FASER: ForwArd Search ExpeRiment at the LHC (1901.04468v1)

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 in large numbers in the far-forward region and then 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 describe the FASER program. In its first stage, FASER is an extremely compact and inexpensive detector, sensitive to decays in a cylindrical region of radius R = 10 cm and length L = 1.5 m. FASER is planned to be constructed and installed in Long Shutdown 2 and will collect data during Run 3 of the 14 TeV LHC from 2021-23. If FASER is successful, FASER 2, a much larger successor with roughly R ~ 1 m and L ~ 5 m, could be constructed in Long Shutdown 3 and collect data during the HL-LHC era from 2026-35. FASER and FASER 2 have the potential to discover dark photons, dark Higgs bosons, heavy neutral leptons, axion-like particles, and many other long-lived particles, as well as provide new information about neutrinos, with potentially far-ranging implications for particle physics and cosmology. We describe the current status, anticipated challenges, and discovery prospects of the FASER program.

• The paper introduces a forward search method at the LHC, targeting MeV–GeV particles like dark photons and axion-like particles.
• It details a compact detector placed 480 meters from ATLAS, optimized for identifying long-lived particle decays in a low-background environment.
• Simulations predict FASER’s potential for advancing beyond Standard Model physics and exploring novel neutrino and dark matter interactions.

Insights into the Forward Search Experiment at the LHC (FASER)

The document presents a comprehensive outline of the Forward Search Experiment (FASER) at the Large Hadron Collider (LHC), designed to explore a less-examined domain of particle physics. Unlike traditional experiments seeking high-mass particles with substantial couplings, FASER targets lightweight, extremely weakly interacting particles, anticipated to emerge in the LHC’s forward region. This innovative approach taps into the LHC's high luminosity to uncover particles in the MeV to GeV mass range, such as dark photons, dark Higgs bosons, and axion-like particles, which have eluded detection due to their low transverse momentum and weak Standard Model (SM) interactions.

Experimental Configuration and Mechanisms

FASER is strategically placed 480 meters downstream of the ATLAS interaction point, nestled within an underutilized side tunnel (TI12), making it optimal for detecting long-lived particles (LLPs) that evade interaction over vast distances before decaying into identifiable SM particles. The initial stage of FASER comprises a compact, cost-efficient design: a 0.047 m³ decay volume encased in a magnetic dipole setup to facilitate charge separation of decay products and precise momentum measurements through associated tracker modules. Subsequent plans for FASER 2 envision a larger apparatus with increased sensitivity, capturing a broader mass spectrum and enhancing detection capability.

Particle Production and Discovery Potential

The research emphasizes FASER’s effectiveness at leveraging the immense production rates inherent to the LHC’s forward physics. The 14 TeV collision energy facilitates abundant production of light mesons and their associated decay pathways, crucial for generating a detectable flux of hypothetical particles. Simulations predict FASER's capacity to discern dark photon signals in the mass range of 100 MeV to GeV, alongside prospective sensitivity to the intricate dynamics of dark Higgs bosons, particularly those emerging from $B$ meson decays within its geometric acceptance.

FASER and its future iteration, FASER 2, articulate a robust discovery potential for various other particle interaction models, solidified through extensive projections and parameter space mappings. Such investigations promise to bridge gaps in the current understanding of weakly interacting light particles, essential for broadening the physics landscape and illuminating potential ties to dark matter phenomena.

Implementation and Strategic Context

The paper also discusses the location and strategic fit of FASER within the existing LHC infrastructure. The TI12 and TI18 tunnels provide a natural low-background environment, enhancing the prospects for clean detection of LLP decay processes. This strategic placement within the LHC framework constitutes a smart utilization of existing facilities, aligning innovation with logistical pragmatism.

Theoretical and Practical Implications

As FASER advances from concept to implementation, its contributions to particle physics are dual: extending the LHC’s exploratory reach into lesser-chartered territory and potentially offering new insights into neutrino physics. The anticipated neutrino interactions at unprecedented energies could offer additional scientific dividends, reinforcing FASER’s status as a multi-faceted endeavor.

While the primary thrust remains toward establishing novel insights into BSM phenomena, the extension to neutrino physics and cosmic ray studies suggests a versatile application spectrum. With FASER 2 predicted to scale the investigation into higher luminosity runs, these projects align seamlessly within the broader physics agenda, potentially invigorating subsequent collider-era pursuits.

Concluding Considerations

The FASER project represents an essential shift towards nuanced and inclusive particle searches at the LHC, advocating for meticulously optimized experiments with compact foundational costs. As FASER moves through its operational phases, careful assessment of challenges, particularly concerning scaling issues linked to detector size and required infrastructural modifications for FASER 2, will be pivotal. The collaboration's interdisciplinary vision and integration of experimental and theoretical expertise underpin an endeavor poised to substantively augment the scientific returns from the world’s premier particle collider.