The paradigm of thermal freeze-out offers a straightforward testable solution to the puzzle of DM abundance generation in the early Universe. However, the increasing sensitivity of DD experiments is placing increasingly stringent constraints on the magnitude of interactions between DM and SM states. This raises a fundamental question: would negative results at future DD facilities allow us to completely rule out the thermal paradigm, thereby indicating the necessity of alternative production mechanisms to solve the DM puzzle?

We first consider the simplified s-channel model with scalar DM. When the DM relic density is primarily determined by pair annihilations into SM fermion pairs, the thermally averaged cross section can be reliably approximated using the so-called velocity expansion,³

We do not consider, in our study, values of the mass close to s-channel resonances or opening thresholds of annihilation channels, corresponding to the conventional cases where the velocity expansion is not valid [38].

Using the velocity expansion, an analytical estimate of the relic density is given by [39] Ωχh2≃1.07×109  GeV-1xfg*1/2MPl(a+3b/xf),(6)where a and b represent the s- and p-wave terms in the velocity expansion, respectively. Assuming mχ≪mS and annihilations predominantly into bottom quark pairs, we can write (Ωχh20.12)≈19(λχ2cS2)-1(mχ100  GeV)2(1/5mχ/mS)4.(7)

This equation can be solved for λχScS and the result substituted into the analytical expression for the DM scattering cross section with protons, σχpSI=μχp2πmp2mχ2mS4(λχS)2cS2fp2,(8)where fp=∑q=u,d,sfqp+627fTG≈0.3. In this way, it is possible to obtain the following prediction for the DM scattering cross section as a function of its mass: σχpSI≈1.1×10-42(100  GeVmχ)4(0.12Ωχh2)-1.(9)

Repeating the same procedure for the other s-channel portals, we have, in the case of the fermionic DM with spin-0 mediator, ⟨σv⟩(ψ¯ψ→f¯f)≈∑f3ncfv24πgψ2cS2mf2vh2mψ2(4mψ2-mS2)21-4mf2mχ2θ(mψ-mf)=3ncfv24πgψ2cS2mf2vh21-4mf2mχ2θ(mψ-mf)×{mψ2mS4mS≫mψ116mψ2mS≪mψ.(10)

For the vector portal, we have ⟨σv⟩(ψ¯ψ→f¯f)=|gψV|2πmψ2(4mψ2-mZ′2)2∑fnfc|gfV|21-4mf2mψ2θ(mψ-mf)=|gψV|2π∑fnfc|gfV|21-4mf2mψ2θ(mψ-mf)×{mψ2mZ′4mZ′≫mψ116mψ2mZ′≪mψ(11)and σψpSI=μψp2π|gψV|2|2guV+gdV|2mZ′4(12)for the DM annihilation and scattering cross sections, respectively. Considering the hierarchy mψ≪mZ′, we can translate the requirement of correct relic density into a relatively simple equation, (Ωψh20.12)≈3.5×10-2(∑fncf|gψV|2|gfV|2)-1(mψ100  GeV)2(1/5mψ/mZ′)4,(13)whose result can be related to the following prediction for the scattering cross section with protons: σψpSI≈2.3×10-40  cm2|2guV+gdV|2∑fnfc|gfV|2(100  GeVmψ)2(0.12Ωψh2).(14)

The analytical results just discussed are illustrated comprehensively in Fig. 2. The figure shows the (mDM,σDMpSI) parameter space, where the solid lines correspond to the expected DM scattering cross section (with different colors corresponding to different portal models as displayed directly on the plot) when the DM couplings are fixed to achieve the correct relic density, assuming mχ,ψ=1/5mS,Z′. The dashed lines have been obtained in an analogous fashion but considering mχ,ψ=5mS,Z′.

The results clearly demonstrate that, when a strong correlation between DM relic density via annihilation into SM quarks and DM scattering with nucleons is enforced, the freeze-out paradigm is already in strong tension with experimental data. An absence of detection at future facilities would seemingly exclude DM coupled with quarks below the TeV scale.

