Transcript Poster_HallAFew-Body_copy7_FINAL

Hall A Experiments on Nuclear Few-Body Form Factors*
E. Khrosinkova, M. Katramatou, M. Petratos (Kent State U.), M. Olson (St. Norbert College), A. Camsonne, J. Gomez (JLab) on behalf of the Hall A Collaboration
The Hall A Facility of Jefferson Lab is pursuing an experimental program to measure the “Form Factors” of the lightest, simplest nuclear systems in nature, the nuclei of the
deuterium and helium atoms. The form factors provide fundamental information on the size, structure, and internal dynamics of these nuclear systems.
The nucleons inside nuclei constantly interact with each
other. The force between two nucleons, the nucleonnucleon force, is what holds the nucleons together to form
the nucleus. One of the goals of nuclear physics research
is to study all aspects of this force.
The Few-Body Nuclear Systems
All matter is made up of atoms. Each atom is comprised of
a positively charged nucleus consisting of protons (p) and
neutrons (n), collectively called nucleons, and negatively
charged electrons orbiting around the nucleus. Each
nucleon is made up of 3 quarks, which are held together by
massless particles, the gluons, acting like springs.
The form factors of the few-body nuclear systems give us,
in general, information on the internal structure and
dynamics of these systems, and in particular on their
shape, size, composition, quantum mechanical wave
function, and on the nucleon-nucleon force that holds their
mutually interacting nucleons together.
The deuterium nucleus is comprised of two nucleons: one
proton and one neutron. It is the simplest nuclear system,
the “two-body” system, also called the deuteron. Deuterium
exists in very large quantities in sea water.
The Few-Body Nuclear Systems
A stable helium nucleus can contain either three or four
nucleons:
• Two protons and one neutron making up a He-3 nucleus,
a “three-body” nuclear system. He-3 is rare on earth but
is abundant on the surface of the moon.
• Two protons and two neutrons making up a He-4 nucleus,
a “four-body” nuclear system. Twenty five percent of the
Universe is He-4. Supermarket Helium balloons contain
He-4 gas!
The He-3 nucleus is a spinning round object. In simple
terms, it rotates about itself much the same way that the
earth rotates about itself. The spin can be in two distinct
orientations, “up” or “down”. Any nucleus with these two
spin orientations has two form factors: a charge one and a
magnetic one.
The charge form factor provides
information on the distribution of the electrically charged
protons in the nucleus, and the magnetic form factor on
the magnetization created by the moving charged protons
(a moving electrically charged particle always creates a
magnetic field!).
The He-4 nucleus is a round object, which has no spin.
This is because the spinning motion of the two protons
cancels the spinning motion of the two neutrons. Like all
nuclei with no spin, it has only one form factor, a charge
form factor.
Measuring the Form Factors
The form factors of the few-body systems are measured in
experiments where high energy electrons collide with
deuterium or helium target nuclei. (The currently running
experiment in Hall A, E04-018, is measuring the form
factors of He-3 and He-4.)
The high energy electrons are provided by the Jefferson
Lab Continuous Electron Beam Accelerator Facility
(CEBAF).
The target nuclei are those of deuterium or helium atoms,
which are stored in gaseous forms in metallic cans under
high pressure and low temperature, the cryogenic targets.
The deuterium nucleus can exist in three different spin
states. This results in the deuteron having three form
factors, a charge form factor, a magnetic form factor, and a
quadrupole form factor. The existence of the quadrupole
form factor is ultimately due to the fact that the deuteron is
not exactly a round object but close to an egg-like object.
What Do the Form Factors Tell Us?
Collectively, the deuteron, the He-3 and He-4 nuclei, and
another nucleus made up of two neutrons and a proton, the
triton, are the “few-body” systems of nuclear physics.
* Work supported in part by the National Science Foundation (Grant PHY-0355181)
Nuclei, as other subatomic objects are described by
quantum mechanical wave functions.
These are
mathematical functions of space and time, in other words
mathematical tools used to describe the state and
whereabouts of these objects.
View of the target inside
the scattering chamber,
and the scattering
chamber at the pivot.
Hall A Experiments on Nuclear Few-Body Form Factors
Most of the time, the incident electrons do not interact with
the target nuclei, and end up mostly undisturbed at the end
of the beam line in a water cooled damp.
State of the art, specialized nuclear electronics devices and
high speed computers receive the information (“data”) for the
particles identified by the detectors. The collection of the
experimental data is directed and monitored by scientists in a
“Counting House”, above the underground Hall A. Typically,
it takes two years for a small group of scientists to analyze
these data and publish the results.
Very rarely, the electrons do interact with the target nuclei
resulting in elastic or inelastic collisions:
•
In elastic collisions, the incident electrons bounce
(scatter) off the heavy nuclei, which in turn break up
from the atoms and recoil with fairly large speeds. In
inelastic collisions, the target nucleus breaks up into
fragments (smaller nuclei and/or free protons or
neutrons and possibly other subatomic particles like
pions).
The form factors are extracted from measurements of elastic
collisions.
In this case, both scattered electrons and
recoiling nuclei are detected with the two state-of-the art
High Resolution Spectrometers of Hall A.
HRS recoil spectrometer
Counting House
Experimental Outcome
Previous experiments on the form factors of the deuterium
and helium nuclei have established their shape and size.
Both are tiny objects, about one thousand million millions
(1,000,000,000,000,000) times smaller than the size of a pingpong ball or an egg!
Calorimeter detector of the electron spectrometer
The Hall A Spectrometers are made up of large
superconducting electro-magnets, which gather the
scattered electrons and recoiling nuclei, and focus them
(like optical lenses focus light) onto two sets of subatomic
particle detectors housed in the so-called shielded “huts”.
Every minute, CEBAF can deliver a beam with up to one
thousand
million millions (1,000,000,000,000,000)
electrons per second onto a cryogenic target.
The electrons encounter in their path in the target, many
many nuclei, for example, in the case of He-3,
approximately
fifty
thousand
billion
billions
(50,000,000,000,000,000,000,000) of He-3 nuclei.
Scintillator detector of the recoil spectrometer
The goal of the JLab experiments is to “look” now inside the
helium and deuterium nuclei, with finer precision and
resolution and study the shapes of the form factors at large
collision impacts, which are a measure of how hard the
target nuclei are hit by the incident electrons. JLab is the
only place in the world where experiments such as these
can be performed! Form factors may also provide us with
valuable input into examining the light nuclei as systems of
quarks and gluons and ultimately lead us to a complete
understanding of the simplest, lightest nuclei in Nature!