Beam Dumps and Hidden Sectors: From PS191 to SHiP

The standard way to look for new particles at a high-energy facility is the collider configuration: bring two beams into collision and measure the produced particles’ decay products. This is what the LHC does and what most discovery-oriented experiments are organised around. But there is an alternative configuration that targets a fundamentally different physics regime: the beam dump.

In a beam-dump experiment, an intense proton beam is directed into a thick passive target — tens of metres of dense material engineered to absorb essentially all produced particles within the target volume. Most of the energy goes into hadronic and electromagnetic cascades that thermalise inside the dump. The very few particles that emerge are the long-lived neutral ones: neutrinos, of course, but also any hypothetical hidden-sector particles whose weak coupling to ordinary matter lets them traverse the dump without interacting. A downstream detector measures the surviving flux.

The technique trades cross-section sensitivity (the production rate in a thick dump is far lower than in a clean-collision facility) for filter quality (the dump suppresses backgrounds by twelve to fifteen orders of magnitude). The trade-off is well-matched to searches for particles with very small couplings to the Standard Model and decay lifetimes of microseconds to milliseconds — exactly the parameter space where heavy neutral leptons (HNLs), dark photons, axion-like particles (ALPs), and other hidden-sector candidates are expected to live.

This post is about how beam-dump experiments work, what the historical search programme has constrained, and what the next generation — SHiP at CERN being the flagship — will reach.

The basic principle

A proton beam at energy $E_p$ striking a thick target produces a hadronic cascade. At GeV-scale proton energies, the dominant secondaries are pions and kaons; at the higher beam energies of the SPS or LHC, charmed mesons and B mesons become abundant. Charmed-meson decays and tau-lepton production from kaon and pion decay provide the rare sources of heavy-mass states.

Most produced particles either interact further in the dump or decay to short-lived secondaries that also interact. The surviving particles after the dump are:

• Neutrinos at the kaon and pion-decay branching ratios.
• Photons from rare processes that escape the dump.
• Hypothetical hidden-sector particles with cross-sections much smaller than weak: heavy neutral leptons mixing with ordinary neutrinos at the level of $|U{|}^2\sim 10^{-8}$ to $10^{-10}$, dark photons with kinetic-mixing parameters $ϵ\sim 10^{-5}$ or smaller, ALPs with couplings to gauge bosons.

The dump is followed by a decay volume — typically a tens-of-metres-long evacuated region — where the hidden-sector particles, if they exist, can decay before reaching the spectrometer. A heavy neutral lepton with mass 1 GeV and lab-frame Lorentz factor $\gamma \sim 100$ has a decay length

$L_{\mathrm{decay}}=\beta \gamma c\tau \sim 30\mathrm{m}\cdot {\left(\frac{|U{|}^2}{10^{-8}}\right)}^{-1}{\left(\frac{1\mathrm{GeV}}{m_N}\right)}^5$

so the decay volume length matches the sensitivity to the mixing parameter at the level the experiment targets.

The detector at the end of the decay volume looks for the visible decay products of the hidden-sector particles — typically a charged lepton pair plus possibly a charged hadron, with the parent’s invariant mass reconstructable from the decay-product kinematics. The signature is essentially background-free given a sufficiently long upstream dump.

PS191: the original

PS191 ran at CERN in the early 1980s as the first dedicated beam-dump experiment optimised for heavy-neutral-lepton searches. It used the 19.2 GeV proton beam from the CERN Proton Synchrotron, delivered $\sim 10^{19}$ protons-on-target over its run, and viewed the decays of any produced HNLs in a 12-metre-long decay volume followed by a charged-particle spectrometer.

The dominant production mechanism at PS191’s beam energy was charged-pion and charged-kaon decay: ${\pi }^{+}\to {\mu }^{+}+{\nu }_{\mu }$ and $K^{+}\to {\mu }^{+}+{\nu }_{\mu }$ both have a small branching ratio for the muon-neutrino to be replaced by a heavy partner of the same flavour, with the branching modified by the mixing parameter $|U_{\mu N}{|}^2$. The HNL produced then decays back to detectable particles via the same mixing.

PS191 set the first laboratory limits on $|U_{\mu N}{|}^2$ in the mass range 0.05 to 0.4 GeV, reaching values around $10^{-7}$ depending on the mass. These limits remained the world’s best for nearly two decades and are still cited in compilations of HNL constraints.

NuTeV and the GeV scale

In the 1990s, NuTeV at Fermilab used the 800 GeV Tevatron proton beam (later 120 GeV from the Main Injector) to extend the HNL search into higher-mass and lower-mixing regions. The much higher beam energy enabled the production of heavier states up to the few-GeV scale, accessing parts of parameter space PS191 could not reach.

NuTeV’s primary physics was precision neutrino-nucleon deep inelastic scattering, but its detector configuration also allowed dedicated HNL searches in the decay-volume mode. The resulting limits, combined with PS191 at lower energies, define the current laboratory constraints on the HNL mixing parameter as a function of mass.

The current landscape

A handful of contemporary experiments contribute to the broader hidden-sector search programme.

NA62 at CERN, primarily designed for ultra-rare kaon decays, has also produced HNL constraints by analysing its high-statistics kaon-decay sample for the corresponding HNL signatures. NA62 reached limits comparable to PS191 in the GeV mass range with substantially better systematic control.

T2K and NOvA near-detector data have been reanalysed for HNL signatures in tau and muon-neutrino decays, contributing modest improvements over the historical limits.

LHCb in dedicated runs has searched for displaced-vertex signatures of long-lived particles, including HNLs from B-meson decays. The displaced-vertex topology in LHCb’s vertex detector is exquisitely sensitive to lifetimes in the picosecond-to-nanosecond range, complementing the beam-dump approach at shorter lifetimes.