A possible way out, even within simplified setups such as those under consideration, would be to consider scenarios where the DM relic density relies essentially on annihilations into pairs of mediators (this implicitly requires heavier DM relative to the s-channel mediator). In this way, one can achieve effective DM annihilations without necessarily implying a scattering cross section in tension with experimental data (for a detailed study of this scenario, the reader may refer to [40]).

In this “secluded” regime, assuming the hierarchy mχ,ψ≫mS,Z′ so that final state masses can be neglected, the relic density depends only on the DM mass and a single coupling. Considering the following cross sections, ⟨σv⟩(χχ→SS)≈(λχS)464πmχ2,⟨σv⟩(ψ¯ψ→SS)≈364πgψ41mψ2v2,⟨σv⟩(ψ¯ψ→Z′Z′)=|gψV|416πmψ2,(15)we obtain the following predictions: λχS≈0.24(mχ100  GeV)1/2(0.12Ωχh2)1/4,gψ≈0.50(mψ100  GeV)1/2(0.12Ωψh2)1/4,gψV≈0.22(mψ100  GeV)1/2(0.12Ωψh2)1/4.(16)

Although it does not directly impact the relic density, the coupling between the mediator and the SM is subject to the requirement of ensuring DM thermalization in the early Universe. The minimal value ensuring this condition can be estimated via the following condition: ⟨σv⟩(DM DM→f¯f)nDM,eqH|T=Ts.f.o.=1,(17)with Ts.f.o. being the standard freeze-out temperature.

The two panels of Fig. 3 show contours in the (mDM,σDMpSI) parameter space. These correspond to the prediction of the DM scattering cross section for the three s-channel portals under scrutiny, with coupling values according to Eqs. (15) and (16). The different lines in each plot correspond to different mass ratios: mχ,ψ=3mS,Z′ (solid lines), mχ,ψ=5mS,Z′ (dashed lines), and mχ,ψ=10mS,Z′ (dot-dashed lines).

Beyond the secluded regime, other possibilities allow for a less tight connection between relic density and direct detection. Indeed, one can have scenarios in which the effective coupling between the DM and nucleons is suppressed. Such suppression can be “accidental” as due to the so-called “blind spots” (in such a case, the inclusion of effects emerging at one or more loop levels is crucial to assess the capability of DD experiments to test this class of models) or more intrinsic of the model under scrutiny. A relevant example of this last kind of scenario is represented by models in which the DD is due to operators emerging at the one-loop level.

First, we can have suppression of the effective couplings between DM and nucleons in the presence of so-called blind spots. However, blind spots normally correspond to very specific assignments of model parameters and are not stable under radiative corrections. The inclusion of effects emerging at one or more loop levels is crucial to assess the capability of DD experiments to test this class of models. The corresponding cross sections have been computed in detail, for example, in [19,31], for electroweakly interacting and t-channel mediator DM, respectively. Another relevant example is represented by DM interacting with s-channel simplified mediators coupled to leptons rather than quarks. We will discuss this type of scenario later in the paper. The capability of next-generation experiments of probing concrete models with suppressed DD has been recently discussed in [41].

Another interesting possibility would be represented by the case that the thermally produced DM represents just a fraction of the total DM component of the Universe. This kind of scenario will be the object of future work.

The case of study of the present paper is instead represented by single component DM produced, possibly nonthermally, in nonstandard cosmological conditions. The correct relic density will be, in general, achieved in different regions of the parameter space with respect to the conventional freeze-out paradigm. One of the main purposes of the present study will be to assess whether at least some of such regions will be accessible to future experiments.

We first consider the case customarily dubbed early matter domination (EMD), corresponding to ω=0. In this scenario, the exotic component Φ behaves as pressureless matter, leading to a modified expansion history where H∝a-3/2 rather than the standard radiation-dominated scaling H∝a-2. This altered expansion rate has profound implications for both the freeze-out dynamics and any nonthermal DM production processes.

