Extended Data Fig. 3: Reconstructed invariant mass spectra.

a, b, Reconstructed muon-pair invariant-mass spectra for the liquid hydrogen (a) and liquid deuterium (b) targets. In the lower mass region, the predominant signal is produced by J/ψ → μ⁺μ⁻ decay, followed by μ⁺μ⁻ decay of ψ′. The prominence of the J/ψ peak provides a calibration point for the absolute field of the solid iron magnet. At invariant masses above 4.5 GeV/c², the Drell–Yan process becomes the dominant feature. The data are shown as red points. Additionally, Monte Carlo (MC) simulated distributions of Drell–Yan, J/ψ and ψ′, along with measured random-coincidence and empty-target backgrounds, are shown. The sum of these is shown in the blue solid curve labelled ‘MC sum’. The normalizations of the Monte Carlo and the random background were from a fit to the data.