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H. A. Tanaka
Photons and Quantum Electrodynamics
The Photon• Apart from we need some other particle/object with definite Lorentz
transformation properties to make Lorentz invariants• What would we do with the “vector” term to get a Lorentz
scalar?• Recall the photon:
• Classically, we have Maxwell’s equations:
• Recall that we can re-express the Maxwell equations using potentials:
• these can in turn be combined to make a 4 vector:• Likewise for the “source” terms ρ and J:
��
⇥�µ⇥
⌅ · E = 4�⇥ ⌅ · B = 0
⌅⇤E +1cB = 0 ⌅⇤B� 1
cE =
4�
cJ
E = �⇥�
Aµ = (�,A)Jµ = (c�,J)
B = ⇥�A
Maxwell’s Equation in Lorentz Covariant Form• All four equations can be expressed as:
• The issue is that A is (far) from unique:• Consider:
• the last terms cancel, so the “new” Aµ is also a solution to Maxwell’s solution
• they are physically the same, so we can make some conventions:• “Lorentz gauge condition”:• “Coulomb gauge”
⇥µ⇥µA⇥ � ⇥⇥(⇥µAµ) =4�
cJ⇥
Aµ � Aµ + ⇥µ�
�µAµ = 0
A0 = 0
⇥µ⇥µA⇥ =4�
cJ⇥
⇥ · A = 0
⇥µ⇥µ(A� + ⇥��)� ⇥�(⇥µ(Aµ + ⇥µ�) =⇥µ⇥µA� � ⇥�(⇥µ(Aµ) + ⇥µ⇥µ⇥��� ⇥�⇥µ⇥µ�
Fµ⇥ = �µA⇥ � �⇥Aµ =�
⇧⇧⇤
0 �Ex �Ey �Ez
Ex 0 �Bz By
Ey Bz 0 �Bx
Ez �By Bx 0
⇥
⌃⌃⌅
Solutions to the Maxwell Equation in Free Space:
• “Free” means no sources (charges, currents): Jµ=0• Find solution as usual by ansatz:
• Now check:
• Conclusions:• Photon is massless• Polarization ε is transverse to photon direction:
• it has two degrees of freedom/polarizations
�µ�µA⇥ = 0
Aµ(x) = a e�ip·x�µ(p)
⇥µA⇥(x) = �ipµ a e�ip·x�⇥(p)
⇥µ⇥µA⇥(x) = (�i)2pµpµ a e�ip·x�⇥(p) = 0 p2 = m2c2 = 0
A0 = 0� �0 = 0
⇥µAµ = 0� pµ�µ(p) = 0
⇥ p · � = 0
Making a “scalar” object:
• In the end, these spaces must collapse:• In Lorentz space, this happens by contracting indices:• In spinor space, products of adjoint spinors with spinors (with gamma
matrices possibly in between): Г=(product of g matrices)• but some expressions have structure in both:
gµ⇥aµb⇥ = aµbµ
u1�v2
M = � g2e
(p1 + p2)2[u(3) �µ v(4)] [v(2) �µ u(1)]
sum over μ collapses the Lorentz structure
�⇥ = S�1�µS⇥x⇥
⇥xµ⇥
sum over μ collapses the Lorentz structure
Product of 4x4 matrices in spinor space
Contracted in spinor space, but not in Lorentz
Same here
Reminder of Dirac Spinors
• We can now construct the column vector u:
u1 = N
�
⇧⇧⇤
10
pzc/(E + mc2)(px + ipy)c/(E + mc2)
⇥
⌃⌃⌅ u2 = N
�
⇧⇧⇤
01
(px � ipy)c/(E + mc2)�pzc/(E + mc2)
⇥
⌃⌃⌅
Use “positive” energy solutions
Use “negative” energy solutions
positrons
electrons
�v2 ⌘ u3 = N
0
BB@
pz
c/(E +mc2)(p
x
+ ipy
)c/(E +mc2)10
1
CCA v1 ⌘ u4 = N
0
BB@
(px
� ipy
)c/(E +mc2)�p
z
c/(E +mc2)01
1
CCA
A second look at Dirac spinors
• “s” labels the spin states (two for electrons/positrons)• The exponential term sets the space/time = energy/momentum• Let’s look at the “spinor” part u,v which determines the “Dirac structure”:
• If we insert ψ into the Dirac equation, we get:
• If we take the adjoint of these equations, we get:
�(x) = ae�(i/�)p·x us(p) �(x) = ae(i/�)p·x vs(p)electrons positrons
(i��µ⇤µ �mc)⇥ = 0⇥ (�µpµ �mc)⇥ = 0⇥ (�µpµ �mc)us(p) = 0
(i��µ⇤µ �mc)⇥ = 0⇥ (��µpµ �mc)⇥ = 0⇥ (�µpµ + mc)vs(p) = 0
“momentum space Dirac equations”
us(�µpµ �mc) = 0 vs(�µpµ + mc) = 0
Orthogonality and Completeness of Spinors:
• From the explicit form of our u/v spinors, we can also show:
• We can also show:
uivj = viuj = 0uiuj = 2mc �ij
�
s=1,2
usus = (�µpµ + mc)�
s=1,2
vsvs = (�µpµ �mc)
vivj = �2mc �ij
�
⇧⇧⇤
a1
a2
a3
a4
⇥
⌃⌃⌅ (b1, b2, b3, b4) =
�
⇧⇧⇤
a1b1 a1b2 a1b3 a1b4
a2b1 a2b2 a2b3 a2b4
a3b1 a3b2 a3b3 a3b4
a4b1 a4b2 a4b3 a4b4
⇥
⌃⌃⌅
Photon Polarizations and Orthogonality:• We showed that the polarization 4-vector εμ with the Lorentz and Coloumb
gauge conditions must satisfy:
• We noted that this allows two degrees of freedom corresponding to transversely polarized electromagnetic fields. • We need to two orthogonal ε basis vectors to span the space
• For example, if the photon is moving in the z direction, we can choose:
• The polarization vectors satisfy orthogonality/completeness relations:
p · � = 0
�1µ =
�
⇧⇧⇤
0100
⇥
⌃⌃⌅
⇥i�µ ⇥µj = ��ij
�
s=1,2
⇥si ⇥
s�j = �ij � pipj
�2µ =
�
⇧⇧⇤
0010
⇥
⌃⌃⌅
The Feynman Rules: External Lines
• First right down the Feynman diagram(s) for the process and label the momentum flow• use p’s for external lines, q’s for internal (Griffiths convention).• Note that there are two flows:
• “particle/antiparticle”• momentum • These are separate
• Now the components of the expression• External Lines:
• Electrons: incoming outgoing• Positrons: incoming outgoing• Photons: incoming outgoing
p1
p2
p3
p4
q
us(p) us(p)
vs(p) vs(p)
�µ(p) ��µ(p)
The Feynman Rules: Vertices and Propagators:
• For each QED vertex:• where as before, momentum is “+” incoming, “-” outgoing from vertex
• Internal lines:• electron/positron propagator
• Photon propagator• indices match vertices/polarization
• Integral over momentum:
• Finally: cancel the overall delta function, what remains is -iM
p1
p2p3
p4
q
ige�µ
i(�µqµ + mc)q2 �m2c2
�igµ⇥
q2
d4q
(2�)4
(2⇥)4�4(k1 + k2 + k3)
Example:
• Order matters due to Dirac matrix structure (photon part doesn’t care)• Griffiths: go backward through the fermion lines:
• In the “final state”: • In the “initial state”:• Throw in the internal photon propagator:
u(3) ige�µ v(4)
v(2) ige�� u(1)
�igµ⇥
q2
1(2�)4
�d4q
(2⇥)4�4(q � p3 � p4)(2⇥)4�4(p1 + p2 � q)
[u(3) �µ v(4)] gµ⇥ [v(2) �⇥ u(1)]i(2⇥)4�4(p1 + p2 � p3 � p4)⇥g2
e
(p1 + p2)2
M = � g2e
(p1 + p2)2[u(3) �µ v(4)] [v(2) �µ u(1)]
p1
p2p3
p4
q
Example: e+ + e- → e+ + e-
p1
p2
p4
p3
q
e
e
e
e
p3
q
p4p1
p2
u(3) ige�⇢ v(4) v(2) ige�
� u(1)�ig⇢�
(p1 + p2)2
u(3) ige�µ u(1) v(2) ige�
⌫ v(4)�igµ⌫
(p1 � p3)2
(2⇡)4�4(p1 + p2 � p3 � p4)