Transcript Nuclear Physics at Richmond What is Nuclear Physics? Nuclear science began by studying the structure and properties of atomic nuclei, the cores of atoms.

Nuclear Physics at Richmond
What is Nuclear Physics?
Nuclear science began by
studying the structure and
properties of atomic nuclei,
the cores of atoms. Today, its
reach extends from the quarks
and gluons that form the
substructure of the onceelementary protons and
neutrons, to the most dramatic
of cosmic events—
supernovae. At its heart,
nuclear physics attempts to
understand the composition,
structure, and properties of
atomic nuclei.
These two diagram show the
structure within the atom. The
left shows how much smaller a
quark is than an atom. The top
tier is the atom, which is 100,000
times bigger than the nucleus.
The quarks inside the protons
and neutrons within the nucleus
are at least 100,000 times smaller.
The figure above shows how the
quarks make up the nucleons
and how the nucleons form to
make the nucleus and atom.
What is the Universe Made Of?
The Frontier
The Standard Model is a theory that describes and explains subatomic particles and
interactions. Matter as we know it is comprised of six quarks, six leptons and the force
carriers. Quarks and leptons are called fermions with half-integer spin and the force carriers
are known as bosons with integer spin.
One of the major frontiers in nuclear physics is understanding why quarks
and gluons act the way they do to create everyday matter. We still do not have
a quantitative understanding of how quarks unite to form nuclei.
No one has ever seen a lone quark. This property, called confinement, means
quarks cannot be isolated. The quarks are bound with other quarks by the
color force to form pairs or triplets. The force between quarks increases as the
distance between them increases (see left-hand figure below), so no quarks can
be found alone.
Bosons are force carrier particles which create the interactions between the matter
particles and defines the properties of matter. There are three known forces; Strong or
Color, Electroweak, and Gravity. The strength of the force depends on the mass of the force
carrier particle as well as the distance between particles.
Gluons act on quarks and other gluons and bind atomic nuclei together. They keep the
quarks from being able to be separated by conserving color charge. Quarks are confined by
a force equivalent to 22 tons. This allows no quark to be seen alone.
The Electroweak force acts on quarks and leptons. It causes heavier particles to decay
into the lighter ones. This explains why most of our matter is comprised of up and down
quarks (including neutrons and protons) and electrons.
Gravity, though most familiar to people, is the particle we know the least about. It is the
weakest force, (Think about it, you need something as big as a planet to notice a difference)
and the Graviton, though speculated to exist, has never been observed.
Fermions make up most of the matter around us.
There are two groups, the first is comprised of
leptons and their corresponding neutrinos. The
neutrinos are much smaller than the other leptons
and interact with other matter via the weak force and
gravity.
There are 6 quarks that make up matter. As far as
we know, quarks can only be found in groups called
hadrons. There are two known types of hadrons,
baryons which are made from three quarks, and
mesons which are made from a quark and an antiquark. An example of a baryon is a neutron or a
proton, however it is interesting to note that their
mass may be made up mostly from the forces that act
The Real Table of the Elements.
on the quarks and not the bare quarks themselves.
The potential between two quarks rises
linearly as a function of separation. This
allows the quarks not to be separated
The reasons for quark confinement are complicated. The best theory is
quantum chromodynamics (QCD) and there is no analytic proof that QCD
should be confining, but intuitively confinement is due to the force-carrying
gluons having color charge. As two electrically-charged particles separate, the
electric fields between them diminish quickly, allowing electrons to become
unbound. However, as two quarks separate, the force experienced by the quark
remains constant regardless of its distance from the other quark. The total
energy increases linearly with distance. This is one of the major challenges in
nuclear physics since quarks can not be studied independently.
Thomas Jefferson National Accelerator Facility, Newport News Va
Racetrack-shaped
Accelerator
(underground)
Hall B
Thomas Jefferson National Accelerator Facility (JLab) is a US
Department of Energy national laboratory and the newest precision
accelerator in the United States. It is a giant electron microscope used
for illuminating the core of an atom. Its focus is on determining the
nature of quarks and how they bind together to make neutrons and
protons and most of the matter we see today.
The Continuous Electron Beam Accelerator Facility (CEBAF) is the
central particle accelerator at JLab. CEBAF is capable of producing
electron beams of 2-6 GeV. The racetrack shaped accelerator is about
7/8 mile around and 25 feet underground.
The electrons are
accelerated through the straight sections and then bent around the
curves using magnets. The electrons travel up to five times around the
track at relativistic speeds and are then sent to one of the three
experimental halls where the beam collidess with the target.
Lattice QCD has confirmed the
existence of flux tubes between
static massive charges.
Bottom: CLAS Detector in Hall B.
Upper Right: UR students visiting JLab:
Rusty Burrell, Nate Lawrence, Brent Follin and Kuri Gill.
CLAS – The CEBAF Large Acceptance Spectrometer,
located in Hall B at JLab, is used to detect electrons,
proton, pions and other subatomic particles. CLAS is a 45ton, $50-million radiation detector with around 40,000
detecting elements and nearly a 4π coverage. CLAS is
able to detect most particles scattered off in a nuclear
collision because it covers a large range of angles. The
particles go through each region of CLAS leaving behind
signals that are collected and stored on tape. Using the
information gained from the layers, the collisions are
reconstructed and the intensity pattern reveals the forces
and structures of the colliding particles. CLAS is able to
determine the mass and velocity of the particles and can be
used to reconstruct the events so that we can better
understand what is happening. The event rate is about
3000 Hz (or 15 MegaBytes/s). The initial analysis is done at
JLab and we analyze those results more deeply at the
University of Richmond.