The 28th Mazurian Lakes Conference on Physics, which took place from 31 August to 7 September 2003 in Krzyze, Poland, looked at the atomic nucleus as a laboratory for fundamental processes.
The Mazurian Lakes meeting, held in a picturesque region with thousands of lakes and forests, has evolved from a bi-annual school on nuclear physics into the conference it is nowadays. At the same time, nuclear physics has also evolved, so that it now contains many more subfields than the traditional areas of nuclear structure and reactions (figure 1). The 2003 meeting covered several of these subfields and included important excursions into astrophysics (neutron stars and supernovae) and particle physics (neutrinos).
There was a special anniversary touch to the subjects of the 2003 conference. In 1953 the first hypernucleus was observed in Warsaw by Marian Danysz and Jerzy Pniewski, followed by the discovery of a double hypernucleus by the same two physicists. Fifty years later, the physics of hypernuclei is experiencing a large-scale revival, in particular with the FINUDA program at DAFNE in Frascati and the PANDA project at GSI in Darmstadt, both of which were the subject of presentations at the conference. Andrzej Wróblewski from Warsaw gave a historical review, describing – with many previously unknown or neglected details – the turbulent history of hypernuclei and strangeness, their discovery and progress in the field during the past half century.
Strange behaviour
Numerous hypernuclei containing a single-Λ particle and a few double-Λ hypernuclei have now been discovered and studied experimentally. However, compared with the enormous amount of data and knowledge that has been accumulated over the years for normal nuclei, relatively little is still known about hypernuclei. The reasons, of course, lie in the difficulties related both to producing them and studying them within the narrow window of their lifetimes. At the meeting, Yusuke Miura from Tohoku talked about the recent important progress in the γ-spectroscopy studies of hypernuclei. The basic strengths of Λ-Λ versus nucleon-Λ interactions, which were discussed by Abraham Gal from Jerusalem, are still being debated, but some conclusions can be drawn from the simplest binding-energy differences between single-Λ and double-Λ hypernuclei.
Both Wanda Alberico of Torino and Hyoung Chan Bhang of Seoul discussed the weak non-mesonic decay rates of hypernuclei. The long-standing and still-debated issue here is the so-called Γn/Γp puzzle, i.e. the fact that the observed widths for Λ + n → n + n decays seem to be much larger relative to Λ + p → n + p than those that were obtained theoretically. However, the recent progress, both in theory and in experiment, seems to yield more convergent results.
Helmut Oeschler from CERN and Helena Bialkowska from Warsaw presented some theoretical and experimental studies that aim at seeing the quark-gluon plasma (QGP) through the “lens” of strangeness production in high-energy heavy-ion collisions. If a flavour-equilibrated fireball of quark matter is produced in energetic nucleus-nucleus collisions, then the production of strange and non-strange particles should be comparable. This can be quantified in the form of the Wróblewski factor, λS = 2ssbar/(uubar + ddbar), and studied through the production of strange mesons, strange baryons, or hidden strangeness. However, experimental maxima in λS, which reach values of approximately 0.6, can also be explained in a statistical model by a kinematical cut through the T-µB plane (where T is temperature and µB is baryon chemical potential). These maxima therefore do not necessarily signal the presence of the QGP. Other signals, such as an atypical energy dependence for strangeness production, may also be invoked but must be confronted with data from hadron-hadron and hadron-nucleus collisions.
Nuclear medium and nuclear matter
The question of how to measure the hadron mass in the nuclear medium was discussed by Piotr Salabura from Cracow. He argued that the two-body decays into e+e– pairs could be used to measure the invariant mass of a decaying hadron directly, because leptons travelling through the nuclear medium are not perturbed in their final states. Dalitz decays, in which the e+e– pair is accompanied by a hadron, can also be used; the problem here lies in the necessity of disentangling contributions coming from various decaying hadrons. The HADES experiment at GSI in Darmstadt is directed at studies of this kind.
