First Search for Lorentz and CPT Violation in Double Beta Decay with EXO-200

A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of ^(136)Xe has been performed using an exposure of 100  kg⋅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 −2.65×10^(−5) GeV < â^((3))_(of) < 7.60×10^(−6) 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.