LHCb discovers a new tetraquark state

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In a recent paper submitted by the LHCb collaboration at arXiv, they have reported a tetraquark state composed of charm quarks and antiquarks. The statistical significance of the data is more than 5σ.

The abstract:
Using proton-proton collision data at centre-of-mass energies of √s=7, 8 and 13TeV recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9fb−1, the invariant mass spectrum of J/ψ pairs is studied. A narrow structure around 6.9GeV/c2 matching the lineshape of a resonance and a broad structure just above twice the J/ψ mass are observed. The deviation of the data from nonresonant J/ψ-pair production is above five standard deviations in the mass region between 6.2 and 7.4GeV/c2, covering predicted masses of states composed of four charm quarks. The mass and natural width of the narrow X(6900) structure are measured assuming a Breit--Wigner lineshape.
An article on this paper:
https://www.symmetrymagazine.org/article/lhcb-discovers-a-new-type-of-tetraquark

A tweet by the LHCb collaboration:
 
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Answers and Replies

  • #2
Drakkith
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Interesting. How many other tetraquark states are known? A quick look at the tetraquark page on wiki didn't give me any conclusive answers.
 
  • #3
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That depends on who you ask.
Z(4430) is well established.
Zc(3900) looks like a tetraquark, too.
Then there are the three older LHCb candidates, X(4274), X(4500) and X(4700)
Now there is X(6900), which looks like (charm, charm, anticharm, anticharm) object.
There is also X(3872), which doesn't fit into the traditional models and might be a tetraquark.

There are a few more candidates where it's unclear if they exist - or if their existence is clear it's unclear if they are tetraquarks (or similar).
 
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  • #4
Vanadium 50
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The a0(980) and f0(980) are almost certainly tetraquarks. This goes back to a 1975 paper by Ken Johnson and Bob Jaffe.
 
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  • #5
ohwilleke
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There are definitional issues concerning what counts as a tetraquark too, which complicate the business of making a definitive list.

In sensu stricto a tetraquark is a hadron in which all quarks involved are bound directly to each other via gluon mediated strong force interactions.

But, sometimes a "meson molecule" , is also called a tetraquark in the less strict sense. In this example, if it is not a "true" tetraquark, it could be two charmonium mesons bound to each other by something akin to the residual strong force mediated mostly by pions that binds nucleons in an atomic nucleus to each other.

Experimentally distinguishing between the two possibilities is a subtle matter, even if you can agree that a bound structure is made of four valence quarks of particular types.
 
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What is the status of lattice QCD results on these tetraquarks?
 
  • #7
ohwilleke
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Lots of papers, no consensus.
 
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