The system of Boltzmann equations (18) can be solved analytically in this case via the following change of variables [49,50]: Φ=ρϕa3Λ,NDM=nDMa3,a=AaI.(20)This transformation allows us to factor out the terms linear in the Hubble expansion rate, recasting the system in the following more tractable form: dΦdA=-ΓΦHA1/2aI3/2Φ,dNDMdA=A1/2aI3/2HΛbDMmϕΓϕΦ-⟨σv⟩HA-5/2aI-3/2(NDM2-NDM,eq2),dTdA=(1+T3dheffdT)-1{-TA+ΓϕΛmϕ(1-bDMEDMmϕ)TsHA-5/2aI-3/2Φ+2EDMsHA-11/2aI-9/2⟨σv⟩(NDM2-NDM,eq2)},(21)where H is defined as H≡(aIA)3/2H=(ΛΦ+ρR(T)A3aI3+EDMNDM3MPl2)1/2.(22)As already pointed out, the equation for the entropy density as been translated into an equation for the temperature T.

Unless differently stated, we will assume initial conditions (corresponding to A=1 in the new variables) consisting of a value of 106  GeV for the temperature (or, equivalently, for the energy density) of radiation and to a value ρϕ=12mϕ2MPl2, with mϕ=106  GeV for the initial energy density of Φ.⁴

The chosen initial conditions imply a fixed initial ratio between the initial energy density of the radiation and of the exotic component Φ. One could consider the latter as an additional free parameter, see, e.g., [9] with potential impact to the DM relic density. We will leave this possibility to future study.

Let us first consider the case bDM=0. A first illustration is provided by Fig. 4. It shows, for the portal models, the evolution of the DM yield, for a benchmark assignation of the DM and mediator masses and for the value of 1 GeV of the reheating temperature. In each panel, the different lines correspond to the different value of the couplings between the DM and the mediator. By looking at the shape of the different curves, one can notice that for λχ=0.01÷1, the DM abundance at early times (i.e., low value of x) grows until it matches the shape of an equilibrium curve (namely, the value of Yχ is an equilibrium distribution function assumed for the DM). For 1≲x≲100, the DM abundance decreases with x (notice that, for the parameter choice in the figure, x=100 corresponds to the reheating temperature). Such decrease is interpreted as the dilution effect due to the entropy injection from the decay of the Φ field. Once the decay of the field is completed, for x>100, the comoving number density gets a plateau besides a small change for x≈1000 (corresponding to T≃100  MeV, due to the variation of the relativistic degrees of freedom, geff, heff, as consequence of the QCD phase transition). In this setup, the dependence of the final relic density to the couplings of the DM is analogous to the standard freeze-out case, with the relic density increasing as the value λχ decreases and vice versa. For λχ<0.01, on the contrary, the DM is never capable to get in thermal equilibrium. For x<1, the comoving abundance progressively increases, analogous to the conventional freeze-in, due to productions from interactions with the particles of the thermal plasma, until the aforementioned depletion effect becomes effective. Again, the final relic density (i.e., the plateau shape on the right portions of the plots) is reached for x>100, after the decay of the field occurs and the DM production from annihilations of thermal bath SM particles is no longer effective. In this setup, similar to the conventional freeze-in, the DM relic density increases as the coupling λχ increases and vice versa. In summary, studying the dependence of the DM abundance on the coupling, we envisage two limiting regimes for the solutions, a freeze-out-like regime and freeze-in-like regime. For fixed values of the DM and mediator masses, the two regimes correspond to different ranges of the DM couplings. One can finally infer that, in an EMD, one would need, to match the correct DM relic density, smaller couplings, with respect to the case of standard cosmological history, in the case of freeze-out and, instead, bigger couplings, always compared to the standard scenario, in the case of freeze-in. This would allow one, in the former case, to evade the constraints from direct detection while, in the latter case, would allow the model under scrutiny to approach the experimental sensitivity, something normally very difficult in the standard freeze-in.

