Dr. Bill Pezzaglia Particle Physics Updated: 2010May20 Modern Physics Series 1 ROUGH DRAFT

Particle Physics (aka “high energy physics”) A.Fundamental Forces B.Classical Particles C.Nuclear Force D.Leptons E.More Particles F.Quark Model G.The Standard Model 2

A. Fundamental Forces 1)Gravity 2)Electromagnetism 3)Strong (Nuclear) Force 4)Weak Force (beta decay) 3

1. Gravity & EM Electrical Force is infinite in range, mediated by massless “photon”. Dominates atomic/molecular Gravitational Force is infinite in range, mediated by massless “graviton” (unconfirmed). Dominates in the large (because macroscopic matter is neutral) Einstein attempts to unify these two (Unified Field Theory) 4

B. Classical Particles 1)Classical period (up to 1930) 2)Spin, Pauli Exclusion Principle 3)Antimatter 5

B. Classical Particles 1)Up to 1930, atoms and spectra explained by: Electron(1897) Photon (1905) Proton (1911) Neutron (1932) 6

B1a. The Electron 1891 Stoney proposes “electron” as fundamental electric charge 1897 Thomson discovers the electron. Three experiments on “cathode rays” 1)deflected by magnetic field 2)Deflected by electric field 3)Measures e/m 7

B1b. The Proton 1886 Goldstein discovers “canal rays” which move in opposite direction as “cathode rays” 1918 Rutherford’s experiment demonstrates small size of Hydrogen nucleus, which is 1800x more massive than electron Rutherford calls it the “proton” (greek word “protos” for “first”)

B1c. The Neutron 1920 Rutherford proposes neutral particle in nucleus (thought it was a proton combined with electron) to explain nuclear masses (e.g. helium is mass of 4, but only has charge of +2 protons) 1932 Chadwick discovers neutron (Nobel prize!) Slightly heavier than proton; spin ½ like proton, even though it is neutral, it has a significant magnetic moment! 9

B2. Spin 1922 Stern Gerlach Experiment shows 2 spin states 1924 Pauli introduces “spin” quantum number Pauli Exclusion principle: “fermions” (half integral spin) obey it, but “bosons” (integral spin) do not. 10

B.3. Antimatter Every particle has an “antiparticle”, which is analogous to the particle moving backwards in time 1927 Paul Dirac predicts “anti-electron” 1931 Anderson finds it (“positron”) 1955 Segre & Chamberlain discover the “antiproton” (at UCB !) 1956 the “anti-neutron” is discovered at UCB ! 11

C. Nuclear Force 1)Yukawa Potential 2)Pi Meson Prediction 3)Pion Reactions mediate nuclear force 12

1. Nuclear Force and Yukawa Potential Electromagnetic force is mediated by the massless “photon” and has infinite range “Strong Force” holds the nucleus together, but has range of only about 1.5x meters Yukawa proposes mediated by a massive particle, which limits range 13

2 The Pion (Pi-Meson) Yukawa estimates mass of particle (“meson”) from equating range to its Compton wavelength Mass estimated to be 130 MeV 1947 the “pion” is discovered (140 MeV) Has zero spin (“boson”) Three types:  +  0  - 14

3. Pion Reactions Can change a proton to neutron, etc  + + n  p  - + p  n p  p +  0 n  n +  0 So, a neutron can decay to proton, emits a pi- which is absorbed by a proton, turning into a neutron. This reaction creates an attractive force between the nucleons. 15

D. Leptons and Weak Interaction 1.Three “generations” of the electron (muon, tau) 2.Three types of neutrinos Electroweak Theory (1983 Vector Boson Discovery) 16

E. More Particles 1.“Strange” Mesons 2.More Baryons 3.The 8 fold way 17

1. More Mesons “Strange” Kaon particles 18

2. Strange Baryons 19

Murray Gell-Mann 1969 Nobel Prize (for quark model) 1962 The 8 fold way Predicts a particle that had not yet been found 20

Even More Baryons 21

F. Quark’s Model (1963) 22

2a. Quark model of Baryons 23 All Baryons made of 3 quarks (one of each “color”, so that they can all three be in the same “1s” orbital and not violate pauli exclusion principle) Proton is an “uud”, which adds up to plus charge

2b. The Neutron 24 The neutron is a “udd” combination, which has net zero charge. The “beta” decay of a neutron into a proton is hence due to one of the “d” quarks decaying into an “u” quark

More Quarks “c” Charmed Quark 1977 “b” Bottom (beauty) quark 1995 “t” Top (truth) quark

Charmed Baryons: Spin 1/2 26 C=+2 C=+1 C=0

Charmed Baryons: Spin 3/2 27 C=+2 C=+1 C=0 C=+3

2c. B Baryons (Fermilab) 28

2c. Mesons in quark model 29 All mesons are made of a quark-antiquark pair. Pi plus would be “up” plus “antidown” quark

Charmed Mesons Spin 0 Hexadecimet 30

3. Gluons 1.Gluons mediate the strong force 2.They hold the quarks together 3.i.e. the ‘squiggle” between u and d quarks 31

G. The Standard Model 1. Three generations 3 isospin doublets of quarks Matches 3 generations of lepton doublets Matches (?) 4 fundamental forces? 32

Fundamental Particles and Interactions 33

3. CPT Symmetry 1951 Schwinger suggests that physics is invariant under a CPT transformation Parity: The laws of physics would be the same in a “mirror” universe (weak interactions violate this, neutrinos are only left circularly polarized, antineutrinos are right circularly polarized). Time Reversal: The laws of physics should be valid if we run the movie backwards (problems with entropy) Charge Conjugation: Replace all particles with their antiparticles. Is physics the same? 34

Summary The standard model is an empirical set of rules. There is no theory that yet gives: Masses of quarks Why quarks all decay to u & d the number of generations as being fixed to 3 why there are only 4 forces why only mesons (qq) and baryons (qqq) are allowed. String Theory was our hope to produce this, but so far it has not succeeded. Many feel that this “theory of everything” is actually a “theory of nothing”. 35

References/Notes 36