XYZ spectroscopy and the existence of possible 4-quark states

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Discussion Overview

The discussion centers on XYZ spectroscopy and the potential existence of 4-quark states, specifically focusing on the particles Z(4430)^- and Z_c(3900)^+. Participants explore the implications of their decay channels and the significance of their discoveries in the context of particle physics.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that the Z(4430)^- provides unambiguous evidence for 4-quark states due to its decay into c\bar c and its charged nature, suggesting a quark content of c\bar c d\bar u.
  • Others argue that the Z_c(3900)^+ shares similar features but is not considered as strong evidence for 4-quark states, citing differences in the clarity of its discovery.
  • One participant questions the criteria for determining the significance of particle discoveries, referencing the Z(3900)'s observation in multiple decay channels with high statistical significance.
  • Concerns are raised about the reliability of measurements and the potential for systematic uncertainties to affect the interpretation of results, referencing past examples in particle physics.
  • Another participant highlights the importance of the Argand diagram used in the LHCb discovery, which demonstrates the behavior of the peak in the mass plot as indicative of a particle rather than background noise.
  • A question is posed regarding the lack of phase analysis for the Z(3900) by other experiments, with a suggestion that it may be due to the requirement of a large number of events for such analysis.

Areas of Agreement / Disagreement

Participants express differing views on the significance of the Z(4430)^- compared to the Z_c(3900)^+. While some believe the LHCb discovery is more solid, others contend that both discoveries are equally valid based on their observed peaks. The discussion remains unresolved regarding the criteria for significance in particle discovery.

Contextual Notes

Participants note that systematic uncertainties may not follow normal distributions, which could impact the interpretation of discovery significance. The discussion also highlights the technical challenges involved in performing phase analysis for particle identification.

Einj
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Hi everyone, I've been studying the so-called XYZ spectroscopy and the existence of possible 4-quark states.

The LHCb collaboration recently confirmed the existence of a particle called [itex]Z(4430)^-[/itex]. This particle is the unambiguous evidence for the existence of 4-quark states. From what I understood the reason is that this particle decays as [itex]Z(4430)^-\to \psi' \pi^-[/itex]. Since it decays in a [itex]c\bar c[/itex] it must contain such quarks as valence quarks. Moreover it is charged and therefore its minimal quark content can only be [itex]c\bar c d\bar u[/itex].

My question is: some time ago was also discovered another particle, the [itex]Z_c(3900)^+[/itex] decaying in [itex]J/\psi \pi^+[/itex]. Why this is not considered as an evidence for 4-quark states? It seems to me that it follows the same criteria as the [itex]Z(4430)^-[/itex].

Does anyone know something about it?

Thanks
 
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This particle is the unambiguous evidence for the existence of 4-quark states.
It is an unambiguous evidence for something beyond the established categories.

The Zc(3900)+ has the same features, but the discovery of this particle was not as clear as the observation of the Z(4430).
 
What do you mean exactly? If I remember correctly the Z(3900) was safely observed in two different decay channels with significantly more than 5 sigma. What other factors define the "significance" of the discovery?
 
I don't have the time to check the original references now, but systematic uncertainties do not follow a normal distribution. See the Opera neutrino measurement for example - they could have observed 50 sigma and still be wrong because of an unaccounted measurement error. It is easy to get a model wrong (and therefore see a particle that does not exist), especially in areas with so many open question as in XYZ states. Two different experiments don't make that less likely if they use the same models.
 
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Understood. So, at the end of the day what makes the LHCb discovery so important is that it is more solid that the others right?
 
I disagree - there's nothing "better" about the 4430 discovery than the 3900. Both have an unambiguous peak. What the LHCb result adds is something called a Argand diagram showing the phase at the mass pole: this is highly technical, but it demonstrates that the peak in the mass plot behaves as a particle and not some sort of bizarre background.
 
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Do you know if there is any particular reason that prevent the other experiment to perform the same phase analysis for the Z(3900)? I mean, it was discovered a few years ago and no one did that. Is that because of it was impossible for the other experiments or what?
 
Among other reasons, it takes a lot of events.
 

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