![]() ![]() even though they are composite objects and the interaction can be visualized as being between their constituent quarks. They can be drawn for protons, neutrons, etc. Particle interactions can be represented by diagrams with at least two vertices. Forms of primitive vertices for these three interactions are Keep in mind that time progresses upward, and that a downward arrow is not a particle progressing downward, but an antiparticle progressing upward (forward in time).Īfter being introduced for electromagnetic processes, Feynman diagrams were developed for the weak and strong interactions as well. A backward arrow represents the antiparticle, in these cases a positron. Other electromagnetic processes can be represented, as in the examples below. The vertical direction indicates the progress of time upward, but the horizontal spacing does not give the distance between the particles. The time and space axes are usually not indicated. Here two electrons enter, exchange a photon, and then exit. Only lines entering or leaving the diagram represent observable particles. All electromagnetic interactions can be described with combinations of primitive diagrams like this one. Virtual particles are represented by wavy or broken lines and have no arrows. ![]() (Particle physicists often reverse that orientation.) Particles are represented by lines with arrows to denote the direction of their travel, with antiparticles having their arrows reversed. The time axis points upward and the space axis to the right. Each vertex must conserve charge, baryon number and lepton number.ĭeveloped by Feynman to describe the interactions in quantum electrodynamics (QED), the diagrams have found use in describing a variety of particle interactions. Each point at which lines come together is called a vertex, and at each vertex one may examine the conservation laws which govern particle interactions. The internal state of a hadron is viewed as composed of a fixed net number of quarks, but with a dynamic cloud of gluons and quark-antiquark pairs in equilibrium.įeynman diagrams are graphical ways to represent exchange forces. The property of interaction with each other is very different from the other exchange particles, and raises the possibility of gluon collections referred to as "glueballs". Within their range of about a fermi, the gluons can interact with each other, and can produce virtual quark-antiquark pairs. The photon does not carry electric charge with it, while the gluons do carry the "color charge". These properties contrast them with photons, which are massless and of infinite range. The range of the strong force is limited by the fact that the gluons interact with each other as well as with quarks in the context of quark confinement. The gluon exchange picture there converts a blue quark to a green one and vice versa. The gluons are in fact considered to be bi-colored, carrying a unit of color and a unit of anti-color as suggested in the diagram at right. Note that thegluon generates a color change forthe quarks. Gluon interactions are often represented by a Feynman diagram. That strong interaction was modeled by Yukawa as involving an exchange of pions, and indeed the pion range calculation was helpful in developing our understanding of the strong force. That short-range nucleon-nucleon interaction can be considered to be a residual color force extending outside the boundary of the proton or neutron. The gluon can be considered to be the fundamental exchange particle underlying the strong interaction between protons and neutrons in a nucleus. ![]() The gluon is considered to be a massless vector boson with spin 1. In this case a proton loses its positive charge therefore it is carried away to the electron via the W + boson transforming the electron into an electron-neutrino.Gluons are the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles. The electron-neutrino in this diagram is drawn with a shallow gradient to show that it travels close to the speed of light.įinally the boson mediating the interaction is added as a wave between the two points of interaction. It should be recognised that baryon conservation means a baryon becomes another baryon and lepton conservation means that a lepton is transformed from one type of lepton into another. The particles after the interaction are drawn at the top of the Feynman diagram. Both the proton and electron in this diagram are drawn with a steep gradient to show their relatively small velocity compared to the speed of light. The particles prior to the interaction are drawn first at the bottom of the Feynman diagram. ![]()
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