Decide that the ability to provide the first order Feynman will be important. Therefore the students should be able to draw the diagrams for any interaction. The focus will be primarily on the quark and lepton level but strong interactions will be mediated on the long-range scales by quark exchange. Simplest model for proton neutron interactions is pion exchange.


Following excerpted from Stanford website

   Left-to-right in the diagram represents time; a process begins on the left and ends on the right.

   Every line in the diagram represents a particle; the three types of particles in the simplest theory (QED) are:

NOTE: (There are different conventions for the direction of time. Some choose to have time develop vertically and so the diagrams are rotated.





Particle Represented

f-electron.gif (164 bytes)

straight line, arrow to the right

electronGlossary Term

f-positron.gif (159 bytes)

straight line, arrow to the left


f-photon.gif (211 bytes)

wavy line


f-a.gif (344 bytes)

An electron emits a photon



f-b.gif (335 bytes)

An electron absorbs a photon



f-c.gif (333 bytes)

A positron emits a photon



f-d.gif (344 bytes)

A positron absorbs a photon



f-e.gif (322 bytes)

A photon produces an electron and a positron (an electron-positron pair)



f-f.gif (331 bytes)

An electron and a positron meet and annihilate (disappear), producing a photon











Need to identify internal lines and external lines when diagrams are assembled to describe processes. Lines that ultimately enter or leave are real particles and must have a physical mass corresponding to the particle type. Internal lines are mediating the interaction. That is they are exchanged in order to have a momentum and energy transfer or interaction between two physical particles. These field particles do not have to have a correct mass based on the particle type. At all vertices the momentum and energy are conserved. Sum of all the incoming is equal to the outgoing for the three (four) particles that combine to from a vertex.



So the QED vertex

One can rotate any arm as follows:



We will need to examine which rotations are significant. In order to do this we need to identify internal and external lines. Therefore we draw the lowest order diagram that represents a process:




Rotating external lines is only significant if the line move across the 90o line. This causes the particle to turn into its antiparticle and the reaction changes.

A antiB + C + D as shown above.

Moving particle from one side to the other or rotating external arms is based on crossing symmetry. If one is actually performing a calculation the question remains do elements of the underlying calculation change when one explores the crossing symmetry related diagrams.


  1. change the sign of the four momentum so that the particle is now a negative energy state: p is the four momnetum

A(p1)+B(p2)C(p3)+D(p4) same amplitude A(p1) C(p3)+D(p4) +antiB(-p2)

  1. fermions have an additional factor of -1 for the amplitude


Rotating internal lines simply changes the time ordering of the vertices. All such time orderings are assumed to be included so only one of the diagrams is drawn.



Therefore there is only the need to draw on of these drawings to describe the process A+BC+D


However there may be other drawings. These can be found by deciding which of the external lines are meeting at a vertex.


If we look at the possibilities and calculate the 4-momentum given to the internal line then we have:


A B vertex

PAμ + PBμ

s channel

A C vertex

PAμ - PCμ

t channel

A D vertex

PAμ - PDμ

u channel

The allowable vertices depend on the types of particles.


e+ + e- e+ + e- (E&M)


Allows two of the diagrams above, the incoming particles (A,B) can form a vertex or outgoing (A,C) can form a vertex but not (A,D).



Weak vertices


ud, cs, νe- , e+ νbar

These emission graphs are W+

du, sc, e- ν , νbar e+

These emission graphs are W-

uu, dd, ν ν , e+ e+

These emission graphs are Zo

ud, cs, νe- , e+ νbar

These absorption graphs are W-

du, sc, e- ν , νbar e+

These absorption graphs are W+

uu, dd, ν ν , e+ e+

These absorption graphs are Zo

ubard, cbars, e- νbar

These production graphs are W-

dbaru, sbarc, e+ ν

These production graphs are W+

ubard, cbars, e- νbar

These annihilation graphs are W-

dbaru, sbarc, e+ ν

These annihilation graphs are W+

ubaru, cbarc, ν νbar, e- e+

These annihilation graphs are Zo

ubaru, cbarc, ν νbar, e- e+

These production graphs are Zo

Due to quark mixing the any d,s,b can be replaced by a d,s,b.



Here are some weak processes from the lepton sector. The external lines show the fundamental fields to be the (ee) and (μ,ν μ). For the weak interaction there is no difference between electron and neutrino. They are manifestations of the same particle. The interactions will need W+, W- ,Zo to mediate the interaction so they will be included.

(ASIDE: Also to carefully define the states the electron and muon need to have right and left handed versions. eR, eL, μR, μL. These are spin projection that are chosen by projecting onto the momentum direction. Only left handed leptons are present in the above diagrams. This detail can be neglected without difficulty when establishing and overview of the interactions.)

Most of the hadrons will interact over long distances by pion exchange because color singlets are not allowed. Typically we can follow these by showing the quark lines. The gluons have not been shown in the following diagrams.

Very close range interaction Standard nuclear force



Diagrams on the left are not color singlets so they must be very short range. While the diagrams on the right exchange a particle that is a color singlet.

Very close range interaction Standard nuclear force



Weak interaction



fundamental vertex spectator quarks added





fundamental vertex spectator quarks added


The QCD interactions are complicated because the quarks come in three colors but the color structure of objects that make up composite systems is not typically shown. In principle one can simply add gluons in the same manner that one adds photons. The only change will be in the color which is not shown. The usefulness of the Feynman diagrams requires a bit more care. Since there can be appreciable gluon exchange an expansion in terms of graphs is only appropriate in some situations.