Observation of the doubly charmed heavy proton Ξcc+

3/18/2026 at 8:12:41 PM

This is significant because binding 2 heavy quarks together is very hard to do because they decay so fast.

All the particles made from just 3 light quarks have been found. And I think all of the ones made with 1 heavy quark and 2 light ones have also been found.

This is only the 2nd particle made from 2 heavy quarks that has ever been found.

It's not a coincidence that both of those were made with charm quarks, because charm quarks are the lightest of the heavy ones.

None made of 3 heavy quarks has been found (yet).

Also: A particle made with a top quark is not possible - it decays too quickly.

by ars

3/18/2026 at 1:31:16 PM

Are there are other flavours of proton still to be discovered? Can you (theoretically) build a proton with any two quarks selected from {up, charm, top} and one selected from {down, strange, bottom}?

by steve_gh

3/18/2026 at 4:34:18 PM

Yes, also mixes like up+top+strange. There is a list in https://en.wikipedia.org/wiki/List_of_baryons but note the note:

> Particles with † next to their names have been predicted by the Standard Model but not yet observed.

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There are some technical details, because QM rules makes everything more complicated and you can potentially have a mix of quarks. I (mis)remember something weird in barions, but I can't find what it was.

But there is something weird in mesons. For example the pi_0 is 50% up+antiup and 50% down+andidown . More details inhttps://en.wikipedia.org/wiki/List_of_mesons

by gus_massa

3/18/2026 at 4:23:54 PM

Good questions…

Another one: Is this newly discovered particle stable, or does it only exist for a short time before transforming in some way? I didn’t see any mention of that in the article.

Is it theoretically possible to have atoms with 4x the proton mass, starting with hydrogen at atomic weight 4? Pu-239 could theoretically have an atomic weight of 521 instead of 239… Wild!

by Recurecur

3/18/2026 at 5:43:30 PM

Haven't read this article but I think the one I read yesterday about this said it was short lived on the millionth of a millionth of a second scale.

by rigrassm

3/18/2026 at 8:21:20 PM

It's not stable, and no it's not theoretically possible.

A proton is the lightest stable baryon, and thus the only only stable one. It's not a coincidence - in particle physics if a lighter elementary particle is possible the heavier one will ALWAYS decay into it. "Whatever is not forbidden is mandatory." (Combination particles like atoms are more complicated because there are other things that might force the particle to exist.)

by ars

3/19/2026 at 4:47:28 PM

I agree.

The closest example I can remember is that you can have atoms with muons instead of electrons for a shot time ~2.2E-6 seconds, that is a pretty long time for for an unstable particle. You can do some chemistry in that "long" time. (Can you put a muon around a heavy atom like gold and get some extra time for special relativity corrections?)

If you want to replace protons, I guess you can try with "strange" particles instead of "charmed" particles. The difference of mass is small, like only a 10% more instead of a x4 increase. In particular, the sigma particle (up+up+strange) has a half life of ~2E-10 seconds that is shorter than the half life of a muon but much longer that the half life of this new particle. (I can find the number, but let me handwave a ~~~1E-22 seconds(???).)

by gus_massa

3/18/2026 at 8:14:55 PM

It's not actually a proton. And yes you can built a particle made from any combination. I posted elsewhere in the thread with a bit more details.

by ars