Complementary Search of Fermionic Absorption Operators at Hadron Collider and Direct Detection Experiments

Instead of the energy recoil signal at direct detection experiments, dark fermion appears as missing energy at hadron colliders. For a fermionc dark sector particle that coupled with quarks and neutrino via absorption operators, its production at collider is accompanied by an invisible neutrino. We study in details the mono-$X$ (photon, jet, and $Z$) productions at the Large Hadron Collider (LHC). We start from the quark-level absorption operators to make easy comparison between the collider and direct detection experiments. In other words, we study the model-independent constraints on a dark fermion with absorption operator. In addition, the interplay and comparison with the possible detection at the neutrino experiments, especially Borexino, is also briefly discussed. We find that light nuclear target can provide the stronger constraints on both spin-dependent and spin-independent absorption operators.

💡 Research Summary

The paper investigates a novel class of interactions between a fermionic dark‑sector particle (denoted χ) and Standard Model (SM) quarks and neutrinos, mediated by so‑called absorption operators. Unlike the usual WIMP paradigm where dark matter particles are produced in pairs and give rise to missing transverse energy (MET) at colliders, the absorption operators allow a single χ to be “absorbed” by a nucleus, converting its mass and kinetic energy into a neutrino and nuclear recoil. This leads to a distinct experimental signature: at colliders the process pp → χ ν + X (with X = γ, jet, or Z) produces a mono‑X final state plus MET, while in low‑energy experiments the same operator induces χ + nucleus → ν + nucleus, i.e. a nuclear recoil accompanied by an invisible neutrino.

The authors adopt an effective field theory (EFT) framework, writing down all dimension‑6 four‑fermion operators that couple a left‑handed neutrino, a Dirac χ (with both chiralities), and a light‑quark bilinear. The five independent Lorentz structures are:

• Scalar (OS): ( \bar q q )( \bar ν_L χ_R )
• Pseudoscalar (OP): ( \bar q iγ5 q )( \bar ν_L χ_R )
• Vector (OV): ( \bar q γμ q )( \bar ν_L γμ χ_L )
• Axial‑vector (OA): ( \bar q γμγ5 q )( \bar ν_L γμ χ_L )
• Tensor (OT): ( \bar q σμν q )( \bar ν_L σμν χ_R )

Each operator carries a cutoff scale Λ_i, which parametrises the mass of the heavy mediator that has been integrated out. The EFT description is valid as long as Λ_i (or the mediator mass) is well above the partonic centre‑of‑mass energy √ŝ. The authors discuss a concrete UV completion involving a dark vector V′ and a scalar mixing parameter Θ, and argue that for perturbative couplings (gχ ≈ gq ≈ 4π) the mixing can be as small as Θ ≳ 6 × 10⁻³ without conflicting with existing bounds.

Collider analysis:
The paper focuses on mono‑X signatures at the LHC (13 TeV, 139 fb⁻¹). The relevant partonic processes are q \bar q → γ χ \bar ν and its charge‑conjugate, as well as analogous processes with an initial‑state jet or a Z boson. Analytic expressions for the differential partonic cross sections are derived (Eqs. 3.1a‑c), showing characteristic collinear (∝ sin θ_γ) and soft (∝ 1/(ŝ − m_X²)) singularities. To regularise these, the authors impose cuts on the photon transverse momentum (p_T^γ > 200 GeV) and on the invariant mass of the χ–ν system (m_X).

Monte‑Carlo simulations are performed with MadGraph5_aMC@NLO interfaced to FeynRules models of the five operators. The dominant irreducible background is pp → γ Z(ν \bar ν). Additional reducible backgrounds (γ W with lost lepton, γ Z with charged leptons, γ + jets with mis‑measured jets) are suppressed by the MET cut. The simulation is validated against the ATLAS mono‑photon analysis by reproducing the missing‑energy spectrum after applying an overall normalisation factor derived from the background comparison. The resulting 95 % CL limits on Λ_i are of order 1 TeV, essentially independent of the dark‑fermion mass for m_χ ≲ 100 GeV. Projections for the High‑Luminosity LHC (3 ab⁻¹) and a possible 27 TeV High‑Energy LHC indicate that the reach could extend to Λ_i ≈ 3–5 TeV, especially if tighter p_T cuts are employed.

Direct‑detection and neutrino‑experiment analysis:
In the low‑energy regime, the same operators mediate χ absorption on nuclei: χ + N → ν + N. The recoil energy spectrum differs from conventional elastic scattering because the outgoing neutrino carries away a sizable fraction of the initial χ mass. The authors compute spin‑independent (SI) cross sections for OS and OV, and spin‑dependent (SD) cross sections for OA and OT, including nuclear form factors. They find that light nuclear targets (e.g., He, C, Si) provide the strongest constraints because the kinematic suppression ∝ q²/m_N² is milder.

Neutrino detectors such as Borexino can be sensitive to the emitted neutrino via inverse‑beta‑like processes or through the detection of the accompanying recoil electron. By recasting Borexino’s solar‑neutrino measurements, the paper derives limits on Λ_i that are competitive with, and in some cases stronger than, those from traditional direct‑detection experiments (e.g., CRESST, DarkSide). The tensor operator exhibits interference between SI and SD components, which the authors explore in detail.

Conclusions:
The study demonstrates that absorption operators offer a complementary probe of dark‑sector fermions that is distinct from the usual pair‑production searches. Collider mono‑X analyses and low‑energy nuclear‑absorption experiments probe overlapping but not identical regions of parameter space, providing a robust, model‑independent test of the EFT. Light nuclear targets emerge as especially powerful for both SI and SD operators, suggesting that future direct‑detection experiments should consider incorporating such materials. The projected sensitivities of the HL‑LHC and HE‑LHC indicate that collider searches will continue to improve the bounds on Λ_i, potentially reaching the multi‑TeV regime. Overall, the paper highlights the synergy between high‑energy collider physics, underground direct‑detection, and neutrino observatories in the quest to uncover the nature of dark matter or broader dark‑sector particles.