I generally get the difference between matter particles (leptons and quarks) and “force carrying” particles (bosons). But I still do not understand how the “exchange” of fundamental / elementary bosons (e.g., photons and gluons) bind or ‘glue’ matter particles together as well as repel matter particles — as in attraction of oppositely charged particles and repulsion of identically charged particles (and the photon flux density in electromagnetic fields around charged particles and magnets). The typical Feynman diagram merely shows the interaction of a single photon (real or virtual, eh) between two electrons . I get lost in the equations, of course. And most interesting cases may not even be solvable. On my wish list is a visualization at field scale  which advances an explanation for mere mortals (and does not involve playing catch with balls).
[I think that I get the Periodic Table and how sharing electrons between atoms (valence shell electrons) makes molecules (and therefore all stable matter), but not the Standard Model table and how “sharing” of bosons makes for attractive and repulsive forces.]
I’m fascinated by condensed matter physics without grasping the mathematics of Bose–Einstein statistics. Most of us grew up hearing about superfluids.
Lasers are possible because photons are bosons — indistinguishable particles in the same (quantum) state — and can pile up on each other. Frictionlessly sociable [“no significant interaction between the particles”]. But what’s the photon density in a laser pointer, eh?
Atoms exist — and the universe and mankind — because electrons are leptons and not bosons. In normal situations electrons cannot be packed together in the same state — they like some personal or private space.
And composite bosons are even trickier. It’s all about quantum spin.
So, this ThoughtCo article caught my attention: “What Is a Boson?” by Andrew Zimmerman Jones (May 27, 2019).
Photon – Known as the particle of light, photons carry all electromagnetic energy and act as the gauge boson that mediates the force of electromagnetic interactions.
Gluon – Gluons mediate the interactions of the strong nuclear force, which binds together quarks to form protons and neutrons and also holds the protons and neutrons together within an atom’s nucleus.
W Boson – One of the two gauge bosons involved in mediating the weak nuclear force.
Z Boson – One of the two gauge bosons involved in mediating the weak nuclear force.
… there are other fundamental bosons predicted, but without clear experimental confirmation (yet) … [Higgs, graviton, bosonic superpartners]
Mesons – Mesons are formed when two quarks bond together. Since quarks are fermions and have half-integer spins, if two of them are bonded together, then the spin of the resulting particle (which is the sum of the individual spins) would be an integer, making it a boson.
Helium-4 atom – A helium-4 atom contains 2 protons, 2 neutrons, and 2 electrons … and if you add up all of those spins, you’ll end up with an integer every time. Helium-4 is particularly noteworthy because it becomes a superfluid when cooled to ultra-low temperatures, making it a brilliant example of Bose-Einstein statistics in action.
 This several year old Physics Stack Exchange thread makes the point: “Feynman diagram for attractive forces.”
When you do this it doesn’t matter whether the lines look like they’re attracting or not, because you can deform the diagram any way you like (as long as you keep the same external lines). The thing that tells you whether the force is attractive or repulsive is the math; if you use the electron-positron diagram to calculate the potential energy you will find that it corresponds to an attractive Coulomb potential; if you reverse the positron arrow so it now represents another electron (without moving the lines at all!), you will now find that the potential is repulsive.
The upshot here is that so-called “virtual particles”, which are internal lines in a Feynman diagram (in your examples those would be the photon, the gluon and the pion), are not actual particles being exchanged. They’re just a neat picture that helps visualizing the process, but in reality the particles are interacting through their quantum fields, and these fields are very hard (maybe even impossible) to understand intuitively. But remember that the diagrams in your post are what we call “tree level”. They’re the simplest diagrams for the given processes, but in reality there is an infinite number of them, with ever growing number of vertices and lines, and the more diagrams you calculate the more accurate your results will be.
Feynman diagrams do not intent to show attraction nor repulsion. They are just a bookkeeping graphical tool [“eye candy”] for calculating such amplitude. You may use them though to find out if the interaction is attractive or repulsive.
 And visualizing somehow, despite limitations of any analogy or metaphor, the momentum transfer in field gradients.
2 thoughts on “A boson by any other name would …”
I’m seeing more clearly how Feynman was a “particles guy,” as characterized by a few physicists. His purpose in quantum electrodynamics (QED) was calculation (“… just do the math”), and his diagrams were a novel and powerful way to organize the equations. And his sum over all possible paths (path integral formulation) may reflect that we are really talking about fields. Matter “particles” as quantum oscillations or localized vibrations within those omnidirectional fields. And interactions as perturbations of those pervasive continuous fields.
Wiki > Feynman diagram
Wiki > Path integral formulation
This week’s science news cycle contains articles about some physics research on exotic matter in microgravity aboard the International Space Station (ISS). Particularly Bose-Einstein condensates (BECs), which exhibit some quantum behaviors.
These articles (below) helped clarify why not all bosons are massless. And why not all bosons are fundamental particles. (It’s all about spin.) And highlight mainstays of quantum research: rubidium atoms, laser cooling, magnetic containment (trap).
A Wiki graphic for this topic shows the energy density profile (velocity-distribution data) of gaseous rubidium atoms when cooled to near absolute zero.
There are six (known) states of matter. Three are familiar collections of neutral atoms: solid, liquid, gas (as for water). The 4th is a collection of ionized atoms: plasma. The 5th a collection of integer spin particles: Bose-Einstein condensate. The 6th a collection of spin 1/2 particles: Fermionic condensate (e.g., Cooper pairs).
Essentially, a single wave function or quantum state characterizes the entire condensate.
 Phys.org >”Quantum ‘fifth state of matter’ observed in space for first time” by Patrick Galey (June 11, 2020).
 Space.com > “Scientists create exotic matter on space station to explore the quantum world” by Charles Q. Choi (June 11, 2020).
 Forbes > “What Are The Fifth And Sixth States Of Matter?” by Ethan Siegel (June 9, 2020).
Comments are closed.