International Union of Pure and Applied Physics

WG.9: Working Group on International Cooperation
in Nuclear Physics (ICNP)

Five Key Questions for Nuclear Physics and their Relation to Existing and Proposed Nuclear Physics Facilities

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  Nuclear physics is driven by fundamental investigations of the origin, evolution, structure and phases of strongly interacting nuclear matter. This is a challenging mandate that requires a balanced program of experimental and theoretical effort to address a series of key questions which carry to the larger scientific community. There is broad international consensus on these more important questions as these are given in nearly identical formulations in both the Nuclear Science Advisory Committee (NSAC) report in 2002 and in the Nuclear Physics European Collaboration Committee (NuPECC) report in 2004. The five key questions and the major facilities where these questions can be answered are:

1) Can the structure and interactions of hadrons be understood in terms of QCD?

Nucleons are composite particles made up of quarks and gluons. There exists to date partial answers on how the quarks are distributed and move within the nucleon and the 2004 Nobel Prize in Physics was awarded for the discovery of asymptotic freedom within the context of perturbative Quantum Chromo-Dynamics (QCD). However, QCD is unsolved in the confinement regime where the quark coupling strength is too large to permit perturbative methods to be used. One of the central problems in nuclear physics remains the connection of the observed properties of the hadrons to the underlying theoretical framework of QCD. The solution requires advances both in theory and experiment. In a few specific cases, where chiral coefficients are well known, recent advances in lattice QCD, in combination with chiral perturbation theory, have allowed one to extrapolate full lattice simulations to physical quark masses; thus enabling a direct comparison with experimental observables. The objectives are:

  • a tomographic view of the valence quarks and their motion within the nucleon;
  • a detailed understanding of how QCD gives rise to the properties of the lighter hadrons; and how these properties are modified when they are placed in a nuclear environment.
  • Experiments designed to make detailed comparisons with QCD predictions are high-priority endeavors for research at facilities across the USA , Japan , and Europe . In particular, J-PARC of KEK and JAEA, FAIR at GSI and the 12 GeV upgrade at Jefferson Lab, which will operate in the near term, were all designed (to varying degrees) to address this question in detail.

In addition one should mention the important program of study using colliding polarized protons on polarized protons at RHIC, the current program at Jefferson Lab and there are plans under development at Jefferson Lab and RHIC to extend the earlier work at DESY, namely the study of the structure of the sea of the nucleon. Jefferson Lab and RHIC, as well as possibly CERN, are developing plans to extend these studies to the structure of the sea of atomic nuclei.

2) What is the structure of nuclear matter?

The original and central role of nuclear physics is to understand the properties of nuclei and nuclear matter. This is a formidable task, which, at least at the present time, seems better approached in steps: from the basic equations of QCD through effective field theories to nucleon-nucleon interactions, few-body systems and very-light nuclei; and further on to the many approaches used to describe nuclear structure, ranging from methods such as Green's Function Monte Carlo (GFMC) to the shell model and density functional theory. While calculations based on "realistic" nucleon-nucleon interactions, supplemented with three-body forces, have achieved quantitative success in reproducing the features of light nuclei, detailed agreement is still lacking for heavier nuclei. This is a problem not restricted to the description of heavy nuclei but is common to the description of other complex systems, such as proteins.

In nuclear physics the development of a comprehensive, predictive theory of complex nuclei is a key goal for nuclear physics. Worldwide this has driven the development of high-quality and multi-faceted radioactive beams, as these allow one to move from a one-dimensional picture, where the mass of the nucleus varies, to a two-dimensional picture where both proton and neutron numbers vary over a wide range. Rare isotope beams are obtained either through the well known ISOL process or through in flight fragmentation. There exists at present a plethora of small and large facilities of both kinds, the larger ones being NSCL at Michigan State University , ISOLDE at CERN, ISAC (I and II) at TRIUMF, and SPIRAL1@GANIL in Caen , France . The near-future facilities are RIKEN in Japan , SPES in Italy , GSI-FAIR in Germany and SPIRAL2@GANIL in France . Still in the planning stage are EURISOL in Europe and RIA in the USA . These facilities are also engaged in studying current nuclear structure problems. The quest for the super-heavy elements is an ongoing effort at JINR in Dubna, GSI in Darmstadt , and RIKEN in Saitama.

3) What are the phases of nuclear matter?

