THE STANDARD MODEL






Note that most of the normal matter in the universe (80% of normal matter) has never been inside a star, and never will be. This fact indicated to physicists in the 1950s that the entire universe must at one time have been dense and hot enough to fuse neutrons and protons into helium, otherwise there was no way for the normal-matter universe to consist almost entirely of hydrogen and helium.



The strong force depends on a quantum number called color, which is the equivalent of charge for the strong interaction. Each kind of quark comes in three different colors, red, green and blue. Any bound state has NO COLOR, in other words has to consist of equal amounts of red, green and blue particles. The virtual bosons of the strong force, called gluons, carry a color and an anticolor.  Yellow is the anticolor to blue, cyan is the anticolor to red, and magenta is the anticolor to green.  Thus if a red up quark absorbs a blue-cyan gluon, it becomes a blue up quark.


Suppose a green quark emits a green-cyan gluon. This leaves the quark red. Now suppose that same (virtual) green-cyan gluon is absorbed by a red quark. That leaves the quark green.




To summarize, quarks form two main kinds of particles: (1) BARYONS, of which the proton and neutron are the least massive examples, are composed of three “valence quarks,” one of each color. All baryons are fermions. (2)MESONS, which are composed of a valence quark-antiquark pair. All mesons are bosons. All baryons and mesons (collectively called HADRONS) are colorless, and held together by the strong interaction. Of all the hadrons, only the proton appears to be stable.



Every particle has an antiparticle, although in some cases the particle and antiparticle are identical (for example, the photon).  Pair production and annihilation in the Standard Model: a photon in the Coulomb field of  a nucleus converts into (for example) an electron and its antiparticle, the positron.  Particles and antiparticles are always produced in pairs and annihilated in pairs.  A pair will annihilate into two photons.  If a single-particle description of these processes is adopted, an anti-particle can be viewed as a particle travelling backward in time.

Weak decay of a neutron in the Standard Model: a down quark changes to an up quark by emitting a W- boson, which in turn creates an electron-antineutrino pair... or converts a neutrino moving backward in time into an electron moving forward in time.  Thus pair annihilation of an electron and positron can be viewed as a collision of an electron with two photons, the second collision knocking it backward in time.

The very early universe presumably consisted entirely of bosons, but if matter resulted from pair production by these bosons, why is there not an equal amount of matter and antimatter in the universe?  In fact, the observable universe consists entirely of matter.  Processes are known where the probability of creating a particle is very slightly greater than the probability of creating an antiparticle, but this happens only for a few processes involving the weak interaction.  Whatever processes took place in the very early universe, we can see today, by comparing the number of photons in the universe to the number of electrons, that about one time in 109, a particle was created without an accompanying antiparticle.


Timeline of the Early Universe (317L)

The Origin of the Mass of Ordinary Matter!