Rutgers University Department of Physics and Astronomy
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Nuclei are prime examples of fermion many-body systems of interacting neutrons and protons. Light nuclei on the other hand fall in the category of few-body problems.
Nuclei can also be used to test aspects of elementary particle theory. For example, the nature of the neutrino can be studied via double beta decay of certain heavy nuclei. One very exciting research area involves heavy ion collisions at high energies. When two relativistic nuclei collide, hadronic matter at very high temperatures and density can be produced, possibly resulting in new forms of matter such as a quark-gluon plasma or a condensate of pi-mesons, as they may have existed in the initial stages of the universe after the big bang.
Professor David Harrington
There is much convincing evidence that "elementary" particles, such as nucleons and mesons, are actually composites of quarks, anti-quarks, and gluons (known collectively as partons). This compositeness changes the interaction of the elementary particles with nuclei because they can be excited on one nucleon, then de-excited or excited further on another. I have calculated the influence of these processes on the total cross sections for the scattering of a nucleon from heavy nuclei at very high energies. This calculation requires, however, the amplitudes for transitions from one excited state to another, for which there is no direct experimental information. I am now trying to see if coherent diffraction scattering from the deuteron can put constraints on these transition amplitudes, and if a composite model for nucleons which takes into account the excitation energies, can be used to calculate these effects.
Professor Willem Kloet
From high energy experiments the substructure of nucleons in terms of quarks and gluons is evident. On the other hand at low energy a description of nuclei with nucleon degrees of freedom is prefered. How should nuclear processes in the transition region be modeled? For example the annihilation of nucleons and antinucleons into mesons is inherently a very short range process at any energy and models based on nucleon degrees of freedom are inappropriate. By constructing models for this annihilation process using other degrees of freedom, one can get insight in the substructure of the nucleon for relatively low energy processes.
Professor Aram Mekjian
My research is concerned with understanding the collision between two heavy ions of high energies. By looking at these collisions, we are trying to find new phenomena that may occur when nuclei are compressed to high densities. These phenomena include the production of quark matter, pion condensation, and the appearance of a density isomer or Lee-Wick matter. Using statistical mechanics and thermodynamics, the distribution of products in relativistic heavy ion collisions has been studied. The formation of composite nuclei has been formulated in the same framework that accounts for the formation of the elements under explosive conditions as in supernovae explosions and in the big bang.
Professor Larry Zamick
My recent and current research topics include:
I have constructed a model which consists of simply setting all T=0 two-body interaction matrix elements to zero and keeping those with T=1 as they were. This model leads to partial dynamical symmetries and corresponding degeneracies, which we have attempted to explain. Surprisingly this model gives fairly reasonable spectra for even-even nuclei in the f-p shell. We find that the restoration of the T=0 matrix elements is needed to explain staggering of high spin states in odd-even nuclei, the isovector scissors mode strengths, and to bring the nuclei somewhat away from the vibrational limit and towards the rotational limit.
It has been often said that states of different isospins have nothing to do with each other but this is not true--there is the constraint of orthogonality. Exploiting this fact I am able to greatly simplify the expression for the number of J=0 pairs in a mixed system of neutrons and protons.
I have constructed a new topic"Companion Problems in Isospin and Quasispin" in which I note that the mathematics that is used for a system of identical nucleons (e.g. only neutrons) can be used for a different problem which involves neutrons and protons (e.g. diagonalizing a six-j symbol). In the identical particle case this leads to a quasispin result concerning the number of states of a given seniority. In the companion case of mixed neutrons and protons it leads to the above mentioned simplification of the expression for the number of J=0 pairs.
The general interest in the field of nuclear structuure has shifted to nuclei far from stability--either proton rich or neutron rich--, with a mind to understanding how the heavy elements were originally formed. I have been collaborating on the magnetic moment measurements of excited states , both with the local experimental group and one from Bonn, and we are indeed going to heavy unstable nuclei e.g. 68Ge and we are planning to go further.
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