A very interesting idea to study the equilibration process in nucleus-nucleus collisions – that is, the extent to which they come to equilibrium in an intermediate state – was discussed by Norbert Herrmann from Heidelberg. By using projectiles and targets that have different isospin compositions, one can, in a sense, tag the nucleons that originate from the projectile or target, to see if they bounce off each other, equilibrate, or pass each other. Experimental data obtained at GSI at the heavy-ion synchrotron, SIS, at energies of 100-200 A MeV clearly indicate that the colliding systems are never fully stopped and that the transparency increases with incident energy. Thus, at least in this case, we have a clear experimental indication that the systems are not really equilibrated.
At a completely different scale of energies of 130 and 200 GeV, studied at Brookhaven’s Relativistic Heavy Ion Collider (RHIC), Terry Awes from Oak Ridge compared the nuclear modification factors for Au + Au and d + Au collisions in order to pin down the so-called jet-quenching effect. At these energies, jets of particles are produced when the projectile and target quarks collide and create flux tubes that then break into colourless hadrons. The nuclear modification factor gives a rate of the production of a given hadron in a nucleus-nucleus collision relative to that for a proton-proton collision. It is supposed to tell us how much the medium in which the particle is produced influences the observed outflow of particles after the collision. For a fairly wide region of transverse momenta, the values of the d + Au modification factor are close to 1, while those for Au + Au are suppressed at about 0.3. This is a strong indication that a new kind of medium (possibly the QGP) is created in the Au + Au collisions.
Neutron stars and neutrinos
In the session on astrophysics, Karlheinz Langanke from Aarhus talked about how the properties of stellar objects may depend strongly on detailed nuclear-structure properties. First he showed the impressive results of large-scale shell-model calculations for the Gamow-Teller strength distributions. These calculations agree incredibly well with the newly measured data (obtained with 100 keV resolution), indicating that the low-energy nuclear properties are kept well under control by using two-body interactions in a restricted valence space. Then he showed how similar calculations, performed in heavier nuclei within the Monte Carlo shell model, modify simplistic electron capture rates, which have been used up till now to model supernovae explosions. The effect is truly dramatic because the capture on nuclei now turns out to be more important than the capture on protons as assumed previously. During a special session, Piotr Magierski from Warsaw, the winner of the 2003 Zdzislaw Szymanski Prize, also talked about neutron stars when he gave a lecture on the thermodynamic properties of the neutron star crust.
At the frontier between astrophysics and neutrino physics, there were two interesting talks about stellar objects viewed through their neutrino emission. Matthias Liebendörfer from Toronto has investigated the time and energy characteristics of neutrinos emitted during a supernova explosion. If such an explosion happens again nearby, we may be able to learn from the observed neutrino flux about how such an event proceeds, provided we have a good model at hand. Dima Yakovlev from St Petersburg and Pawel Haensel from Warsaw both talked about neutron-star cooling due to neutrino emission and strange particles in the core. Since the cooling process crucially depends on details of occupations near the Fermi surfaces, and thus on correlations, proton and neutron pairing in the neutron star matter may strongly influence the rate of cooling. Experimental data seem to suggest that proton pairing may be preferred over neutron pairing.
Among several talks on neutrino physics, Yuri Kamyshkov of Oak Ridge and Joanna Zalipska of Warsaw presented experimental studies performed at the KamLAND and Super-Kamiokande facilities, respectively. They discussed neutrino oscillation phenomena studied by the observations of reactor electron antineutrinos and atmospheric muon neutrinos, and the so-called large mixing angle solution for the neutrino mass difference and mixing angle.
