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First Search for Quirks at the LHC with FASER

Using 2022–2024 LHC data with an integrated luminosity of 186 fb⁻¹, the FASER experiment conducted its first search for quirks and, finding no events, established the first exclusion limits for quirks with masses above the weak scale across a broad range of infracolor confinement scales.

Original authors: FASER Collaboration

Published 2026-07-30

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: FASER Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic LEGO set. For decades, scientists have been building models with the pieces they know: the tiny bricks of the Standard Model that make up everything from your coffee cup to the stars. But there's a nagging feeling that the instruction manual is missing a few pages. Why is gravity so weak compared to the other forces? What is that invisible "dark matter" holding galaxies together? To answer these questions, physicists often imagine a hidden, parallel world of particles and forces that are too shy to show up in our everyday experiments. One of the most intriguing characters in this hidden world is a hypothetical particle called a "quirk." Think of a quirk not as a single brick, but as a pair of dancers tied together by an invisible, super-strong elastic band. Unlike normal particles that zip around at the speed of light, these dancers are heavy and sluggish. Because they are tied together, they don't just fly straight; they wobble and oscillate as they move, leaving a strange, slow trail that looks nothing like the usual suspects in the particle zoo. Finding them would be like discovering a new species of animal that moves in a way no one has ever seen before, proving that the universe is far more creative than our current blueprints suggest.

This is exactly what the FASER collaboration set out to do in their latest paper, "First Search for Quirks at the LHC with FASER." They took a giant, high-speed particle accelerator—the Large Hadron Collider (LHC)—and used it to smash protons together, hoping to create these elusive quirk pairs. The team didn't look for quirks right at the crash site, though. Instead, they placed a specialized detector called FASER 480 meters down the tunnel, hidden behind a massive shield of rock and magnets. This setup acts like a cosmic sieve: it blocks almost all the ordinary debris from the collisions, letting only the most stubborn, fast-moving particles (like neutrinos and muons) through, along with any potential quirks.

The researchers analyzed a massive amount of data collected between 2022 and 2024, equivalent to 186 "inverse femtobarns" of proton collisions at an energy of 13.6 TeV. They were looking for a very specific signature: pairs of particles that arrive late (because they are slow) and leave a double charge signal (because there are two of them moving together). They used the detector's scintillators—sensors that light up when charged particles pass through—to measure exactly when the particles arrived and how much charge they carried. By comparing the timing of the arrival against the speed of light, they could spot any "slow motion" intruders.

The result? The detector saw nothing. Not a single quirk. The search was incredibly clean, with almost no background noise to confuse the results. Because they found zero events where they expected to see some if quirks existed, the team was able to draw a very firm line in the sand. They have now ruled out the existence of quirks with masses above the "weak scale" (a specific range of heaviness) for a wide variety of invisible force strengths, specifically for confinement scales between 300 eV and 100 keV. In simple terms, if quirks exist, they are either much heavier or behave in a way that is completely different from the models the team tested. While this isn't the discovery of a new particle, it is a major step forward in mapping the unknown: by proving that quirks aren't hiding in this particular corner of the universe, the FASER team has helped scientists narrow down the search for the next big breakthrough in physics.

Technical Summary: First Search for Quirks at the LHC with FASER

Problem and Motivation
The Standard Model (SM) of particle physics, while successful, fails to resolve the gauge hierarchy problem and lacks a viable particle candidate for cold dark matter. Extensions involving new non-Abelian gauge groups are theoretically motivated, particularly by "neutral naturalness" models where non-colored top partners charged under a hidden gauge group can ameliorate the hierarchy problem. A specific class of such extensions involves "quirks"—hypothetical matter particles charged under a new, QCD-like gauge group known as infracolor (IC). While IC glueballs are viable dark matter candidates and the framework addresses the hierarchy problem, direct collider searches for quirks have been sparse. Prior searches were limited to the Tevatron (DØ Collaboration), and no dedicated search has been conducted at the Large Hadron Collider (LHC), despite proposed strategies for ATLAS, CMS, and LHCb.

Methodology
This analysis presents the first LHC search for quirks using data collected by the FASER detector between 2022 and 2024. The study utilizes an integrated luminosity of 186 fb⁻¹ of proton-proton collisions at a center-of-mass energy of $\sqrt{s} = 13.6$ TeV.

