Also not knowing anything about this topic, I'd assume it wasn't normal because we're talking about mass close to zero, and mass must be greater than zero.
The mass in their result is 80433.5±9.4 MeV/c^2. The result of the experiment is a Gaussian like distribution. If you consider a Gaussian distribution with μ=80433.5 and σ=9.4, the probability to get a result that is less than 0 is 4E-15899105.
I filled this widget https://www.wolframalpha.com/widgets/view.jsp?id=53fa34c5c66...
And got this result https://www.wolframalpha.com/input?i=mean%3D%5B%2F%2Fnumber%...
Note in the graphic that σ is 10000 smaller than μ so the probability to get a negative result is almost zero and you can just ignore it.