Fermilab announces new tetraquark state

[websterling]

Nearly five years after the Tevatron was decommissioned the experimental collaborations are still analyzing data and today Fermilab announced that DZero discovers a new particle consistent with a tetraquark

This candidate is different from previous discoveries in that it contains four different flavours of quarks whereas previous candidates contain a quark-antiquark pair of the same flavour.

The paper at the arXiv: Observation of a new $B_s^0 π^\pm$ state

 

[mfb]

Uhm... I guess there is a reason that paper took so long to be made public.

That "cone cut" described on page 4 heavily biases the shape towards events near the threshold. Looking at figure 3, the peak without cone cut looks much more reliable than the one with it.

"The $B^0_s \pi^\pm$ background with a real $B^0_s$ meson is modeled using a Monte Carlo (MC) simulation [9] of events containing a $B^0_s$ meson and additional pions tuned to reproduce the $B^0_s$ transverse momentum distribution in data."
The $B^0_s$ transverse momentum distribution in data is not the critical point here. The pions are low-energetic and background comes from the poorly understood soft QCD processes in the interaction. Okay, at least they checked the $B^0_s$ sidebands.

Without cone cut, the sidebands (empty markers in figure 2) seem to have a different distribution than the peak region (figure 3b). This is also discussed in the text, but without explanation. Also, they get a lower number of events in the $B^0_s \pi^\pm$ peak without cone cut.

There is clearly something not understood, and it looks like a mass peak, so it could be a new particle, but the analysis could have been done more carefully. Also, where are checks with other decay modes of the $B^0_s$? That particle is long-living, so the peak should look very similar in other decay modes. The purity or signal yield might be a bit lower, but $J/\psi \phi$ is not the only relevant decay channel. Also, why should it be a tetraquark? Excited states of a $B^0$ could decay to $B^0_s \pi^\pm$. Would be odd, but not impossible. A check of the $B^0 \pi^\pm$ spectrum would help.

Anyway, I guess LHCb can quickly check this with data recorded already. They have the samples of $B^0_s \to J/\psi \phi$ for mixing studies anyway, larger than the Tevatron samples.

 

[Vanadium 50]

I agree with mfb - a cone cut is a terrible thing to use. First, when you make an angular cut and a momentum cut, you make a mass cut. Figure 2 shows this. Worse, the cut takes effect exactly at the peak. That maximally sculpts the spectrum. The fact that the yield gets all wonky with and without this cut is probably related to this. The significance is probably questionable, and the mass and width certainly are.

 

[websterling]

Apparently some members of the LHCb collaboration have some of the same concerns and are doing some checks on their data to see if they've missed something.

Since the paper has been submitted to PRL, I wonder how critical the referees will be. It might be interesting to compare this draft to anything eventually published.

 

[websterling]

Apparently the D0 result is not confirmed by LHCb- from the LHCb site:

The LHCb Collaboration reported today a result of a similar analysis using a sample of Bs0 mesons 20 times higher than that used by the D0 Collaboration. The Bs0π invariant mass spectrum is shown in the figure using the $B_s^0$ mesons decaying into J/ψ and φ mesons or into $D_s$ and π mesons. No structure is seen in the region around the mass of 5568 MeV. Hence the LHCb analysis does not confirm the D0 result.

More details will be presented at Moriond QCD.

 

[mfb]

A PDF of the presentation is here (direct link). Absolutely nothing.

 

[dukwon]

Since the paper has been submitted to PRL, I wonder how critical the referees will be. It might be interesting to compare this draft to anything eventually published.

I'm not sure if this is a faux-pas, but I have actually spoken to one of the PRL referees for this paper. It's probably going to be rejected...

 

[websterling]

I find it a little sad that the D0 collaboration was so problematical with their analysis.

It's a bit reminiscent of their "discovery" of the $\Omega^-_b$.

 

[websterling]

It's probably going to be rejected...

This was published in PRL today- Evidence[/PLAIN] for a $B_s^0 π^\pm$ State

It's v3 of the original paper on the arXiv
The titles are different. I haven't had a chance to compare the two versions of the paper. I don't think that LHCb has published their non-finding.

There was also this published in PRL yesterday- Decoding the X(5568) as a Fully Open-Flavor $su\bar b\bar d$ Tetraquark State (arXiv version)