Nuclear Physics
Nuclear physics began with the discovery of radioactivity, transmutation
of matter, and the discovery of the nucleus. The latter two discoveries
were made by Sir Ernest Rutherford. McGill University's long and strong
tradition of excellence in nuclear physics began with Rutherford's tenure
at McGill between 1898 and 1907 during which he discovered the transmutation
of matter. The same tradition of excellence continues on to this day.
Today, nuclear physics encompasses a wide range of modern physics. The
traditional study of nuclei and their reactions is still a vibrant part of
modern nuclear physics. In the latter part of the 20th century, however,
a new and exciting field of nuclear physics started to emerge, the study of
nuclear matter under extreme conditions.
![[phase diagram]](i.nuc/phase.jpg)
The phase diagram of nuclear matter
Soon after the advent of Quantum Chromodynamics (QCD), the theory of the strong
nuclear force, physicists began to realize that at extreme temperatures of
trillions of Kelvin, the protons and neutrons in nuclei should, in effect,
melt, and the released quarks and gluons should form a completely new phase of
matter. The hunt for this new state of matter, dubbed the Quark-Gluon Plasma
(QGP), soon began and the powerful relativistic heavy ion colliders at the
Brookhaven National Laboratory and at CERN have now confirmed that under
this extreme and highly relativistic condition, QGP is indeed the phase of
the nuclear matter. Yet, many properties of the produced QGP, such as the
lowest viscosity ever measured, were completely unexpected.
To put QGP in perspective, this kind of temperature (about a billion times
hotter than the surface of the sun) existed in nature only when the Universe
was about a micro-second old, about 1 cubic millimeter of QGP contains
enough energy that it could power current Canadian economy for few hundred
million years, yet it flows more freely than the superfluid helium!
The study of QGP is the new frontier of modern nuclear physics. The
Nuclear Theory Group at McGill has long been playing a central role in the
development of this exciting new field. The group currently consists of two
professors (Charles Gale and Sangyong Jeon) and more than a dozen students
and postdoctoral fellows. The group also has strong ties to researchers in
the high energy theory group at McGill and collaborators in the US, Europe
and Asia. The main focus of our study is QGP and the relativistic heavy ion
collisions in which it is made. The research topics vary widely from purely
theoretical to numerical simulations. What ties all of our efforts together
is the question, How does one use heavy ion collision phenomenology to learn
about QGP? This calls for a comprehensive model of the full evolution of
heavy ion collisions.
![[jet evolution]](i.nuc/jets.png)
Evolution of jets in QGP
To achieve the extreme conditions necessary to produce QGP, heavy nuclei
such as gold or lead are accelerated to almost the speed of light and
made to collide with each other. The produced QGP then cools as it expands
and eventually turns back into ordinary matter. To accurately describe and
predict the behavior of these processes requires understanding of the initial
nuclei, energy and entropy release during the collision, formation of QGP,
expansion and cooling, and finally the phase transition back to ordinary
nuclear matter. While all these are happening, high energy quarks (called
the jets) may traverse QGP shedding some of its energy, and photons from
black-body radiation are being produced at each stage.
To understand all of the above is a challenging task to say the least. Yet,
the goal of the our group is nothing short of building a comprehensive model
of the full heavy ion collision and QGP evolution encompassing the essence
of all of the above!
To achieve this goal, some of us are working on applying string theory
techniques to the study of QGP, some of us are studying quantum field
theories at extremely high temperatures, some are building the most advanced
hydrodynamic models of the QGP evolution, and some are studying the effect
of QGP on ultrarelativistic particles that are traversing it. Yet, there
are many important un-answered questions such as What is the nature of the
initial conditions? How does the QGP form so quickly? that are waiting for
bright minds.
To add excitement, the LHC has started to produce a copious amount of new
heavy ion collision data which contains more surprises that await
theoretical resolution. Our group is fully engaged in studying all aspects
of these issues. This is an exciting time to be a nuclear physicist,
especially at McGill!
![[hydro simulation]](i.nuc/hydro.png)
A hydrodynamic simulation of QGP evolution
The formation of the elements that make up our universe, from the remnants
of the big bang that created it, continues to be a fascinating mystery. It
is thought that at least part of the production of the heavier elements
took place during explosive astrophysical events (supernovae, x-ray bursts
etc.) that are powered by nuclear reactions among short-lived, radioactive
nuclides at the limits of nuclear binding. The atomic masses of these nuclei
are essential to understanding these processes because they determine the
energy released and determine the path of the nuclear reaction chains that
take place in these events. Furthermore, the atomic masses of nuclei that
participate in super-allowed beta-decay provide a unique opportunity for
tests of fundamental symmetries in the standard model for particle physics.
Nuclear mass measurements are done using the Canadian Penning Trap Mass
Spectrometer (CPT) at the Argonne national Laboratory that collects short
life nuclei produced in reactions at the
ATLAS heavy-ion accelerator. With
this system, nuclear masses of isotopes with lifetimes as short as 30
milliseconds are measured with very high accuracy and sensitivity. Nuclear
mass measurements are also performed using the
TITAN facility at
TRIUMF in
Vancouver where the unstable nuclei are produced by a different process;
nuclear spallation.
In recent years, techniques originally used for atomic spectroscopy have been
applied to measure such nuclear properties as spin, electric and magnetic
moments, and the change of charge-radius between neighboring isotopes. These
techniques are based on the precise measurement of atomic hyperfine structure
in the interaction of laser beams with atomic beams obtained from isotope
separators. The laboratory has pioneered in the development of a number of
high sensitivity techniques for such studies.
Our group participates in a program of such measurements at the
ISAC
radioactive beam facility at TRIUMF. Using a spectroscopic method known as
collinear fast beam laser spectroscopy and by making use of existing facilities
such as the TITAN ion trapping system and material science beta-NMR and
beta-NQR it is possible to perform spectroscopy measurements on ion beams
with intensities as low as a few tens of ions per second.
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