CORDIS - EU research results

Baryons in extreme QCD matter

Periodic Reporting for period 1 - XQCDBaryons (Baryons in extreme QCD matter)

Reporting period: 2017-10-01 to 2019-09-30

Quantum chromodynamic (QCD) is a part of the Standard Model of elementary particles and their interactions. It is this theory that governs the strong interactions between quarks and gluons. At very large momentum scales (or small distances), the QCD coupling parameter becomes small and perturbative expansion is applicable. QCD has been tested throughly in this regime. However, at somewhat larger distances the strong interactions bind quarks and gluons into hadrons, such as the pion and the proton. This is the region where numerical simulation of the underlying theory (lattice QCD) is required. Experimentally there was a lot of progress recently, discovering new resonances that contain a charm quark and a charm antiquark (charmonia). More recently, at the LHCb at CERN, new baryons containing two charm quarks and a light quark have been discovered as well as many baryons containing the even heavier bottom quark. The properties of these particles and resonances need to be derived from QCD, which then will also guide experimental searches. An understanding of the world of elementary particles and of our place in the world is important for society as a whole. The objectives of this project are mainly to develop calculational techniques that enable us to do so but also to achieve some predictions that can directly be compared to experiment.
Lattice simulations (including a continuum limit extrapolation) were started and concluded, clarifying the spectrum of baryons containing two charm quarks and a strange quark, as a follow-up on preparatory work studying baryons with two charm quarks and a light quark. Subsequently, also the spectrum of mesons and baryons containing both a bottom quark and a charm (anti)quark was determined. Of these states only one has been discovered experimentally so far (and was confirmed in these new set of simulations). The results will guide future experimental searches, e.g. of the LHC experiments.

Another line of research was directed to the spectroscopy of charmonia. If these states become heavy, which is the case for radial excitations, these heavy-heavy mesons can decay into pairs of heavy-light mesons. Such short-lived resonances can be characterized by a pole in a complex Riemann sheet (rather than by just one real mass parameter). It is possible to extract the position of these poles from lattice simulations by varying the box size and also by varying the average momentum. Such scattering analyses had been carried out successfully in the past for hadronic ground states. In the charm sector several technical complications exist. One is the fact that radial excitations need to be resolved too, which requires a large basis of interpolators and novel methods (stochastic distillation). The other issue is a large number of decays into different pairs of mesons. For instance, an excited charmonium may not only decay into two heavy-light mesons but also into a lower lying charmonium state plus a light meson. Last but nor least, on the lattice the continuum symmetry is reduced and the spin identification becomes non-trivial, in particular at non-zero total momentum (in flight). All these was addressed and resolved in a string of publications and finally applied to a situation where the light quark masses are still unphysically heavy, to manage the computational cost.

The results were published in a number of scientific articles and presented at several international conferences and workshops. In particular, the Fellow was invited to present a plenary review talk on the research field at the annual conference of the Lattice Field Theory community, Lattice 2018 at Michigan State University.
The project excelled the state-of-the-art in several aspects, some mentioned above. Some of the results will be used to guide experimental searches, others are relevant with respect to interpreting experimental findings. With the help of such calculations, our theoretical tools become sharpened and the precision of Lattice QCD results steadily increases. Another major aspect was the development of novel methods to address strongly decaying resonances that appear as final states in many situations of great phenomenological interest. The Fellowship started a collaboration that is still ongoing and we expect to achieve results for the charmonium part of the project at physical quark masses in the near future.
Spectrum of baryons and mesons, containing bottom and charm quarks: predicted and observed.
The charmonium threshold spectrum, at (as yet) unrealistic quark masses.