# Lecture 2 Dynamics.pdf - H . A . TA N A K A LECTURE 2: PA R...

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L E C T U R E 2 :PA R T I C L E D Y N A M I C SH. A. TANAKA
A N N O U N C E M E N T SWe are in MP1115!Problem set 1 is posteddue Tuesday 6 October.There is a drop box (#7) in the basement of MPOffice hours today (Thursday) 1500-1600
O U T L I N E A N D G O A L SReview qualitatively some elementary properties of thefundamental interactions and particlesIntroduce Feynman diagrams as a way to describe interactionsIn the future, they will correspond to mathematical expressionsthat allow us to calculate thingsFor now, it will be a way of determining how certain reactionscan (not) proceed, and deduce some basic propertiesWon’t really get to what they fundamentally mean; that is QFTDetermine which reactions are allowed, which are notwhich interactions are involved or are dominantget an idea for the “strength” of an interaction.
N O T EDetails on particle masses, properties, etc. can befound on the Particle Data Group site:pdg.lbl.govMost (all?) of the relevant information is also in tablesin the textbook.
T H E S TA N D A R D M O D E LThe fermions make what we usuallycall “matter”neutrons, protons, electronsthe “bosons” correspond to what wesometimes call interactions or forceselectromagneticweakstrongThey also differ in a a fundamental way:• particles are endowed intrinsically with angular momentum• “spin”• fermions have half integer spin: 1/2, (3/2, 5/2, . . .)• bosons have integer spin: 0, 1, (2, 3, . .. . )+2/3-1/30-1
M A S S E SThe particles span an enormous range of masses:091.2 GeV/c280.4 GeV/c200.511 MeV/c2105.6 MeV/c21777 MeV/c2<1 eV/c2<1 eV/c2<1 eV/c23 MeV/c21.2 GeV/c2174 GeV/c24.3 GeV/c20.12 GeV/c27 MeV/c2124 GeV/c2
Thus far, we have specified reactions by specifying theinitial and final state, e.g.reactions that start with one particle and result inmany are called “decays”reactions that involve the interaction of more thanone initial state particle are called “scattering”We will talk now about what happens during “i.e. we will talk more about the reaction “itself”R E A C T I O N Snp+e-+ ¯νe+μ++μe++e-μ++μ-p+ ¯pW-+X+
At a McDonald’s in the US midwest:Person: Why do you have Feynman diagrams allover your van?Feynman: Because I AM Feynman!Person: “Ahhh . . . . . “
F E Y N M A N D I A G R A M SThe basic building blocks are “vertices”defines a basic interactionusually read from left-to-right in time“e came in,γemitted, e came out.”Direction of arrows matter! reverse implies antiparticleThe vertex itself has a number implied called the “coupling constant.”This characterizes the “strength” of the interaction: ~1/137 for EME,pconserved at each vertex (i.e E,pflow in = E,pflow out)The vertices can be arranged in any way:eeγeeγeeγeeγ

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Term
Summer
Professor
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Tags
Quark, Weak interaction, Elementary particle