Figure 5 illustrates instead the behavior of the DM relic density by varying the reheating temperature. For each model (the different rows in the figure) we have considered three representative values of the DM couplings, corresponding to the WIMP and FIMP regimes, as well as to an intermediate regime among the two.

For λχ=1, as already discussed before, the curves displayed in the panels in the first column of Fig. 5, the DM abundance tracks the equilibrium value for x<1, gets depleted by the entropy injection for 1≲x≲xR with xR=mχTR and then gets the final value for x≫xR. In this regime, the dependence of the relic density from the reheating temperature can be described by the following analytical approximation [55]: ΩDMEMD=TR3Ts.f.o.Tf.o.4ΩDMT,(24)where ΩDMT represents the value of the relic density, for the given assignation of DM/mediator masses and couplings, assuming the conventional freeze-out in a standard cosmological history with Ts.f.o. being the standard freeze-out temperature. Tf.o. is the actual freeze-out temperature, which can be estimated via the usual rule of thumb nDM⟨σv⟩(Tf.o.)=H(Tf.o.). The latter definition stems from the fact that, according to the initial energy of the field Φ, the freeze-out of DM annihilation can occur when the Hubble expansion parameter is dominated by the radiation energy density, so that Tf.o.=Ts.f.o. and the DM relic density is just the thermal value divided by a dilution factor due to the entropy injection, or, alternatively, when the Hubble expansion parameter is dominated by ρϕ. (Equations to estimate numerically Tf.o. have been provided, for example, in [43]). Moving to the case of very small couplings, as represented by the right panel of Fig. 5, the DM is never capable of reaching the equilibrium values at early times. An analytical estimate of the relic density in such regime has been provided in [55], although originally derived in [56], and reads ΩDMh2≃0.12⟨σv⟩10-16  GeV-2(100  GeVmDM)(TR1  GeV)7.(25)There is no analytic estimate valid for the intermediate regime between the two limiting regimes just illustrated. In such a case, one can rely just on the numerical solution.

We are, finally, ready to assess the detection prospects of the scenarios of DM production during EMD. We have considered first the scenario of high values of the couplings, corresponding to thermal freeze-out in a standard cosmological scenario. We have performed a parameter scan over the following ranges: The scan ranges are mχ,ψ∈[1,1000]  GeV,mS∈[10  GeV,10  TeV],λχS,gψ∈[10-2,10](26)for the two s-channel simplified models, mψ∈[1,1000]  GeV,mZ′∈[1,10]  TeV,gψV,gfV∈[10-2,10](27)for the spin-1 portal, and mχ∈[1,1000]  GeV,mΦ∈[1,10]  TeV,λχ∈[10-2,10](28)for the t-channel simplified model. For all the models, we have varied the reheating temperature in the range [10  MeV,min(10  GeV,Ts.f.o.)].

The results are shown in Fig. 6. The four panels (corresponding to the four portal models) of the figure show the model points featuring the correct relic density in the (mDM,σDM,pSI) bidimensional plane. The current exclusion bound from LZ and the projected sensitivity by XLZD (assuming 200 ton yr exposure) are reported as well, as, respectively, dashed blue and dot-dashed purple lines. Finally, the region colored in yellow correspond to the neutrino floor. The color code of the model points correspond to the value of the reheating temperature.

Figure 7 shows the results of a broader exploration of the parameter space in which a larger window of values of the DM coupling has been considered. For simplicity, we have focused on just the s-channel portal with scalar DM and the spin-1 portal. In both cases, the coupling has been varied in the [10-10,1] range, while the mass of the mediator has not been varied, contrary to the previous scan, but, instead, kept fixed to the three values of 1, 10, and 100 TeV.

