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24 September 2026: An unexpected particle bsky.app/profile/kopp...

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New particles discovered at the LHC

That makes it hadron number 86 discovered at the LHC. The full list is here www.koppenburg.ch/particles.html /end

koppenburg.ch

So far 1 fundamental particle has been discovered at the LHC.

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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

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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?

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.

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The quark content says it's an excited strange-charmed meson, which usually decay to a D meson and a strange kaon. But that's not allowed as its mass is too low. So the only chance it has is to violate isospin conservation.

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.

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... .

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...

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).

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

Patrick Koppenburg 🐧 @koppenburg.ch · 23/09/2026

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...

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I grew up in a city that starts at 370m above sea level and reaches more than 700m. From any point you see the lake and the mountains in the South. How can you live in a place that is so flat you can't tell any direction? Really scary.

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Patrick Koppenburg 🐧 @koppenburg.ch · 23/09/2026

Avec l'AI il faut poser des questions comme à un humain. Bizarrement Cluade et Gemini n'ont pas les mêmes goûts littéraires.

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Patrick Koppenburg 🐧 @koppenburg.ch · 22/09/2026

C'est quoi ce dictionnaire ? 😬 Le bon échoué bien demain. Le bon échalotes bien demain. Le bon échouer bien demain.

il échoit bien demain. échoit n'est pas reconnu.

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21 September 2026: A completed multiplet bsky.app/profile/kopp...

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But what about the baryon with three charm quarks? Quantum mechanics forces it to have spin 3/2, and thus it is in another multiplet. The Ωccc⁺⁺ is probably already there in the data, but too rare to be seen. We will likely need the second upgrade of the LHCb detector to see it. /end

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Here's the completed multiplet as a model.

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The observation was made using 2024 data collected by the upgraded LHCb detector.

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Then came the singly charged one earlier this year, and finally the Ωcc⁺ now, with two charm quarks and two strange quarks. The particle appears as a clear excess over the background in a mass plot.

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The first floor was completed in the eighties. The doubly charmed baryons in second floor were all discovered by the LHCb experiment, starting with the doubly charged Ξcc⁺⁺ ("Xi-cc-plus-plus") in 2017.

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What if one adds the charm quark, discovered in 1974? One gets more complicated diagrams. The baryon multiplet we know is at the base, and there's a first floor with one charm quark and a second with two.

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That was for spin-1/2 baryons. If one looks at spin-3/2, where the three quarks have all their spin aligned, more possibilities are allowed. At the very bottom the is the Ω baryon "omega" with three quarks, discovered in 1964.

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The same applies for baryons, this time with three quarks. There's a similar one for anti-baryons. The neutron and proton are at the top. All other particles have one or two strange quarks.

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This scheme explained all mesons known at the time as pairs of a quark and an antiquark for all possibilities of up, down and strange quarks. There are 8 possibilities for up, down and strange quarks and antiquarks. Gell-Mann called this the Eightfold way.

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The word quark was coined by Gell-Mann. Zweigs's Aces and Petermann's spinors did not stick.

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