^ 197 _ 80 Hg decays to ^ 197 _ 79 Au via electron capture, with a...

The nuclear reaction for the electron capture process is given by:

^{197}_{80}\text{Hg} + ^{0}_{-1}\text{e} \rightarrow ^{197}_{79}\text{Au} + \nu_{e}$<br><br>In an electron capture process, a proton in the nucleus captures an inner-shell electron and converts into a neutron. To conserve the lepton number, an electron neutrino ($\nu_{e}) is emitted.

Thus, the other particle emitted in this decay is a neutrino.