A joint Fermilab/SLAC publication

High-energy physics beyond E=mc^2

02/13/09

When it restarts, the Large Hadron Collider will be the largest and highest-energy particle collider in the world. The LHC may be the last of its kind in the evolutionary chart of particle colliders, according to a presentation by Craig Dukes, a particle physicist at the University of Virginia, at the AAAS conference.

Accelerators have reached higher and higher levels of energy over the years.

Accelerators have reached higher and higher levels of energy over the years.

For the first time in 70 years of research at the energy frontier, physicists are not planning for the next, bigger version of the collider, Dukes said. Physicists have high hopes for the exciting research to be conducted at the LHC, but it cannot answer all of their questions about the makeup of the universe.

In addition to searching for undiscovered particles with the LHC, physicists are entering an experimental realm known as the intensity frontier. In experiments at the intensity frontier, physicists use a combination of intense particle beams and highly sensitive detectors to observe rare processes in search of new physics. One of the benefits of intensity frontier experiments is that they may reveal massive, undiscovered particles even the LHC cannot create.

In particle colliders like the LHC, physicists use Einstein's famous equation, E=mc2, to detect new particles. They bring beams of lightweight particles to a very high energy then smash them together. Because mass is a highly concentrated form of energy, the kinetic energy in the collision briefly converts into particles that can be more massive than those that make up the colliding beams. The more energy in the collision, the greater the mass of the particles the physicists can create.

That's why they built the LHC, to access higher energy levels than ever before in the hopes of creating theoretical particles that lower-energy colliders cannot create.

Scientists hope to find particles like the Higgs boson, which physicists theorize gives mass to matter. They also hope to find supersymmetrical particles, partner particles that mirror known particles but have much greater mass.

But should the particles that physicists seek prove too massive for even the LHC to pry from the vacuum, physicists have another crack at them at the intensity frontier.

"With the energy frontier, you need the highest possible energies," Dukes said. "What you need with the intensity frontier are the highest possible intensities."

This Feynman diagram shows a virtual top quark and anti-top quark appearing in the interaction between an electron and a positron.

This Feynman diagram shows a virtual top quark and anti-top quark appearing in the interaction between an electron and a positron.

The greater the concentration of particles that physicists can squeeze into a beam in an intensity frontier experiment, the more chances they have to catch a glimpse of virtual particles at work.

According to the Heisenberg Uncertainty Principle, even massive particles can pop briefly in and out of existence as virtual particles. The more massive the particle, the less frequently this happens. When virtual particles drop in, they are much less massive than usual, but physicists can detect them in intensity frontier experiments by the effect they have on interactions between other particles like kaons and muons.

By colliding two concentrated beams of one of these types of particles into one another or by firing them into a target, physicists bring them into proximity in the hopes that they will interact. They then study those interactions with ultra-precise detectors, looking for unusual outcomes. They can identify the presence of virtual particles involved in an interaction by the effects they have.

Even if the LHC grants physicists the chance to observe the particles physicists seek directly, they will need experiments like those at the intensity frontier to make precise measurements of their parameters.

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