Scientists Are on the Brink of Discovering the Fifth Fundamental Force of Nature

We don’t know for sure, it is one of the possibilities why prediction and result don’t line up.

“If the measurements don’t line up with the prediction [calculated with the standard model], that could be a sign that there is some unknown particle appearing in the loops—which could, for example, be the carrier of a fifth force,” particle physicist Jon Butterworth told The Guardian.

The article overstates the case. I posted it because it's interesting, but the headline is hyperbole. It is the usual situation where an anomalous observation could lead to an important new discovery and a revolution in theory, but it may turn out to be an issue with the experimental setup, a confounding factor that no one has thought of, or some new phenomenon that can after all be accommodated without major theoretical upheavals.

Here's another source that reports that a Russian experiment has obtained results that fit with the Standard Model, so the discrepancy could be caused by any of these other factors, not by a deficiency in the Standard Model itself:

Dreams of new physics fade with latest muon magnetism result

(archive link)

So it's far too early to be saying we're on the brink of a scientific revolution due to these anomalous results. All we know is that the anomaly seems intermittent and is so far unexplained.

It’s in the article, the standard model is proving to be incomplete, not covering new phenomena, it works for other things but there’s something missing