Ebook: Strangeness and Spin in Fundamental Physics

The CLXVII Course “Strangeness and Spin in Fundamental Physics” of the “Enrico Fermi” School, held in Varenna from June 19 to 29, 2007, was dedicated to the discussion of the role played by two subtle and somehow puzzling quantum numbers, the Strangeness and the Spin, in Fundamental Physics. They both relate to basic properties of the fundamental quantum field theories describing strong and electro-weak interactions and to their phenomenological applications. In some instances, like the partonic spin structure of the proton, they are deeply correlated.

The concept of Strangeness was introduced in 1953 in order to explain experimental observations in Particle Physics, which could not be interpreted without the introduction of such a new quantum number. Strangeness was the paradigm for the introduction, in the following decades, of other new quantum numbers (Charm, Beauty, …) and may be considered as the cornerstone for the building up of the presently accepted theory of strong interactions, QCD, and of the quark content of matter.

However, it was soon realized that Strangeness plays a crucial role not only in elementary systems, but also in much more complicated many-body ensembles (nuclei, atoms, neutron stars, …). Due to the circumstance that the mass of the s-quark is somewhat between the masses of the light u- and d-quarks (from which our world is built) and those of the heavy c⁻, b⁻ and t-quarks, exotic many-body systems containing s-quarks may be assembled and carefully examined experimentally. The study of their properties is of fundamental importance for understanding their underlying structure. The physics of Strangeness then encompasses the fields of Elementary Particles, Nuclear Physics, Atomic Physics and Astrophysics, with close and important links between them.

The concrete demonstration of the great importance gained by the Strangeness Physics is given by the great number of large experiments partially or fully devoted to it at large Laboratories like RHIC-BNL, TJNAF, DAPHNE, GSI, Nuclotron-Dubna. Even more, the powerful complex of accelerators J-PARC, under completion at Tokai (Japan) which will supply in two years beams of mesons, in particular Kaons, of unprecedented intensity and purity, will dedicate a large amount of time to Strangeness Physics studies.

Half of the lectures and seminars of the Course covered many aspects of the Strangeness Physics, and were given by world-recognized experts in the field: Professors Bendiscioli, Bertini, Bressani, Feliciello, Gal, Guaraldo, Nagae, Nappi and Schramm.

The interest in Spin Physics has enormously grown in the last 15-20 years, both from the experimental and theoretical points of view. In all high-energy experimental facilities there are ongoing or planned spin-dedicated experiments with collisions of polarized particles: HERMES at DESY, STAR and PHENIX at RICH-BNL, several experiments at TJNAF. Even BELLE, at KEK, studies spin effects in the fragmentation of quarks produced in e⁺e⁻ interactions. Collisions between polarized protons and polarized antiprotons have been proposed for future experiments at GSI: these would explore entirely new aspects of the strong interactions and QCD. Experiments with polarized protons are also planned at J-PARC.

The many puzzling results recently obtained by measuring several spin asymmetries have stimulated enormous progress in the study of the spin structure of protons and neutrons. Intense theoretical activity has discovered new features of non-perturbative QCD, like strong correlations between the spin and the intrinsic motions of quarks inside the nucleons. The high-energy spin physics community is aiming at reaching a full understanding of the proton structure, which not only takes into account the longitudinal degrees of freedom, but also transverse motions, spin and orbital angular momenta of quarks and gluons.

The other half of the lectures and seminars of the Course was devoted to many facets of high-energy spin physics, and were given by excellent speakers and active researchers in the field: Professors Anselmino, Avagyan, Boglione, Hash, Jaffe, Leader, Saito and Vogelsang.

The purpose of the Course was that of providing a complete, updated and critical account of the most recent and relevant discoveries in the above fields, both from the experimental and theoretical sides, and was fully achieved. The Course was very timely and important for the Enrico Fermi School, focused on education and information and providing a careful analysis of the available data, predictions for ongoing experiments and suggestions for future plans.

In conclusion the organizers of the Course warmly thank the European Project Hadron Physics of the FP6 framework and the Project HYPERGAMMA of PRIN (Italian Ministry of Research) for the generous financial support to the School, which made its organization possible. Special thanks are due to Miss B. Alzani for the continuous and invaluable help during the Course, as well as to Miss G. Bianchi Bazzi, Miss R. Brigatti, Mr. D. Caffarri of the Villa Monastero organization. Mrs. M. Missiroli and Mrs. C. Vasini are also acknowledged for their precious activity before and after the completion of the Course. Finally special acknowledgements are due to Dr. A. Feliciello and Prof.Ph.G. Ratcliffe, Scientific Secretaries of the Course, for their continuous, patient and precious work at all stages of the scientific and practical organization.

