Trieste focuses on hadrons

The fifth Perspectives in Hadronic Physics conference was held on 22–26 May at the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste. The latest in a series organized every second year by the ICTP and the Italian Istituto Nazionale di Fisica Nucleare (INFN), this year’s conference was also sponsored by the Consorzio per la Fisica, Trieste, and the Department of Physics, University of Perugia. It brought together around 100 theorists and experimentalists for more than 60 plenary talks, focusing on present and future theoretical and experimental activities in hadronic physics and relativistic particle–nucleus and nucleus–nucleus scattering.

A major success of the conference was the joint participation of the hadronic and heavy-ion communities. This was reflected in the wide range of topics, from the structure of the hadron at low virtuality, to the investigation of the states of matter under extreme conditions and the possible formation of quark–gluon plasma in high-energy heavy-ion collisions. This article presents a summary of the broad range covered by the speakers.

Hadrons, in vacuo and in the medium

The first part of the conference focused on the study through quantum chromodynamics (QCD) of free hadrons and the properties of hadrons in the medium – that is, in nuclear matter. It covered a broad spectrum of theoretical approaches and experimental investigations, including hadron structure and the quantities that describe it, namely form factors, structure functions and generalized parton distributions (GPDs). In this context there was much emphasis on the appreciable amount of experimental work undertaken at Jefferson Lab and at the Mainz Microtron (MAMI), for example, casting important light on the role of strange quarks in the nucleon.

MAMI has also obtained values of the masses and widths of mesons in the medium, which appear to differ appreciably from the free case. The possibility of a ω-nucleus bound state was suggested. Scalar and axial vector mesons can be generated dynamically within a chiral dynamics approach, which was presented in detail.

Information from Jefferson Lab on the nucleon-spin structure function from almost real photon scattering to the deep-inelastic scattering (DIS) region was reviewed at the meeting, and recent results were reported on the use of semi-inclusive DIS on the proton and the deuteron as a tool for investigating the up and down quark densities. Quark–hadron duality was discussed both in its theoretical and experimental aspects, illustrating how recent data from Jefferson Lab can be used to extract the higher twist contributions to the moments of parton distribution functions, which are sensitive to the quark–gluon correlations in the nucleon. Several talks presented recent results on the calculations of hadron form factors and cross-sections in terms of relativized quark models. These included the consideration of higher Fock components in the hadron wave functions.

Exclusive hard processes were discussed in terms of a new nonperturbative quantity that describes the hadron-to-photon transition, for instance in virtual Compton scattering in the backward region. The usefulness of this approach was illustrated in forward exclusive meson-pair production in γγ* scattering.

GPDs were the subject of detailed discussion at the meeting, with a report on the impressive experimental results from Hall A and the Deeply Virtual Compton Scattering Collaboration at Jefferson Lab. These experiments have accessed the twist-2 term in the proton, which is a linear combination of GPDs, and they find almost no dependence on momentum-transfer squared, Q². This is in good agreement with the theoretical expectation where the process described by the so-called “handbag” diagram dominates.

Turning to the calculation of GPDs, a meson-cloud model allows their computation at the hadronic scale, while GPDs for a nuclear target have been calculated in a constituent quark model, which shows that nuclear effects prove to be much larger than expected. Theoretical results for Compton scattering and two-photon annihilation into pairs of hadrons within the handbag approach were compared with data from Jefferson Lab and from the Belle experiment at the KEKB facility. The meeting presented indications for the presence and role of two different non-perturbative scales in hadronic structure, while showing that complementary information on the 3D parton structure of the hadron was accessible by measuring multiple parton distributions in hadron–nucleus reactions.

Cold nuclear matter figured in several talks, which presented new experimental and theoretical results. These clearly demonstrated that nowadays our knowledge on nuclei has reached the stage of a quantitative access to nucleon–nucleon correlations.

Eventually it will be possible to explore QCD where the strength of nonlinearities is substantially higher than at DESY’s HERA electron–proton collider.

Mechanisms for quark hadronization and hadron formation in the medium were another important topic. The HERMES collaboration at DESY reported a clear attenuation of various leading hadrons in heavy targets compared with the DIS process on deuterium. Theoretical interpretations of these nuclear effects are based either in terms of inelastic hadron interactions or in terms of quark energy loss. Successful interpretations of the data provide information on the time needed to produce a colour-neutral precursor, which eventually fragments into a leading hadron. Preliminary data at a lower photon energy from the CEBAF Large Acceptance Spectrometer at Jefferson Lab, particularly on the transverse-momentum broadening, are expected to shed new light on these effects owing to the finite formation time of hadrons.

