REVIEW
The LHC is not enough: the LZ High-Recoil Event at FCC-hh and a Muon Collider
High Energy Physics - Phenomenology · Submitted 22 Sep 2026 · 116 references
Pith's verdict: A genuinely useful, unusually candid map of which future colliders can test the LZ-motivated electroweak-multiplet class, worth refereeing and citing — but the headline FCC-hh exclusion intervals rest on a rescaling model that fails its own one cross-region check by a factor of 3.5, so those numbers are conditional until a dedicated simulation appears.
T0 · AI review
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Referee report
The paper
The paper claims
The paper claims the single 248 keV nuclear-recoil event seen by LUX-ZEPLIN, if it is inelastic electroweak dark matter, cannot be tested at the HL-LHC but could be tested at a 100 TeV FCC-hh and at a 10-30 TeV muon collider, and that…
The reading
What carries the argument
Two objects carry the argument. The first is the yield-rescaling model of Eq. (23), which transfers the published wino/Higgsino disappearing-track study of Ref. [63] to all other multiplets by rescaling the signal with gauge couplings, quark-antiquark parton luminosities, mass-dependent fermion-versus-scalar pair cross sections, and the geometric decay-in-track probability P_track = exp(-r_min/lambda_T) - exp(-r_max/lambda_T); the selections and background estimates are kept unchanged. The second is the charged-state sum rule of Eq. (16), Delta_1 + Delta_2 = 2 delta_g + delta_m0, which within the leading splitting-operator effective field theory bounds the lightest charged splitting of any multiplet containing both Q = +1 and Q = -1 states to at most about 167 MeV, thereby setting the long-lived-particle lifetimes that the track searches exploit.
Extended reading notes
Core claim
Within a benchmark class of single approximately pure electroweak multiplets (Dirac fermions or complex scalars with nonzero hypercharge, protected by an exact Z2, with dominant inelastic Z exchange and the full dark-matter abundance set by thermal freeze-out), the LZ event's collider consequences are generic rather than supersymmetric. The neutral splitting required by the recoil lies below the electron-pair threshold for every candidate, so the heavier neutral state decays invisibly; production is governed by gauge couplings; and thermal masses range from 0.58 to 11.5 TeV. Applying the charged-state sum rule of Eq. (16), the lightest charged splitting of the larger multiplets is bounded above near 167 MeV, which motivates long-lived charged partners. Using a validated rescaling model of the signal yield, the paper reports that FCC-hh at 100 TeV with 30 $ab^{-1}$ covers the radiative doublets (adopting the published fermionic reach), excludes the fermionic quadruplet over roughly 70-167 MeV in the single-branch estimate at the lower normalization, covers the fermionic triplet through a lifetime-independent Drell-Yan measurement, and covers the scalar triplet, scalar quadruplet and fermionic quintuplet over more limited charged-splitting intervals extending below the pion mass. Published muon-collider projections cover the fermionic doublet and triplet at 10 TeV and the full fermionic family near 14 and 30 TeV; only the scalar quintuplet escapes every channel examined.
Load-bearing premise
The FCC-hh coverage numbers come from assuming that the published wino/Higgsino disappearing-track selections and backgrounds transfer unchanged to the heavier, longer-lived multiplets, so if the tracking efficiency or backgrounds do not transfer, the reported splitting intervals would shrink or vanish.
Editorial extensions
If this is right
- If the LZ signal persists and survives the solar-capture and sideband constraints, testing the thermal electroweak interpretation would motivate an energy-frontier machine beyond the LHC, with the choice between FCC-hh and a multi-TeV muon collider made concrete by the coverage maps in the paper.
- FCC-hh can decide the fermionic doublet and the fermionic triplet even when the charged state is prompt and leaves no track, because the Drell-Yan and monojet channels do not depend on the charged lifetime.
- A 10 TeV muon collider, the current design baseline, settles the fermionic doublet and triplet regardless of the unknown charged spectrum, while roughly 14 and 30 TeV extend coverage to the quadruplet and quintuplet.
- The fiducial targets require tracking of charged particles with decay lengths from centimeters to meters, which translates into a concrete inner-tracker design requirement at FCC-hh.
- Low-energy electron-positron machines with center-of-mass energy at or below 500 GeV cannot pair-produce any of the eight thermal candidates, so they cannot directly test this interpretation.
Reading between the lines
- The FCC-hh coverage figures are only as strong as the unverified assumption that the published selections and backgrounds transfer to the heavier, longer-lived candidates; the paper's own stated factor-3.5 normalization gap between the two signal regions marks the size of that risk.
- A dedicated FCC-hh study that retains both charged mass eigenstates and their cascades near degeneracy, which the paper explicitly does not implement, could move or erase the ~167 MeV endpoints for the quadruplet and quintuplet.
- If the LZ event turns out to be a background fluctuation, the same collider program still stands as a generic test of thermal electroweak dark matter, because the mass and production predictions come from freeze-out rather than from the event itself.
