# Explanation of the 511 keV line: cascade annihilating dark matter with the \(^8\)Be anomaly

## Abstract

A possible dark matter (DM) explanation about the long-standing issue of the Galactic 511 keV line is explored in this paper. For DM cascade annihilations of concern, a DM pair \(\pi _d^{+} \pi _d^{-}\) annihilates into unstable \(\pi _d^{0} \pi _d^{0}\), and \(\pi _d^{0}\) decays into \(e^+ e^-\) with new interactions suggested by the \(^8\)Be anomaly. Considering the constraints from the effective neutrino number \(N_{\mathrm {eff}}\) and the 511 keV gamma-ray emission, a range of DM is obtained, \(11.6 \lesssim m_{\pi _d^{\pm }} \lesssim 15\) MeV. The typical DM annihilation cross section today is about 3.3 \(\times \) \(10^{-29}\) cm\(^3\) s\(^{-1}\), which can give an explanation about the 511 keV line. The MeV scale DM may be searched by the DM-electron scattering, and the upper limit set by the CMB s-wave annihilation is derived in DM direct detections.

## 1 Introduction

It has been many decades since the observation of the Galactic 511 keV gamma-ray line in 1970s [1] (see Ref. [2] for a review), while the source of it is still unclear. It was demonstrated by INTEGRAL [3] in 2003 that the 511 keV line originates from the electron-positron annihilations in positronium states. The emission was found to be concentrated towards the Galactic bulge region [4], and few point-like sources have been detected by INTEGRAL [5, 6]. A possible explanation of the 511 keV line is from dark matter (DM) annihilations [7, 8, 9, 10, 11] or decays [12, 13], or new atoms binded [14]. Analysis results of Refs. [11, 15] show that the INTEGRAL signals are more peaked towards the center than the DM density profiles, and the decaying DM origin of \(e^+\) is disfavored. For DM annihilations, the interesting scenario of a pair of DM particles annihilating into a pair of \(e^+ e^-\) with the DM mass \(\lesssim \) 7.5 MeV and the annihilation cross section \(\sim \) \(10^{-30}\) cm\(^3/\)s is allowed by the continuum gamma-ray emission [16, 17], while this scenario seems in tension with the present cosmological data [18]. In addition, the signal is difficult to be explained by the known astrophysical sources [2, 18, 19]. Thus, the main source of the 511 keV emission line is unclear yet.

As the transition mechanism between DM and the standard model (SM) particles is unknown, exotic DM sources can still be a reasonable explanation about the 511 keV line, and possible signatures of new interactions may bridge the hidden sector and the SM sector. Recently, an apparent anomaly with 6.8\(\sigma \) discrepancy was observed in \(^8\)Be transitions [20], and a new vector boson *X* was suggested, with the mass \(m_X \simeq \) 17 MeV and the transition form \(X \rightarrow e^+ e^-\). For possible couplings of *X* with quarks and leptons, the vector form was studied in Refs. [21, 22], and the axial vector form was analyzed in Ref. [23] (for more discussions, see e.g., Refs. [24, 25, 26, 27, 28, 29]). For the possible *X* portal DM investigated in Refs. [30, 31, 32, 33], the DM in ten MeV scale is allowed by the constraints. Apparently, the direct annihilation mode of a DM pair \(\rightarrow {X} \rightarrow \) \(e^+ e^-\) is faint for the 511 keV line.

The *X* portal DM could potentially be the source of the 511 keV line, while the injection of the low energy \(e^+\) becomes a crucial question. This is of our concern in this paper. In the case of a pair of DM particles first annihilating into unstable particles, then converting into SM particles, the required low energy \(e^+\) may be obtained in cascade annihilations (for more about this type annihilations, see e.g., Refs. [34, 35, 36]). Here we consider a scenario of three dark pions are in a triplet in the hidden sector, with two dark charged pions being DM candidates and the unstable neutral pion transiting into SM particles via the *X* boson. The main annihilation channel is a pair of dark charged pions \(\rightarrow \) a pair of neutral dark pions which converting into SM \(e^+ e^-\) pairs during DM freeze-out, and this mode is phase space suppressed today. In this scenario, the required low energy \(e^+\) can be obtained for the 511 keV line. In addition, the constraint from the cosmic microwave background (CMB) observation [37, 38] will be considered in DM annihilations.

For DM with the mass \(\sim \) 10 MeV, the dark sector energy release to the big bang nucleosynthesis (BBN) and the effective number of relativistic neutrino \(N_{\mathrm {eff}}\) at the recombination needs to be considered, and the parameter space will be derived with these constraints. For the ten-MeV scale DM, the DM-electron scattering can be employed in the search of DM [39, 40, 41], and the direct detection of DM will be discussed.

