FASER's Physics Reach for Long-Lived Particles

Abstract: FASER,the ForwArd Search ExpeRiment,is a proposed experiment dedicated to searching for light, extremely weakly-interacting particles at the LHC. Such particles may be produced in the LHC's high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons (HNLs); light B-L and $L_i - L_j$ gauge bosons; axion-like particles (ALPs) that are coupled dominantly to photons, fermions, and gluons through non-renormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

• The paper demonstrates FASER’s capacity to detect long-lived particles produced in high-energy LHC collisions.
• It employs detailed analyses of models including dark photons, dark Higgs bosons, heavy neutral leptons, and axion-like particles.
• It predicts significant discovery potential during LHC Run 3 and FASER 2 through low-background forward detection.

FASER's Physics Reach for Long-Lived Particles

The paper "FASER's Physics Reach for Long-Lived Particles" by the FASER Collaboration provides a detailed analysis of the potential of the ForwArd Search ExpeRiment (FASER) to discover new light, weakly coupled long-lived particles (LLPs) at the Large Hadron Collider (LHC). The experiment is strategically placed to exploit the far-forward particle spectrum produced in proton-proton collisions, with a focus on the unexplored parameter space that might yield insights into new physics beyond the Standard Model (SM).

Overview and Methodology

The FASER detector, located 480 meters downstream of the ATLAS interaction point, is designed to detect particles traveling in the forward direction. It operates under the premise that LLPs may be produced in high-energy collisions at the LHC and traverse significant distances without interacting, subsequently decaying into detectable SM particles within the detector volume.

The paper comprehensively explores FASER's sensitivity to various theoretical models predicting the existence of LLPs. These include:

• Dark Photons: Produced through kinetic mixing with the SM photon, yielding a massive vector boson that couples weakly to SM charged particles.
• Dark Higgs Bosons: Emergent from extensions of the Higgs sector, these bosons may have mixings with the SM Higgs, leading to unique decay signatures.
• Heavy Neutral Leptons (HNLs): Often associated with neutrino mass generation mechanisms, these particles typically mix with SM neutrinos and participate in weak interactions.
• Axion-Like Particles (ALPs): Pseudo-scalars that can couple to photons, gluons, and fermions, characterized by non-renormalizable interactions.
• Dark Scalars with Yukawa-like Couplings: Scalars whose interactions mimic the Yukawa couplings of the SM, offering rich phenomenological implications.

Each model is analyzed in terms of production channels, decay modes, and the subsequent signatures detectable by FASER. The authors present detailed calculations of the potential background levels and the anticipated number of signal events under various assumptions of luminosity and running conditions.

Results and Implications

FASER's reach is evaluated for two operational phases: during LHC Run 3 and its potential extension, FASER 2, during the high-luminosity LHC era. For both phases, the research delineates regions in parameter space where FASER can expect to make discoveries or set competitive limits relative to other experiments. One significant advantage of FASER is its environmental setup, allowing it to leverage the large hadron flux in the forward direction while maintaining low-background conditions due to natural and engineered shielding.

The analysis predicts that FASER will achieve discovery sensitivity for dark photons across a broad range of masses and kinetic mixing parameters. Similarly, it can explore regions of HNL parameter space previously inaccessible to other experiments, especially in cases where these particles arise from $B$-meson decays. FASER's capability to observe or constrain ALPs near the MeV to GeV scale further underscores its role in probing new physics motivated by unresolved SM anomalies and dark matter considerations.

Future Prospects

By providing a low-cost, high-reward addition to the LHC's experimental repertoire, FASER epitomizes a strategic shift towards detecting weakly coupled phenomena in particle physics. Its contributions are expected to complement those of larger experiments by targeting the specific niche of highly collimated, energetic yet weakly interacting particle streams.

Looking forward, the success of FASER could inform the design of future detectors aiming to unravel the mysteries surrounding dark sectors and their interactions with the SM. Additionally, it might inspire advancements in theoretical frameworks, potentially steering them toward articulating more subtle, non-conventional signals that might only be detectable at such niche experiments.

Overall, this investigation represents a crucial step toward broadening the understanding of particle interactions at the energy frontier and emphasizes the symbiotic relationship between theoretical innovation and experimental ingenuity in the quest for new physics.