Several presentations showed that eventually it will be possible to explore QCD where the strength of nonlinearities is substantially higher than at DESY’s HERA electron–proton collider. The meeting also discussed ultraperipheral collisions, which will allow the study of structure functions at low Q², and diffraction at very high energies.

From hadrons to quark–gluon plasma

The heavy-ion part of the conference began with an overview of saturation physics from the Relativistic Heavy Ion Collider (RHIC) to the Large Hadron Collider (LHC). Here emphasis was on experimental signals for the so-called colour glass condensate, followed by the theoretical aspects of saturation and shadowing physics at small values of the variable, Bjorken x.

The modification of the jet shapes in the jet-quenching phenomenon at RHIC seems to provide an efficient tool for probing the soft gluon radiation induced by the produced medium. At both RHIC and the LHC, the ratios of heavy to light mesons at large transverse momentum offer solid possibilities for checking the formalisms for energy loss. Photon-tagged correlations have also been proved to be efficient tools for extracting both the vacuum and the medium-modified fragmentation functions, in proton–proton and nucleus–nucleus scattering. The strength of the jet-quenching process depends on the medium transport coefficient qˆ, but this dependence is weakened by the geometrical bias that favours the production of the hard parton at the periphery of the medium. Nonetheless, its precise value is of considerable importance to interpret the present RHIC data and to foresee the amount of quenching in lead–lead collisions at the LHC. A recent non-perturbative estimate of this quantity, using the anti-de Sitter space/conformal field theory correspondence, was presented.

Recent measurements of J/ψ production in deuteron–gold and gold–gold collisions by the PHENIX collaboration at RHIC seem to be consistent with a weak shadowing effect together with the possible inelastic interaction of the J/ψ meson in cold nuclear matter. In the heavy-ion collisions, the J/ψ suppression at RHIC is remarkably similar to that observed at CERN’s Super Proton Synchrotron (SPS), despite the much larger energy density reached at RHIC. The reason for this is not yet clear and could be owing to the formation of J/ψ states from the statistical recombination of charm quarks in the medium.

The soft-physics side reported recent numerical calculations on plasma instabilities, which attempt to determine the behaviour of an anisotropic non-abelian medium on long time scales. Observables measured in two-pion (Hanbury-Brown/Twiss) interferometry and in pion spectra at RHIC are consistent with the emission of pions from a system that has a restored chiral symmetry. The recent preliminary data from the NA60 experiment on r-meson production in indium–indium collisions at SPS energies were discussed. While the measurements compare well with expectations for the broadening of the ρ width, these data tend to exclude the drop in mass expected from Brown-Rho scaling, which predicts the in-medium mass to be proportional to the qbarq condensate. More detailed presentations complemented overviews of heavy-ion collisions at intermediate energies and the physics programme for the ALICE experiment at the LHC.

The conference then focused on spectroscopic studies and the production of exotic states. The new exotic states discovered at 4 GeV by Belle and the BaBar experiment at SLAC can be understood as diquark–antidiquark (qq–qbarqbar) states. The meeting also covered various problems related to dense hadronic matter – in particular, the high-temperature phase of QCD, bifurcations in the physics of strong gluon fields, and the topological structure of dense hadronic matter, and the possibility of measuring the production of “strangelets” at the LHC using the Centauro And STrange Object Research (CASTOR) detector at the CMS detector.

The conference also discussed the formation and properties of ultra-dense quark matter in the stars, from several different aspects. These included the colour-superconducting quark matter that can be formed in the core of compact stars, the various many-body approaches to the treatment of the equation of state of nuclear matter at baryon densities exceeding the density of normal nuclei by several times, and the quark-deconfinement model of gamma-ray bursts.

The last part of the meeting focused on the presentation of the most relevant plans for future experimental facilities. These included overviews on the future Facility for Antiproton and Ion Research (FAIR) at GSI and the broad programme of physics at the Japan Proton Accelerator Research Complex (J-PARC), as well as physics at Jefferson Lab with the CLAS detector. The possibilities offered by a high-luminosity electron–ion collider were also discussed.

The conference closed with a talk from the 2005 physics Nobel prize winner, Roy Glauber from Harvard, who described his pioneering work on quantum optics and its relationship to heavy-ion physics. The hadronic and heavy-ion communities are now looking forward to the sixth Perspectives in Hadronic Physics ICTP conference.