Research

 

The research done at the LBL Nuclear Theory group covers many subfields of nuclear and particle physics, astrophysics and even condensed matter physics.

 

Parton Dynamics

On the basis of a QCD-inspired partonic description, a number of critical issues in heavy-ion collisions at RHIC energies and beyond have been addressed: the initial condition of the dense matter formed, the approach of such dense parton matter to equilibrium, and the probes of the early parton dynamics. While the initial parton density is found to be very high, it is also far from equilibrium and the equilibration time depends sensitively on the initial parton density. A few hard probes, such as charm, J/psi, and jets, have been investigated as indicators of the initial parton density and early parton dynamics. Continuing research will seek to reduce the uncertainty in the initial conditions as well as further explore the associated hard probes.

Hard Probes of QCD Matter

A number of studies have been directed towards heavy-quark production, both in the initial nucleon-nucleon interactions and in the plasma phase. Heavy-quark production in pp collisions has been studied using perturbative QCD at leading order, next-to-leading order, and, near the production threshold, all-order resummation at leading log and next-to-leading log. The knowledge gained from pp physics can help us to understand the level of heavy-quark contribution to the dilepton spectrum at RHIC and LHC although medium effects, such as energy loss by the charm quark moving through the nucleus, may play an important role. This latter effect is being examined in more detail, particularly by including the effects of longitudinal expansion, as is the possibility that e-mu coincidence measurements for open charm decay may provide information on the parton energy-loss mechanism.

Quarkonium production and suppression is an especially interesting topic in light of recent CERN SPS data from NA50 and this data has been addressed in terms of hadronic suppression mechanisms. At higher energies, where plasma production is likely, the high initial temperature of the plasma could lead to Ypsilon suppression or result in the gluons and light quarks gaining an effective mass, thereby enhancing thermal production of heavy quarks in the plasma. Manifestations of higher-twist effects on heavy-quark production are also being pursued, both in the context of the intrinsic charm model and, more recently, in a final-state coalescence model involving the quantum mechanical overlap of the initial- and final-state wave functions on the amplitude level.

Disoriented Chiral Condensates

A number of investigations have been carried out within the confines of the linear sigma model. In particular, the conditions for the occurrence of the DCC phenomenon has been studied in various collision scenarios and it has been shown that the soft pionic modes become unstable only when a sufficiently rapid expansion or cooling takes place. A variety of possible DCC signatures have been studied, including the power spectra of the emerging pions and the distribution of the neutral pion fraction. Wavelet-type analysis techniques have also been developed for the study of domain structure of the pion field.

Dileptons may be a very sensitive probe of soft pionic modes and dilepton spectra have been calculated with different models in order to examine the prospects for using this observable as a DCC diagnostic. It is planned to explore this mechanism in detail, with an emphasis on the role of other hadrons in DCC formation and decay.

Chiral Restoration and In-Medium Effects

Dilepton production at the CERN SPS has been investigated within a hadronic transport model. It was found that although the CERES data can be explained within errors, the present dilepton data do not provide additional sensitivity to the initial hadronic configuration. In-medium effects on the pion form factor and the pion dispersion relation lead to changes in the dilepton yield which, however, are smaller than the errors of present data. This calculation will be continued in order to explore the intermediate mass regime.

Studies in the hadronic phase have shown that the chemical equilibration rate is shortened by the presence of vector mesons and also affected by the onset of chiral restoration. Expansion and finite-size effects will be addressed within the transport model, and it is planned to extend these investigations into the strange sector. Furthermore, it is predicted that the L(1405) has a strongly momentum-dependent in-medium mass shift due to the Pauli principle. The prospects for investigating this effect with pion-induced reactions and stopped kaons are currently being assessed.

The collaborative effort to include the collective spin-isospin modes in transport treatments of heavy-ion collisions is being continued. The formal developments made earlier for nuclear matter have been adapted to the finite, non-equilibrium environment of the colliding system and implemented into an existing nuclear Boltzmann transport code. Dynamical simulations are being carried out in order to ascertain how the pionic collectivity affects experimental observables, such as flow patterns and pion yields.

Chiral models of the nuclear force will be extended to describe hypernuclei and the equation of state for both strange and charmed matter, with relevance for both heavy-ion physics and compact stars. Associated precursor signals for the chiral phase transition in dense matter will be sought and the possible transition to a kaon condensed phase in the core of a neutron star will be also studied.

Transport Theory

With a view towards high-energy nuclear collisions, work has been made on developing a suitable transport description from quantum field theory, using a 1/Nf   expansion, a loop expansion around background fields, and closed time path formalism.

The efforts to take account of quantum effects in dynamical simulations of many-body systems have progressed significantly. Building on earlier work, the quantum fluctuations inherent in wave-packet dynamics have been included into molecular dynamics by means of a quantal Langevin force. The extended description leads to a considerably improved reproduction of the observed intermediate-mass fragment yields for nuclear collisions. The method is quite general and can be employed in other areas of physics as well, for example for atomic clusters where it affects the critical properties of noble gases significantly.

Studies of nuclear dynamics with effective one-body transport theories have recently included the first realistic test application to the multifragmentation of expanding gold nuclei.

Nuclear Astrophysics

Gravity binds nucleons in a neutron star an order of magnitude more strongly than the strong force binds them in nuclei. The Pauli principle is therefore brought strongly into play in distribution the baryon number over many species of baryons and quarks. Suggested observational consequences have so far had little specificity (like cooling for which many different scenarios all fall into only several categories). Several avenues have recently been pursued: formation of low-mass black holes triggered by hyperonization or conversion of the core to quark matter; crystalline structure in the mixed confined-deconfined phase and possible effects on pulsar glitches; and structural changes (such as size and moment of inertia) which will reflect themselves in the time structure of pulsar spin down. Such changes are expected because the density profile changes with angular velocity and centrifugal force, ushering in thresholds for the population of new baryonic species. With each threshold the equation of state will be softened. Therefore the transformations will be reflected in the time-dependence of the pulsar rotation. In particular the signal of a first order phase transition is strongly registered in the braking index of pulsars, a measurable quantity. It is estimated that the signal will be present in about ten of the presently know pulsars if the phase transition does take place.

Macroscopic Nuclear Properties

The long-range aim of this work is to develop a quantitative understanding of the macroscopic properties of nuclei (binding energy, surface energy, density, compressibility, neutron matter, etc.). Using the semi-classical Thomas-Fermi method, we have recently achieved such a description, which yields the binding energy (or mass) of a nucleus as a function of N,Z and the nuclear shape. The model is currently being generalizing to include angular momentum as well. An approximate description of ground-state, super-deformed and fission-isomeric rotational bands, and of the associated moments of inertia has been achieved.

Transition From Order to Chaos

The development of a macroscopic theory of nuclear dynamics continues based on the parallel between a transition from ordered to chaotic nucleonic motions and the transition from an elastic to a dissipative collective nuclear response. Recent work concerns a comprehensive three-way comparison between classical and quantal computer simulations and the Wall Formula for nuclear dissipation, as applied to the excitation of independent particles in a time-dependent potential well rippled at a wide range of frequencies. Four of the most important corrections to the basic Wall Formula have been included. The results are being applied to a renewed comparison of theory and experiments on fission and nucleus-nucleus collisions.