac predicted existence of antimatter 1932 antielectrons (positrons) found in conversion of energy into matter 1995 antihydrogen consisting of antiprotons and positrons produced at CERN In principle an antiworld can be built from antimatter Produced only in accelerators and in cosmic rays

James Joyce Murray Gell-Mann