The Science of ATLAS and CMS

CMS detector
The CMS detector. Image © CERN
ATLAS and CMS are general-purpose particle physics experiments. Designed to see a wide range of particles and phenomena produced in LHC collisions, each involves approximately 2,000 physicists from some 35 countries. These scientists use the data collected from the complex ATLAS and CMS detectors to search for new phenomena, including the Higgs boson, supersymmetry and extra dimensions. They also measure the properties of previously-discovered quarks and bosons with unprecedented precision, and are on the lookout for completely new, unpredicted phenomena.

How do they know what to look for? Physicists have spent decades developing the Standard Model, a set of theories that describe in detail the fundamental particles that make up the universe and the forces at work between them. But physicists know that their understanding of these particles and forces is incomplete. Many theories have been put forth to describe physics beyond the Standard Model, but only experimental results will tell us which theories are wrong and which are on the right track. Physicists also need to keep refining the details of established theories, by precisely measuring the properties and behavior of previously-discovered particles.

How do physicists search for and measure particles and phenomena? When protons collide head-on in the center of the ATLAS and CMS detectors, their inner quarks and gluons may annihilate, creating a burst of pure energy. This energy could transform into new particles, which almost instantaneously decay into particles that exist in everyday life, such as electrons, photons and neutrons. More available energy can translate into heavier new particles, and the proton-proton collisions at the LHC will eventually have an energy of 14 TeV—seven times higher than the most powerful accelerator to come before. The huge ATLAS and CMS detectors completely surround the point where protons collide. The emerging particles leave traces in the detectors that are turned into electronic signals. Physicists convert these signals into energies and paths of the particles, and use this information to reconstruct what happened just after the protons collided.

ATLAS detector
The ATLAS detector. Image © CERN

Origin of Mass

Why do some particles have mass and others none? Theorists believe that the universe is permeated by a mass-generating field called the Higgs field. Different particles feel the Higgs field in different ways, and thus acquire different masses. The long-sought Higgs boson would be the carrier of this field. The Higgs boson, possibly hundreds of times heavier than a proton, could be created in the proton collisions in the center of the ATLAS and CMS detectors.

Supersymmetry and Dark Matter

Supersymmetry, or SUSY, could provide a way to unify three of the four fundamental forces: the electromagnetic, weak and strong forces. This symmetry, which may have existed in the very early, high-energy universe, could be detectable with the ATLAS and CMS experiments.

To prove its existence, ATLAS and CMS physicists search for the range of particles predicted by the varied theories of SUSY. These supersymmetric particles, or sparticles, have the same charge but opposite spin to the particles we’re familiar with, such as photons and electrons. The lightest of these sparticles may be a major part of the cosmic dark matter that we know exists but cannot yet describe or detect.

Extra Dimensions

Particle physics theories exist that predict that there are more dimensions of space than the four—three of space, one of time—that we experience. String theory, one of the leading such theories, predicts seven undiscovered dimensions of space. Testing string theory requires searching for these extra dimensions and exploring their properties.

Extra dimensions would be invisible to us in everyday life, but may become detectable at very high energies. If extra dimensions exist they could manifest themselves in the ATLAS and CMS detectors through the appearance of new particles or an energy leak from our four dimensions into the others. Another possible consequence of extra dimensions may be the creation of microscopic black holes. These black holes, like a heavy particle, would immediately decay into more mundane particles that physicists would measure using the ATLAS and CMS detectors.

Matter vs. Antimatter

Experiments teach us that for every fundamental particle there exists an antiparticle, and the Big Bang almost certainly produced an equal amount of matter and antimatter. So why is today’s universe dominated by matter? Subtle asymmetries between matter and antimatter, observed in many experiments over the past decades, may hold the answer. ATLAS and CMS physicists study bottom quarks and particles made from them, called B mesons, to investigate the differences in behavior between matter and antimatter.

Standard Model Particles

Discovery of a particle is only the first step for physicists. Once the announcement has been made and the celebrations wind down, scientists get down to the business of measuring the particle’s behavior and properties and comparing against the Standard Model. At the LHC, ATLAS and CMS physicists investigate the behavior of known particles, including the W and Z bosons—carriers of the electroweak force—and top and bottom quarks. Any discrepancy from predicted measurements may point to a weakness in the Standard Model.

The Unknown

In addition to using the ATLAS and CMS detectors to test these and many more theories put forth over the past decades, physicists also search for unpredicted signals and phenomena. Some of the most important discoveries in physics over the last century have been completely unexpected, and many scientists expect that more surprises may be waiting at the LHC.