A search for Lorentz- and $CPT$-violating signals in the double beta decay spectrum of ${}^{136}\mathrm{Xe}$ has been performed using an exposure of $100\mathrm{kg}·\mathrm{yr}$ with the EXO-200 detector. No significant evidence of the spectral modification due to isotropic Lorentz-violation was found, and a two-sided limit of $-2.65\times {10}^{-5}\mathrm{GeV}<{â}_{\mathrm{of}}^{(3)}<7.60\times {10}^{-6}\mathrm{GeV}$ (90% C.L.) is placed on the relevant coefficient within the Standard-Model Extension (SME). This is the first experimental study of the effect of the SME-defined oscillation-free and momentum-independent neutrino coupling operator on the double beta decay process.

• Received 31 January 2016

DOI:https://doi.org/10.1103/PhysRevD.93.072001