Tova Holmes Wins Simons Foundation Support For Muon Collider Groundwork

University of Tennessee, Knoxville Assistant Professor Tova Holmes is part of a scientific trio that won $1 million from the Simons Foundation to break new ground in particle physics and train young scientists to explore it.

The proposal is one of only two the foundation funded this year through its Targeted Grants in Mathematics and Physical Sciences program. The two-year grant allows Holmes and her co-investigators to do crucial groundwork for building a muon collider. This next-generation facility is part of the nation's particle physics roadmap, designed to deliver the energy upgrades needed to untangle mysteries about dark matter and what holds the universe together.

To reach this goal, the grant supports young researchers working at the intersection of experimental particle physics and accelerator physics. Scientists have spent decades developing instruments to study matter's building blocks. Now those tools are commonly used in fields like medical imaging and making semiconductors. Holmes and her colleagues (Isobel Ojalvo of Princeton University and Karri DiPetrillo of the University of Chicago) want universities to play a bigger role in educating scientists who understand this technology to ensure its progress for science and society.

Catching Muons While You Can

Holmes met Ojalvo and DiPetrillo working at the Large Hadron Collider (LHC), the world's most powerful particle accelerator. Revving up to just about the speed of light, the LHC collides two beams of particles at different spots along a circular scientific racetrack. Experiments at the collision points detect and analyze the results, looking for escapees, debris, new particles, and extra dimensions. The LHC was home to breakthrough science confirming the Higgs boson, the particle whose associated field gives other fundamental particles their mass.

For many scientists (including Holmes and her colleagues) the muon collider is the next frontier for particle physics. Typical colliders rely on proton or electron beams. Protons are composite particles and when they collide, only fractions of their energy (carried by quarks and gluons inside them) can be used to make new particles. Like muons, electrons don't have any smaller constituents, but they are much lighter, making it impossible to accelerate them to high energies. Muon beams offer higher collision energy, producing more data and taking up less space. There's a catch, however.

Holmes explained that stable particles like electrons and protons are plentiful and easy to manipulate into beams.

"Muons are not like that," she said. "They're constantly being produced in our atmosphere but they are not just sitting around. They only live two-millionths of a second … so you need to create them and then harness them before they disappear."

Creating them isn't the tough part. Corralling them is.

"If you take a bunch of protons and slam them into a target you can make muons," Holmes explained. "But trying to gather them up, get them all aligned into a really tight beam and then manipulate, accelerate, focus, (and) collide them: that part hasn't been done before. That is a really unique challenge."

Dark Matter and the Fate of the Universe

Muons might be problematic, but Holmes has two targets in mind that make them worth the trouble: understanding dark matter and the life of the universe.

"As soon as people realized that there was dark matter out there they started trying to hypothesize what kind of particles this could be," she said. "We haven't been able to build something sensitive enough, and high-energy enough, to access it. I think we have a pretty good shot at that with a muon collider."

A muon collider could also help explain what the Higgs boson is up to and what that means for the life (and maybe collapse) of the universe. It could mass produce collisions so energetic that they spawn multiple Higgs bosons, with interactions between those identical particles giving scientists a deeper understanding of how it works.

"The Higgs boson seems to be extremely essential to understanding the birth of our universe and possibly its death," Holmes said.

The Higgs is like a play with three starring roles: The boson, the field, and the potential. Scientists know that the scenery includes a well, with a curve at the bottom.

"When the Higgs field fell into that well, something fundamental changed in the universe," Holmes said. "Before that, everything was massless. Everything could interact on even footing. All of a sudden, particles acquired different masses. You have preferential interactions. That's what creates our protons and our neutrons. That's what creates all of matter."

The Higgs potential might undo all that.

"Think about potentials as rolling hills," Holmes explained. "You put something at the top; it rolls down and finds a minimum potential and it settles there."

In a real landscape there might be another valley below, but if there were a hill between the two, nothing would happen. Unfortunately, Holmes added, "in quantum mechanics there's a mechanism for tunneling through that hill. If that happens and the Higgs field finds a new potential minimum, it completely disrupts everything about the matter that we have around us. The universe would suddenly and dramatically completely reorder itself."

Holmes said the beginning and end of the universe are tied to the shape of the Higgs potential and a "multi-Higgs factory is the only way that you can address those questions."

Next-Gen Accelerator Scientists

The Simons Foundation grant will help Holmes and her co-investigators mentor young scientists who will help draw this factory's blueprint. All three will take on postdocs and students, and they'll jointly supervise their work. Specifically, they'll work at the junction of experimental particle physics and accelerator physics.

"Accelerator physics is based in fundamental physics and is really driving a huge fraction of science today," she said. "Accelerators have become massively useful to everyone," across not only different fields of physics but also in the private sector.

While accelerators have their foundation in particle physics, Holmes said most students working in the field don't get experience working with them because the research is centered primarily at national labs rather than universities.

"What we have is a field that everybody's relying on that doesn't have a pipeline of people to become the next generation who are able to create these incredible machines," she explained. "We have a great program in the U.S. but it doesn't have the number of people it should."

The grant will fund students working on challenges in both the particle physics and accelerator aspects of building a muon collider, part of what Holmes calls the "pre-work" in mapping out its future. While the collider itself would takes years and substantial public investment, she said they want to demonstrate that the most technically challenging physical pieces of it are actually possible, with plenty to discover along the way.

"Even before you make the final collider there's a lot of interest in thinking about what kind of experiments you could set up with that intermediate beam," she said. "If you look somewhere you've never looked before, you don't know what you're going to see."

Provided by University of Tennessee at Knoxville