FASER Rules Out Broad Range of Hypothetical 'Quirk' Particles in First LHC Search
Three years of data from the detector 480 meters from ATLAS found no sign of heavy, slow-moving pairs that could explain why gravity is so much weaker than the other forces.
A collaboration at the Large Hadron Collider has run the first dedicated LHC search for "quirks," hypothetical heavy particles proposed to explain why gravity is so much weaker than the other fundamental forces. It found none, but the null result rules out a wide range of possibilities and tests a new way of looking.
The analysis, by the FASER Collaboration, is published in Physical Review Letters. It uses data collected between 2022 and 2024. The target is a puzzle called the gauge hierarchy problem: for reasons the Standard Model cannot explain, the weak nuclear force is some 10^24 times stronger than gravity. One proposed fix, developed by Markus Luty and Junhai Kang at the University of Maryland in 2008 and building on ideas from the 1980s, adds a new family of particles, the quirks. Some versions of the theory allow them to interact with the Higgs boson in a way that stops quantum effects from pushing the Higgs mass far higher than observed.
Quirks would feel a new force called "infracolor," which resembles the strong nuclear force. "This force could hold quirk-antiquirk pairs together with a nearly unbreakable string," said Eli Welch of the University of California, Irvine, the paper's primary author. "This would cause them to oscillate as they travel and make them difficult to simulate and detect." The first dedicated search for quirks was done at Fermilab's Tevatron collider in 2010. Searches at the LHC since then have either tried to trace the particles' complicated paths or looked for quirks that stop and annihilate in the detector.
FASER takes a different approach. It sits 480 meters (1,575 feet) downstream of the ATLAS experiment and is shielded by more than 100 meters of rock, so very few background particles reach it. The experiment was built to detect neutrinos from collisions and to search for new elementary particles. "By measuring their delayed arrival and slow speed, together with the unusually large energy deposits from both particles, we can identify quirks without having to trace their paths," Welch said. "This makes our search far less sensitive to their unusual oscillations than conventional searches."
The data showed no excess. "Our results rule out a broad range of possibilities within the quirk model we considered, including masses of particular interest to theories of physics beyond the Standard Model," Welch said. The collaboration says the result gives useful guidance for the hunt and could inspire similar searches at other colliders. The central point is that the detection strategy works: it does not depend on following a quirk's erratic trajectory.
The Standard Model has survived every test physicists have thrown at it, yet hints of incompleteness remain, from dark matter to the hierarchy of forces. The FASER team hopes its initial results lay the groundwork for an expanded search with more data. Narrowing the quirk model does not solve the hierarchy problem, but each excluded mass range is a region of theory space physicists no longer need to wonder about.
