Doubly charming tetraquark

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TL;DR Summary
LHCb discovered ##T_{cc}^+##, with two charm quarks and two light antiquarks
LHCb has discovered a couple of tetraquarks and pentaquarks in the last years. Recently they added one more, but this one different from the previous discoveries.

Tetraquarks have two quarks and two antiquarks. If all of them are light (up/down/strange) it's basically impossible to measure them and experimentally confirm that they actually have four quarks, so people mainly focus on tetraquarks with at least one heavy quark. LHCb has found two with one charm quark, but most of them have a charm quark and an anti-charm quark (all other quarks are light in each case). Charm and anticharm are produced together by the strong interaction anyway, so it's a likely combination. One discovered tetraquark consists of two charm quarks and two anti-charm quarks.
Last month LHCb has announced the discovery of ##T_{cc}^+##, which has two charm quarks, an anti-up and an anti-down quark (and its antiparticle, which is always included here). It's the most charming tetraquark ever!

With two heavy quarks of the same type it is relatively easy to describe for theorists. The two charm quarks almost behave like classical particles in this system, and the light quarks don't change that much. Unlike for other exotic particles there were useful predictions for its mass, and the comparison with the measured mass helps refining the models. The ##T_{cc}^+## can decay to two D0 mesons and a pion and its mass is only a little bit above their combined mass. That puts it close to the sum of masses of ##D^{*+}+D^0##, so theorists study if the particle can be described as a molecule made out of these two mesons. It's a common pattern that tetraquarks can often be found close to the sum of masses of two mesons and pentaquarks near the sum of masses of baryon and meson. If the molecules are a good description then we are seeing some sort of "nuclear chemistry" at work.

Similar to ##T_{cc}^+##, there should be ##T_{bb}## with two bottom quarks instead of charm quarks. It is predicted to be bound quite tightly and with an energy too low to decay via the strong interaction. It would have to decay via the weak interaction, which means it should be long-living and fly maybe a few millimeters before decaying. That would give unprecedented insight into the properties of tetraquarks - you can find all associated particles, you can look for many more decay modes and so on. Unfortunately bottom quarks are rare compared to charm, and you need two of them.

LHCb announcement
Detailed article by Tommaso Dorigo
 
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  • #2
snorkack
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Can you summarize known polyquarks?
There is one known and stable light hexaquark, that behaves plainly as a nucleus consisting of two separate nucleons. Deuteron also is longer lived than one of the component nucleons.
Protonium also possibly counts as a hexaquark. Is protonium bound by electromagnetic or strong forces?

Now, there don´t seem to be any light pentaquarks. Interactions between nucleons and pions do not seem to form bound nuclei. Neither do there appear to be any light tetraquarks - interaction between two pions does not seem to form bound nuclei either.

Which heavy tetraquarks, pentaquarks and hexaquarks are confirmed to exist?
 
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Protonium has a binding energy of less than 1 keV, so it's two well-separated hadrons. Deuterium can be described nicely as bound state of proton and neutron. In a true hexaquark the quark content should be more important.
Now, there don´t seem to be any light pentaquarks.
Or we just can't find them. They are probably very short-living, and even if you find any resonance, how do you distinguish some wide ##uddd\bar{u}## resonance from a wide ##udd# resonance? There are tons of poorly measured light quark resonances. Same for tetraquarks.
Which heavy tetraquarks, pentaquarks and hexaquarks are confirmed to exist?
Patrick Koppenburg maintains a list of all discovered particles at the LHC. It doesn't include the X(3872) because that was discovered by Belle before. I think everything else is included.
 
  • #4
snorkack
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Protonium has a binding energy of less than 1 keV, so it's two well-separated hadrons. Deuterium can be described nicely as bound state of proton and neutron. In a true hexaquark the quark content should be more important.Or we just can't find them. They are probably very short-living, and even if you find any resonance, how do you distinguish some wide ##uddd\bar{u}## resonance from a wide ##udd# resonance? There are tons of poorly measured light quark resonances. Same for tetraquarks.
And they are just that - resonances. While the one confirmed light hexaquark is stable.
Yes, light diquarks decay - all three of them - but even π0 has just 8 eV width.
The two non-resonance light baryons form one hexaquark and the other two combinations (diproton and dineutron) are well known to be unbound, with decay/scattering widths in MeV range.
How about the three light mesons? How well are pion-nucleon and pion-pion scattering processes known?
I suspect that absence of light tetraquark and pentaquark states with width in the order of shorter lived component lifetime or more can be well verified.

Also, I should think the expression "mesonic molecule" misleading. Mesons, like nucleons and hyperons, are hadrons and subject to strong force. Just like a bound system of two or more baryons, some or all of which are hyperons, is a "hypernucleus", not a "hyperonic molecule", a bound system of two or more hadrons, some or all of which are mesons, could well be described as "mesonucleus".
 

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