Good question. It's mostly the third — but let me unpack that.
The Standard Model predicts a specific value for the weak mixing angle, which determines the electron's vector coupling. My measurement at SLAC, along with other SLD measurements, consistently preferred a slightly different value than what LEP (the European competitor experiment) found using a different technique.
The key word there is "different technique." SLD used a polarized beam of electrons — a completely novel approach at the time — which gave us direct access to the left-right asymmetry without needing to untangle final-state effects. LEP extracted the same parameter from b-quark forward-backward asymmetry. Two fundamentally different methods probing the same physics, with different systematic exposures, giving different answers.
Both experiments had good resolution. We spent enormous effort characterizing the systematics, and they're small compared to the statistical uncertainty. But the two most precise determinations of this parameter disagreed at roughly the 3-sigma level — and that disagreement has never been explained. The world average splits the difference, and the Standard Model prediction is consistent with that average, so you could say "the SM is fine" if you squint. But nobody knows why the two experiments don't agree with each other.
It could be an unidentified systematic error in one experiment. It could be that something beyond the Standard Model is subtly shifting one measurement and not the other. That ambiguity is exactly what makes it a "dangling thread" rather than a resolved question.
The Standard Model predicts a specific value for the weak mixing angle, which determines the electron's vector coupling. My measurement at SLAC, along with other SLD measurements, consistently preferred a slightly different value than what LEP (the European competitor experiment) found using a different technique.
The key word there is "different technique." SLD used a polarized beam of electrons — a completely novel approach at the time — which gave us direct access to the left-right asymmetry without needing to untangle final-state effects. LEP extracted the same parameter from b-quark forward-backward asymmetry. Two fundamentally different methods probing the same physics, with different systematic exposures, giving different answers.
Both experiments had good resolution. We spent enormous effort characterizing the systematics, and they're small compared to the statistical uncertainty. But the two most precise determinations of this parameter disagreed at roughly the 3-sigma level — and that disagreement has never been explained. The world average splits the difference, and the Standard Model prediction is consistent with that average, so you could say "the SM is fine" if you squint. But nobody knows why the two experiments don't agree with each other.
It could be an unidentified systematic error in one experiment. It could be that something beyond the Standard Model is subtly shifting one measurement and not the other. That ambiguity is exactly what makes it a "dangling thread" rather than a resolved question.