Simplified dark matter models with loop effects in direct detection and the constraints from indirect detection and collider search

We reexamine the simplified dark matter (DM) models with fermionic DM particle and spin-0 mediator. The DM-nucleon scattering cross sections of these models are low-momentum suppressed at tree-level, but receive sizable loop-induced spin-independent contribution. We perform one-loop calculation for scalar-type and twist-2 DM-quark operators and complete two-loop calculation for scalar-type DM-gluon operator. By analyzing the loop-level contribution from new operators, we find that future direct detection experiments can be sensitive to a fraction of parameter space. The indirect detection and collider search also provide complementary constraints on these models.


I. INTRODUCTION
Although the existence of Dark Matter (DM) has been established by substantial cosmological and astronomical observations, the microscopic nature of DM particles is still unknown. An appealing candidate of DM is the Weakly Interacting Massive Particle (WIMP) arising from various extensions of the Standard Model (SM). The experimental searches for WIMP consist of four main categories, i.e. the direct detection (DD) of possible scattering between DM and SM target materials, the indirect detection (ID) looking for signals of DM annihilation/decay products from the sky, the collider searches for signals from DM productions at high energy accelerators, and the gravitational and/or cosmological effects originating from the DM in the early and/or the current Universe.
Among the aforementioned four categories, the DD experiments have achieved significantly improved sensitivity in the past two decades, but yield null results up to now and very stringent bounds on the WIMP-nucleon scattering cross section. A natural explanation of the absence of a confirmed DM signal is that the scattering rate is highly suppressed by the small typical value of transfer momentum of the process and/or the relative velocity between DM and nucleon. A simple but compelling scenario resulting in the suppressed rate at tree-level is that fermionic DM particles χ scatter off the target nucleon N through a pseudo-scalar mediator in the t-channel scattering process [1,2]. The corresponding tree-level DM-nucleon contact interaction can reduce down to a non-relativistic contact operator in the non-relativistic limit. Here, s χ (s N ) is the DM (target nucleon) spin, and the scattering exchange momentum q is only of order 10 MeV. As a result, this scenario leads to a momentumsuppressed spin-dependent (SD) scattering cross section and thus an undetectable signal rate.
The suppression of tree-level scattering rate makes it appealing to further scrutinize the highorder effect from one-loop induced processes (see early discussions in e.g. [3,4]) and the possibly detectable signal at the upgrades of DD experiments. Integrating out the one-loop diagrams can induce distinct scalar-type operators giving non-momentum-suppressed spin-independent (SI) scattering cross section. The enhancement of loop-level SI cross section by the squared total nucleon number in a nucleus competes with the loop suppression and may dominate the WIMPnuclei cross section over the suppressed tree-level scattering. This one-loop effect in direct DM detection has been investigated in both simplified frameworks and UV complete models [2,[5][6][7][8][9][10][11][12].
Recent progresses on the pseudo-scalar mediator scenario go beyond the one-loop processes for scalar-type DM-quark operator m qχ χqq and include the dedicated contributions from two-loop scattering diagrams for scalar-type DM-gluon operator αs πχ χGG after integrating out both heavy quarks and the mediator. It has been shown in Refs. [13,14] that the full two-loop calculation deviates sizably from the result obtained by the conventional relation between the scalar-type current of heavy quarks and that of the gluon. This discrepancy is caused by the failure of the quark momentum expansion for heavy quarks and an ignored two-loop diagram for gluon emission when one utilized the relation for DM-gluon scattering.
In this work we revisit the loop effect in the DD of simplified DM models including but not limited to a pseudo-scalar mediator. We consider the hypotheses with one spin-0 mediator only coupled to the SM quarks and fermionic DM particles, giving momentum-suppressed WIMPnuclei scattering cross sections at tree-level. The latest approaches of dedicated loop calculations are utilized in the high-order contributions to the cross sections, together with the estimate of the Renormalization Group Equations (RGE) running effects. We also take into account the constraints from other DM detection categories, e.g. the DM relic abundance, the ID constraint in terms of gamma-ray emission, as well as the current status of collider search. These synergistic discussions are regarded as a more complete improvement of the recently appeared works [13,14].
The paper is outlined as follows. In Sec. II we describe the simplified dark matter models. Then