Up to now the simplified model with electroweak interacting DM has not been considered. The reason is that the DM annihilation cross section depends on the SM gauge couplings and, for the considered range of masses, it leads to an underabundant density if the conventional thermal production is assumed. Consequently, the correct relic density can never be achieved in the case of a cosmology with an early matter domination, unless an extra nonthermal source of DM is assumed, as will be evidenced in the next section.

Let us now consider the case bDM≠0, i.e., the possibility of nonthermal DM production from the decay of the field Φ.

In this section, we explore the implications of modified cosmological histories on dark matter production, focusing on scenarios where the Universe undergoes accelerated expansion during the relevant epochs. We examine both the thermal freeze-out regime, where dark matter was initially in thermal equilibrium, and the freeze-in regime, where dark matter production occurs through feeble interactions. Our analysis demonstrates how nonstandard cosmologies can reconcile theoretical predictions with observational constraints, particularly for models that face challenges in the standard cosmological framework.

Along these lines, we hence consider the case in which the exotic component Φ can trigger an accelerated expansion of the Universe [58], ρΦ∝a-(4+n),n>0.(33)Among the various possibilities, quintessence [59–61] (corresponding to n=2, ω=1) is particularly popular.

Neglecting the case of nonthermal DM production, the nonstandard cosmological history affects the freeze-out of the DM, setting it at an earlier stage with respect to standard cosmology. The two temperatures are related by the following approximate equation: xs.f.o.e-xs.f.o.=xf.o.e-xf.o.(xf.o.xr)n/2,(34)where xs.f.o. corresponds to the freeze-out in a standard (i.e., radiation-dominated) cosmological history, while xr=mχ/Tr. The temperature Tr represents a reference temperature marking the end of the nonstandard cosmological history. It is defined as the temperature at which the energy densities of radiation and Φ coincide, so that for T<Tr the Universe returns to being radiation dominated, H=ρ(T)3MPl2ρ(T)=ρR(T)[1+g*(Tr)g*(T)(g*,s(T)g*,s(Tr))(4+n)/3(TTr)n].(35)

Semianalytical solutions of the Boltzmann equations can be obtained in the case n≥2. Considering an s-wave-dominated annihilation cross section ⟨σv⟩=σ(s=4mχ2)=a, the DM abundance can be approximately written as Yχ(x)≃xrmχMpla[2xf.o.+log(xxf.o.)]-1,n=2,Yχ(x)≃xrn/22mχMpla[xf.o.n/2-2+xn/2-1n-2]-1,n>2.(36)

One of the key aspects of thermal freeze-out during accelerated expansion of the Universe is that the DM relic density is enhanced with respect to standard cosmology. Higher annihilation cross sections are needed to match the experimentally favored value. This setup offers a further solution to accommodate the correct relic density for SU(2)-multiplet DM with masses below the TeV scale. We have therefore determined the viable parameter space for the simplified model with SU(2)-doublet DM as shown in Fig. 12.

The figure shows, in the (mχ,Tr) parameter space, the isocontours corresponding to the correct relic density for n=2 (blue curve), n=3 (magenta curve), and n=4 (purple curve).

The case of a fast-expanding Universe is also of particular interest for feebly interacting particles. As evidenced in [62], a general analytical solution cannot be achieved, as different processes can contribute to the generation of the relic density. A general feature is, nevertheless, that DM production is suppressed with respect to a radiation-dominated Universe; consequently, comparatively higher couplings are needed. This feature is particularly interesting, as it would potentially put a freeze-in scenario within the reach of next-generation detectors. We provide a (semi)analytical insight by considering, as a case of study, the s-channel simplified model. A similar reasoning can be applied to the other portal models. The freeze-in regime can be achieved for this model by considering the regime mχ≪mS and very small values of the λχS,cS parameters. In this setup, one can straightforwardly adapt the expression provided in [62] for the case of pair production of DM from annihilations of SM states by taking |MSMSM→χχ|2=(λχS)2cS2mf2vh2mχ4mS4,(37)so that one obtains Yχ(x)≃13510256π8(λχS)2cS2g*3/2mχ3MPlmS4mf2vh2xr-n/2∫0xdx′K1(x′)x′2+n/2.(38)

