Effective field theory interactions for liquid argon target in DarkSide-50 experiment

We reanalyze data collected with the DarkSide-50 experiment and recently used to set limits on the spin-independent interaction rate of weakly interacting massive particles (WIMPs) on argon nuclei with an effective field theory framework. The dataset corresponds to a total $(16660\pm 270)\mathrm{kg}\mathrm{d}$ exposure using a target of low-radioactivity argon extracted from underground sources. We obtain upper limits on the effective couplings of the 12 leading operators in the nonrelativistic systematic expansion. For each effective coupling we set constraints on WIMP-nucleon cross sections, setting upper limits between $2.4\times {10}^{-45}{\mathrm{cm}}^2$ and $2.3\times {10}^{-42}{\mathrm{cm}}^2$ ($8.9\times {10}^{-45}{\mathrm{cm}}^2$ and $6.0\times {10}^{-42}{\mathrm{cm}}^2$) for WIMPs of mass of $100\mathrm{GeV}/{\mathrm{c}}^2$ ($1000\mathrm{GeV}/{\mathrm{c}}^2$) at 90% confidence level.

Figure 1

Expected recoil-energy spectra of argon nuclei in DarkSide-50 from the interaction of $100\mathrm{MeV}/{\mathrm{c}}^2$ WIMPs with the standard halo model velocity distribution for five different EFT operators. Spectra include the acceptance of the detector and are arbitrary normalized. Curve labeled (3) shows the standard spectrum corresponding to the SI operator, i.e., the form factor $M$ in the adopted notation. The other four curves correspond to (1) the nuclear response function $M$ times the factor $v^{\perp 2}{q}^{-4}$, (2) ${\mathrm{\Phi }}^{″}$ times $q^{-2}$, (4) $M$ times the factor $q^2$, and (5) ${\mathrm{\Phi }}^{″}$ times $q^4$.

Figure 2

DarkSide-50 90% C.L. exclusion curves on the cross section parameter ${\sigma }_i$ for the 12 EFT terms as defined by Eq. (7). Going from top to bottom, we see a group of four curves that correspond to the nuclear response function ${\mathrm{\Phi }}^{″}$ times $q^4$, $q^2$, 1, or $q^{-2}$; then a group of eight curves corresponding to the nuclear response function $M$ times $q^2$, 1, $q^2{v}^{\perp 2}$, $q^{-2}$, $v^{\perp 2}$, $q^{-2}{v}^{\perp 2}$, $q^{-4}$, or $q^{-4}{v}^{\perp 2}$. The solid black curve represents the standard spin-independent limit that corresponds to the current limit published in Ref. [25].