Can the LZ nuclear-recoil event be explained by dark matter without LHC conflict?

A 248 keV recoil sitting 1.5σ from the nuclear-recoil band

The LUX-ZEPLIN experiment reported a single event with a recoil energy of 248±23 (stat)±23 (syst) keV, lying about 1.5σ away from the nuclear-recoil band. The paper notes this event is difficult to accommodate within the expected background, which is what makes a dark matter interpretation worth testing. The same paper records that the event carries a global significance of 2.6σ, while the Lagrangians it studies were selected because they reach a local significance of 3σ or higher for at least one dark matter mass.

Those two numbers answer different questions and should not be swapped. The 2.6σ figure is the global significance of the observed event; the 3σ-or-higher criterion is a local significance threshold used to pick which effective interactions are worth carrying into the collider analysis. The paper also states that the absence of additional events at lower recoil energies around the keV scale disfavors conventional elastic scattering with momentum-independent interactions, which is why momentum-dependent and spin-dependent operators matter here.

Momentum-dependent operators suppress low-energy recoils

If dark matter scatters elastically off nucleons through a momentum-independent interaction, the same coupling that produces a 248 keV recoil should also produce events at lower recoil energies, and those are not seen. The paper's route around this is momentum dependence: interactions whose scattering rate falls at low momentum transfer can still reproduce the observed high-energy recoil while staying quiet where the data are empty. The authors frame this as the elastic alternative to inelastic interpretations, where a dark-sector mass splitting suppresses low-energy recoils through kinematics instead.

The paper restricts its elastic analysis to Lagrangians that yield a local significance of 3σ or higher for some dark matter mass, drawn from the 20 possible effective Lagrangians for Dirac fermionic dark matter listed in the non-relativistic effective theory of direct detection. Preferred dark matter masses in these fits typically lie in the TeV range, which is precisely the scale the LHC can probe.

From Lagrangian to a preferred LZ band

The direct-detection side starts from relativistic Lagrangians written with a cutoff scale Λ and coefficients d, then performs a non-relativistic expansion retaining leading terms in momentum transfer. This produces the standard set of non-relativistic operators built from Galilean-invariant and Hermitian blocks, and the dark matter–nucleus transition probability separates into dark matter response functions and nuclear response functions. For elastic scattering off an approximately charge-parity and parity even ground state, the possible nuclear responses reduce to eight independent types including interference terms, and the authors adopt updated nuclear structure calculations implemented in dmscatter.

The differential rate integrates over the dark matter velocity distribution, assumed Maxwell-Boltzmann with a cutoff at the escape velocity, using a local dark matter density of 0.3 GeV/cm^3 and astrophysical parameters fixed to recommended conventions. Detector effects are folded in with an energy resolution parameterized as ΔEr = 1.46 sqrt(Er/keV) keV, inferred from the 23 keV statistical uncertainty at 248 keV, together with a 2.84 ton-year exposure and a detection efficiency. The resulting best-fit regions, bounded by red dashed lines in the paper's figures, are what the authors call the LZ band. In the direct-detection analysis Λ is fixed to 246.2 GeV and the nucleon mass to 0.938 GeV, so the coefficients act as proxies for the energy scale probed by direct detection.

Recasting ATLAS monojet data onto nucleon operators

On the collider side the conventions change, and this matters for reading the results. The authors set the coefficients to 1 and take the nucleon mass equal to Λ, so Λ becomes a new physics parameter alongside the dark matter mass rather than a fixed normalization. The relevant dark matter–quark interactions are implemented in FeynRules, with quark-level couplings chosen to reproduce the intended isoscalar and isovector structures at the nucleon level; matching for scalar, pseudoscalar, vector, and axial-vector currents follows standard prescriptions, while tensor-current interactions are matched using nucleon matrix elements of the quark tensor current.

