[Paper Review] A facility to Search for Hidden Particles (SHiP) at the CERN SPS

SHiP Collaboration, M. Anelli+234 more|arXiv (Cornell University)|Apr 20, 2015

Particle physics theoretical and experimental studies110 references269 citations

TL;DR

This paper proposes the SHiP facility at CERN's SPS accelerator to search for hidden particles—exotic, long-lived, weakly interacting particles predicted by theories beyond the Standard Model. Using a high-intensity 400 GeV proton beam, SHiP employs a dedicated detector system with a long vacuum decay tank and spectrometer to detect rare decays of hidden particles, achieving sensitivity to mixing parameters (U²) down to 10⁻⁹ for masses below 10 GeV/c², surpassing existing and planned experiments by orders of magnitude.

ABSTRACT

A new general purpose fixed target facility is proposed at the CERN SPS accelerator which is aimed at exploring the domain of hidden particles and make measurements with tau neutrinos. Hidden particles are predicted by a large number of models beyond the Standard Model. The high intensity of the SPS 400~GeV beam allows probing a wide variety of models containing light long-lived exotic particles with masses below ${\cal O}$(10)~GeV/c$^2$, including very weakly interacting low-energy SUSY states. The experimental programme of the proposed facility is capable of being extended in the future, e.g. to include direct searches for Dark Matter and Lepton Flavour Violation.

Motivation & Objective

• To explore a broad class of hidden sector particles predicted by models beyond the Standard Model, including long-lived exotic states and low-energy SUSY candidates.
• To address the challenge of detecting very weakly interacting particles with masses below 10 GeV/c², which are inaccessible to collider-based experiments.
• To design a facility capable of probing new physics with high sensitivity, including future extensions to dark matter and lepton flavour violation.
• To provide a dedicated experimental platform with superior background suppression and geometric acceptance compared to existing fixed-target experiments.
• To enable precision measurements of tau neutrino properties and search for rare decays involving hidden particles.

Proposed method

• Utilize the high-intensity 400 GeV proton beam from the CERN SPS to produce exotic particles in a fixed-target interaction region.
• Employ a long vacuum decay tank (up to 100 m) to allow long-lived particles to decay before detection, enabling identification via displaced vertices.
• Implement a spectrometer with particle identification detectors to reconstruct decay products and distinguish signal from background.
• Apply advanced radiation protection and beamline design to ensure safe operation and efficient beam delivery.
• Use a modular detector setup with high granularity and low material budget to maximize detection efficiency and minimize background.
• Leverage existing CERN infrastructure and beam-sharing schemes to minimize cost and maximize feasibility.

Experimental results

Research questions

• RQ1 What is the sensitivity reach of SHiP for detecting long-lived hidden particles with masses below 10 GeV/c² and mixing parameters U² down to 10⁻⁹?
• RQ2 How does SHiP's sensitivity compare to existing fixed-target experiments like NA62 and future facilities such as FNAL's PIP-II beamline?
• RQ3 Can SHiP achieve near-zero background levels for HNL decays, particularly below the kaon mass threshold?
• RQ4 What is the expected yield and detection efficiency for hidden particles decaying into visible final states in a 100 m decay volume?
• RQ5 What are the technical and cost implications of adapting the SPS beamline and infrastructure for a dedicated SHiP facility?

Key findings

• SHiP achieves a statistical sensitivity reach in U² of approximately 10⁻⁹ for hidden neutral leptons (HNLs) with masses below 10 GeV/c², surpassing existing experiments by more than a factor of 15.
• The annual integrated proton intensity at SHiP is estimated at 4×10¹⁴ protons per spill, resulting in a total of 4×10¹⁶ protons per year, which is 40 times higher than NA62’s current limit.
• The geometric acceptance for HNLs in SHiP is about 8 times larger than in NA62 when the detector is moved to 80 m, due to the longer decay path and optimized geometry.
• The sensitivity of SHiP to HNLs is more than 10 times better than that of the FNAL PIP-II beamline, even assuming zero background, due to superior beam intensity and lower background levels.
• The sensitivity of colliding-beam experiments like those at the LHC is estimated to be about 200 times worse than SHiP for HNL detection, due to lower yield and higher background.
• The facility is designed to allow future extensions to direct dark matter searches and lepton flavour violation studies, with a scalable and modular detector architecture.