Detecting and Studying High-Energy Collider Neutrinos with FASER at the LHC (1908.02310v2)

Abstract: Neutrinos are copiously produced at particle colliders, but no collider neutrino has ever been detected. Colliders, and particularly hadron colliders, produce both neutrinos and anti-neutrinos of all flavors at very high energies, and they are therefore highly complementary to those from other sources. FASER, the recently approved Forward Search Experiment at the Large Hadron Collider, is ideally located to provide the first detection and study of collider neutrinos. We investigate the prospects for neutrino studies of a proposed component of FASER, FASER$\nu$, a 25cm x 25cm x 1.35m emulsion detector to be placed directly in front of the FASER spectrometer in tunnel TI12. FASER$\nu$ consists of 1000 layers of emulsion films interleaved with 1-mm-thick tungsten plates, with a total tungsten target mass of 1.2 tons. We estimate the neutrino fluxes and interaction rates at FASER$\nu$, describe the FASER$\nu$ detector, and analyze the characteristics of the signals and primary backgrounds. For an integrated luminosity of 150 fb$^{-1}$ to be collected during Run 3 of the 14 TeV Large Hadron Collider from 2021-23, and assuming standard model cross sections, approximately 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos will interact in FASER$\nu$, with mean energies of 600 GeV to 1 TeV, depending on the flavor. With such rates and energies, FASER will measure neutrino cross sections at energies where they are currently unconstrained, will bound models of forward particle production, and could open a new window on physics beyond the standard model.

Detecting and Studying High-Energy Collider Neutrinos with FASER at the LHC

The paper introduces the Forward Search Experiment (FASER) at the Large Hadron Collider (LHC), specifically its FASER$\nu$ sub-detector, which aims to detect neutrinos produced in high-energy collisions—a feat never accomplished at colliders previously. Despite the copious production of neutrinos during particle collisions, their detection has been remarkably challenging due to their weak interactions. FASER$\nu$ is strategically placed 480 meters downstream from the ATLAS interaction point along the beam collision axis, which allows it to capitalize on the high flux of neutrinos originating from the LHC.

The neutrino flux encompasses all flavors—electron, muon, and tau neutrinos—and offers a unique complement to experiments detecting neutrinos from other sources, such as cosmic rays or nuclear reactors. The detector, designed with emulsion films interleaved with tungsten plates totalling 1.2 tons, is optimized for detecting interactions where neutrinos will produce charged particles observable in emulsion films, leveraging the exceptional spatial resolution of such detectors.

Numerical Results and Findings

The FASER$\nu$ detector is expected to interact with approximately 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos over an integrated luminosity of 150 fb$^{-1}$. This affords the opportunity to probe neutrino energies ranging between 600 GeV and 1 TeV where existing constraints on cross section measurements from other sources are sparse or non-existent. Importantly, these interactions allow for the exploration of neutrino cross sections at energy scales previously uncharted, providing an opportunity to further test the predictions of the Standard Model concerning lepton universality and cross sections in neutrino interactions.

Challenges and Background Mitigation

The paper acknowledges the challenges posed by numerous high-energy backgrounds, primarily originating from muons that overlap the neutrino interaction zone. These backgrounds can produce secondary particles upon interaction with surrounding materials. However, thanks to the detector’s configuration and stringent criteria—specifically requiring neutral vertices with multiple outgoing charged particles and considering the directional alignment with the initial neutrino path—the authors propose that backgrounds may be effectively distinguished from signal interactions. Additionally, the incorporation of scintillators and potentially coupling the setup with the FASER spectrometer could enhance background rejection capabilities and energy resolution.

Broader Implications and Future Prospects

The ability to detect neutrinos at a collider and measure their properties in detail holds profound implications for particle physics, potentially paving the way for comprehensive neutrino programs at future collider experiments, including the High-Luminosity LHC and proposed future colliders. Such programs would offer insights into heavy flavor neutrino interactions with charm and beauty quarks, which can be critically influenced by new physics models suggesting lepton universality violations.

The experiments set to be conducted by FASER$\nu$ will not only signify a major milestone—the detection of LHC collider neutrinos—but also provide significant data that could inform high-energy astrophysical experiments, such as those conducted by neutrino observatories like IceCube, bolstering their theoretical models regarding atmospheric neutrino production.

In summary, the paper of collider neutrinos at the LHC enabled by FASER$\nu$ establishes a promising new direction in neutrino research, offering complementarity to the intensity frontier by exploring neutrino properties at the energy frontier. The anticipated success of FASER$\nu$ may well lay the groundwork for large-scale neutrino studies at future collider experiments, heralding a new era of inquiry into fundamental particle interactions.