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

L E C T U R E 2 :
PA R T I C L E D Y N A M I C S
H. A. TANAKA

A N N O U N C E M E N T S
•
We are in MP1115!
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Problem set 1 is posted
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due Tuesday 6 October.
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There is a drop box (#7) in the basement of MP
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Office hours today (Thursday) 1500-1600

O U T L I N E A N D G O A L S
•
Review qualitatively some elementary properties of the
fundamental interactions and particles
•
Introduce Feynman diagrams as a way to describe interactions
•
In the future, they will correspond to mathematical expressions
that allow us to calculate things
•
For now, it will be a way of determining how certain reactions
can (not) proceed, and deduce some basic properties
•
Won’t really get to what they fundamentally mean; that is QFT
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Determine which reactions are allowed, which are not
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which interactions are involved or are dominant
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get an idea for the “strength” of an interaction.

N O T E
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Details on particle masses, properties, etc. can be
found on the Particle Data Group site:
•
pdg.lbl.gov
•
Most (all?) of the relevant information is also in tables
in the textbook.

T H E S TA N D A R D M O D E L
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The fermions make what we usually
call “matter”
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neutrons, protons, electrons
•
the “bosons” correspond to what we
sometimes call interactions or forces
•
electromagnetic
•
weak
•
strong
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They 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
-1/3
0
-1

M A S S E S
•
The particles span an enormous range of masses:
0
91.2 GeV/c 2
80.4 GeV/c 2
0 0.511 MeV/c 2
105.6 MeV/c 2
1777 MeV/c 2
<1 eV/c 2 <1 eV/c 2 <1 eV/c 2
3 MeV/c
2
1.2 GeV/c
2
174 GeV/c
2
4.3 GeV/c 2 0.12 GeV/c 2 7 MeV/c 2
124 GeV/c 2

•
Thus far, we have specified reactions by specifying the
initial and final state, e.g.
•
reactions that start with one particle and result in
many are called “decays”
•
reactions that involve the interaction of more than
one 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 S
n → p + e
-
+ ¯ ν
e
⇤
+
→ μ
+
+ ⇥
μ
e
+
+ e
-
→ μ
+
+ μ
-
p + ¯ p → W
-
+ X
+

At a McDonald’s in the US midwest:
Person: Why do you have Feynman diagrams all
over your van?
Feynman: Because I AM Feynman!
Person: “Ahhh . . . . . “

F E Y N M A N D I A G R A M S
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The basic building blocks are “vertices”
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defines a basic interaction
•
usually read from left-to-right in time
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“e came in, γ emitted, e came out.”
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Direction of arrows matter! reverse implies antiparticle
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The vertex itself has a number implied called the “coupling constant.”
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This characterizes the “strength” of the interaction: ~1/137 for EM
•
E, p conserved at each vertex (i.e E, p flow in = E, p flow out)
•
The vertices can be arranged in any way:
e
e
γ
e
e
γ
e
e
γ
e
e
γ