Elementary Particles

Democritus (~2500 years ago) What is the universe made of? How did the universe come to be the way that it is?

Recall the four fundamental forces: GRAVITY: All particles are influenced. Relative strength: 10 −38 WEAK: All particles are influenced. Relative strength: 10 −7 E & M: Excludes neutrinos. Relative strength: 10 −2 STRONG: Excludes leptons. Relative strength: 1

How do we classify particles? According to their masses. LEPTONS, “lightweights” (e.g, electrons, neutrinos) MESONS “middleweights” (e.g., pions, kaons) BARYONS “heavyweights” (e.g., protons, neutrons) According to the types of forces that affect them. According to their intrinsic spin (ang. mtm.) According to the Pauli exclusion principle.

Leptons Are not affected by the STRONG force. Lepton #, L, is conserved in all reactions. Electron Muon Tau Associated Neutrinos 𝑒 − 𝜇 − 𝜏 − 𝜐 𝑒 𝜐 𝜇 𝜐 𝜏 𝐿 𝑒 = +1 𝐿 𝜇 = +1 𝐿 𝜏 = +1 Antiparticles 𝑒 + 𝜇 + 𝜏 + 𝐿 𝑒 =−1 𝐿 𝜇 =−1 𝐿 𝜏 =−1 Furthermore, Lepton # for each family is independently conserved.

Example m+  e+ + ne + anti-nm Le: 0  (- 1) + 1 + 0 Le: -1  0 + 0 + (-1) Le: 0  0 + 0 + 0

Another Example p+  m+ + n To what family does the neutrino belong? Muon family Is the neutrino a particle or an antiparticle? Particle What is the lepton number of the neutrino? Lm = +1 (Le = Lt = 0)

Hadrons (Mesons & Baryons) Particle reactions governed by STRONG force. Baryons: B = +1 & Antibaryons: B = –1 Baryon Number is conserved in all reactions.

Example Applying Conservation 1) p + p  p + p + p 2) p + n  p + p + p ALLOWED 3) p- + p  n + p What is the charge of the p-meson in each case? 1) p0 2) p- 3) p0

Another Example p + n  p + p + n + X What is “q” for the unknown particle, X? What is “B” for the unknown particle, X? Using the same unknown from above… X + p  p + p + p- + p- ALLOWED? X + p  p- + p- + p+ + p+ + p0 ALLOWED? q = –1 B = –1 No! Yes!

Allowed or Not Allowed? p+ + p  p + p + p- NEVER HAPPENS! (Why?) (Electric charge is not conserved.) p+ + n  L0 + K+ HAPPENS! p+ + n  L0 + K+ + n NEVER HAPPENS! p + n  L0 + K + + n HAPPENS! p + n  L0 + K+ NEVER HAPPENS! The reactions that do occur conserve BARRON #.

How to explain? Time to define a new property… “STRANGENESS” K+ has S = +1 p, n, p’s have S = 0 L0, K–, S+ have S = –1 Strangeness is conserved in ALL interactions involving the strong force.