REVIEW
Spin-independent scattering of pseudoscalar-mediated dark matter
High Energy Physics - Phenomenology · Submitted 11 Sep 2026 · 49 references
Pith's verdict: A careful, comprehensive EFT analysis of pseudoscalar-mediated DM direct detection — technically sound but not surprising.
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Referee report
The paper
The paper claims
Pseudoscalar-mediated dark matter yields detectable SI scattering only with sub-TeV new physics.
The reading
What carries the argument
The central calculation is the one-loop matching of the pseudoscalar EFT onto the low-energy scalar and twist-2 operators that generate spin-independent DM–nucleon scattering. The identities are the loop functions $F_0$, $F_1$, $F_2$ for quark diagrams, $I_0$, $I_1$ for gluon diagrams, and the two-loop function $F_3$ for heavy-quark contributions. These functions encode the dependence on $m_\chi$ and $m_a$, and their evaluation directly sets the SI cross section in terms of the Wilson coefficients and the UV scale $\Lambda$.
Extended reading notes
Core claim
In a sub-electroweak effective field theory with a pseudoscalar mediator, the one-loop spin-independent scattering of Dirac fermion dark matter can generate cross sections above the neutrino floor. The dominant contributions come from operators quadratic in the pseudoscalar ($a^2 \bar{q} q$ and $a^2 GG$). After imposing the observed relic abundance and indirect-detection bounds, the UV cutoff $\Lambda$ must be $\lesssim 640$ GeV for these operators to yield an observable signal. For linear operators ($a \bar{q} \gamma_5 q$ and $a G \tilde{G}$), observability requires $\Lambda \lesssim 140$ GeV, a regime already severely constrained by collider searches.
Load-bearing premise
The load-bearing assumption is that the effective field theory reliably describes the DM relic abundance for all parameter points, including those where the DM mass exceeds the EFT cutoff, an acknowledged limitation for the $a\bar{q}\gamma_5q$ scenario.
Editorial extensions
If this is right
- If the $a^2 \bar{q} q$ or $a^2 GG$ operator dominates, future experiments (e.g., XLZD) may detect a spin-independent signal for $m_\chi > m_a$ and $\Lambda \lesssim 640$ GeV.
- For the $a \bar{q} \gamma_5 q$ or $a G \tilde{G}$ operators, the observable parameter space requires $\Lambda \lesssim 140$ GeV, forcing any UV completion to contain particles at or below the electroweak scale.
- In the regime $m_\chi > m_a$, the relic abundance is set by $p$-wave annihilation $\bar{\chi}\chi \to aa$, decoupling the direct-detection signal from indirect-detection constraints.
- For $m_\chi < m_a$, indirect-detection limits from Fermi-LAT already exclude the parameter space for the linear operators, and the quadratic operators cannot yield the observed relic abundance without additional new light states.
Reading between the lines
- If future direct-detection experiments reach the neutrino floor without a signal, the most optimistic pseudoscalar-mediated scenarios considered here would be ruled out for $\Lambda \lesssim 640$ GeV, pushing new physics to higher scales or stronger couplings.
- The similar $\Lambda$ reach of the $a^2 \bar{q} q$ and $a^2 GG$ scenarios implies that any UV completion generating comparable coefficients for these operators will face a common direct-detection test.
- The paper's focus on one operator at a time leaves open the possibility that interference between operators or additional CP phases could alter the SI cross section; a full analysis of simultaneous operators might shift the discovered boundaries.
Original abstract
Dark matter with pseudoscalar couplings provides a well-motivated scenario in which direct-detection signals are suppressed at tree level, since the scattering off nuclei is both spin-dependent and momentum suppressed. While spin-independent scattering is absent at tree level, it arises at one loop and can provide the leading direct-detection signal. We revisit this scenario in a general sub-electroweak effective field theory with a light pseudoscalar mediator, including interactions through to mass-dimension-six. We compute the matching onto the quark and gluon operators relevant for direct detection to determine whether this scenario could be detectable at future experiments, while also requiring consistency with the observed dark matter relic-abundance and indirect-detection limits. We find that while models with a pseudoscalar mediator can generate spin-independent cross sections above the neutrino floor, this generally requires additional new physics below the TeV scale.
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
Dark matter with pseudoscalar couplings provides a well-motivated scenario in which direct-detection signals are suppressed at tree level, since the scattering off nuclei is both spin-dependent and momentum suppressed. While spin-independent scattering is absent at tree level, it arises at one loop and can provide the leading direct-detection signal. We revisit this scenario in a general sub-electroweak effective field theory with a light pseudoscalar mediator, including interactions through to mass-dimension-six. We compute the matching onto the quark and gluon operators relevant for direct detection to determine whether this scenario could be detectable at future experiments, while also requiring consistency with the observed dark matter relic-abundance and indirect-detection limits. We find that while models with a pseudoscalar mediator can generate spin-independent cross sections above the neutrino floor, this generally requires additional new physics below the TeV scale.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Dark matter with pseudoscalar couplings provides a well-motivated scenario in which direct-detection signals are suppressed at tree level, since the scattering off nuclei is both spin-dependent and momentum suppressed. | background 1.00 | 0.05 | 0.03 | asserted 0.99 |
| s1 | While spin-independent scattering is absent at tree level, it arises at one loop and can provide the leading direct-detection signal. | background 0.96 | 0.14 | 0.07 | asserted 0.86 |
| s2 | We revisit this scenario in a general sub-electroweak effective field theory with a light pseudoscalar mediator, including interactions through to mass-dimension-six. | contribution 0.53 | 0.79 | 0.03 | asserted 0.83 |
| s3 | We compute the matching onto the quark and gluon operators relevant for direct detection to determine whether this scenario could be detectable at future experiments, while also requiring consistency with the observed dark matter relic-abundance and indirect-detection limits. | contribution 0.60 | 0.95 | 0.06 | hedged possibility 0.44 |
| s4 | We find that while models with a pseudoscalar mediator can generate spin-independent cross sections above the neutrino floor, this generally requires additional new physics below the TeV scale. | result 0.99 | 0.96 | 0.91 | asserted 0.87 |
What was examined, and what was not
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Figures
Showing 5 of the paper's 10 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2609.12336 by the authors.
References
Discussion
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