Your thread title is overstated and misleading.
That is... weird.
I say that because that mass is pretty low. At that mass, particle accelerators should have been creating these things by the dozens, for years. To only see two is very strange.
Or, it could be a very tiny elephant named Bingbong. I am unimpressed by such statements to the media. This sounds flashy, but this result has far too little verification to support such speculation.
Neither. This is one team (who, by the way, seem to have announced new particle finds in the past, and then retracted those announcements without explanation), with only a couple of detections. That's not a new force, or new science, yet. It seems that no other teams have replicated the results. We'll want far more detections to happen to nail down that the thing is real, and not some anomaly in their mechanisms, or other form of error, before we start really considering this to be solid new science.
Most physicists think the Standard Model is incomplete.
Although it's too soon to claim a fifth force, it cannot be dismissed by an over-hyped title. That's bad science.
And, I didn't dismiss it based on the title. I gave some very clear reasons why this particular claim should be viewed with high skepticism. I made exactly zero statements rejecting a fifth force, in general. So... bit of a strawman, there.
On the other hand, there are experiments that look for specifically this sort of thing (not the LHC which produces relatively few high-energy collisions, but other experiments that produce very many lower-energy ones), and they haven't found it.
It's not error of the other experiments as much as they just haven't necessarily accessed the right parameter range yet necessarily. But, yes, the test is reproducing the results, both in similar experiments by other groups and different types of experiments that should be able to detect a new particle of the proposed type in a distinct way.Such a low-energy quantum could lie within the acceptable error of the experiments. The true test is when someone besides the original experimenters try to reproduce the same results.