Figure 13 shows the outcome of a parameter scan of the simplified s-channel (both scalar and fermionic DM) and t-channel models over the following ranges: mχ∈[1,1000]  GeV,mS∈[1,10]  TeV,cSλχS∈[10-10,10-3],mψ∈[1,1000]  GeV,mS∈[1,10]  TeV,cSgψ∈[10-10,10-3],mχ∈[1,1000]  GeV,mΦ∈[1,10]  TeV,λχ∈[10-5,10-1].(39)In each case, the temperature associated with the end of the nonstandard cosmological phase has been varied as Tr∈[10  MeV,10  GeV] with, as customary, the implicit requirement Tr<Ts.f.o. For each of the considered models, the scan has been repeated 3 times, corresponding to the values 2, 3, and 4 of the parameter n. Each row, one for each of the considered simplified models, shows plots displaying the parameter assignations giving the correct DM relic density in the (mχ,λχScS,gψcS,λχ) parameter space, following a color pattern determined by the value of Tr. In each row, the different panels correspond to the different assignations of n. The distribution of the model points in each panel can be qualitatively understood as follows. The range of the scans for each model has been chosen so that the DM mass is always lighter than the one of the mediator. In this setup, as shown by Eq. (38) (a similar result would be obtained also for the other simplified models), the DM relic density increases with the DM mass. In order to keep the value around the experimental determination, for fixed (Tr,n) it is then necessary to lower the coupling. Furthermore, the DM relic density decreases Tr. For fixed values of the DM and mediator masses, it is then necessary to choose higher values of the coupling. The decrease of the relic density with Tr is stronger as the value of n increases. Comparing the different panels of the single rows, one can consequently see that the maximal values of the couplings compatible with the correct relic density are obtained for n=4. The results demonstrate that nonstandard cosmological histories can significantly modify the viable parameter space for dark matter models, opening new regions that would be excluded in standard cosmology. This is particularly relevant for models with weak couplings, where the enhanced expansion rate during the nonstandard epoch can compensate for the reduced interaction strength, allowing these models to achieve the correct relic density while remaining compatible with experimental constraints.

The implications of these results extend beyond the specific models considered here. Nonstandard cosmological histories provide a general framework for reconciling dark matter theory with observation, offering new pathways to the observed relic density that can evade current experimental bounds. This is particularly important for models that face the WIMP miracle problem, where the cross sections required for thermal freeze-out are in tension with direct detection limits. By modifying the cosmological context, we can maintain the elegance of thermal production while avoiding the most stringent experimental constraints.

Our systematic examination of simplified dark matter models demonstrates that the parameter space accessible to next-generation experiments remains remarkably rich, even in the face of the irreducible neutrino background. The interplay between thermal freeze-out and freeze-in production mechanisms opens multiple avenues for discovery, with each pathway offering distinct phenomenological signatures that can guide experimental strategies. Particularly compelling are scenarios involving momentum-dependent interactions and blind-spot configurations, which naturally suppress direct detection cross sections while maintaining cosmologically viable relic densities.

The exploration of electroweakly interacting multiplets and t-channel mediator models reveals that loop-induced effects often dominate the detection prospects for theoretically well-motivated scenarios [19,63]. This finding underscores the critical importance of higher-order calculations in establishing reliable theoretical predictions for ultimate sensitivity experiments. Our analysis shows that many models previously considered beyond experimental reach may become accessible through careful consideration of radiative corrections and multiloop contributions.

