Accelerator-Based Neutrino Beams

TL;DR

Accelerator-based neutrino beams enable controlled studies of neutrino oscillations, including $\delta_{CP}$-driven CP violation and mass ordering, through horn-focused conventional beams and long-baseline experiments. The paper details beamline components, high-fidelity simulations, and in situ flux constraints that underpin precise flux predictions and systematic control. It catalogs historical, current, and planned facilities (BNL, CERN, Fermilab, T2K, NuMI, BNB, LBNF/DUNE) and discusses upgrades and future concepts (DAE$\delta$ALUS, IsoDAR, MOMENT, ESSnuSB, NuSTORM, neutrino factories, ENUBET) that could push sensitivity and explore new physics. Together, these developments position accelerator-based beams at the forefront of measuring CP violation, probing beyond-Standard-Model phenomena, and refining cross-section physics for neutrino interactions.

Abstract

Over the past six decades, accelerator-based neutrino beams have revolutionized particle physics. Neutrinos created with accelerators have been used to discover the muon neutrino and tau neutrinos was discovered and to confirm the existence of neutrino oscillations. More recently, long-baseline experiments have offered the first experimental hint of CP violation in the neutrino sector. Building and operating such beams is an enormous technical challenge, yet they remain our most versatile tool for studying neutrinos. With new experiments such as DUNE and Hyper-Kamiokande, and ideas such as neutrino factories, the next generation of beams will address open questions about neutrino mass ordering, CP violation, and possible physics beyond the standard model.

Figures (5)

• Figure 1: Schematic of the upstream portion of the LBNF neutrino beamline showing the major components. From left to right (in the direction of the beam): the horn-protection baffle, three magnetic focusing horns, and the entrance to the decay pipe. The bulk steel shielding of the target chase is shown in gray and brown, while the water-cooling panels are depicted in light blue. The core elements involved in focusing and transporting the secondary particles toward the decay pipe are shown in this detailed layout. Image courtesy of Fermilab.
• Figure 2: NuMI Horn 1, with a red arrow indicating the current flow direction in FHC mode. Trajectories of pions with different initial momenta. Red: 5 GeV/$c$, green: 10 GeV/$c$, blue: 20 GeV/$c$, black: A 10 GeV/$c$ pion with opposite charge ("wrong-sign")is defocused by the horn running in the FHC mode. The beam travels from left (upstream) to right (downstream) in this figure. Figure adapted from Ref. q6l6-wywy.
• Figure 3: Neutrino flux predictions at a selection of current and future neutrino detectors.
• Figure 4: Muon neutrino flux uncertainties at the DUNE far detector in neutrino mode (top left) and antineutrino mode (top right). Also shown are breakdowns of the hadron production (bottom left) and focusing uncertainties (bottom right) for muon neutrinos in neutrino mode. Figure reproduced from DUNETDR and courtesy Luke Pickering.
• Figure 5: Time line of past, present and future conventional neutrino sources.