II. SIMPLIFIED DARK MATTER HYPOTHESIS
In this work, we focus on the simplified DM frameworks which consist of Majorana fermion DM χ and a spin-0 mediator a coupled to χ and the SM quarks with strength g χ and g q respectively.
We consider each of the following three scenarios at a time Here the aqq coupling is also scaled by the SM-like Yukawa coupling with v 0 = 246 GeV being the SM Higgs vacuum expectation value. The model D4 with a being a pure pseudo-scalar is designated as the pseudo-scalar mediator DM model in most of literatures. Models D2 and D3 are induced by more specific UV complete models with CP violation [14][15][16][17][18][19] and correspond to the cases with specific CP angles in Ref. [14].
Based on the DM interactions with quarks and gluons at tree-level, the DM-nucleon contact interactions are described by the effective Lagrangians as follows where the tree-level coefficients are defined as The coefficient C tree N (D 3 ) is equal to C tree N (D 4 ) as models D3 and D4 share the same quark bilinear formqγ 5 q. Here ∆ N q , f N q and f N G are quark/gluon-nucleon form factors as numerically used in micrOMEGAs [20]. The quark-level constant is defined as C q = g χ g q mq v 0 . Consequently, their differential DM-nucleus scattering cross sections read as where m T is the nucleus mass, v is the DM speed in Earth's frame, E R is the nuclear recoil energy and F are the form factors defined in Ref. [21]. The tree-level WIMPnucleus scattering cross sections of the above simplified models are all dependent on the transfer momentum q = √ 2E R m T . As seen above, they are suppressed by m 2 N q 2 /m 4 a , m 2 χ q 2 /m 4 a and q 4 /m 4 a for models D2, D3 and D4, respectively.

III. LOOP EFFECT IN DIRECT DETECTION
In this section, we derive the loop-level effect in direct DM detection of the above simplified models, followed by the estimate of the scale effects in terms of renormalization group evolutions.

