SHiP

SHiP

Overview

The Search for Hidden Particles (SHiP) is a proposed fixed-target experiment at the Super Proton Synchrotron intended to probe portals to physics beyond the Standard Model and to search for feebly interacting particles predicted in extensions such as the Higgs-portal, axion-like scenarios, and heavy neutral leptons. Conceived within the context of European particle-physics initiatives, the project aims to exploit the high-intensity, slow-extracted proton beam of the CERN facility to produce rare long-lived states from interactions on a dense target, with a downstream detector system optimized for displaced decays and weakly interacting particles. SHiP bridges efforts from intensity-frontier programs exemplified by experiments at the Brookhaven National Laboratory, Fermilab, and proposals associated with the European Strategy for Particle Physics.

Physics Goals

SHiP targets several theoretically motivated portals and candidate particles: heavy neutral leptons related to the seesaw mechanism and baryogenesis via leptogenesis, light dark-sector mediators coupling through the vector portal such as dark photons, scalar mediators connected to the Higgs portal, and pseudo-Nambu–Goldstone bosons including axion-like particles postulated in Peccei–Quinn theory. The experiment also aims to search for light dark matter candidates produced in proton-target collisions and to test models that address anomalies reported by experiments like NuTeV and measurements related to the muon g−2 tension. By targeting masses in the MeV–GeV range and lifetimes corresponding to macroscopic decay lengths, SHiP complements collider searches at the Large Hadron Collider and precision flavor experiments such as LHCb and Belle II.

Experimental Design

The facility design uses a high-density target and a hadron absorber followed by a muon shield to reduce backgrounds from prompt charged particles and penetrating muons produced in the target. Protons from the Super Proton Synchrotron strike a heavy target, producing a flux of secondary mesons (including pion, kaon, and charm hadrons) whose decays can produce hidden-sector states. A long evacuated decay volume downstream provides a low-background region for displaced decays into visible Standard Model final states. Background mitigation strategies are informed by experience from beam-dump experiments such as CHARM, NuCal, and modern designs like NA62 in beam-dump modes. The conceptual layout balances acceptance for long-lived particle decays, shielding constraints similar to those considered for the CERN Neutrinos to Gran Sasso program, and integration with existing CERN infrastructure.

Detector Components

Key instrument systems include a magnetic spectrometer incorporating tracking stations and precision timing detectors to reconstruct charged-particle trajectories from displaced vertices, electromagnetic and hadronic calorimetry for energy measurement and particle identification, and muon identification systems modeled on designs used in ATLAS, CMS, and LHCb. A dedicated upstream veto and timing layer suppresses backgrounds from residual upstream activity, while the vacuum decay vessel reduces interactions with air. Particle identification makes use of ring-imaging Cherenkov techniques analogous to those in COMPASS and Belle II, and silicon pixel or strip trackers benefit from developments pioneered for the ALICE and ATLAS inner detectors. Radiation-hard readout electronics and triggerless or streaming data acquisition architectures draw on technology transfer from CERN experiments and from upgrades implemented at Fermilab facilities.

Data Analysis and Sensitivity

Analysis strategies focus on the identification of displaced vertices with minimal upstream activity, employing multivariate classifiers and likelihood-based fits to separate signal topologies from residual background sources including random crossings, neutrino interactions, and rare Standard Model decays. Sensitivity projections are derived from full simulation chains incorporating beamline particle production models benchmarked to results from NA61/SHINE, hadronic production measurements from HARP, and charm-production data informed by SELEX. Expected exclusion and discovery reaches cover large regions of parameter space for heavy neutral leptons, dark photons, scalar mediators, and axion-like particles when evaluated against benchmark models developed in the theoretical community working on portals and hidden sectors associated with groups such as those at CERN Theory Department and universities involved in flavor physics.

Collaboration and Timeline

The SHiP collaboration comprises institutes and laboratories across Europe and worldwide, including participants from national laboratories and universities with experience in fixed-target, neutrino, and collider experiments. The collaboration interacts with coordination bodies like the CERN management and the European Strategy for Particle Physics process for prioritization. Project milestones include conceptual design reviews, technical design reports, and integration studies with the Super Proton Synchrotron beam schedule and facility upgrades. Timelines have aimed for construction and commissioning phases contingent on community prioritization, funding approvals from national agencies, and alignment with CERN accelerator availability windows shaped by operations of the Large Hadron Collider and its upgrade programs.

Impact and Related Experiments

SHiP would complement discovery potential pursued by intensity-frontier and collider searches including LHCb, Belle II, dedicated beam-dump concepts at J-PARC, and dark-sector initiatives like SeaQuest and proposed experiments at Fermilab (for example, SHiP-like proposals and successor projects). The experiment promises to inform theoretical efforts on neutrino mass generation, dark matter model-building, and solutions to outstanding anomalies, while contributing techniques applicable to future long-lived particle searches at the High-Luminosity Large Hadron Collider and proposed facilities such as the Future Circular Collider and long-baseline neutrino programs like DUNE.

Category:Particle physics experiments