Comagnetometer Bounds on a Background Axial Torsion Field

If spacetime carries a background axial torsion — a preferred spatial direction coupled to spin, left over from cosmology or generated by distant polarised matter in theories where torsion propagates — the most sensitive detectors for it are comagnetometers, which compare the precession of two spin species to cancel magnetic noise while retaining sensitivity to spin-only couplings. This paper converts representative comagnetometer sensitivities into torsion bounds. A potassium / helium-3 instrument resolving anomalous spin energies of 1e-26 eV bounds a homogeneous axial torsion at 5.07e-20 per metre; a neutron / mercury instrument at 1e-25 eV bounds it at 5.07e-19 per metre. For scale, the torsion that Earth's rotation would source if bulk rotation counted as spin density — it does not in Einstein-Cartan theory, where only intrinsic spin polarisation sources torsion — is 3.36e-22 per metre, a factor of about 151 below the potassium / helium-3 bound: the instruments are within two orders of magnitude of even that misconceived benchmark, while the torsion genuinely predicted around an unpolarised Earth is zero. The paper also states what the bound does not do: it does not constrain the contact torsion inside polarised matter, which no background-field search can see. Computed from stated assumptions, not measured.