The rest of this paper is organized as follows. The interactions in the new sector will be presented, and the annihilations of DM will be discussed in the next. Then, the numerical analysis with constraints are elaborated. The last part is the conclusion.

## 2 Interactions in the new sector

*X*is obtained, and the light dark quarks \(u_d\) and \(d_d\) are assumed with equal masses. At the energy below the dark QCD-like scale \(\Lambda _N\), the dark chiral symmetry of the light dark quarks is broken and three dark pions are generated, with the two dark charged pions \(\pi _d^{\pm }\) being DM candidates and the neutral dark pion \(\pi _d^{0}\) being unstable. Here the value \(\Lambda _N \sim f_{\pi _d}\) is taken in the dark sector, and \(f_{\pi _d}\) is the dark pion decay constant. For more about a similar case, see the dark pions considered in Ref. [57]. In addition, SM particles are singlets under SU(N)\(_d\). The SM fermions suggested by the \(^8\)Be anomaly [20] couple to the

*X*boson, and the effective interaction mediated by

*X*is taken as \(\mathcal {L}_I^i\) = \(- X_\mu J^\mu _{\mathrm {SM}}\), where the SM fermion current \(J^\mu _{\mathrm {SM}}\) is

*X*, both vector and axial couplings are included for SM fermions. For the excited states of Beryllium, the anomaly was observed in the transition of isoscalar \(^8\mathrm {Be}^*\), while the isovector \({^8\mathrm {Be}^*}^\prime \) was not. This can be naturally explained by the primary axial couplings of

*X*to quarks [21, 22, 23], and the couplings suggested by the \(^8\)Be anomaly are [23]

*X*. In the process \(\pi _d^{0}\) \(\rightarrow e^+ e^-\), the corresponding matrix element is taken as

*v*is \(v \approx \) 246 GeV, and \(\theta \) is a parameter with \(|\theta | \ll 1 \).

Here the particles we focus on are relevant to the main interactions between DM and SM particles, and we should keep in mind that there may be more particles in the dark sector.

## 3 Annihilations of DM

*s*is the total invariant mass squared, and the factor \(\beta _0\) is \(\beta _0 =\) \( \sqrt{1 - 4 m_{\pi _d^{0}}^2 / s}\). Here we consider that the \(\sigma _0^{} v_r\) gives the main contribution to the annihilation cross section during DM freeze-out, and the value of \(f_{\pi _d}\) can be obtained with the DM relic density inputted. The DM cascade annihilation is phase space suppressed today. In the nonrelativistic limit, one has

*s*= 4\(m_{\pi _d^{\pm }}^2 + m_{\pi _d^{\pm }}^2 v_r^2 + \mathcal {O} (v_r^4)\), and

*T*of the universe cooling to \(m_{\pi _d^{0}}\). In this case, the number densities of dark pions are of their equilibrium values before DM freeze-out. At \(T \sim \) \(m_{\pi _d^{0}}\), the effective lifetime \(\tau _{eff}\) of \(\pi _d^{0}\) is

*T*. Thus, we can derive a limit (see e.g., Ref. [62] for more)

## 4 Numerical analysis

### 4.1 DM mass with constraints of \(N_{\mathrm {eff}}\)

For DM with the mass \(m_{\pi _d^{\pm }}\) \(\lesssim \) 15 MeV, the main annihilation product in SM sector is \(e^+ e^-\). For thermal freeze-out DM, the value of the parameter \(x_f = m_{\pi _d^{\pm }} / T_f\) is about 20, where \(T_f\) is the freeze-out temperature of DM. This means that the electron-photon plasma in the early universe was heated by the energy injected from dark pions after the decoupling of the electron neutrinos (a typical decoupling temperature \(T_d\) for neutrinos is \(T_d \sim \) 2.3 MeV [64]). The abundance of light elements from BBN and the CMB power spectra at recombination time can be altered by the energy injection. These effects can be considered in terms of the effective number of the relativistic neutrinos \(N_{\mathrm {eff}}\), and the expectation value is \(N_{\mathrm {eff}}\) = 3.046 in the standard cosmological prediction [65, 66]. Considering the constrains on \(N_{\mathrm {eff}}\) with the Planck data [37], a lower bound \( N_{\mathrm {eff}} \gtrsim \) 2.9 is adopted [30]. Moreover, as the mass \(m_X \gg T_d\) and the lifetime of *X* \(\ll \) 1 second, the contribution from *X* boson is negligible.

