The article doesn't indicate what the physical manifestation of negative-frequency wave would be. It only reports the observation of a positive-frequency wave t

The article doesn't indicate what the physical manifestation of negative-frequency wave would be. It only reports the observation of a positive-frequency wave that seems to require the transient existence of a negative-frequency intermediate. This is similar to how particles like the Higgs boson are not directly detected in particle accelerators, but are inferred from the directly-observable particles that they produce. The difference is that physicists are (I think) pretty sure that the Higgs actually physically exists, if only for a short time, while the same is not clear for the negative-frequency intermediate wave described here.