Experimental Methods

Accelerators and Beams

There are three sources of high energy particles:

Natural or controlled radioactivity: Radioactive sources are commonly used for testing and calibrating detectors. Large numbers of neutrinos are a by product of nuclear reactors. Both of these sources are limited in energy.
Cosmic rays: The earth is continually bombarded by a mixture of particles from the sun and distant stars. The energies can be considerable, but the mixture of particles is not controlled, and the directions are random. However, as demonstrated by the discovery of neutrino mixing and mass, there is a considerable amount that has been learned studying cosmic rays, both about particles, and about their sources.
Particle accelerators: We can create beams of particles by accelerating charged particles with electric fields. The charged particles can be held in a circular orbit in a uniform magnetic field. In modern use, a number of accelerators are connected together in an accelerator complex at a lab. All accelerator complexes begin with either electrons (extracted from a cathode by heat or light) or protons (extracted from hydrogen gas). These initial beams are accelerated, and can be collided with a target to produce other particles (positrons, antiprotons, pions, kaons, lambdas, sigmas, muons, photons, or neutrinos) which are manipulated to produce secondary beams. In particular, positrons and antiprotons, being stable, can be stored, accelerated, and formed into beams for e⁺e^- or p-pbar collisions.

Particle Measurements

There are literally hundreds of different particle detectors out there, far too many for me to discuss in this course. Luckily, they are based on a small number of possible interactions of particles with bulk matter, and a small number of quantities that can be measured. I will focus on these interactions and how they are used to measure quantities about particles.

Assume a particle is passing through your experimental apparatus (detector). What information can you measure about that particle? Position, direction, charge, momentum, energy, mass, point of origin, point of decay, spin orientation.

Interactions of Particles with Bulk Matter

We detect particles using sensors made from bulk matter: gases, liquids, and solids. Let's consider the general types of processes that can lead to "signals" in bulk matter, and how the travel of a particle is effected. I'll limit the discussion to relativistic particles, since this is generally the case in particle physics.

Process	Description
ionization	passing particle ionizes an atom, leaving behind an electron and positive ion.
scintillation	passing particle excites (or ionizes) an atom which emits light (visible or u.v.) as it returns to an unexcited state
showers (electromagnetic and hadronic)	cascade of secondary particles created when particles that interact electromagnetically and strongly pass through high A/Z material.
Cerenkov radiation	emission of light (visible and u.v.) when the speed of a particle, βc, exceeds the speed (phase velocity) of light in a medium, c/n where n is the index of refraction (analogy of sonic boom for a vehicle exceeding the speed of sound)
transition radiation	emission of light (visible and u.v.) when a particle passes from one medium to another of differing EM properties

In the list of interactions I neglect several novel ones, for simplicity. These include phonon excitations in a crystal at low temperatures, and radio frequency Cerenkov radiation.

particle	lifetime
e⁺, e^-	infinite
p, pbar	infinite
γ	infinite
π⁺, π^-	2.6×10^-8 s
K⁺, K^-	1.2×10^-8 s
K⁰_S	0.89×10^-10 s
K⁰_L	5.2×10^-8 s
Λ, Λbar	2.6×10^-10 s
Σ^±, Ξ, Ω	about 10^-10 s