The constraint itself comes from recasting the ATLAS search for new phenomena in events with an energetic jet and missing transverse momentum in proton-proton collisions at a center-of-mass energy of 13 TeV, based on 139 fb^-1 of integrated luminosity. The paper notes that CMS has performed a similar search at 13 TeV with 101 fb^-1, but the analysis here uses the ATLAS result. The signature is monojet plus missing transverse energy, which is what dark matter pair production recoiling against a jet would look like.

LHC limits push viable masses above roughly a TeV

For most of the effective interactions considered, the paper finds that LHC constraints exclude the dark matter interpretation of the LZ event for dark matter masses below roughly a TeV, subject to the validity of the effective field theory. In the representative case of the isovector Lagrangian L10^v, the LZ band marks the parameter space consistent with one observed event, while the region above the green line is ruled out at 95% confidence level by the collider analysis. The blue dotted line at mχ = Λ serves as an indicative boundary for effective field theory validity, with the theory expected to remain valid below it.

The paper describes the resulting preference as dark matter masses above a few TeV, disfavoring elastic-scattering interpretations of the LZ event below a few TeV. It reports similar outcomes for most of the other effective Lagrangians, summarized in its table. The authors also state that for dark matter masses below a few TeV these constraints are orders of magnitude stronger than existing direct-detection bounds, and that the corresponding parameter space may be extremely challenging to probe even in future direct-detection experiments.

First LHC constraints on explicit momentum-dependent Lagrangians

The paper emphasizes two claims of novelty. First, it presents what the authors describe as the first study of current LHC constraints on explicitly momentum-dependent Lagrangians, including L10, in this context. Second, it argues that the complementarity between direct detection and collider constraints had not previously been demonstrated for the recently reported LZ event. The authors also note that the strong LHC constraints are qualitatively consistent with expectations from earlier studies of collider probes of momentum- and spin-dependent interactions, so the surprise is not the existence of strong limits but their application to this specific event.

The broader implication the authors draw is that the result extends beyond interpreting the LZ event itself. Because the operators favored by the LZ event are predominantly momentum- and spin-dependent, and because the collider limits on them are severe at sub-TeV masses, the same parameter space becomes hard to reach with direct detection alone. That reframes the question from whether LZ saw dark matter to which experimental frontier can still test the operators that would explain it.

The exclusion holds only where the EFT remains valid

The central exclusion is conditional, and the condition is stated in the result itself: the LHC constraints exclude the dark matter interpretation for masses below roughly a TeV subject to effective field theory validity. The paper's own validity marker is the line at mχ = Λ, below which the effective description is expected to hold. Readers should therefore treat the sub-TeV exclusion as an EFT-scoped statement rather than a model-independent one, and should not extend it to masses or mediator scenarios the paper does not analyze.

Two further boundaries are worth keeping in view. The analysis is restricted to elastic scattering of Dirac fermionic dark matter, and it is restricted to Lagrangians reaching a local significance of 3σ or higher for some dark matter mass; inelastic interpretations, which the paper explicitly sets aside, are not constrained by this collider recasting. The collider side also uses a single ATLAS monojet search at 13 TeV with 139 fb^-1, so the reach quoted here is tied to that dataset and that signature rather than to the LHC in general.

Sub-TeV elastic explanations are squeezed from both sides

The practical upshot is a threshold rather than a verdict. If the LZ event is elastic dark matter scattering through one of the effective interactions studied here, the viable dark matter mass sits above roughly a TeV, because below that the monojet search already excludes the required parameter space. That is the comparison readers should carry away: not direct detection alone, and not colliders alone, but the overlap region where the LZ band and the ATLAS exclusion meet.

The most consequential detail is that for many of these interactions the collider bound is orders of magnitude stronger than existing direct-detection bounds below a few TeV, which means future direct-detection experiments may struggle to probe the same parameter space. The paper's own priority is therefore the momentum-dependent operators, and L10 in particular, where the LHC supplies the leading constraint on an interpretation that direct detection cannot yet close.

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