FASER at the LHC, in the very-forward direction of ATLAS, accesses long-lived particles produced in proton-proton collisions whose decay products travel through hundreds of metres of rock before reaching FASER’s spectrometer. The same long-lived sensitivity that detects FASER’s forward-physics neutrinos also enables HNL searches at GeV-scale masses.

These together have tightened the HNL bounds across the relevant parameter space, but the most powerful future improvements will come from dedicated next-generation beam-dump facilities.

SHiP: the flagship future facility

SHiP — Search for Hidden Particles — is the proposed CERN beam-dump experiment that would push the hidden-sector frontier by several orders of magnitude. The proposal, originally submitted in 2013, has gone through extensive technical refinement and was approved for preliminary construction in 2023.

SHiP uses the 400 GeV proton beam from the CERN SPS, delivering $2\times 10^{20}$ protons-on-target over a planned five-year run. The dump is 30 metres of molybdenum followed by 50 metres of tungsten and iron shielding. The decay volume is 50 metres long and 5 metres in radius, evacuated to limit interaction backgrounds. The downstream detector includes a spectrometer with magnetised tracking, electromagnetic and hadronic calorimetry, and muon identification.

The expected production rates at SHiP are unprecedented. The 400 GeV beam in the dump produces approximately $10^{17}$ to $10^{18}$ charmed mesons over the run — orders of magnitude more than any previous beam-dump experiment. Charmed-meson decays are the dominant source of GeV-scale HNL production through their semileptonic and leptonic channels.

SHiP’s projected sensitivity to the HNL mixing parameter reaches $|U{|}^2\sim 10^{-10}$ to $10^{-12}$ across the relevant mass range — three to four orders of magnitude beyond current limits. This is the parameter space favoured by low-scale leptogenesis and resonant baryogenesis scenarios where right-handed neutrinos with masses in the GeV range produce the baryon asymmetry of the universe through CP-violating decays in the early universe.

A discovery at SHiP would constitute the first laboratory confirmation of a heavy neutral lepton — a direct test of the seesaw mechanism — and would tie the laboratory result to the cosmological matter-antimatter asymmetry.

Other hidden-sector targets at SHiP

Beyond HNLs, SHiP’s clean signature is well-matched to several other hidden-sector candidates:

Dark photons produced by Bremsstrahlung off the target and decaying to lepton pairs in the decay volume. SHiP would push the mass-mixing sensitivity by orders of magnitude into the parameter space relevant to dark-matter-mediator scenarios.

Axion-like particles produced through gluon and photon couplings, decaying to gauge-boson pairs in the decay volume. SHiP would extend the ALP sensitivity in coupling-mass space significantly.

Light dark-matter particles produced in association with visible Standard-Model particles and detected through their scattering in a dedicated downstream module. Several SHiP physics analyses target this channel.

The breadth of the physics programme is one of SHiP’s distinctive features: the same facility addresses multiple beyond-Standard-Model scenarios with a single beam-dump configuration.

The complementary picture

The HNL and broader hidden-sector search programme has multiple complementary approaches:

• Beam-dump experiments (PS191, NuTeV, SHiP) probe long-lived neutral particles at GeV scales with hadronic-cascade production.
• Collider displaced-vertex searches (LHCb, ATLAS, CMS, FASER) probe shorter lifetimes through the displaced-vertex signature at higher production rates.
• Lepton-collider experiments (Belle II, future FCC-ee) probe the same parameter space through clean-collision processes.
• Future facility proposals (CODEX-b, MATHUSLA, ANUBIS) extend the displaced-vertex sensitivity to longer lifetimes.

Each technique covers a different part of the lifetime-coupling parameter space. The combination is broadly complementary, with SHiP being the most ambitious dedicated beam-dump effort and the LHC-based displaced-vertex experiments providing complementary coverage at smaller couplings and lifetimes.

For neutrino physics, the most important specific target is the HNL parameter space at masses $m_N\sim 0.1$ to $10$ GeV and mixings $|U{|}^2\sim 10^{-9}$ to $10^{-12}$, which is the region predicted by the $\nu$MSM minimal extension to the Standard Model that explains neutrino masses, dark matter, and baryogenesis simultaneously. SHiP’s projected reach covers a substantial fraction of this region.

Summary

Beam-dump experiments search for long-lived weakly-coupled particles by directing intense proton beams into thick passive targets that absorb essentially all ordinary particles, leaving only the very rare hidden-sector candidates to traverse a downstream decay volume. The technique pioneered at CERN’s PS191 in the 1980s established the first laboratory limits on heavy-neutral-lepton mixing in the MeV-to-GeV mass range, and has been refined by NuTeV at Fermilab, NA62 at CERN, FASER at the LHC, and several other current experiments. The flagship next-generation facility, SHiP at CERN, will deliver $2\times 10^{20}$ protons on target over five years to a 30-metre molybdenum dump followed by a 50-metre evacuated decay volume and downstream spectrometer. SHiP’s projected sensitivity reaches HNL mixing parameters $|U{|}^2\sim 10^{-10}$ to $10^{-12}$, several orders of magnitude beyond current limits and into the parameter space favoured by low-scale leptogenesis and the $\nu$MSM scenario for explaining neutrino masses, baryogenesis, and dark matter simultaneously. A discovery at SHiP would constitute the first direct laboratory confirmation of a heavy neutral lepton, tying the laboratory result to the cosmological matter-antimatter asymmetry and providing one of the most consequential beyond-Standard-Model discoveries possible at any current or planned facility.