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Physics of Neutrino Scattering

Defining our terms

The overall goal of the CCFR collaboration was to look at the results from neutrino-nucleon deep-inelastic scattering (or "DIS"). What does this mean? Let's break the phrase down into its individual words:

In a scattering experiment, scientists aim a beam of particles at a target. The particles in the beam are said to "scatter" off of the particles that make up the target. Scientists study what happens to the beam's particles after they scatter. Perhaps the beam will be deflected or transformed in some way. By examining these changes, scientists can investigate the properties of the particles that make up the beam, the particles that make up the target, or both.

In an elastic collision, the total momentum and total kinetic energy of the particles are the same before and after the collision. In an inelastic collision, the total momentum is the same but the total kinetic energy is changed after the collision.

At the relativistic energies of modern particle physics, the kinetic energy and momentum of most particles are almost the same, so for our purposes we have different definition: In an elastic collision, the beam and target particles are unchanged after scattering; in inelastic scattering, the beam and/or target particles are fragmented by the scattering. At the energies of this experiment, the target particle (e.g., a proton) is fragmented and transformed into many other particles, and so the interaction is called "deep" inelastic scattering.

The beam in this experiment is made up of neutrinos. The neutrino is small, electrically neutral, and has little (if any) mass. It only interacts via the weak nuclear interaction, a force whose properties are currently believed to be well-understood.

The target in this experiment is made mostly of iron. The most massive part of an iron atom is its nucleus, which is composed in turn of "nucleons" -- protons and neutrons. It is the protons and neutrons that are considered to be the target of the beam, and hence it is referred to as "neutrino-nucleon" scattering.

Another term for protons and neutrons is baryons (it's a matter of context: they're called nucleons when they're inside an atomic nucleus, and baryons when they're considered as a class of particles). Baryons are made out of quarks. The flavor of quarks inside a baryon determine what its type; a proton is made of two "up" quarks and two "down" quarks, while a neutron is made of two "down" quarks one "up" quark. The force that holds the quarks together is the strong nuclear force.

One of the goals of the CCFR experiment is to understand proton structure. We can do this because, while the neutrino is inelastically scattering off the nucleon, it elastically scatters off of the quarks inside the nucleon. To get some insight how this works, we must look more closely at deep-inelastic neutrino scattering.

The neutrino interaction

There are two different types of neutrino-nucleon scattering: charged-current and neutral-current. To see the difference between these two, we must take a close-up view of the particle interaction. A charged-current interaction looks like this:

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