“In search of a life” of supersymmetric particles at CERN

A team of researchers from the University of Chicago recently embarked on the search for a life, or rather, the search for the life of long-lived supersymmetric particles.

Supersymmetry is a proposed theory to expand the standard model of particle physics. Similar to the periodic table of the elements, the standard model is the best description we have for subatomic particles in nature and the forces acting on them.

But physicists know that this model is incomplete: it leaves no room for gravity or dark matter, for example. Supersymmetry aims to complete the picture by coupling each standard model particle with a supersymmetric partner, opening up a new class of hypothetical particles to detect and discover. In a new study, UChicago physicists have discovered the limits to the properties these super partners, if any, might have.

“Supersymmetry is really the most promising theory we have for solving as many problems as possible in the standard model,” said Tova Holmes, an assistant professor at the University of Tennessee, Knoxville, who worked on the experiment as a post-graduate researcher. PhD from UChicago. “Our work fits into a larger effort at the Large Hadron Collider to reconsider how we look for new physics.”

The Large Hadron Collider, located in Europe at CERN, accelerates the protons to nearly the speed of light before forcing them to collide. These proton-proton collisions produce a slew of additional particles in which researchers hope to find new physics.

“But at the Large Hadron Collider, new physics events are extremely rare and difficult to identify in the colliding particle debris,” said Prof. Young-Kee Kim, chair of UChicago’s physics department and co-author of the study. effort led entirely by women.

The UChicago team sought the production of sleepers – hypothesized superpartners of existing leptons of electrons, muons and tau – using data collected in ATLAS, a particle detector at CERN. In the supersymmetry model tested, sleepers are theorized to have a long life, which means they can travel long before they decay into something detectable by ATLAS.

“One of the ways we can lose the new physics is if the particle doesn’t decay promptly when it is produced,” Holmes said. “Typically, we are blind to long-lived particles in our searches, because we essentially eliminate anything that doesn’t look like standard immediate decay in our detector.”

Sleepers are expected to eventually decay into their normal lepton partners. But unlike conventional decays, these leptons will be displaced, which means they won’t return to the original proton-proton collision point. It was this unique feature that physicists were looking for.

In four years of ATLAS data collection, however, the UChicago researchers found no lepton shifted events. This lack of discovery has allowed them to establish what is called a limit, ruling out a range of masses and lives that could have long-term sleepers.

“We’re at least 95% sure that if a sleeper in this model exists, it doesn’t have the masses and lives in the shady parts of this plot,” said Lesya Horyn, Ph.D. recently minted. from UChicago who recently completed his thesis on this measurement.

Does a null result disappoint the team? Not at all.

“Finding nothing tells you so much,” Horyn said. Knowing that long-lived sleepers don’t have certain masses and human lives informs researchers where to focus future research.

“From my point of view, this research was the number one thing the theorists were asking to cover,” said Holmes. “It looked like we could do it, and we did!”

The result stimulated the team to push the boundaries further. At some point in the next decade, the Large Hadron Collider will enter its periodic shutdown, leaving plenty of time for the ATLAS hardware upgrade.

“This was a first step in analysis, so there are definitely places to improve,” Horyn said.

An urgent update will be a renewal of the activation system, which selects whether events should be saved or deleted. The trigger is currently optimized to store short-lived particle decays, not long-lived sleepers critical to this supersymmetry quest.

More immediate improvements can be made without waiting for shutdown.

“Future steps could include finding the same model using more reliable data from subsequent executions of the Large Hadron Collider,” said Xiaohe Jia, a Harvard graduate student who worked on the experiment as a UChicago undergraduate student. Another path to explore, he said, could be using similar techniques to expand the search for long-lived particles beyond mere sleepers.

For now, the completion of the standard model remains a mystery, but the team is proud to have conducted an early search for this supersymmetry model in ATLAS.

“Discovering new physics is like finding a needle in a haystack,” Kim said. “While we haven’t seen anything in the current data, there are great opportunities for the future!”