NO LESS: Novel Opportunities for Light Exotic Searches at the SPS

TL;DR

The paper evaluates how reconfiguring the NA62 detector complex for a future CERN ECN3 beam-dump facility (BDF) can probe feebly interacting particles in the MeV–GeV range. Using the Alpinist framework, it compares multiple detector geometries from minimal to SHiP-like configurations, under a zero-background assumption, to quantify sensitivity to dark photons, Higgs-like scalars, heavy neutral leptons, and axion-like particles. The key finding is that even modest detector rearrangements (BDF0/BDF1) yield competitive reach, while a full SHiP-like setup (BDF4) offers the strongest overall sensitivity, particularly for forward-produced FIPs and long decay volumes. This work informs detector-design choices and demonstrates the potential of leveraging existing NA62 infrastructure to advance FIP searches shortly after LS3.

Abstract

A powerful way to test models with feebly interacting particles in the MeV to GeV mass range is through proton beam-dump experiments. In this paper, we compare the current sensitivity of CERN's NA62 experiment running in beam-dump mode with that of a hypothetical experiment using the same detectors in a future CERN ECN3 beam-dump facility. When optimising such an experiment, the geometric setup is particularly relevant for the specific new-physics scenario under study, since different production mechanisms can generate different angular distributions of new particles. We show that even the most minimalistic reconfiguration of the existing NA62 experiment's detectors can already provide a very competitive sensitivity and collect data immediately after the beam is available.

Figures (11)

• Figure 1: Location of the detectors and the decay volume along the $z$-axis with respect to the centre of the target in meters for BDF 0 (left) and BDF 3a (right) configurations. The locations of the STRAW stations 2 and 3 are indicated in grey.
• Figure 2: Ratio of the acceptance for setup BDF 3a to that for setup BDF 0, for a dark scalar decaying into $e^+ e^-$ produced by proton bremsstrahlung (left) and by B meson decays (right).
• Figure 3: Location of the detectors and the decay volume along the $z$-axis with respect to the centre of the target in meters for the BDF 4 configuration.
• Figure 4: Exclusion regions at 90% CL in the plane (mass, coupling) for the dark photon (BC1) benchmark. In the absence of a FIP signal, the expected exclusion from NA62 NA62:2025yzs at its final statistics Jerhot:2936260 re-weighted to the updated Bremsstrahlung production is shown as a black line. For comparison, the grey line shows the expected sensitivity for the same statistics but estimated with the toy MC. The expected exclusions of the "minimal post-LS3 scenario" (BDF 0) and the "maximal post-LS3 scenario" (BDF 3a) based on rearrangements of the present NA62 detectors can be compared to that of the originally proposed SHiP design (BDF 4) as brown, red and blue lines, respectively. Since the goal is to focus on the future sensitivity of experiments in ECN3, the sensitivity projections of DarkQuest Apyan:2022tsd, FASER FASER:2022hcn, LHCb Gorkavenko:2023nbk and other experiments operational in the relevant timescale are not displayed.
• Figure 5: Exclusion regions at 90% CL in the plane (mass, coupling) for the Higgs-like scalar (BC4 and BC5) benchmarks. For details, see the caption of Fig. \ref{['fig:BC1']}.