Probing hidden symmetry via nonlinear transport in an altermagnet candidate Ca₃Ru₂O₇

X-ray and neutron diffraction are foundational tools for structure determination; however, their resolution limits can lead to misassignments in materials with subtle distortions. Here we demonstrate that nonlinear transport provides a powerful complementary approach to uncover hidden crystal symmetries, using Ca₃Ru₂O₇ as a case study. Below the magnetic transition at T_S = 48 K, our experiment reveals a previously overlooked lower-symmetry phase. This is evidenced by the emergence of longitudinal nonlinear resistance (NLR), indicating combined translational and time-reversal symmetry breaking, and thus rendering Ca₃Ru₂O₇ an altermagnetic candidate in terms of symmetry classification. DFT calculation suggests that the lower-symmetry phase arises from an extremely subtle lattice distortion (~0.1 pm) below T_S, below the detection limit of conventional diffraction. Moreover, NLR is accompanied by nonlinear Hall effect, both enhanced by the large quantum metric associated with Weyl chains. Our findings establish nonlinear transport as a sensitive probe of hidden symmetry breaking and highlight an alternative route to discovering altermagnetic states.