Physicists at Indiana University are part of a global scientific collaboration preparing to install new particle-detector components at one of the world's most powerful scientific instruments, an upgrade meant to help researchers better understand the makeup of the fundamental building blocks of the universe.
The instrument is the Large Hadron Collider (LHC) near Geneva, Switzerland, the world's largest particle accelerator. Operated by CERN, the European Organization for Nuclear Research, the collider works by smashing protons together at nearly the speed of light and studying what flies out of the collisions.
IU is one of roughly 245 institutions from more than 40 countries that make up ATLAS, one
Professor of Physics Harold "Hal" Evans.of the LHC's two general-purpose detectors, where thousands of scientists work together to catch and measure those collisions. Harold "Hal" Evans, Professor of Physics in the College of Arts and Sciences at IU, is the deputy project manager overseeing the United States' roughly $300 million share of that upgrade. IU has been part of the ATLAS experiment since it began operating in 2009.
The LHC just finished a multi-year stretch dubbed "Run 3," and in this period alone recorded more twice the collision data than it had Runs 1 and 2 combined, going back to when it first switched on in 2009.
"The LHC is now entering a roughly three-year shutdown so crews can rebuild it to produce far more intense beams of protons," said Professor Evans. "IU and its partners are using the pause to install detector upgrades scientists have spent years designing and building."
Physicists' current explanation for how the universe works, called the Standard Model, correctly predicts the results of experiments with remarkable precision. But it cannot explain dark matter, the invisible substance that makes up most of the universe's mass, or the strange behavior of particles called neutrinos, so scientists know the Standard Model is incomplete.
A theory that fills those gaps could also explain why the universe is made of matter at all, rather than the equal mix of matter and antimatter that should have wiped each other out shortly after the Big Bang. The LHC may help answer these and other questions.
Evans noted that people often ask why this kind of research is worth the investment, and he sees the benefits falling into two categories.
The first is technology that spins off in the short term. The World Wide Web, for example, was invented at CERN so physicists could share data more easily. CERN research has also driven advances in proton therapy for cancer treatment and detector technology used in medical imaging.
"It's hard to overstate how important the downstream impacts have been to society, to technology, to the economy," Evans explained.
Once the upgrade is complete, he said, the detector will need to sort through roughly 10 times more collision data per second than it does now. At IU Bloomington, which has deep faculty expertise in physics and quantum mechanics, Evans's team is designing algorithms to run on specialized computer chips that sit on the detector itself, using machine learning to make decisions in billionths of a second about which collisions are worth keeping and which to discard.
"This research could eventually apply beyond ATLAS to any technology that needs to filter huge amounts of fast-arriving data in real time," Evans said.
The second category of benefit is longer term, pointing to the history of quantum mechanics, a set of ideas that seemed purely theoretical when physicists developed it in the early 1900s. Today, Evans said, technology built on quantum mechanics accounts for somewhere between two-thirds and three-quarters of the United States' economic output, depending on how it's measured.
"Understanding the universe at an infinitesimally small or subatomic level has proven to be very economically beneficial in the long term," he said.
Among the long-term payoffs that drive the physics side of the upgrade is a question Evans, his IU colleagues, and scientists worldwide are chasing: whether the particles now considered fundamental building blocks of matter, such as quarks and the Higgs boson, which was discovered at the LHC in 2012, are truly fundamental, or whether they are made of something even smaller still than a subatomic particle.
In this light, Evans is working on detecting events in which two Higgs bosons are produced
Visual reconstruction of a di-Higgs-like event recorded by a previous iteration of the ATLAS particle detector, the type of events for which the IU group is searching. Precise measurement of the rate at which these extremely rare events occur will help to reveal the fundamental structure of our universe." src="https://news.iu.edu/live/image/gid/17/width/500/height/330/31285_ATLAS_HH-4b-VBF.png" title="ATLAS HH-4b-VBF" srcset="https://news.iu.edu/live/image/scale/2x/gid/17/width/500/height/330/31285_ATLAS_HH-4b-VBF.png 2x, /live/image/scale/3x/gid/17/width/500/height/330/31285_ATLAS_HH-4b-VBF.png 3x" data-max-w="3784" data-max-h="2496" loading="lazy" data-optimized="true"/>
Visual reconstruction of a di-Higgs-like event recorded by a previous iteration of the ATLAS particle detector, the type of events for which the IU group is searching. Precise measurement of the rate at which these extremely rare events occur will help to reveal the fundamental structure of our universe.from a single collision between protons, something so rare it happens far less often than a single Higgs boson does. A single Higgs boson turns up in roughly one out of every billion proton-proton collisions.
Why pursue such knowledge? Because, Evans said, "This may reveal where things like supersymmetry could be hiding."
Supersymmetry is a theory that predicts a heavier partner particle for every known particle. If it exists, it could also help explain what the composition of dark matter actually is. Physicists have been searching for evidence of supersymmetry for decades without success, but Evans said the more powerful, upgraded LHC could improve the odds of spotting it, or of finding some other clue pointing beyond the Standard Model.
"We have a reasonably good idea of what the building blocks of the universe are right now at a certain level," Evans said. "The question the upgraded collider will help answer is simple to ask and hard to answer: are there more of those building blocks that we haven't seen yet? And what does this mean for our understanding of quantum mechanics and how the universe works?"
