[Paper Review] SHADOWS (Search for Hidden And Dark Objects With the SPS)

W. Baldini, A. Balla+21 more|arXiv (Cornell University)|Oct 15, 2021

Dark Matter and Cosmic Phenomena18 references12 citations

TL;DR

SHADOWS proposes a beam-dump experiment at CERN's SPS using a 400 GeV proton beam to search for feebly-interacting particles (FIPs) in the MeV–GeV mass range, leveraging the existing P42 beam line and NA62 infrastructure. It aims to probe previously unexplored parameter space, improving sensitivity to FIP couplings by 1–4 orders of magnitude compared to current limits, with potential discovery of dark sector particles or light mediators.

ABSTRACT

We propose a new beam-dump experiment, SHADOWS, to search for a large variety of feebly-interacting particles possibly produced in the interactions of a 400 GeV proton beam with a high-Z material dump. SHADOWS will use the 400 GeV primary proton beam extracted from the CERN SPS currently serving the NA62 experiment in the CERN North area and will take data off-axis when the P42 beam line is operated in beam-dump mode. SHADOWS can accumulate up to a ~2 x10^19 protons on target per year and expand the exploration for a large variety of FIPs well beyond the state-of-the-art in the mass range of MeV-GeV in a parameter space that is allowed by cosmological and astrophysical observations. So far the strongest bounds on the interaction strength of new feebly-interacting light particles with Standard Model particles exist up to the kaon mass; above this threshold the bounds weaken significantly. SHADOWS can do an important step into this still poorly explored territory and has the potential to discover them if they have a mass between the kaon and the beauty mass. If no signal is found, SHADOWS will push the limits on their couplings with SM particles between one and four orders of magnitude in the same mass range, depending on the model and scenario.

Motivation & Objective

• To explore the parameter space of feebly-interacting particles (FIPs) in the MeV–GeV mass range, particularly above the kaon mass, where existing constraints weaken.
• To search for dark matter candidates, axion-like particles (ALPs), heavy neutral leptons, and light scalars that may explain cosmological and particle physics puzzles.
• To exploit the high-intensity 400 GeV proton beam from the CERN SPS in beam-dump mode to achieve up to 2×10^19 protons on target per year.
• To significantly improve existing limits on FIP couplings with Standard Model particles by 1–4 orders of magnitude in the 1–5 GeV range.
• To establish a new experimental frontier in FIP searches using existing accelerator infrastructure and detector technologies.

Proposed method

• Utilize the P42 beam line at CERN SPS, currently serving NA62, in beam-dump mode with a high-Z target to produce FIPs via proton interactions.
• Deploy a four-station muon system with scintillating tiles and SiPM readout to achieve 250–300 ps time resolution per tile and >99.8% detection efficiency.
• Implement a magnetic muon sweeping system and detector-based background suppression to reduce muon-induced backgrounds.
• Use iron filters (~3.0λ_I thick) to absorb secondary particles and enhance signal purity in the decay region.
• Integrate an upstream veto system with high-granularity scintillating tiles to tag residual muons and reduce background.
• Design a multi-component detector including tracking, timing, electromagnetic and hadronic calorimeters, and a segmented muon system for full event reconstruction.

Experimental results

Research questions

• RQ1 Can SHADOWS discover new feebly-interacting particles (FIPs) with masses between the kaon and beauty quark masses, where current constraints are weak?
• RQ2 To what extent can SHADOWS improve the sensitivity to coupling strengths of FIPs with Standard Model particles in the MeV–GeV range?
• RQ3 How effectively can the combination of beam-dump operation, magnetic muon sweeping, and active veto systems suppress muon-induced backgrounds?
• RQ4 What is the achievable physics reach of SHADOWS for dark sector particles such as axion-like particles (ALPs), light scalars, and heavy neutral leptons?
• RQ5 Can SHADOWS probe models addressing the strong CP problem, neutrino mass generation, and baryon asymmetry through FIP production and detection?

Key findings

• SHADOWS can accumulate up to 2×10^19 protons on target per year, enabling high-precision searches in the MeV–GeV FIP mass range.
• The experiment is expected to improve limits on FIP couplings with SM particles by 1–4 orders of magnitude, depending on the model and scenario.
• A time resolution of 250–300 ps per scintillating tile with SiPM readout has been demonstrated in prototype tests at INFN Frascati and Bologna.
• The muon system, using 16- or 32-tile modules in a staggered chess-like layout, ensures minimal dead zones and balanced mechanical load.
• Background suppression is achieved through a combination of magnetic muon sweeping and active veto systems, reducing muon-induced backgrounds to manageable levels.
• The tentative schedule targets installation during Long Shutdown 3, with a pilot run at the start of Run 4 and integration of 5×10^19 protons on target by Run 5.