Discovery article (also here)
Note: use of second magnet (with opposed field sign) in order to cross-check momentum measurement.
This prevents the possibility of background from ψ and its resonances (multiple scattering or other sources of widening).
Software/apparatus check: mix muons belonging to different events. If the peak is a spurious effect due to enhanced/depressed bins, you should see it also in these fake pairs.
Check that the signal is related to the target: runs with the target out.
Yoh's recollection
Decay muons contribute to ~ 90% of the events with muon pairs, but they can be subtracted by looking at same-sign pairs.
In order to reduce multiple-scattering effect on the resolution: put the densest absorber closest to the interaction point, and only low-Z absorbers afterwards. This leads to smearing of production angle, but not so much of momentum.
(Reason: the dense absorber is before the magnet.)
Using muons instead of electrons has an advantage: with the absorbers you get rid also of the neutrals. The reason why Ting's experiment only detected in a small angle was to avoid the central region, where the background was too high. With muons they could extend the acceptance by 3-5 times.