*y*is equal to \(p_{\pi _d} / T_{\gamma }\). For dark pions, the degree of freedom is \(g_B^{}\) = 3. The result of the effective number \(N_{\mathrm {eff}}\) as a function of \(m_{\pi _d} / T_d\) is depicted in Fig. 1. Considering the lower bound \(N_{\mathrm {eff}} \gtrsim \) 2.9, we obtain the result \(m_{\pi _d} / T_d \gtrsim \) 5.8 for dark pions. As the neutrino decoupling is not a sudden process [64, 65, 66, 69], a lower bound \(T_d \gtrsim \) 2 MeV is adopted here. Now, taking \(m_{\pi _d^{\pm }}\) as the value of \(m_{\pi _d}\), the mass range of DM is

### 4.2 Parameters for the 511 keV line

Now, we turn to the constraint from CMB. The CMB constraint on the annihilation mode of a DM pair \(\rightarrow \) \(e^+ e^-\) was derived in Ref. [38]. For the DM of concern, the revised upper limit of the similar mode can be obtained via the DM mass multiplied by two and the corresponding upper limit doubled compared with that in Ref. [38], i.e., the annihilation cross section of the process \(\pi _d^{+} \pi _d^{-}\) \(\rightarrow \) \(\pi _d^{0} \pi _d^{0}\) should be \(\lesssim \) (6−7)\(\times 10^{-30}\) cm\(^3\)/s at the recombination. In the DM temperature \(\sim \) 0 limit, \(\Delta / m_{\pi _d^{\pm }} \lesssim 5 \times 10^{-10}\) is adopted. The dark charge coupling parameter \(\alpha _d^{}\) (like \(\alpha \) in SM) is related to the mass difference \(\Delta \) [57, 58, 59, 60, 61], and this means \(\alpha _d^{} \lesssim \) \(10^{-13}\) with the constraint of CMB. Thus, we neglect the contribution via dark charge couplings in DM annihilations and self-interactions.

### 4.3 Direct detections of DM

*q*is of order \(\alpha m_e\), with the recoil energy of a few eV. The form of DM-electron scattering given by Ref. [73] is adopted. Here, the DM-electron scattering cross section is

## 5 Conclusion

A DM interpretation of the Galactic 511 keV line has been demonstrated in this article, i.e., a DM pair \(\pi _d^{+} \pi _d^{-}\) annihilates into unstable \(\pi _d^{0} \pi _d^{0}\), and \(\pi _d^{0}\) decays into \(e^+ e^-\) with new interactions suggested by the \(^8\)Be anomaly. Considering the constraint from \(N_{\mathrm {eff}}\) and the 511 keV gamma-ray emission, a mass range of DM is obtained, \(11.6 \lesssim m_{\pi _d^{\pm }} \lesssim 15\) MeV. For the DM annihilations today, the typical DM annihilation cross section is about 3.3 \(\times \) \(10^{-29} \mathrm {cm}^3 \mathrm {s}^{-1}\), which is of the order required by the 511 keV line. Thus, the DM of concern can give an explanation about the 511 keV line.

The MeV scale DM can be searched by the DM-electron scattering. In this paper, we consider the DM-electron scattering mainly mediated by \(\phi \), and an upper limit of this scattering is set by the CMB s-wave annihilation (for the DM s-wave annihilation mediated by \(\phi \)), with \(\bar{\sigma }_e \lesssim 10^{-44}\) cm\(^2\). The Ge detector at low temperature of a few K can be employed to search for MeV scale DM, with the scattering cross section \(\bar{\sigma }_e\) reaching about (5−8)\(\times 10^{-45}\) cm\(^2\) in a 1 kg-year exposure [75]. For the DM of concern, the lower detection bound from the neutrino background may be reached in about 50 kg-year exposure by the Ge detector. Moreover, if there is a vectorial interaction between scalar DM and *X*, as discussed in Ref. [30], the CMB constraint on DM-electron scattering may be relaxed. Briefly, the DM p-wave annihilation mediated by *X* should be minor during DM freeze-out, e.g., with the *X*’s contribution \(\lesssim \) 10%. In this case, the DM-electron scattering cross section becomes less than/or similar to 10% of the scalar DM-electron scattering case obtained by Ref. [30], i.e., a more broad range of the cross section allowed. We look forward to the MeV scale DM search via the future DM-electron scattering experiment.

## Notes

### Acknowledgements

This work was supported by the National Natural Science Foundation of China under Contract No. 11505144, and the Longshan academic talent research supporting program of SWUST under Contract No. 17LZX323.

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