Report for the Academic Year 1992-1993 - The Institute Libraries ...

Institute for advanced study
In response to the recent discovery of a significant population of primordial
binaries in globular clusters. Prof. Hut has made a study of binary-binary scatter-
ing experiments, partly numerically and partly through analytical calculations,
in collaboration with Douglas Heggie from the University of Edinburgh. In
addition, together with Steve McMillan, he continued a series of N-body simula-
tions of tidally limited star clusters in which the influence of various populations
of primordial binaries was investigated. With Jun Makino and Lars Hernquist,
from the University of California at Santa Cruz, he completed a comparative
study of the error propagation of various hardware and software limitations on
collisional stellar dynamics calculations.
Professor Hut also took part in a seminar on Science Studies, organized by
Professor Geertz in the School of Social Science.
FRANK WILCZEK's scientific activity over the past year, as in the previous
year or two, falls into a few main categories:
QCD Phase Transition: Professor Wilczek's student, Krishna Rajagopal, and he
worked out the consequences of the renormalization group applied to the QCD
phase transition. This phase transition can be studied theoretically and numeri-
cally for various numbers of quark flavors and masses. The analysis indicates
that the phase transition associated with chiral symmetry breaking is likely to be
second order for two massless flavors but first order for three or more. In the
case of the second order transition one can exploit universality to make many
quantitative predictions, some of them quite striking. In laboratory experiments
there is less flexibility, and one must live with a particular spectrum of quarks.
It appears that for purposes of the transition the real world is close to the case of
two massless quarks (u and d) while the strange quark plays a less important
role, although it is not impossible that one actually lies close to the tricritical
point which marks the boundary between 2 light flavor (second order) and 3
light flavor (first order) dynamics. In any case the actual u and d quarks are
not precisely massless and the behavior of thermodynamic quantities near the
transition therefore will be rapid but not truly singular. It is possible that some-
thing close to quark-gluon plasma in thermal equilibrium at a temperature above
the QCD phase transition temperature can be produced in the course of heavy
ion collisions. Indeed, recent calculations indicate this rather strongly for RHIC
and LHC energies. Of course the plasma once produced will cool rapidly, and
the question arises whether the dramatic events that have taken place leave
dramatic signatures. Wilczek and Rajagopal explored the possibility that large
correlated regions of misaligned chiral vacuum, similar to domains in a magnet,
could develop. They found that such regions were likely to develop under condi-
tions of a quench, that is when the hot plasma is suddenly put in contact with a
much colder heat bath. While a quench is at best a very crude idealization of
what actually happens in a heavy ion collision, it may serve to indicate qualitative
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