Hadrons in Munich: from light mesons to heavy ions

26 August 2011

Highlights from the HADRON 2011 conference.


Hadron 2011, the 14th International Conference on Hadron Spectroscopy, was the latest in a long series that started in 1985 in Maryland. Originally conceived as a conference on light meson spectroscopy, it now covers all aspects of hadron physics, although spectroscopy and hadron production are still the topics that characterize the meeting. This year, 37 plenary talks, 128 presentations in parallel sessions and 37 posters offered ample possibilities to find out about the latest developments and results, from hypernuclear physics to meson and quarkonium spectroscopy, and from nucleon structure and the meson-baryon interaction to heavy-ion physics.

The conference began by looking at issues related to light mesons, with a summary of recent theoretical progress and experimental tests in chiral dynamics and low-energy ππ-scattering phenomena. There were new results on light meson spectroscopy from the BESIII experiment in Beijing and COMPASS at CERN. While COMPASS impressively confirmed previous findings on π1(1600), an exotic meson seen in high-energy diffraction, new structures have been observed in radiative J/Ψ decays that point towards new and narrow meson states between 1.8 and 2.5 GeV/c2, the details and nature of which have still to be unravelled.

Size and structure

Even after many years of precision experiments, the size of the proton is still a hot topic. New findings in laser spectroscopy of muonic hydrogen, which give the proton radius as more than 5σ smaller than previously determined, have opened the hunt for new explanations, although theory cannot offer effects large enough to solve the puzzle.


Research into nucleon structure has for years shifted to spin degrees of freedom. After precision measurements on the helicity contribution of quarks in polarized nucleons, COMPASS has also set new limits on spin effects resulting from polarized gluons. These findings are confirmed by spin experiments at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven. With this, the focus now turns towards transverse-spin degrees of freedom (transversity). Noncollinear treatment of partons inside the nucleon offers a large number of new observables, which can link to quark angular momenta. Both COMPASS and RHIC have new physics programmes on transverse polarization effects, and measurements of Drell-Yan processes using polarized targets are also on the way. Hopes are high that the unexpected single-spin asymmetries that have been observed in pion production at RHIC may finally be understood.

On the low-Q2 side, big efforts at various laboratories – such as Bonn, Mainz, Jefferson Lab etc – are offering real or virtual photon beams. These allow a coherent set of (double-) polarized scattering and production experiments, also with many-body final states. Using the complete set of polarized measurements, the puzzle of baryon resonances, their identification and quantum numbers seem now to be within reach via new and sophisticated partial-wave analyses.

Quarkonium spectroscopy and the hunt for further quarkonium-like states that seem not to fit the qq picture of the meson have been and still are highlights in hadron physics. Precision experiments finally allowed the BELLE and BaBar experiments at KEK and SLAC, respectively, to observe missing quarkonium states such as hb(1P), hb(2P), as well as ηc (1S) and ηc(2S). More precise determination of the mass and width, as well as unexpected decay patterns were revealed also by BESIII, which has observed about 109 J/Ψ decays. The puzzle of the mass and width of the D(Ds) meson states is on the way to being settled with their spin assignments being resolved. The conference also heard about the remarkable progress in achieving a comprehensive and unified theory description of quarkonium properties at zero and finite temperature in an effective field-theory framework.

The biggest puzzle currently in hadron physics concerns the large number of exotic quarkonium-like states with narrow widths and high excitation energies, as compared with the open-flavour meson channel. New work was reported on the X(3872) and other, partly new states. Theoretical investigations offer a rich choice of possibilities. The X(3872) has a good chance of just being the radial excitation of the χc state, but there is also a beautiful effective field-theory description in the molecular-interpretation case. However, further stunning observations were reported from the beauty sector. Two charged quarkonium-like states found by BELLE lie close in mass to the open b-threshold and have been dubbed Zb, in analogy to the charm sector.

Lattice calculations have shown huge progress with new algorithms, allowing the extraction of excited baryon and meson-state energies. A report from the Flavianet Lattice Averaging Group presented lattice results for kaon and pion physics with the aim of making them accessible to the community. There are also new calculations of hadron structure, the baryon and meson form-factors and the g-2 factor.

First and impressive results were reported from all of the LHC experiments. In particular, CMS and LHCb – offering the best mass resolutions – have confirmed the potential of hadron machines in this field. In addition to the usual quarkonium states, exotic states have also been observed and the elusive Bc mesons have already been seen. At this stage, the focus is on the production cross-section of heavy quarkonia, which can now be understood at LHC energies, assuming colour octet contributions and next-to-leading order (NLO) processes to be relevant. The descriptions follow data up to transverse momenta as high as 20 GeV/c. One of the uncertainties comes from unknown polarization effects that influence acceptance calculations. On the theoretical side, huge progress has been achieved with the full NLO calculation of the J/Ψ cross-section in non-relativistic QCD (NRQCD) and a combined global data analysis of all existing experiments that hints at the universality of the long-distance NRQCD matrix elements.

Hadron machines are unique in the production of b-baryons and Fermilab’s Tevatron has so far been leading this field. The CDF collaboration reported on recent progress with the observation of excited Σb states and a radially excited Λc. CDF and DØ also presented new precision measurements of the mass and width of other charmed baryons.


A thermal medium, of the type generated in heavy-ion collisions at the LHC, can modify hadron properties, especially in the case of quarkonia. The theory of such modifications was reviewed and first results of lead–lead collisions at the LHC presented. Results from ATLAS and CMS show the striking effects of jet-quenching and also the melting of the excited Y-states as compared with the ground-state partner. At lower energies, mass shifts and absorption cross-sections of vector mesons have been studied in the medium. Mass shifts – a long-standing issue, where many predictions have stimulated experimental efforts – have not been observed but small effects have been reported by the HADES experiment at GSI, Darmstadt, on the width of ω mesons in nuclei.

Recent and impressive progress in light meson and quarkonium spectroscopy is in good part a result of recent high-luminosity experiments, which offer 10–100 times the statistical sample of their predecessors. Heavy-meson physics, for long the domain of lepton colliders, is now seeing LHC experiments starting to compete in an impressive way and using their low luminosity data from 2010 to catch up with the Tevatron experiments. An interesting future lies ahead with even further increases in luminosity and precision being offered by future experiments such as BELLE II, the SuperB facility and the PANDA experiment at the Facility for Antiproton and Ion Research.

Two impressive summary talks concluded the conference. Stefano Bianco of Frascati/INFN reviewed the experimental situation, a challenging task in view of the large number of new results presented. On the theoretical side, Chris Quigg of Fermilab gave an inspiring outlook on hadron physics. He recognized the enormous diversity and reach of experimental programmes, which offer insights from unexpected quarters, while remarkable progress has been achieved in theory with the emergence of lattice QCD. However, many puzzles remain, leaving ample opportunities and much work to do, as there are still “simple” questions that the field cannot answer.

Participants enjoyed the coffee breaks in the sunny and secluded courtyard of the Künstlerhaus, a building erected more than 100 years ago for artists to meet and enjoy social events. Long and intense discussions also offered vital scientific exchange around the poster session, making this event a pleasant ending to the day. Long hours of sitting were compensated on Wednesday afternoon with a bicycle tour through the old town of Munich and the English garden, with refreshing drinks in the beer garden. Last but not least, the conference enjoyed a guest talk on neutrino physics by Thierry Lasserre of Saclay, who discussed the mass determination from flavour oscillation and reported fresh results from T2K on hints of νμ→νe oscillation.

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