Spring is coming... Chicago, March 2014

Exotic Quarkonia -- Alexander von Humboldt Fellow

The scientific community has witnessed what is called the golden age for heavy quarkonium physics, dawned a decade ago and initiated by the confluence of exciting advances in Quantum Chromodynamics (QCD) and an explosion of related experimental activity.

On the experimental side, everything started in 2003 when the Belle Collaboration reported the discovery what is now called the X(3872) (with more than 1000 citations, this is the most quoted result of the B-factories). Since then, BABAR, BES, CLEO, and Fermilab have joined the search and the number of new states has increased dramatically. More than a dozen charmonium- and bottomonium-like XYZ states have forced an end to the era when heavy quarkonium was considered as a relatively well established bound system of a heavy quark and antiquark (see Figure below). New forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks have been proposed. Experiments focused on the abundant production and systematic studies of the XYZ states are needed. Preferably, these should be performed using hadronic probes because the cross sections are expected to be very large. Europe is situated in a privileged position with the current LHCb experiment and the promising Antiproton Annihilations at Darmstadt (PANDA) experiment at the Facility for Antiproton and Ion Research (FAIR).

From a theoretical point of view, heavy quarkonium physics experienced a breakthrough with the development of effective field theories. After the birth of nonrelativistic QCD (NRQCD) in 1986 and, in particular, of potential nonrelativistic QCD (pNRQCD) in 1998, one has been able to give a model-independent, QCD based approach to this field of research, where just the right degrees of freedom appear. These theories have become an important tool to compute physical observables incorporating corrections systematically through a suitable power counting.

Many of the new XYZ states are located close to or above threshold, making this energy region the most interesting one for theoretical and experimental studies. Because situation changes drastically at or above the open-flavor threshold, no EFT description has yet been constructed nor have the appropriate degrees of freedom been clearly identified for most of the new states. Concerning lattice studies, the threshold regions remain troublesome, calculations of excited states have been only recently pioneered and the full treatment of bottomonium on the lattice seems to be tricky. All this together explains why many of our expectations for these states still rely on potential models.

The aim of this research project is to work out a systematic, model-independent and QCD-based description of the XYZ states. We are still taking advantage of the nonrelativistic nature of these states and the appearance of a hierarchy of energy scales to construct adequate Effective Field Theories from the ones that we know at present. For instance, NRQCD is a good EFT for states close to and just above threshold, at least when their binding energies remain much smaller than the heavy-flavor mass.

Elastic and Transition Form Factors -- Argonne Fellow

Elastic form factors are of fundamental interest and widespread value because they express the distributions of charge, magnetization and spin within the non-pointlike hadrons that QCD is supposed to generate. Their measured forms are therefore a benchmark test for phenomenology and theory within QCD; and also crucial inputs to calculations and experiments in nuclear structure.

The large number of nucleon-to-resonance transition form factors, with their diverse array of features, ensures that entirely new windows on hadron structure are opened by studying the Q2-dependence of these transitions. The CLAS12 detector in Hall B is a unique facility worldwide, capable of determining transition ╬│NN* electrocouplings of all prominent excited nucleon states in the region of Q2 up to 12 GeV^2. It is important to remind that the N* structure is expected to be dominated by dressed-quark degrees-of-freedom in the region of very large photon momenta.

Finally, CLAS12 will also afford access to parton distributions in an excited nucleon, and enable the concept of GPDs to be applied to the transition of a nucleon to its excited state. New high precision hadro-, photo-, and electro-production data off the proton and the neutron will stabilize coupled channel analyses and expand the validity of reaction models, enable searches for baryon hybrids and investigation of their structure, establish a repertoire of high precision spectroscopy parameters, and measure light-quark flavor-separated electrocouplings over an extended Q2-range for a wide variety of N* states. Including this body of results in the combined analyses will greatly expand both our knowledge and our understanding of the baryon sector, and the transition from the non-perturbative to the perturbative regime of QCD.

Conventional Quarkonia -- Spanish Research Training Fellow

An exhaustive study of conventional heavy meson properties within a nonrelativistic constituent quark model is pursued. Within the heavy quark sector, we focus on the spectroscopy and on the electromagnetic, strong and weak decays and reactions. The description of the approaches used and the discussion of our results, comparing them with the experimental data and also with the results coming from different theoretical approaches can be followed along our publications.

A quite reasonable global description of the heavy meson spectra is reached. Some tentative assignments of the XYZ mesons has been done explaining other properties of the assigned states.

Certain modifications to the original quark model have been suggested. The inclusion of one-loop QCD corrections to the spin-dependent terms of the OGE has served to explain in part the lower mass of the Ds0*(2317) meson as a canonical cs structure. The renormalization with boundary conditions applied to the constituent quark model allows us to disentangle the physics of the ground state to that of the excited states.

Concerning strong decays, we have performed a calculation of the OZI-allowed decay widths of the heavy mesons using two different decay models, the 3P0 model and a microscopic one. We have used the QCDME approach to calculate OZI-suppressed transitions between charmonium vector states and also between botomonium vector states.

Concerning weak decays, we have performed a calculation of the branching fractions for the semileptonic decays of B and Bs mesons into final states containing orbitally excited charmed and charmed-strange mesons, respectively. An analysis of the nonleptonic B meson decays into D(*)DsJ is also included.

The most relevant results are listed below. The new assignment of the psi(4415) as a D-wave state leaving the 4S state for the X(4360). The dependence on the scale of the 3P0 strong decay model. The description of the Ds1(2536) meson as a jq=3/2 cs state is necessary to get a simultaneously explanation of its decay properties.