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I'll be honest, I only have a Highschool education, so there might be something obvious I'm overlooking. However particle physics is of massive Interest to me.

My question is, I know there are unstable quark-antiquark pairs that form Mesons, but are there any Baryons that are not wholly made up of quarks or antiquarks? i.e. "up, up, anti-down"

If the answer is no, why not?

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    $\begingroup$ I don't think you can have $qq\overline{q}$ and respect colour confinement. $\endgroup$ – jacob1729 Oct 11 at 15:23
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    $\begingroup$ "I'll be honest, I only have a Highschool education" ... that flashback to trying to make sense of a description of basic QCD ideas from some popular science library book in the early 80s and no Internet (I didn't even understand these were new ideas, not something discovered before WWII). $\endgroup$ – David Tonhofer Oct 12 at 21:03
  • $\begingroup$ Idk what I'd do without the internet to be honest. $\endgroup$ – Stefan Oct 13 at 0:56
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    $\begingroup$ @Stefan , these days EVERYONE has only "thin knowledge" (due to the internet) $\endgroup$ – Fattie Oct 13 at 21:47
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No, a three-quark baryon can not be be made out of two quarks and one anti-quark (and vice versa) as this would necessarily give the particle color.

Each quark carries one of three colors (red, blue, green) and each anti-quark respectively carries anti-color. Color is an additive quantity when constructing particle and the result must be color-neutral, i.e. it either is made out of for example red + anti-red or red + blue + green quarks. Although we can not observe the "color" of the constituents of a particle directly (as all observable particles must be color-neutral), we can measure its effects indirectly via certain cross-sections.

With this being said, we can now clearly see that there is no way in which we can construct particles of two-quarks and one anti-quark as any possible combination would not be color-neutral. Hence, color-neutrality forbids observable three-quark particles composed out of quarks and anti-quarks.

Penta-quarks however can and must contain quarks and anti-quarks as mentioned in a previous answer.

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    $\begingroup$ Color neutral means the total color must be "white". If you mix all colors (think of an inverse rainbow) you get white. Color and anticolor also add up to white. $\endgroup$ – infinitezero Oct 12 at 0:41
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Pentaquarks contain three quarks and a quark-antiquark pair, and they are baryons, since baryons are defined as having an odd number of valence quarks.

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    $\begingroup$ So there aren't any known 3 quark baryons which have 2 quarks and an antiquark or vice versa? $\endgroup$ – Stefan Oct 11 at 15:16
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    $\begingroup$ They are neither theoretically possible nor known. $\endgroup$ – infinitezero Oct 12 at 13:12
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    $\begingroup$ Pentaquarks not just can but must contain a quark-antiquark pair. $\endgroup$ – Hearth Oct 12 at 15:30
  • $\begingroup$ @Hearth That is correct, thank you. I edited my answer. $\endgroup$ – Danny Oct 13 at 19:57
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Yes, there are some oddball baryons that contain four quarks and one anti-quark. They're called pentaquarks. These could be thought of as a baryon with three quarks plus a meson with a quark anti-quark pair, but sticking to each other more than they should. There are few hadrons that survive more than a tiny fraction of a second, and only the proton and neutron live longer than one second. The "sticky" baryon-meson combinations are found through analysis of scattering data at the major particle accelerators.

The known pentaquarks are: $P_c(4450)^+$, $P_c(4380)^+$ ([CERN 2015][1], [Arxiv paper][2]) and $P_c(4312)^+$ ([CERN 2019][3]) - oh but wait, the 4450 one seems to really be two pentquark peaks close together in mass - 4440 and 4457.[CERN][4] BTW, the subscript 'c' indicates that one of the constituent quarks is a c (charmed) quark.

There's a tetraquark - two quarks and two anti-quarks in one bag - with mass 4430, named $Z(4430)^+$ (nothing to do with the Z boson). [CERN][5] Can it be described as two mesons being "sticky" with each other or is it better described as a more tightly-knit group of four of the little buggers?

Another possible exotic hadron is made of six quarks all stuck together. This is a "dibaryon". When I was an undergrad in physics, I worked with Dr. Yokosawa at Argonne National Laboratory on finding bumps in polarized proton-proton scattering. That was an interesting experience. As of last time I checked the literature, there was [only one found][6] that stood up to analysis, but always there are candidates due to unexplained wiggles in cross-section vs energy plots. These usually turn out to be something less exotic. Interesting, but since anti-quarks aren't involved, never mind!

[1]: https://home.cern/news/press-release/cern/cerns-lhcb-experiment-reports-observation-exotic-pentaquark-particles CERN’s LHCb experiment reports observation of exotic pentaquark particles (14 JULY, 2015)

[2]: https://arxiv.org/abs/1507.03414 Observation of J/ψp resonances consistent with pentaquark states in Λ0b→J/ψK−p decays (LHCb collaboration)

[3]: https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark LHCb experiment discovers a new pentaquark

[4]: http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Pentaq 26 March 2019: Observation of new pentaquarks

[5]: http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Z%284430%29 9 April 2014: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model.

[6]: https://arxiv.org/abs/1610.05591 On the History of Dibaryons and their Final Observation, H. Clement

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