Calculating the nucleus
Wolfram Weise from Trento presented studies at the triple frontier between quantum chromodynamics (QCD), the hadronic medium and nuclear structure. Recent developments in this field are fascinating because we may be witnessing the birth of derivations of nuclear forces from first principles, and an explanation of nuclear binding based on QCD. By applying ideas based on chiral symmetry breaking, the chiral condensate and effective field theory (EFT), we can describe nucleon-nucleon (NN) scattering and finite nuclei almost directly from low-energy QCD considerations. One starts by postulating the chiral Lagrangian of nucleons and pions, then adding symmetry-dictated contact terms that are supposed to describe all unresolved high-energy effects. Such a result shows that the short-distance NN repulsion does not need to be modelled by any kind of hard-core potential or heavy-meson exchange potential, but is a generic feature of these unresolved high-energy effects. As Weise explained, one may perform in-medium chiral calculations and derive the energy-density functional, which within the relativistic-mean-field approximation is directly applicable to finite nuclei. At the expense of fitting one parameter – the EFT cut-off energy – the correct saturation energy, saturation density and symmetry energy can be obtained. From there, standard nuclear-structure calculations lead to describing nuclear masses (only for N = Z nuclei at present) with a precision of about 1 MeV.
Witek Nazarewicz of Oak Ridge and Warsaw talked, among other things, about recent progress in the exact calculations for low-energy states in light nuclei. There, the necessity for NNN interactions has been convincingly shown. Moreover, the NNN forces may also be responsible for a known inadequacy of the G-matrix method to derive the shell-model interactions. Nazarewicz discussed the challenges that nuclear-structure theory faces in describing exotic systems such as those with very large neutron or proton excess, very large angular momentum, or very large mass. In view of the important projects to study these exotica in experiments (RIA, GSI, RIKEN, EURISOL, etc), theoretical efforts in these domains must also be adequately expanded.
In two more talks, Marek Ploszajczak from GANIL and Krzysztof Rykaczewski from Oak Ridge discussed other aspects of exotic nuclei. Ploszajczak presented methods that combine advanced descriptions of bound nuclear states with equally advanced descriptions of scattering states. For weakly bound nuclei, such combined methods are essential. Unfortunately, however, they remained neglected for too long a time because of the necessity to treat expertly two fairly different physical situations. The so-called Gamow shell model has recently been devised to remedy this through a shell-model-like treatment of the particle continuum. Rykaczewski showed that, on the other side of the mass table, i.e. for proton-unstable nuclei, proton emission could be used as a fantastically efficient probe of nuclear states. By a careful analysis of proton radioactivity in deformed nuclei, we can explicitly see that the initial proton really is in a deformed state. This is one of the nicest examples of how spontaneous symmetry breaking works in finite systems.
Recent advances in experimental verifications of the Standard Model of particle physics were also presented at the conference, in the talk by Krzysztof Doroba of Warsaw. He described experiments that precisely measure the mass, width and other characteristics of the Z and W bosons, and convinced us that once these basic physical constants are measured many things can be calculated rigorously within the Standard Model. Some recent novelties were also reported, with Hideki Kohri from Osaka talking about the observation of the pentaquark baryon, Θ.
Looking to the future of experiments in nuclear physics, Peter Senger from Darmstadt gave a very interesting account of the international accelerator facility planned for GSI. He described the main scientific directions in which the facility will aim, namely hadron spectroscopy, the structure of nuclei far from stability and compressed baryonic matter, which will make it a true nuclear-physics facility! This is a superb project that will provide a tremendous amount of data and boost our knowledge of nuclear systems. We all hope that the missing 25% of European funding will be found, and that the project will go ahead at full steam as rapidly as possible.
Altogether about 100 participants – experienced lecturers as well as young PhD students – attended the conference, coming from 14 countries; mostly European but also from China, Israel, Russia and the US. Whilst evolving, the school/conference has maintained most of the traditions for which it is widely known. Matching the scientific programme were a number of social highlights. Two of these are hallmarks of the Mazurian meetings: the first-class chamber music in the local church (the Warsaw string quartet, Camerata, with Samuel Barber, Karol Szymanowski and Johannes Brahms) and of course the regatta, with the the sailing race being won by a woman, Krystyna Wosinska, for the first time. The conference was organized by the Andrzej Soltan Institute for Nuclear Studies in Swierk and Warsaw University, and thanks to the support of the European Physical Society, young European physicists could apply for grants to attend.
