Search for Boosted Dark Matter in COSINE-100

We search for energetic electron recoil signals induced by boosted dark matter (BDM) from the galactic center using the COSINE-100 array of NaI(Tl) crystal detectors at the Yangyang Underground Laboratory. The signal would be an excess of events with energies above 4 MeV over the well-understood background. Because no excess of events are observed in a $97.7\mathrm{kg}·\mathrm{yr}$ exposure, we set limits on BDM interactions under a variety of hypotheses. Notably, we explored the dark photon parameter space, leading to competitive limits compared to direct dark photon search experiments, particularly for dark photon masses below 4 MeV and considering the invisible decay mode. Furthermore, by comparing our results with a previous BDM search conducted by the Super-Kamionkande experiment, we found that the COSINE-100 detector has advantages in searching for low-mass dark matter. This analysis demonstrates the potential of the COSINE-100 detector to search for MeV electron recoil signals produced by the dark sector particle interactions.

Figure 1

(a) An example of a saturated event due to the limited dynamic range (12 bit, 4096 for 2.5 V) is presented as a black solid line. Reconstruction of the saturated event (red dashed line) is achieved by comparison with the template from unsaturated events (thin blue solid line). In this example, a 6.02 MeV energy is reconstructed. (b) The measured energy spectra before (black-solid line) and after (red dots) reconstruction of the saturated events are presented. The reconstructed energy spectrum is calibrated with 7.6 and 7.9 MeV $\gamma$-rays from the neutron capture of iron and copper, respectively, as shown in the blue dotted line.

Figure 2

The summed energy spectra for the five crystals (black solid circles) and the best fit (green solid lines) with BDM signal of $m_1=6$, $m_X=13\mathrm{MeV}$, ${\gamma }_1=20$, $\epsilon =8\times {10}^{-4}$ are presented for the single-hit events (a) and the multiple-hit events (b). Fitted contributions to the background from $\beta /\gamma$ radiation, neutron-capture, and muon mis-tag are indicated. The green bands are the 68% confidence level (C.L.) intervals of the uncertainties obtained from the likelihood fit. For presentation purposes, we draw the BDM signal shapes assuming BDM parameters of $m_1=10\mathrm{MeV}$, $m_X=1\mathrm{MeV}$, ${\gamma }_1=50$, $\epsilon =3\times {10}^{-4}$ and $m_1=6\mathrm{MeV}$, $m_X=13\mathrm{MeV}$, ${\gamma }_1=20$, $\epsilon =8\times {10}^{-4}$ with $\times 30$ amplification of the signal amplitude.

Figure 3

Interpretation of the BDM searches with 1.7 years of COSINE-100 data in terms of the dark photon mass (${\mathrm{m}}_X$) and coupling ($\epsilon$) parameters is presented. (a) When $m_X<2m_1$, our limits are compared with the currently excluded parameter space (shaded region) for the visible decay mode, including results from E141 [68], NA48 [69], NA64 [70], Babar [71], as well as bounds from the electron anomalous magnetic moment [72] and our previous IBDM search with 59.5 days of COSINE-100 data [24]. (b) When $m_X\ge 2m_1$, our limits are compared with the currently excluded parameter space (shaded region) for the invisible decay mode from NA64 [73] and Babar [74] experiments.

Figure 4

Interpretation of the BDM searches with $97.7\mathrm{kg}·\mathrm{years}$ of COSINE-100 data in terms of the relic dark matter mass ($m_0$) and $\epsilon$ parameters is conducted, assuming $m_1=20$ and $m_X=2\mathrm{MeV}$. This result is then compared with limits from $161.9\text{kiloton}·\mathrm{years}$ of SK IV data, assuming $m_1=200$ and $m_X=20\mathrm{MeV}$ [23]. To ensure a proper comparison with the SK result, we assume the coupling constant between ${\chi }_1$ and $X$ through elastic scattering to be 0.5.