M. Anselmino and T. Bressani

The more important steps in hypernuclear physics are briefly summarized, mostly from the experimental point of view. Last results on different items, like spectroscopy, weak-decay modes and other more exotic systems are discussed.

Analyses of strong-interaction data consisting of level shifts, widths and yields in strange atoms of K⁻ mesons and Σ⁻ hyperons are reviewed. Recent results obtained by fitting to comprehensive sets of data across the periodic table in terms of density-dependent optical potentials are discussed. The introduction of density dependence generally improves significantly the fit to the data, leading to novel results on the in-medium hadron-nucleon t matrix t(ρ) over a wide range of densities up to central nuclear densities. A strongly attractive K⁻-nuclear potential of order 150–200 MeV in nuclear matter is suggested by fits to K⁻-atom data, with interesting possible repercussions on condensation and on the evolution of strangeness in high-density stars. The case for relatively narrow deeply bound K⁻ atomic states is made, essentially independent of the K⁻ potential depth. In view of the recently reported inconclusive experimental signals of deeply bound states, dynamical models for calculating binding energies and widths of -nuclear states are discussed. Lower bounds on the width, MeV, are established. For Σ⁻ atoms, the fitted potential becomes repulsive inside the nucleus, in agreement with recently reported (π⁻,K⁺) spectra from KEK, implying that Σ hyperons generally do not bind in nuclei. This repulsion significantly affects calculated compositions and masses of neutron stars.

Ultra-relativistic heavy-ion collisions are believed to provide the extreme conditions of energy densities able to lead to a transition to a short-lived state, called Quark-Gluon Plasma (QGP), where the quarks are no longer bound inside hadrons. The studies performed so far, formerly at SPS (CERN) and later at RHIC (BNL), allowed to achieve a multitude of crucial results consistent with the hypothesis that a new phase of the QCD matter has been indeed created. However, the emerging picture is that of the formation of a strongly interacting medium with negligibly small viscosity, a perfect liquid, rather than the ideal perturbative QCD parton-gas predicted by most theorists. The head-on collisions between lead nuclei at the unprecedented energies of the forthcoming Large Hadron Collider (LHC) at CERN, due to start in 2008, will allow to measure the properties of compressed and excited nuclear matter at even higher initial densities and temperatures, far above the predicted QCD phase transition point. The longer duration of the quark-gluon plasma phase and the much more abundant production of hard probes, which depend much less on details of the later hadronic phase, will likely provide a consistent and uncontroversial experimental evidence of the QGP formation. Among the signals that witness the change in the nature of the state of nuclear matter, the chemical equilibrium value of the strangeness plays a key role since it is directly sensitive to the matter properties and provides information on the link between the partonic and the hadronic phases. The aim of this course is to overview the underlying goals, the current status and the prospect of the physics of the nucleus-nucleus collisions at ultrarelativistic energies. Among the experimental methods adopted to investigate the challenging signatures of the QGP formation, emphasis on those related to the strangeness flavour will be given.

In these lecture notes, the role of strangeness in relativistic astrophysics of compact stars is addressed. The appearance of strange particles, as hyperons, kaons, and strange quarks, in the core of compact stars is examined and common features as well as differences are presented. Impacts on the global properties of compact stars and signals of the presence of exotic matter are outlined for the various strange phases which can appear in the interior at high densities.

A high-intensity proton accelerator complex, J-PARC, is now under construction in Japan. We expect that the first beam would be delivered to the Hadron Experimental Hall around the end of 2008. The world highest intensity K⁻ beams will be available, which will open new opportunities to conduct various types of strangeness nuclear physics experiments. In this lecture several examples from the initial experimental program are introduced.

This paper reports an overview of experimental results and theoretical predictions about the annihilation at rest on nuclei. It stresses that the annihilation on ⁴He creates an environment very favourable to the formation of strange quarks and describes results of recent analyses on this process made on data collected with the spectrometer Obelix at the LEAR accelerator of CERN. The results concern the high strangeness production in specific reaction channels, double-strangeness production and signals for the production of the pentaquark Θ⁺(1539) and antikaon-few-nucleon bound states like K⁻ pn,K⁻ pnn and K⁻d.

