Lecture13-16

B fermions must be created in pairs with antiparticle

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Unformatted text preview: stent with being made from three cons;tuent par;cles became apparent. u༇  Quarks were ul;mately “seen” by an experiment much like Rutherford’s. 56 Elementary Par;cles 57 Elementary Par;cles Electrons are point par;cles (as well as we can measure). u༇  The nucleus of atoms is made of protons and neutrons. u༇  o  Neutron life;me about 12 minutes outside the nucleus. Protons and neutrons are made from u and d quarks. u༇  Quarks are point par;cles (as well as we can measure). u༇  o  Measurements analogous to Rutherford scaFering. o  Limited by energy of accelerator. 58 Small Number of Elementary Par;cles u༇ For the first ;me, the number of most basic par;cles is quite small. u༇ If we don’t count (broken) symmetries, only 9 in this picture. o  3 force carriers (spin 1) o  6 maFer par;cles (spin 1/2) u༇ First genera;on accounts for all stable maFer. 1 4 6 8 5 7 9 3 2 Three genera;on of maFer par;cles is not explained. 59 Ques;on: MaFer Par;cles u༇  Which A) There are three generations of spin 1/2 matter particles. statement about the B) Electrons are elementary pointlike particles. elementary par;cles of C) Quarks are elementary pointlike particles. maFer is D) The first generation of matter particles false. accounts for all stable matter. E) None of the above. 60 The Four Forces (pre 1970) u༇  Electromagne;sm o  Mediated by massless photon u༇  Strong (nuclear) interac;on o  Strong short range forces bind nuclei o  Very complex, no good theory u༇  Weak interac;ons o  Seen in nuclear decay o  Mediated by very massive bosons •  Massive boson theories have problems u༇  Gravity (GR: geometry) o  No quantum gravity 61 Three Gauge Theories u༇  Strong and Weak interac;ons were understood to be gauge theories, like E&M around 1970. o  All vector fields o  Bigger symmetries than E&M u༇  Strong interac;ons had been confusing because quarks had not been found before. o  Quarks are confined u༇  Weak interac;ons were confusing because the symmetry was spontaneously broken. o  Vector boson becomes massive in broken symmetry. 62 Phase (Gauge) Symmetry in QED iλ ( x ,t ) ψ ( x,t ) → e ψ ( x,t ) u༇  Phase symmetry in electron wavefunc;on corresponds to gauge symmetry in E&M vector field A. o  One requires the other for terms to cancel in Schrödinger equa;on. o  Electron’s phase symmetry requires existence of photon. u༇  Gauge symmetry assures charge is conserved and that photon remains massless. 63 2 Par;cles With the Same Mass... 1 2 u༇  Imagine 2 types of electrons with the same mass, spin, charge…, everything the same. u༇  The laws of physics would not change if we replaced electrons of type 1 with electrons of type 2. u༇  We can choose any linear combina;on of electrons 1 and 2. This is called a global SU(2)...
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