Anomalous Resonances As Hadron Molecules

A new pre-print quickly summarizes the hypothesis that most scalar mesons and axial vector mesons and a variety of other anomalous resonances not easily explained with two or three valence quarks are all basically "molecules" of pairs of mesons and/or baryons. For example:

There are many states that can be described from hadron-hadron interaction. Some well-known examples are the scalar mesons obtained from pseudoscalar-pseudoscalar interaction in S-wave and coupled channels: the a0(980) from KK¯ and πη in isospin 1, the f0(980) from KK¯ and ππ in isospin 0, and the f0(500) (σ meson) from ππ scattering in isospin 0. In the strange sector, from vector-pseudoscalar interaction one can describe the f1(1285) as a K∗K¯ + c.c. molecule. In the charm-strange sector there is the D∗ s0 (2317), which can be described as a DK bound state. Similarly, one of the most famous examples in charm sector is the X(3872) which can be explained as a DD¯ ∗ + c.c. molecule. These are just a few cases from meson-meson interaction. On the other hand, in meson-baryon interaction the best example would be the Λ(1405), which is widely accepted [1] as a quasi-bound state between the KN¯ and πΣ thresholds, generated mostly from the KN¯ scattering.

It doesn't acknowledge that this is not a consensus interpretation or what supports this interpretation relative to the alternatives, however. 

In other hadron physics news, the BFKL theorem, named after the authors of the paper that proposed it in the 1970s to explain how the strong force interactions of high energy particles change at high energies in a subtle but important way, has now largely been proven and refined.

One striking feature of particles which are strongly interacting (like the proton) is that if two of them are approaching each other, the chance of them actually colliding increases as the energy of the particles increases. This behaviour was well-known experimentally, and was modelled in a precursor to the Standard Model called “Regge theory”. Amongst other things, the BFKL approach offered, for the first time, a chance of understanding this behaviour from first principles using the Standard Model. . . .

The scattering probability for electrons and protons is generally expressed in terms of mathematical objects called structure functions, and the BFKL predictions said that one particular structure function should rise very rapidly as the fraction of the proton’s momentum involved in the collision got smaller. 

We measured that structure function, and it did rise. But there were problems to sort out before declaring BFKL vindicated. The structure function did not rise as quickly as might have been expected by BFKL. It was also possible to explain the rise using different calculations – not featuring their sums. Most importantly, none of these calculations, by BFKL or others, was very precise, and nor were the data. We were in a grey area. 

Over the years, many more data have come in, and better calculations have been made, by a generation of theorists and experimentalist wrestling with some formidable challenges. The qualitative impact of the BFKL sums is not now expected to be as dramatic as the initial calculations indicated, but it is still there, and still important.

A global analysis published on the arXiv this year by physicists from Amsterdam, Edinburgh, Genoa, Oxford and Rome pulls lots of this work together and makes the qualitative statements about the BFKL sums quantitative. Including these sums (in their newer and more precise form) gives a significantly better description of the data than is the case if they are omitted. 

What this means is that we have pushed our understanding of the strong force into a new, previously unobserved region, and verified a qualitatively new emergent behaviour. 

The formidable mathematics behind these calculations connects a deceptively simple underlying theory with a ubiquitous and counter-intuitive observational fact: scattering probabilities rise at high energies. This has implications for our understanding of many things, from the collisions at the Large Hadron Collider at CERN to the propagation and detection of high energy particles in cataclysmic cosmological events. It may even be important in understanding possible new strongly-interacting theories that may still to be discovered beyond the Standard Model.