A High Precision Measurement

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A High Precision Measurement of the Electrical
Charge Inside the Neutron The Hall A and GEn
Collaborations Jefferson Lab Experiment 02-013
G. Cates, N. Liyanage, B. Wojtsekhowski
(spokespeople)
Over 130 scientists from 15 different countries
worked on this experiment. Some of the
scientists are pictured above (back row, left to
right)Nilanga Liyanage (Univ. of Virginia),
Robert Feuerbach (Coll. of William Mary),
Vladimir Nelyubin (Univ. of Virginia), Gordon
Cates (Univ. of Virginia), Seamus Riordan
(Carnegie Mellon), Aidan Kelleher (Coll. of
William Mary), (front row, left to right)
Sergey Abrahamyan (Yerevyan, Armenia), Ameya
Kolarkar (Univ. of Kentucky), Jonathon Miller
(Univ. of Maryland), Bogdan Wojtsekhowski
(Jefferson Lab)
What's Inside a Neutron?
Our electron detector, BigBite
The neutron makes up roughly half the mass of
every object that you see or feel. The neutron
is a neutral particle. But, the neutron is made
up of smaller particles that have electrical
charges. How do these charges cancel out to give
us a neutral particle?
Called BigBite because it has an acceptance
(field of view) over 30 times larger than other
experiments.
Polarized 3He Target
How Do We Do It?
We capture scattered electrons in our detector.
To make sure that we have captured an electron
that scattered from the neutron we capture the
neutron, too. Studying how electrons scatter is
like taking a picture, but instead of capturing
the light in a camera, we capture the particles
in our detectors.
We are measuring a quantity that tells us how
charge is distributed inside the neutron (charge
density).
Our experiment scattered electrons from
polarized helium-3 gas. This same polarized
helium-3 gas can be used to image lungs at the
University of Virginia hospital.
Medical Technology Spin Off
Our experiment developed techniques that speed
the production of polarized gas. We can polarize
gas five times faster than before for lung
imaging.
6 meters (19.7 feet)
For our target, we want a polarized neutron.
We can polarize helium-3. Most of the time the
neutrons and protons in a helium-3 (sometimes
written 3He) atom line up so that polarizing 3He
is just like polarizing a neutron.
The movement and location of the particles inside
a neutron (quarks) are determined by forces
described by a theory called Quantum Chromo
Dynamics. These forces determine the charge
density in the neutron (among other things). By
measuring this charge density, our measurement
provides insight into these fundamental forces.
Our neutron detector called BigHAND, captured
neutrons that scattered from the target. The
detector was almost 20 feet tall and weighed 80
tons.