For the s-channel scalar portal model, the relationship between relic density and direct detection sensitivity is governed by the thermally averaged annihilation cross section, ⟨σv⟩(χχ→f¯f)≈∑f3nfc16π(λχS)2cS2mf2vh2mχ2(4mχ2-mS2)2⁢[1-4mf2mχ2]θ(mχ-mf),(40)which translates to a predicted spin-independent scattering cross section of σχpSI≈1.1×10-42  cm2(100  GeVmχ)4(0.12Ωχh2)-1.(41)

One of the most significant insights from our work concerns the dramatic impact of nonstandard cosmological histories on the relationship between relic abundance and direct detection prospects. Early matter domination scenarios and fast-expanding Universe models can fundamentally alter the traditional freeze-out dynamics, enabling viable dark matter candidates with properties that differ substantially from conventional expectations [9,10,14].

In the early matter domination regime, the dark matter yield satisfies the modified Boltzmann equation system with the critical condition for reannihilation, ⟨σv⟩>1.96×10-29  cm3 s-1bDM-1(1  GeVTR)4/3,(42)where TR is the reheating temperature and bDM parametrizes the branching ratio for dark matter production from the exotic component decay.

These mechanisms provide natural explanations for suppressed direct detection signals while maintaining the correct cosmological abundance, effectively extending the viable parameter space well into regions that will be probed by ultimate sensitivity experiments. The freeze-in paradigm emerges as particularly promising in this context, as it naturally accommodates the ultraweak couplings characteristic of models approaching the neutrino floor.

For freeze-in production in fast-expanding cosmologies with equation of state parameter ω=-1-n/3, the dark matter yield is approximately Yχ(x)≃13510256π8(λχS)2cS2g*3/2mχ3MPlmS4mf2vh2xr-n/2⁢∫0xdx′K1(x′)x′2+n/2,(43)where xr=mχ/Tr marks the transition temperature.

The approach to the neutrino floor necessitates a fundamental shift in experimental strategy, moving beyond simple sensitivity improvements toward sophisticated background discrimination techniques [1,4]. Our benchmarks highlight the critical importance of multitarget approaches, directional sensitivity, and statistical separation methods in breaking the degeneracy between neutrino- and WIMP-induced nuclear recoils. The complementarity between different experimental approaches—direct detection, indirect detection, and collider searches—becomes increasingly crucial as individual techniques approach their fundamental limits.

Current experiments such as LUX-ZEPLIN have achieved sensitivities approaching 10-47  cm2 for spin-independent WIMP-nucleon scattering [2], while next-generation detectors like DARWIN/XLZD promise to extend this reach to ∼10-48  cm2. The neutrino floor represents a fundamental barrier where coherent elastic neutrino-nucleus scattering creates an irreducible background that mimics WIMP signals.

Emerging technologies, including quantum sensing approaches and gravitational wave detectors, offer unprecedented opportunities to probe previously inaccessible parameter regions. These novel detection strategies can potentially circumvent the traditional neutrino floor limitations by targeting different interaction mechanisms or exploiting ultrahigh precision measurements that were previously impossible.

While our analysis focuses primarily on WIMP-like candidates, the broader implications extend to the entire landscape of dark matter theories. The recognition that traditional thermal freeze-out models may be fundamentally limited by neutrino backgrounds has catalyzed renewed interest in alternative production mechanisms and nonminimal dark sectors [7,8]. Following an analogous approach as, for example, [41,63], we will consider DM masses in the 1 GeV–1 TeV range.

For electroweakly interacting dark matter multiplets, the Lagrangian takes the form L=g24[n2-(2Y+1)2]χγμψ-Wμ++g24[n2-(2Y-1)2]χγμψ+Wμ--ig2YcosθWχγμηZμ,(44)where n represents the SU(2)-multiplet dimension and Y the hypercharge assignment [16,20].

Multimessenger approaches, combining information from gravitational waves, γ-ray observations, and neutrino detections, promise to open entirely new windows into the dark sector. The development of comprehensive theoretical frameworks enables systematic exploration of models with similar phenomenological predictions, which will prove essential for interpreting potential signals in the complex landscape near the neutrino floor.

