Particle Physics Background

Particle physics, also called high energy physics, is the study of the elementary constituents of matter and radiations, and the interactions between them. It examines the fundamental aspects of nature that are common to everything, and attempts to determine the true building blocks of the universe. Building on early experiments that attempted to understand the nature of matter and the world around us, modern particle physics uses some of the most powerful and precise instruments ever conceived to delve deep into the sub-atomic world. Just like astronomers use telescopes to study starts and galaxies, high energy physicists use particle accelerators to investigate quarks and leptons..

Experiments during the late 19th century and the early 20th century cemented our understanding of the atom with the works of J.J. Thompson, E. Rutherford, and others giving us our first insight into the make-up of atoms. They revealed to us the existence of light, negatively charged electrons surrounding a densely packed nucleus consisting of positively charge protons and neutrally charged neutrons. This relatively simple view of the nucleus gave only a hint as to the mysteries that were still to come.

The experiments of Ernest Rutherford, in which he investigated the internal configuration of gold atoms using Alpha radiation, set the tone for future experimentation in particle physics. The Rutherford scattering experiments involved a radioactive source that emitted a stream of alpha particles at a sheet of very thin gold foil positioned in front of a screen. Alpha particles would make small flashes of light on the screen wherever they hit. Particle physics experiments today use the same basic elements as Rutherford: a particle beam is shot at a target and the manner in which the beam is affected is observed and monitored by detectors positioned around the event. This encompasses even the most basic of observation techniques: looking at something.

In human vision, the retina acts as a detector for photons that have arrived at the eye after being emitted by a source and reflecting or interacting with any number of objects. The human brain can gather much information regarding what the light has interacted with by determining things like the direction the light has come from and the wavelength of the light (the colour). The problem with using this same technique to observe increasingly small objects is that due to the wave nature of photons (and all matter), the quality of the image that can be produced is limited by the wavelength of the light being used. A basic rule of thumb (that can be justified with a lot of mathematics and hand-waving), is that the smallest distance than can be probed by a particle is equal to its wavelength. To examine smaller objects, you need a smaller wavelength. This isn’t a problem in our everyday lives because the wavelength of light that our eyes are sensitive to is in the range of a few hundred nanometers (or about 200 times less than the thickness of a human hair). This makes ordinary light suitable for examining objects down to about the size of single cell, but for physicist who want to probe down to the atomic level, which is about 10 thousand times smaller, ordinary light just won’t cut it. To make things worse, particle physics often involves examining the nucleus of the atom, which has a diameter 10 thousand times smaller again! It becomes obvious that a very different approach is required.

Fortunately, the solution lies with the something mentioned previously: that all matter possesses wave properties. In 1923, a graduate student at the University of Paris named Louis de Broglie put forth a revolutionary hypothesis that stated just that: if evidence pointed to light having both particle and wave properties, might electrons and other particles exhibit similar behavior? His hypothesis, which has been confirmed experimentally, states that a particle’s wavelength is inversely related to its momentum. By merely accelerating a particle, and consequently increasing its momentum, the wavelength of the particle can be shortened. This particle-wave duality means that physicists need not limit themselves to using photons and light waves to probe smaller and smaller regions, but can use matter itself accelerated to high speeds. The particle accelerator like the International Linear Collider (ILC) is the tool used by physicist to do just that.

With particle accelerators, the alpha particles of Rutherford’s scattering experiment are replaced with any number of particles ranging from protons to heavy ions to electrons. Whatever particle is used, it must be first accelerated to very high velocities. This is accomplished using massive electromagnetic fields produced outside the accelerator, in a manner similar to the domestic microwave, and then channeled into accelerating cavities using waveguides. The cavities produce massive electromagnetic fields that are timed for the precise moment that the particle passes through. This creates an electromagnetic wave that travels along the accelerator pushing charged particles like a surfer is pushed by an ocean wave.

The target that this beam of particles is being aimed at can be a stationary target like the gold foil used by Rutherford, or can be another beam of particles that has been accelerated in the opposite direction. In either case, the beam, which has been aimed and focused using large magnetic fields, will collide with target particles producing a shower of fragment particles, intermediary particles, and other debris. Some of the products of these collisions will be stable, while others will be highly unstable and decay almost immediately into more stable products. With the lifetime of some of these particles being mere micro-seconds, the real challenge becomes keeping track of each of these particles so that their mass and charge can be determined, ultimately leading to the goal of identifying them and understanding the nature of their fundamental interactions with other elementary particles.