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A bit of history of particle physics that takes an unexpected turn today. In 2003 the BaBar experiment at SLAC reported a particle decaying to a Ds meson and a neutral pion, with a mass of 2317 MeV. arxiv.org/abs/hep-ex/0...

They also saw a peak in the combination of a Bs meson, a pion and a photon, but were unsure what it was.

I was at Belle in Japan at the time and we had the data to confirm this observation. So I wrote a quick analysis in the morning and after lunch I had a peak with 340 events (I no longer have the plot, sadly).

Things went very fast. A week later the CLEO experiment confirmed the particle at 2317 MeV and also that at 2460. arxiv.org/abs/hep-ex/0...

And in Summer Belle reported the quantum properties of the two particles arxiv.org/abs/hep-ex/0... and arxiv.org/abs/hep-ex/0... .

So what is this particle at 2317 MeV? The decay to Ds and pion says it's a strong interaction process. But that should be forbidden as it does not respect isospin, a symmetry between light quarks that the strong interaction conserves.

Quantum mechanics doesn't like that and prevents it by making it long lived. Well, it's 10⁻²² seconds "long", but that's 10 times more than usual strong decays. Experimentally what you see is that the peak is rather narrow.

The exact nature of the two mesons is still disputed. Are they tetraquarks? And are they unique? Can similar states be found in the b-quark sector?

LHCb @lhcb.social.bsky says YES ! We report the observation of a narrow particle decaying to a Bs meson and a neutral pion. The exact same thing but with a beauty quark replacing the charm quark. arxiv.org/abs/2609.25178