- The paper's claim that heavier multiplets are out of reach at the HL-LHC rests on production rates only; lighter, subdominant multiplets would be easier to find, so the negative HL-LHC statement is specific to the full-abundance thermal assumption.
Original abstract
The high-energy nuclear-recoil candidate reported by LUX-ZEPLIN could point to dark matter beyond the reach of the LHC. We examine how future colliders could test this interpretation, starting with the thermal Higgsino and extending to a broad class of electroweak multiplets containing neutral and charged particles. We assume an approximately pure multiplet protected by an exact stabilizing symmetry, with dominant inelastic Z exchange and the full dark-matter abundance set by gauge-dominated thermal freeze-out, where mixing and additional interactions affecting production or freeze-out are negligible. In this setting, heavy thermal masses, rare production and invisible neutral transitions make the Higgsino's main collider obstacles generic across the class, independently of supersymmetry. Charged lifetimes vary, but the benchmark spectra studied, including radiative doublet splittings, remain beyond the projected HL-LHC reach. We show how FCC-hh and a high-energy muon collider can overcome these obstacles. Our FCC-hh estimates show sensitivity to the doublets and several larger multiplets through disappearing charged tracks and complementary channels that do not depend on the charged lifetime. Muon-collider projections cover the fermionic doublet and triplet at 10 TeV and the full fermionic family considered here at 30 TeV, subject to the stated spectrum and systematic assumptions. This connects a possible underground signal to a concrete program for testing thermal electroweak dark matter at future colliders.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The high-energy nuclear-recoil candidate reported by LUX-ZEPLIN could point to dark matter beyond the reach of the LHC. We examine how future colliders could test this interpretation, starting with the thermal Higgsino and extending to a broad class of electroweak multiplets containing neutral and charged particles. We assume an approximately pure multiplet protected by an exact stabilizing symmetry, with dominant inelastic Z exchange and the full dark-matter abundance set by gauge-dominated thermal freeze-out, where mixing and additional interactions affecting production or freeze-out are negligible. In this setting, heavy thermal masses, rare production and invisible neutral transitions make the Higgsino's main collider obstacles generic across the class, independently of supersymmetry. Charged lifetimes vary, but the benchmark spectra studied, including radiative doublet splittings, remain beyond the projected HL-LHC reach. We show how FCC-hh and a high-energy muon collider can overcome these obstacles. Our FCC-hh estimates show sensitivity to the doublets and several larger multiplets through disappearing charged tracks and complementary channels that do not depend on the charged lifetime. Muon-collider projections cover the fermionic doublet and triplet at 10 TeV and the full fermionic family considered here at 30 TeV, subject to the stated spectrum and systematic assumptions. This connects a possible underground signal to a concrete program for testing thermal electroweak dark matter at future colliders.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The high-energy nuclear-recoil candidate reported by LUX-ZEPLIN could point to dark matter beyond the reach of the LHC. | background 0.97 | 0.06 | 0.12 | hedged possibility 1.00 |
| s1 | We examine how future colliders could test this interpretation, starting with the thermal Higgsino and extending to a broad class of electroweak multiplets containing neutral and charged particles. | contribution 0.95 | 0.69 | 0.06 | asserted 0.68 |
| s2 | We assume an approximately pure multiplet protected by an exact stabilizing symmetry, with dominant inelastic Z exchange and the full dark-matter abundance set by gauge-dominated thermal freeze-out, where mixing and additional interactions affecting production or freeze-out are negligible. | method 0.92 | 0.25 | 0.04 | asserted 0.93 |
| s3 | In this setting, heavy thermal masses, rare production and invisible neutral transitions make the Higgsino's main collider obstacles generic across the class, independently of supersymmetry. | result 0.49 | 0.66 | 0.08 | asserted 1.00 |
| s4 | Charged lifetimes vary, but the benchmark spectra studied, including radiative doublet splittings, remain beyond the projected HL-LHC reach. | result 0.92 | 0.76 | 0.06 | asserted 0.99 |
| s5 | We show how FCC-hh and a high-energy muon collider can overcome these obstacles. | contribution 0.93 | 0.74 | 0.32 | asserted 0.99 |
| s6 | Our FCC-hh estimates show sensitivity to the doublets and several larger multiplets through disappearing charged tracks and complementary channels that do not depend on the charged lifetime. | result 0.89 | 0.91 | 0.22 | asserted 1.00 |
| s7 | Muon-collider projections cover the fermionic doublet and triplet at 10 TeV and the full fermionic family considered here at 30 TeV, subject to the stated spectrum and systematic assumptions. | result 0.93 | 0.84 | 0.17 | asserted 0.94 |
| s8 | This connects a possible underground signal to a concrete program for testing thermal electroweak dark matter at future colliders. | interpretation 0.96 | 0.59 | 0.58 | asserted 0.89 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:2609.26870 by the authors.
References
Discussion
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