I'm really excited about the results of Fermilab and J-PARC on the measurement of $(g-2)_\mu$, that is, the anomalous magnetic moment of the muon. The current value of $g-2$ is

\begin{align}
a_\mu^\mathrm{exp}&=0.001\;165\;920\;91(63)\\
a_\mu^\mathrm{SM}&=0.001\;165\;917\;64(52)
\end{align}
where $\mathrm{SM}$ is the full Standard Model prediction, and the uncertainty $(52)$ is mostly just hadronic. There is a $4\sigma$ deviation between theory and experiment, which leads to three possible explanations:

The experimental result is wrong: the errors are underestimated or there are undetected systematic errors in the measurement.

The theoretical calculation is wrong: there is a lot of research about the hadronic contribution because it is very difficult to estimate from first principles. There is a (IMHO, high) chance that the hadronic contribution is miscalculated.

Beyond Standard Model physics: there are unknown particles that contribute to $a_\mu$ (e.g., supersymmetric particles).

There are many planed experiments to constraint the second possibility$^1$, and Fermilab and J-PARC intend to rule out the first one, so that we are certain that the third case is the right one. Therefore, after Fermilab and J-PARC we will probably have the **first quantitative evidence of BSM physics!**

Fermilab is supposed to be running from 2017 to 2018 and will present the results soon after. AFAIK, there is no announced date for J-PARC, but it is expected to begin in the late 2010s.

For more info, see http://arxiv.org/abs/1512.00928

$$ $$ $$ $$

The Fermilab muon ring:

$^1$ and I hope there'll soon be definitive lattice calculations that will settle the uncertainty.

This post imported from StackExchange Physics at 2016-04-20 16:45 (UTC), posted by SE-user AccidentalFourierTransform