What is the order parameter of hidden order in URu2Si2
In plain words
Below 17.5 K, URu2Si2 enters an ordered phase that releases a large amount of entropy, yet nearly forty years of experiments have not identified what orders.
Precise statement
Identify the order parameter of the phase below $T_{\mathrm{HO}} = 17.5\ \mathrm{K}$ in URu2Si2, which has no ordered magnetic moment of matching size and doubles the unit cell along c (wavevector $(0,0,1)$). Candidates include higher-rank multipoles (hexadecapole, dotriacontapole), hastatic order and chirality density waves. An answer gives the symmetry (irreducible representation) and microscopic character, confirmed by a probe that couples directly to it (resonant x-ray scattering, Raman, NMR).
What would settle it
A direct detection of the proposed order parameter whose temperature dependence follows $T_{\mathrm{HO}}$ and vanishes under pressure-induced antiferromagnetism.
Status in the literature
Raman data (2015) show an A_2g mode suggesting chirality-density-wave order, while reports of in-plane fourfold symmetry breaking disagree between probes.