If physicists weren't jumping up and down with excitement in April at the announcement that an unknown particle had been glimpsed at Fermilab, they are now.

The news of a possible particle sighting in the debris of proton-antiproton collisions at the Illinois accelerator had been met with a mix of curiosity and scepticism. It was based on an analysis of eight years of data collected by Fermilab's CDF experiment that looked at collisions that produced a W boson, carrier of the weak nuclear force, along with two jets of quarks.

A suspicious bump in the data showed an unexpected rise in the number of these events clustered around 145 GeV – suggesting that they are being produced by an unidentified particle of the same mass. It was immediately clear that whatever the particle was, it was not predicted by the standard model of physics, the leading theory for how particles and forces interact. To add to the mystery, it was clearly not a Higgs boson, the long-sought particle that gives other particles their mass.

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