The FINUDA Experiment, installed at DAFNE, completed its first two data taking campaigns. A selection of the most interesting results is presented.

We review first the parton model formalism for polarized deep inelastic lepton-hadron scattering. Topics discussed include the “spin crisis in the parton model”, the role of the axial anomaly, our knowledge of the polarized gluon number density and attempts to measure it. Secondly, going beyond the simple parton model, we discuss the evolution of parton densities, the generalization of the parton model in QCD, perturbative QCD corrections and scheme dependence. Finally we comment on our knowledge of the polarized strange quark density and attempts to learn about it from semi-inclusive deep inelastic scattering.

The last years have witnessed an impressive effort—in theoretical work, experimental measurements and data analysis—towards a deeper and better understanding of the proton and neutron structure in terms of their elementary constituents, quarks and gluons. Much progress, beyond the usual simple QCD representation of a fast moving nucleon as a bunch of collinear partons, has been achieved. Fundamental issues, like the intrinsic and orbital motion of quarks inside a proton, its coupling to the parton spin and to the parent proton spin, have been raised and investigated. The transverse, with respect to the direction of motion, degrees of freedom of partons, both in spin and motion, are best suited to study the nucleon internal properties. A new QCD picture of protons and neutrons is slowly emerging.

This lecture reviews the strategies for measuring transverse spin phenomena in deep-inelastic scattering. Understanding such transverse polarisation phenomena in hadronic physics is a long-standing and intriguing problem. Despite of a wealth of experimental data on transverse spin asymmetries there is no measurement yet of the quark tranversity distribution which, together with the unpolarised and the helicity distribution, completes the three basic distributions to be measured in order to obtain a full description of the quark structure of the nucleon at leading twist. Modern developments in hadron physics emphasize the role of correlations of transverse momentum of partons and spin. Such spin-orbit correlations are described by a new class of transverse-momentum–dependent distribution and fragmentation functions (TMDs), which generalise the standard parton distributions. The information on spin-orbit correlations together with independent measurements related to the intrinsic motion of quarks will be the key to construct a complete picture of the internal structure of the nucleon going beyond the collinear approximation. Experiments that aim at pinning down various TMDs are currently running at Cern (Compass Collaboration), Desy (Hermes Collaboration), JLab, Kek and Rhic. Here, we will concentrate on the studies of transverse spin phenomena at the Hermes and Compass experiments in semi-inclusive deep-inelastic scattering on transversely polarised nucleon targets. After an overview of the analysis techniques we will present the first exciting results and give an attempt to interpret them.

We discuss spin phenomena in high-energy hadronic scattering, with a particular emphasis on the spin physics program now underway at the first polarized proton-proton collider, RHIC. Experiments at RHIC unravel the spin structure of the nucleon in new ways. Prime goals are to determine the contribution of gluon spins to the proton spin, to elucidate the flavor structure of quark and antiquark polarizations in the nucleon, and to help clarify the origin of transverse-spin phenomena in QCD. These lectures describe some aspects of this program and of the associated physics.

This lecture is delivered to provide an up-to-date picture of the spin structure of the nucleon basing on the recent results from RHIC experiments. A future prospect of the possible experiments at J-PARC is also given.

The concept of spin has been revisited, in a simple way and in the framework of Lorentz symmetry and quantum theory. The basic proof of the existence of spin, given by the results of Stern-Gerlach experiments, is also discussed.

The main goal of experiments proposed for the CLAS12 detector in conjunction with the 12 GeV CEBAF accelerator is the study of the nucleon through hard exclusive, semi-inclusive, and inclusive processes. This will provide new insights into nucleon dynamics at the elementary quark and gluon level. In this contribution we provide an overview of ongoing studies of the structure of nucleon in terms of quark and gluon degrees of freedom and future physics program planned with CLAS and CLAS12.

The first part of this lecture is a general introduction to distribution and fragmentation functions, the “soft” functions which imbed the full information on the polarization structure of the nucleon in terms of its elementary constituents. I then concentrate on the description of two of these objects, yet unknown: the transversity distribution and the Collins fragmentation functions. The determination of these two soft functions will be the focal point of the last part of my lecture, where a global analysis of the experimental data on azimuthal asymmetries in Semi-Inclusive Deep Inelastic Scattering (SIDIS), from the HERMES and COMPASS Collaborations, and in e⁺e⁻→h₁h₂X processes, from the Belle Collaboration, will be presented.