[gus_massa]

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.