ATLAS Experiment - Background

Background

The first cyclotron, an early type of particle accelerator, was built by Ernest O. Lawrence in 1931, with a radius of just a few centimetres and a particle energy of 1 megaelectronvolt (MeV). Since then, accelerators have grown enormously in the quest to produce new particles of greater and greater mass. As accelerators have grown, so too has the list of known particles that they might be used to investigate. The most comprehensive model of particle interactions available today is known as the Standard Model of Particle Physics. With the important exception of the Higgs boson (which most probably has just been observed by the ATLAS and the CMS experiments), all of the particles predicted by the model have been observed. While the Standard Model predicts that quarks, electrons, and neutrinos should exist, it does not explain why the masses of these particles are so very different. Due to this violation of "naturalness" most particle physicists believe it is possible that the Standard Model will break down at energies beyond the current energy frontier of about one teraelectronvolt (TeV) (set at the Tevatron). If such beyond-the-Standard-Model physics is observed it is hoped that a new model, which is identical to the Standard Model at energies thus far probed, can be developed to describe particle physics at higher energies. Most of the currently proposed theories predict new higher-mass particles, some of which are hoped to be light enough to be observed by ATLAS.

ATLAS is designed to be a general-purpose detector. When the proton beams produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies are produced. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. Experiments at earlier colliders, such as the Tevatron and Large Electron-Positron Collider, were designed based on a similar philosophy. However, the unique challenges of the Large Hadron Collider — its unprecedented energy and extremely high rate of collisions — require ATLAS to be larger and more complex than any detector ever built.

At 27 kilometres in circumference, the Large Hadron Collider (LHC) collides two beams of protons together, each proton carrying presently about 4 TeV of energy — enough energy to produce particles with masses up to roughly five times more massive than any particles currently known — assuming of course that such particles exist. When upgraded in 2014, the LHC with an energy seven million times that of the first accelerator, will represents a "new generation" of particle accelerators.

Particles that are produced in accelerators must also be observed, and this is the task of particle detectors. While interesting phenomena may occur when protons collide it is not enough to just produce them. Particle detectors must be built to detect particles, their masses, momentum, energies, charges, and nuclear spins. In order to identify all particles produced at the interaction point where the particle beams collide, particle detectors are usually designed in layers like an onion. The layers are made up of detectors of different types, each of which is designed to observe specific types of particles. The different traces that particles leave in each layer of the detector allow for effective particle identification and accurate measurements of energy and momentum. (The role of each layer in the detector is discussed below.) As the energy of the particles produced by the accelerator increases, the detectors attached to it must grow to effectively measure and stop higher-energy particles. ATLAS is the largest detector ever built at a particle collider .