A. Loop effect from scalar-type quark/gluon operators
The general Lagrangian for the non-momentum-suppressed DM-nucleon SI cross section is given by where O q µν = i 2q ∂ µ γ ν + ∂ ν γ µ − 1 2 g µν ∂ q is the twist-2 operator. For the models we consider, the Wilson coefficients in Eq. (11) are all zero at tree-level but can be generated at loop-level, denoted The coefficients for scalar-type DM-quark operator and the twist-2 operator, i.e. C box q , C (1)box q and C (2)box q , are generated by the box diagrams in the top panels of Fig. 1. The two-loop diagrams in Fig. 1 with only heavy quark Q in the quark loop contribute to the scalar-type DM-gluon operator and the coefficient C box G .
Following the non-relativistic limit used in Ref. [13], we expand the small momentum of valence quarks in the amplitude of the DM-quark scattering box diagrams. The coefficients C box q , C are equivalent to those for pseudo-scalar mediator model as derived in Ref. [13] C box where the loop functions X 001 , X 111 , F are given in Ref. [13] and the references therein. Following the same procedure, for models D2 and D3, we obtain the corresponding Wilson coefficients which are related to those in model D4 The new loop functions here are collected in Appendix. In our numerical calculation, we use Package-X [22] to compute the above loop functions.
Based on the above effective operators for SI DM-nucleon scattering and the corresponding Wilson coefficients, we define the DM-nucleon constant at loop-level where the second moments of the parton distribution functions for quarks q N (2) and anti-quarks q N (2) are taken from the CTEQ PDFs [23]. The SI cross section of the DM interaction with nucleon is thus given by In terms of the form factor function F (N,N ) M (q 2 ), the differential SI cross section of the DM interaction with nucleus with mass m T is matching. The above procedures have been well elaborated in e.g. [24][25][26][27][28] and implemented in packages such as DirectDM [29], runDM [30], Wilson [31] for specific or generic models.
An important difference should be emphasized between the scalar-type and twist-2 operators in Eq. (11). The scalar-type form factors f N q for light quarks q = u, d, s are attributed to the non-perturbative QCD effects with energy scale around or below 1 GeV, and are obtained from the lattice QCD simulations. Thus, the scalar-type Wilson coefficients of light quarks u, d, s and gluon must take values around 1 GeV. Depending on the scale of the first EFT constructed in the whole analysis, e.g. at µ = m Z , this implies the procedures of RGE and threshold matching when calculating the scalar-type operator contributions. At the scale of about 1 GeV, the heavy quarks Q = c, b, t have been integrated out into the scalar-type gluon operator using the full twoloop calculations, as emphasized in Refs. [13,14]. In turn, the twist-2 form factors q N (2),q N (2) in Eq. (11) can be calculated perturbatively using parton PDFs at various scales [3,[24][25][26][27][28], e.g.
1 GeV or m Z . One can choose a convenient scale to calculate the twist-2 contributions, with the proper active field contents (e.g. 5 flavor quarks u, d, s, c, b at µ = m Z ) and the Wilson coefficients and form factors evaluated at that scale. Note that we ignored the negligible contributions from twist-2 gluon operator, since its Wilson coefficient is suppressed by an additional α s /π due to the operator definition [25,26].
We note that in Refs. [13,14], the values of q N (2),q N (2) are evaluated at µ = m Z in the calculations. Since the same (similar) box diagrams and model parameters are used to obtain the scalar-type and twist-2 Wilson coefficients for quarks (gluons), a more consistent implementation should involve the RGE running effects for the scale-type operators from m Z to 1 GeV as discussed above. Since the coupling between the mediator and the SM quarks is chosen to mimic the SM Yukawa structure, the DM-nucleon constant from scalar-type DM-gluon interaction C G f N G in Eq. (22) is dominated by the top quark loop. The constant C G f N G also dominates over the scalartype and twist-2 DM-quark interactions. To have a conservative estimate of the scale effects on the scalar type DM-gluon operator − 9αs 8πχ χG a µν G aµν , we utilize the package DirectDM to perform its RGE running from m Z to 1 GeV. We find that the scale effects give a negative correction of 1% ∼ 2%, and thus do not affect our main conclusions in this work.

IV. OTHER DARK MATTER CONSTRAINTS
In this section, we consider DM constraints on the above simplified models from other categories mentioned in the Introduction, including the relic abundance, indirect detection and collider search.
Assuming that DM particles have frozen out in the early Universe, as standard thermal relics, they acquire their present abundance through annihilation processes. The pairs of DM particle χ in the simplified models can either annihilate into SM quark or gluon pairs via s-channel processes χχ → a → qq, gg or annihilate into two mediators χχ → aa when kinematically allowed [32][33][34][35][36]. The amplitudes of the two annihilation channels are governed by g χ g q and g 2 χ , respectively. We assume all kinematically accessible final states of the DM annihilation and use micrOMEGAs 5.0 [37] to calculate the relic abundance. Note that the WIMP candidate here may account only a fraction of the total DM of the Universe, referred as multi-component DM scenario [38,39]. In this scenario, the DM energy density measured by PLANCK [40] is imposed as an upper limit on the WIMP relic abundance.
Dwarf galaxies are the search targets for DM annihilation into gamma rays. The Fermi Large Area Telescope (LAT) has detected no excess of gamma ray emission from the dwarf spheroidal satellite galaxies (dSphs) of the Milky Way. Thus, an upper limit on the DM annihilation cross section can be placed from a combined analysis of multiple Milky Way dSphs [41,42]. For individual dwarf galaxy target, Fermi-LAT provided tabulated values of delta-log-likelihood as a function of the energy flux bin-by-bin. The gamma ray energy flux from DM annihilation for the jth energy bin and the kth dwarf is given by where J k is the J factor for the kth dwarf. The energy flux only depends on m χ , σv and J k , and is thus calculable for the DM annihilation process from the above simplified models. We use the PPPC4DMID package [43] to obtain the spectrum of photons dN γ /dE. The likelihood for kth dwarf is where L j,k is the tabulated likelihood provided by Fermi-LAT for each dwarf and energy flux. The uncertainty of the J factors is taken into account by profiling over J k in the likelihood below [41] with the measured J factorJ k and error σ k . A joint likelihood for all dwarfs can then be performed as where J is the set of J factors J k . In our numerical implementation, we adopt the corresponding values of L j,k andJ k , σ k for 19 dwarf galaxies considered in Ref. [42].
According to the maximum likelihood analysis adopted by Fermi-LAT, the delta-log-likelihood is given by where σv and J maximize the likelihood at any given m χ , and J maximizes the likelihood for given m χ and σv . The 95% C.L. upper limit on the annihilation cross section for a given m χ is determined by demanding −2∆ ln L(m χ , σv ) ≤ 2.71. We perform the likelihood analysis and obtain the upper limit using Minuit [44]. Once the annihilation cross section calculated by a certain set of model parameters is larger than the limit, we claim the corresponding parameter values are excluded by Fermi-LAT dSphs.
The Large Hadron Collider (LHC) performed the search for DM in association with energetic jet [45,46] or the third generation quarks [47][48][49] for simplified DM models with spin-0 mediator at √ s = 13 TeV collisions. The most severe limits are from final states with tt and missing transverse momentum [50]. For model D4 with pseudo-scalar mediator, assuming unitary couplings g χ = g q = 1, the range of mediator mass between 15 and 25 GeV is excluded [48]. This limit is valid for all DM masses as long as m a > 2m χ and closely related models D2 and D3 should have very similar collider constraint.

