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How a new muon experiment can advance physics


 From left: Doug Glenzinski, Craig Group and Amy Allen work on a "straw detector" test stand at Fermilab. Photo Courtesy of Fermilab.

In recent years, particle physicists have increasingly turned their attention to finding physics beyond the Standard Model description of the building blocks of matter and how they interact.

While many signs point to the existence of physics outside of the realm of current knowledge, a treasure map doesn’t exist, and a new particle’s discovery won’t necessarily include clues to how it fits into the rest of the particle zoo. Many theories exist to explain the origins of suspected “new physics” and extensions to the Standard Model, the current theoretical framework.

A new Fermilab-based experiment, the Muon-to-Electron Conversion experiment, or Mu2e, could shine light on those gray areas, aiding researchers at the Large Hadron Collider in Europe and likely the next-generation of collider experiments.

Answering a fundamental question

Mu2e received initial US Department of Energy approval in late November. It could find indirect signs for new particles and particle interactions up to the energy scale of 10,000 trillion electronvolts, or 10,000 TeV, far beyond the 14 TeV goal of the LHC. Those discoveries would give the next-generation of colliders an indication of the most promising, discovery-laden energy ranges to search.

Physicists already have discovered that two of the three categories of elementary particles–neutrinos and quarks–change into different particles, a process called flavor violation.

Proving the same process in the third particle category, charged leptons, which includes muons, remains a hurdle to understanding why particles in the same family decay from heavy to light mass states. Physicists have searched for this since the 1940s.  Discovering it is central to understanding what physics lies beyond the Standard Model.

“Any sensible theory that tries to go beyond the Standard Model requires some kind of charged-lepton flavor violation,” says Bob Bernstein, co-spokesperson.

Going beyond the Standard Model will help scientists unify the forces of nature, key to explaining how the universe changed from only free-flowing energy and particles to include solid matter, such as people and plants.

At the most simplistic level, understanding muon-to-electron conversion will clarify how particles created at the beginning of the universe broke down into lighter particles that eventually became the fundamental units  of electricity.

“Electrons are responsible for the electricity that lights our houses and turns on our computers. Muons are some sort of heavier cousin of the electron, but we're not sure just what the relationship is,” Bernstein says.“This experiment will help us understand that relationship, and so understanding muons is part of understanding the electrons that power our society.”

Helping the LHC

With its low energies and large number of muons, Mu2e will directly search for charged-lepton conversion while the LHC is constrained to look for indirect signals.

If the LHC finds new particles, particularly supersymmetric--or SUSY--particles, which are essentially siblings of known particles, Mu2e results will provide the data to put the discovery in context.

“When you add us to the mix, they will be able to pin down what the new physics is,” Bernstein says. “Mu2e both complements and advances what will be done at the LHC.”

If Mu2e physicists find muons morphing into electrons, it will narrow the number of plausible theories for the cause of SUSY. That would give context and insight into an LHC discovery of SUSY particles at low energies.

If they get a “zero” result, meaning they don’t find muons changing into electrons, it will cast doubt on many of the existing SUSY theory models. Physicists would have to substantially rethink their ideas about how the forces of nature unify at higher energies as they believe happened at the time of the big bang.