A team of researchers at the University of Chicago recently embarked on the search of a lifetime—or rather, a search for the lifetime of long-lived supersymmetric particles.
The researchers believe they may have discovered a “brand-new force of nature” at CERN’s Large Hadron Collider that could explain why certain atomic particles behave unexpectedly and which may transform our understanding the rudiments of physics. Authors of the research said this week that their results should “get physicists’ hearts beating just a little faster” after they discovered evidence of a “brand-new” type of particle according to a study shared in a preprint server and confirmed on CERN’s official website.
Since its inception over a decade ago, the Large Hadron Collider (LHC) has sought to delve into the secrets of the universe by studying the smallest discreet particles of matter as they collide at nearly the speed of light.
“But at the Large Hadron Collider, new physics events are extremely rare and difficult to identify in the debris of colliding particles,” said Prof. Young-Kee Kim, chair of the UChicago physics department and co-author of the study, an effort led entirely by women.
The UChicago team searched for the production of sleptons—hypothesized superpartners of the existing electron, muon, and tau leptons—using data collected in ATLAS, a particle detector at CERN. In the tested supersymmetry model, sleptons are theorized to have long lifetimes, meaning they can travel far before decaying into something detectable by ATLAS.
“One of the ways we can miss new physics is if the particle doesn’t decay promptly when it’s produced,” said Holmes. “Typically, we’re blind to long-lived particles in our searches, because we basically cut out anything that doesn’t look like a standard prompt decay in our detector.”
Sleptons are expected to eventually decay into their regular lepton partners. But unlike conventional decays, these leptons will be displaced, meaning they won’t point back to the original proton-proton collision point. It was this unique feature that physicists were hunting for.
In four years of collected ATLAS data, however, UChicago researchers found no displaced lepton events. That lack of discovery allowed them to set what is called a limit, ruling out a range of masses and lifetimes that long-lived sleptons might have.
“We are at least 95% sure that, should a slepton in this model exist, it doesn’t have the masses and lifetimes in the shaded portions of this plot,” said Lesya Horyn, newly minted PhD from UChicago who recently completed her dissertation 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 sleptons don’t have certain masses and lifetimes informs researchers on where to focus future searches.
“Supersymmetry really is the most promising theory we have for solving as many problems as possible in the Standard Model.”
—Tova Holmes, assistant professor at the University of Tennessee, Knoxville
“From my point of view, this search was the number one thing theorists were calling out to have covered,” Holmes said. “It seemed like we could do it—and we did!”
The outcome has energized the team to push the boundaries even further. At some point in the next decade, the Large Hadron Collider will enter its periodic shutdown, leaving ample time for ATLAS hardware to be upgraded.
“This was a first pass at the analysis, so there are definitely places to improve,” Horyn said.
One pressing upgrade will be a revamp of the trigger system, which selects whether events should be saved or thrown away. The trigger is currently optimized to store decays from short-lived particles, not the long-lived sleptons central to this supersymmetry search.
More immediate improvements can be made without waiting for the shutdown.
“Future steps might include searching for the same model using more robust data from the next runs of the Large Hadron Collider,” said Xiaohe Jia, a graduate student at Harvard who worked on the experiment as a UChicago undergrad. Another route to explore, she said, could be using similar techniques to expand the long-lived particle search beyond just sleptons.
For now, the completion of the Standard Model remains a mystery, but the team is proud to have led a first search for this supersymmetry model in ATLAS.
“One of the ways we can miss new physics is if the particle doesn’t decay promptly when it’s produced,” said Holmes. “Typically, we’re blind to long-lived particles in our searches, because we basically cut out anything that doesn’t look like a standard prompt decay in our detector.”
Sleptons are expected to eventually decay into their regular lepton partners. But unlike conventional decays, these leptons will be displaced, meaning they won’t point back to the original proton-proton collision point. It was this unique feature that physicists were hunting for.
In four years of collected ATLAS data, however, UChicago researchers found no displaced lepton events. That lack of discovery allowed them to set what is called a limit, ruling out a range of masses and lifetimes that long-lived sleptons might have.
“We are at least 95% sure that, should a slepton in this model exist, it doesn’t have the masses and lifetimes in the shaded portions of this plot,” said Lesya Horyn,
“Discovering new physics is like finding a needle in a haystack,” Kim said. “Although we did not see anything in the current data, there is great opportunity for the future!”
Aside from the discovery of the Higgs Boson — a previously theoretical particle that gives matter mass — the project has so far failed to shed much light on how the building blocks of matter function.
According to the standard model of particle physics, particles known as quarks, many of which are unstable and exist only for a split-second, can form heavier particles such as protons and neutrons.
Quarks can also decay into other particles known as leptons — including so-called “beauty” quarks — which were discovered at the LHCb in 2014.
The anomaly spotted at the time was that the quarks appeared to also decay into another type of lepton — muons — less often than they decayed into electrons.
The standard model predicts that beauty quarks would decay into muons at the same rate as they do into electrons.
“The only way these decays could happen at different rates is if some never-before-seen particles were getting involved in the decay and tipping the scales against muons,” wrote the authors of this week’s research on The Conversation site.
Until recently there had not been enough data to say for sure what was occurring inside the LHC.
In 2019, scientists re-ran the 2014 experiment on beauty quarks again with additional input gathered in the intervening years.
The dataset has now doubled, and the team poring over it worked “blind” — they couldn’t see the result until all procedures had been reviewed — in order to avoid any accidental interpretation bias.
When the result finally came out, the data showed that there were around 85 muon decays for every 100 electron decays.
The authors said there was only a one-in-a-thousand chance of the result occurring randomly — not enough to prove the existence of an as-yet unknown particle, but strong evidence in favour of its existence.
Part of larger puzzle
The authors offered several possible explanations.
First, the varying decay rate could be the result of a “Z prime” particle, essentially a new force of nature.
“This force would be extremely weak, which is why we haven’t seen any signs of it until now, and would interact with electrons and muons differently,” they wrote.
Another possibility is the currently hypothetical “lyptoquark”, which can decay quarks and leptons simultaneously and which “could be part of a larger puzzle that explains why we see the particles that we do in nature”.
Chris Parkes, LHCb spokesman from the University of Manchester and CERN, said that if the results were confirmed “it would require a new physical process, such as the existence of new fundamental particles or interactions”.
The researchers said that further experimentation was needed to confirm the results.
But if the data are borne out, they said they would have evidence of “part of the larger picture that lies beyond the standard model, which ultimately could allow us to unravel any number of established mysteries.”