Signal Model: The analysis adopts a benchmark model where quirks transform under the representation $(1, 1, -1, N_{IC})$ of $SU_C(3) \times SU_L(2) \times U_Y(1) \times SU(N_{IC})$ , with $N_{IC} = 2$ . This corresponds to SM gauge charges identical to right-handed charged leptons. The model is defined by the quirk mass ( $m_Q$ ) and the IC confinement scale ( $\Lambda$ ). Quirk pairs are produced via Drell-Yan processes (mediated by photons or $Z$ bosons) and, for $\Lambda \ll m_Q$ , remain bound by the IC force. They oscillate around their common center of mass, resulting in trajectories that are distinct from SM particles. Crucially, their high mass and the IC force redirect velocity into transverse oscillations, reducing their longitudinal velocity significantly below the speed of light ( $c$ ).
Detector and Simulation: FASER, located 480 m downstream of the ATLAS interaction point, is shielded from most SM particles by LHC dipole magnets and over 100 m of rock, making it ideal for detecting long-lived, slow-moving particles. The detector includes a tracking spectrometer, a sampling electromagnetic calorimeter, and eight scintillators.
- Simulation: Signal events were simulated using MadGraph5 aMC@NLO interfaced with Pythia 8 (LHCb tune) for parton showering, with normalization corrections applied based on LHCb forward $Z$ boson production measurements. Quirk propagation through material and magnetic fields was simulated analytically for $\Lambda > 3$ keV (where IC forces dominate) and via Geant4 for detector interactions.
- Backgrounds: The primary backgrounds are "through-going muons" (depositing charge in all scintillators) and "kinked" events (muon-induced secondary showers). Cosmic muons and beam-induced backgrounds were found to be negligible.
Selection Criteria:
- Tracking: Events required track segments in upstream and downstream tracker stations. An ellipse was fitted to the tracker-hit distribution to reject shower-like events (requiring the semi-minor axis $< 3$ cm) and ensure the track remained within the fiducial region.
- Charge: A "Quirk-like Charge" (QLC) window was optimized for each scintillator. While a single muon typically deposits charge following a Landau distribution, a quirk pair deposits approximately twice the charge. Signal events were required to exhibit QLC in all eight scintillators.
- Timing and Velocity: Scintillator timing was calibrated to the arrival time of muons traveling at $c$ . A linear regression of arrival time versus position yielded the arrival time ( $t_0$ ) and velocity ( $\beta = v/c$ ). Events were required to have $\beta > 0.6$ and fall outside specific "in-time" regions defined by 2D Gaussian ellipses in the $(t_0, \beta)$ plane. These ellipses excluded the bulk of through-going muons and satellite-bucket backgrounds (protons in neighboring RF buckets).
- Background Estimation: The residual background from through-going muons in the signal region was estimated using a data-driven ABCD method, assuming negligible correlation between charge and timing. The "kinked" background was estimated by extrapolating the power-law tail of non-Gaussian events in the main timing ellipse.

Key Results

Observation: No events were observed in the signal region defined by the analysis.
Background: The estimated background in the signal region was $0.046 \pm 0.010$ events for the 2022–2023 period and $0.082 \pm 0.014$ events for the 2024 period. The total expected background is consistent with zero within uncertainties.
Exclusion Limits: The null result leads to the first exclusion limits for quirks at the LHC. The analysis excludes quirk masses above the weak scale for IC confinement scales in the broad range of $300 \text{ eV} < \Lambda < 100 \text{ keV}$ .

Significance and Claims
The paper claims the first direct search for quirks at the LHC, filling a gap left by previous Tevatron searches and proposed but unexecuted LHC strategies. By utilizing the unique forward geometry and shielding of FASER, the analysis leverages the distinct kinematic signature of quirks—specifically their slow velocity and exotic electromagnetic traces—to achieve a nearly background-free search. The results provide the first experimental constraints on quirk models with masses above the weak scale and confinement scales between 300 eV and 100 keV, significantly advancing the search for hidden non-Abelian sectors and potential solutions to the gauge hierarchy problem.

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