Looking toward the next decade, several key developments will shape the future of dark matter searches at the neutrino floor. The deployment of next-generation multiton detectors will provide unprecedented sensitivity to spin-independent WIMP-nucleon interactions, potentially reaching cross sections as low as 10-48  cm2. Simultaneously, advances in quantum sensing technology and novel detection concepts will enable exploration of previously inaccessible mass ranges, from sub-GeV to ultralight axionlike particles.

The relic density constraint in the standard freeze-out scenario is given by Ωχh2≃1.07×109  GeV-1xfg*1/21MPl(a+3b/xf),(45)where xf=mχ/Tf is the freeze-out parameter, g* is the effective number of relativistic degrees of freedom, and a, b parametrize the s- and p-wave contributions to the velocity expansion.

The theoretical framework developed in this work provides essential guidance for optimizing these experimental efforts. Our benchmarks identify the most promising parameter regions for discovery, highlight the importance of complementary detection strategies, and establish the foundation for interpreting results as experiments push into uncharted territory. The systematic exploration of nonstandard cosmological scenarios and alternative production mechanisms ensures that the theoretical landscape remains rich with possibilities, even as traditional WIMP searches approach their fundamental limits.

The present manuscript is mostly focused on the interplay between relic density and direct detection. We will nevertheless make some remarks in this subsection about indirect detection. On general grounds, the prospects are model dependent as they depend first of all on whether the DM annihilation cross section is s-wave dominated (i.e., velocity/time independent) or p-wave dominated (i.e., velocity/time dependent) and on the specific annihilation final state. Among the benchmark models considered in our study, the s-channel model with fermionic DM and the t-channel portal are characterized by p-wave-dominated cross sections; hence they are not subject to constraints from indirect detection as the present time values of the cross section are much below present and future experimental sensitivity. For the same reason, all the models in the freeze-in (FI) regime are not subject to any constraints. As already pointed out, thermal production during early matter domination and nonthermal production in the direct production regime open regions of parameter space corresponding to value of the DM cross section below the thermally favored value. Present ID constraints are hence expected to affect this scenario to a very modest or even negligible extent. On the contrary, nonthermal production in the reannihilation regime can be efficiently probed by indirect detection.⁶

Scenarios with annihilation cross section of the order or above the thermally favored values are constrained as well by Cosmic Microwave Background, ruling out thermal candidates with s-wave-dominated cross section for masses below 10 GeV [64]. Some early studies on nonthermal production models have been illustrated, for example, in [65,66].

The ultimate success of dark matter detection efforts will likely require a coordinated approach combining multiple experimental strategies, sophisticated theoretical modeling, and innovative technologies that push beyond current paradigms. Our analysis demonstrates that the neutrino floor, rather than representing an insurmountable barrier, marks the beginning of a new era in dark matter searches—one characterized by unprecedented precision, novel detection concepts, and a deeper understanding of the rich theoretical landscape that connects particle physics, cosmology, and fundamental interactions.

The WIMP miracle relation ⟨σv⟩∼3×10-26  cm3/s continues to provide theoretical motivation, even as experimental constraints push models toward increasingly complex scenarios involving loop-induced interactions, nonthermal production mechanisms, and exotic cosmological histories [11,21].

The journey toward understanding the nature of dark matter continues, with the neutrino floor serving not as a destination, but as a waypoint toward even more ambitious goals. The theoretical benchmarks established in this work provide a roadmap for this continuing quest, ensuring that the next generation of experiments will be optimally positioned to make the discoveries that will finally illuminate one of the Universe’s deepest mysteries. As we approach the limits of traditional detection methods, the interplay between theoretical innovation and experimental ingenuity will prove crucial in maintaining momentum toward the ultimate goal of dark matter discovery and characterization.