The Herschel Mirror as a Dish Antenna: An Archival Total-Power Search for a 541.7 GHz Line (Working Note)

Working note, version 1.0 (2026-10-15). AI-assisted research programme; not peer reviewed. We used the 3.5 m primary mirror of the Herschel Space Observatory as a dish antenna and archival HIFI band-1a heterodyne data as the receiver to search for a line at 541.664 GHz (m = 2.240 meV), the only vacuum-induced quantum mass of a gravity framework that ties the galactic acceleration scale to the dark-energy density (fitted coefficient kappa = 1/2). Because a mirror-generated line cancels in reference-subtracted archive spectra, we rebuilt total-power spectra from raw Level-0 counts against Herschel's internal hot and cold loads and stacked them in the spacecraft frame. From 49 observations (7.0 h, 150 spectra) we find no line: a = -6.7 +- 7.7 mK (z = -0.86); 95% upper limit 12.7 mK, i.e. kinetic mixing epsilon < 1.9e-11 for a dish-coupling factor S = 50 (8.6e-12 to 8.6e-11 for S = 10-1000). All pre-registered controls pass (injection, null offsets, IF shuffle, Level-1 anchor) and a mutation test recovers an injected line at 9 sigma. A 3.2 sigma excess at 541.700 GHz is reported as a hint only. The direct, cosmology-independent bound closes the window in which this field could be converted to light at useful efficiency (epsilon about 4-6e-9) by about 200x; it does not reach the framework's falsification level (about 1e-13). Coupling factor, sideband gain and local density are provisional. AI assistance: Anthropic's Claude.