Nuclei are an important manifestation of nuclear matter, since they make up 99.9% of the visible matter in the universe. But it is somewhat humbling to realize the preponderance in the universe of dark matter and dark energy. At the highest densities, yet at still rather low temperatures, the quarks making up the nucleons of nuclear matter may form a new state of nuclear matter, which is color superconducting. As the density rises (but before quark matter of any kind can form) one may also find a large fraction of the matter present is strange and one cannot yet exclude the possibility of kaon condensation. Nuclear matter can also be heated by absorbing energy in a relativistic collision. In this case 'nuclear temperatures' can reach values that represent the state of matter that existed during the first moments after the big bang. The quest for the so-called quark-gluon plasma (or phase transitions in hadronic matter) is an active field of study at international facilities such as GSI in Germany , the LHC at CERN ( ALICE experiment), and RHIC in the USA .

4) What is the role of nuclei in shaping the evolution of the universe?

Primordial nucleosynthesis, nucleosynthesis that occurred during the cooling immediately following the big bang, gave rise to the primordial abundances of H, He, and Li. All other chemical elements in the universe were produced as a result of the nuclear reactions occurring in stars, during supernovae explosions, novae, neutron star mergers, etc. It is another central objective of nuclear physics to explain the origin and abundances of matter in the universe, while nuclear astrophysics must address the many fundamental questions involving nuclear physics issues that remain. The latter include: the origin of the elements, the mechanism of core-collapse in supernovae; the structure and cooling of neutron stars and the presence of strange matter; the origin, acceleration, and interactions of the highest energy cosmic rays; and the nature of galactic and extragalactic gamma-ray sources. Nuclear astrophysics has benefited enormously from progress in astronomical observation and modeling. A new era in nuclear astrophysics has opened up with the advent of radio-active beam facilities dedicated to the measurement of nuclear reactions involving short-lived nuclides of particular relevance to astrophysics. These include measurements of the various nuclear capture processes and the determination of masses, half-lives, and structures of rare nuclei that occur in cataclysmic stellar environments. Most of the rare isotope facilities (mentioned above) have or will have extensive research programs in nuclear astrophysics.

5) What physics is there beyond the Standard Model?

The forces and interactions that were in play in the early stages of the universe have shaped the cosmos as it is known today. Nuclear physicists have long studied the fundamental symmetries of the weak interaction, and probed the Standard Model with precision, low- and intermediate-energy experiments. While the Standard Model has proven remarkably resilient to these tests, there are a few indications of physics beyond the Standard Model. If the breakdown of the Standard Model is confirmed, this could constitute the first indication of super-symmetry, which offers a possible explanation of the dark matter of the universe. A new generation of experiments, designed to push the limits of discovery and precision, can be grouped as follows:

i) The universe has an obvious imbalance between matter and antimatter which the Standard Model is unable to explain. An essential ingredient in the possible solution of this enigma is the presence of new interactions which violate time-reversal-invariance (TRI) and charge conjugation/parity inversion (CP) (if one assumes CPT invariance). There is today a great deal of activity probing for a signal of TRI violation in the properties of mesons, neutrons, and atoms.

ii) Another key question is the nature of the "superweak" forces which disappeared from view when the universe cooled. The Standard Model, as stated above, is one of the better tested theories in physics, but still it is considered to be incomplete. Both nuclear and particle physics experiments are continually searching for indications of additional forces that were present in the initial moments after the big bang. High-energy experiments will probe the TeV scale directly, but high precision experiments at lower energies probe mass scales and parameter spaces not accessible at the high-energy accelerator facilities. Any deviation from the Standard Model discovered at the LHC, for instance, must be reflected in a corresponding rare interaction at lower energy. Jefferson Lab is a prime laboratory for such studies of probing the limits of validity of the Standard Model and of the physics beyond. Other approaches followed are atomic parity violation measurements for which trapping experiments at the rare isotope facilities are essential.

iii) Finally, the resolution of the solar and atmospheric neutrino puzzles by SNO and Super-Kamiokande has opened up the possibilities for exciting discoveries in the neutrino sector, like CP violation. A key question is the nature of the identified neutrino oscillations (long baseline neutrino experiments, e.g. T2K). In this context one can contemplate the possibility of a neutrino factory. The observation of neutrinoless double beta decay would revolutionize the understanding of lepton number in the Standard Model and would determine the mass scale of the neutrino. Clearly, existing and future underground laboratories have an all-important role to play, searching for the decay of the proton, neutrinoless double beta-decay, and not to be forgotten dark matter!

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