V. RESULTS
By combining the theoretical calculations of DM-nucleus scattering cross sections with a certain velocity distribution for DM particles, we can calculate WIMP signal rates for DD experiments. The differential event rate with respect to the recoil energy is given by where ρ χ is the local DM density which is fixed to be 0.3 GeV/cm 3 . We take a cut-off Maxwell-Boltzmann velocity distribution for f ( v) with the escape velocity as v esc = 544 km/s. Together with the differential scattering cross sections obtained above, the differential event rate can be evaluated and in practice we employ DMFormFactor [21,51]  tree-level, in turn, the loop-level spectrum is much greater than that at tree-level for model D4.
Although models D3 and D4 both lead to SD cross section at tree-level, this discrepancy is smaller in model D3 as its tree-level scattering cross section is suppressed by q 2 only. The loop-level spectrum of model D3 dominates over the tree-level one only in the range of low recoil energy.
As the recoil spectrum of the SI scattering induced by loop diagrams is dominant in model D4, the prediction of SI DM-nucleon cross section in Eq. (23) can be compared directly to the limits set by DD experiments to yield a bound on g χ g q . As shown in Fig. 5, for g χ g q ≤ 1 in model D4, the SI scattering cross sections in the red region are below the XENON1T exclusion limit [52,53] but above the neutrino floor. The green region gives cross sections below the neutrino floor. suppressed [35,36]. Above the black contours in Fig. 5, the DM relic abundance satisfies Ωh 2 ≤ 0.12. GeV (right). Assuming g χ = 1, the light blue values are excluded by Fermi-LAT dSphs and the DM relic abundance satisfies Ωh 2 ≤ 0.12 above the black curves.

VI. CONCLUSION
We reexamined the loop-level correction to the WIMP-nucleon scattering cross section in the framework of simplified DM models. A spin-0 mediator is assumed to couple with fermionic DM particle and the SM quarks in each model. The cross sections of these models are low-momentum suppressed at tree-level, but receive sizable loop-induced SI contribution. Following the recent progress on the loop-level correction, we perform one-loop calculation for scalar-type and twist-2 DM-quark operators and complete two-loop calculation for scalar-type DM-gluon operator. By including the loop-level SI cross section, we find that future DD experiments can be sensitive to a fraction of parameter space which gives no detectable signal with only tree-level contribution.
The sensitivity of DD experiments to these models is also complementary to the constraints from ID and collider search.