Hastatic Order in $URu_2Si_2$

The hidden order that develops below 17.5K in $URu_2Si_2$ has eluded identification for twenty-five years. Here we show that the recent observation of Ising quasiparticles in $URu_2Si_2$ suggests a novel ``hastatic order'' {(Latin:\sl spear)},with a two-component order parameter describing hybridization between electrons and the Ising $5f^{2}$ states of the uranium atoms. Hastatic order breaks time-reversal symmetry by mixing states of different Kramers parity; this accounts for the magnetic anomalies observed in torque magnetometry and the pseudo-Goldstone mode observed in neutron scattering. Hastatic order is predicted to induce a basal-plane magnetic moment of order $0.01\mu_{B}$, a gap to longitudinal spin fluctuations that vanishes continuously at the first-order antiferromagnetic transition and a narrow resonant nematic feature in the scanning tunneling spectra.