mrienstra

I don't claim to understand any of this, but it sure sounds like you are correct, looking at the terminology.

Someone had already added the paper to the "Negative mass" article, so I removed it:

https://en.wikipedia.org/w/index.php?title=Negative_mass&dif...

... And added it to the "Effective mass (solid-state physics)" article:

https://en.wikipedia.org/w/index.php?title=Effective_mass_(s...

The bit I removed from the "Negative mass" article said:

In April 2017, researchers at Washington State University claimed to have created a fluid with negative mass inside a Bose-Einstein Condensate.

I expanded it ever so slightly to read:

In April 2017, researchers at Washington State University claimed to have created a fluid with negative effective mass inside a Bose–Einstein condensate, by engineering the dispersion relation.

Slightly awkward, but hopefully at least a slight improvement.

PS: also added a "disambiguation" thingy at the top ("For negative mass in theoretical physics, see Negative mass."), now realizing it may be of dubious utility.