EXO
In the last ten years
neutrino physicists have
found compelling evidence that neutrinos have a tiny mass. However, the
value
of the neutrino mass remains a mystery, with the current upper limit
set at 2.2
eV. The existence of non-zero neutrino mass raises the possibility that
the
neutrino may be its own anti-particle, which is a basic prediction of
many
grand unification theories. Both the particle/anti-particle question
and the
value of the neutrino mass can be addressed by searching for a rare
type of
radioactive decay known as neutrinoless double beta decay. Our group is
collaborating on the EXO experiment, which will soon be the largest
double beta
decay experiment ever attempted (by about a factor of ten). EXO
represents an
opportunity to make a significant discovery in fundamental physics in
the next
few years. We are also constructing a liquid xenon laboratory here at
Maryland
to develop new experimental techniques to apply towards double beta
decay
research in the future.
Go to the EXO
main web page at Stanford
University
Go to the (protected) EXO elog
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Radiative Decay of the Neutron at NIST
The
theory of quantum
electrodynamics predicts that beta decay of the neutron into a proton,
electron, and antineutrino should be accompanied by a continuous
spectrum of
soft photons. While this inner bremsstrahlung branch has been
previously
measured in nuclear beta decay and electron capture decay, it has never
been
observed in free neutron decay, until just recently by a group at NIST
in Gaithersburg, Maryland
in a pilot experiment. Phase II of the
experiment will improve this first measurement by a factor of 10 or
more.
Go to the NIST
group's web page
Go to the main NIST web site
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Fermilab E906: Measurement of quark properties in
nuclei using Drell-Yan production
Nearly all of the
visible mass of universe comes from
protons and neutrons at the core of atomic nuclei, yet their internal
structure
is still not well understood. One specific poorly
understood aspect of the proton is how
anti-up and anti-down quarks are distributed in its quark-gluon sea,
and how
these distributions might change in nuclei. Hints from earlier
experiments
indicate that there is an asymmetry in these distributions that could
influence
other high energy experiments carried out with nuclear targets. A new Fermilab experiment, E906, will use the
Drell-Yan process (two protons annihilate and create two muons). The experiment, which is being led by a
group at Argonne National Laboratory, is planned to run in (or
before) 2009. Our group has recently joined.
Results from prior experiments can be found
at the E866 web site.
Go to the E906 web page
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G0
The goal of the G0 experiment is to measure
parity-violating asymmetries in elastic electron- nucleon
scattering, as a function of momentum transferred to the nucleon. This
momentum dependence translates into probing deeper and deeper into the
interior of the nucleon. The parity-violating asymmetries are sensitive
to
the interference of the neutral weak (Z-exchange) and electromagnetic
forces, resulting in a direct measure of
the neutral weak structure of the nucleon. When combined with known
information about the electromagnetic
nucleon structure, it is possible to decompose the proton's charge and
magnetic structure into contributions
from up, down and strange quarks. From quasi-elastic scattering on
deuterium, we also get information on
the anapole structure of the nucleon.
During Fall 2002, we are embarking on the first
engineering run of the full detector system in Hall C at Jefferon
Laboratory in Newport News, VA. The first data taking will measure
asymmetries at very forward electron angles, where scattered
protons are detected. Then the detector will be turned around and
backward scattered
electrons will be detected from hydrogen and deuterium targets.
Go to the
G0 main web page at the Jefferson
Laboratory.
Click here for a the main G0
web page at the University of
Illinois.
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E96001
:
Eta electroproduction in the Delta region
and the proton's quadrupole
structure
Proton recoil polarization in the p(e,e'p)eta
reaction will be used to study the S11(1535) resonance, which
dominates this reaction, and the nondominant P11(1440) and D13(1520)
resonances. Recoil polarization
provides unique sensitivities to nondominant multipoles that are very
difficult to extract form cross section data
alone. The proposal has been approved but the experiment has not yet
been scheduled.
Find more
information on E96001 .
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E93038: Measurement
of the Neutron Electromagnetic
Form Factors
The neutron electric form factor can
be extracted from neutron recoil polarization measurements using the
d(e,e'n)p reaction in quasi-elastic kinematics. Equivalent information
can be found using a polarized target and polarized beam. The electric
form factor gives detailed information about the internal charge
distribution of the neutron. Two experiments were completed at
Jefferson Lab in early 2002. Another, E02-013
, was completed in 2007, and the data are presently being
analyzed.
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E98108:
Kaon electroproduction at high momentum transfer
This experiment is a higher momentum extension to
E93018, which ran in Hall C at JLab in the mid 1990's. By
using the Hall A spectrometers it is possible to extend measurements of
Kaon electroproduction to higher
momentum transfer, Q2 = 2-3 (GeV/c)2 . These measurements provide
precise data for models that attempt to
describe the production of nucleon excited states which arise from the
production of a pair of
strange-antistrange quarks. These experiments complement those taken in
the CLAS detector in Hall B
because they provide very precise information at specific
kinematics.
A related measurement is the production of hypernuclear states by
electroproduction of kaons. In hypernuclei,
one neutron or proton is replaced by a strange baryon. Production of
these states allows one to learn about the origin of the force between
two baryons as it arises from the interacting quarks and gluons.
Find more information on E98108
and the Hall A Hypernuclear
program.
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E94018 ("t20"):
Deuteron tensor polarization and electromagnetic structure
The deuteron is a spin 1 object, and as a result has
three components to its electromagnetic structure: charge monopole,
charge quadrupole and magnetic dipole. Polarization measurements are
required to separate the three components. E94018
recently provided new information on the charge distribution of the
deuteron through a measure of its tensor polarization in elastic
electron scattering.
Here are two recent publications:
D.
Abbott et al., Phys. Rev.
Lett. 84, 5053 (2000).
D.
Abbott et al., Euro. Phys. Jour A 7, 421
(2000).
More information about the experiment can be found at the JLAB t20 web
page.
This experiment was the thesis project of Kenneth Gustafsson. Kenneth's
thesis can be
found
here.
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E93-018
(ee'K): Kaon electroproduction
Very little data currently exists on the
electroproduction of strange baryons. These data help
constrain models that attempt to describe the flavor dependence of
nucleon excited states.
E93018 provides precise new data on the reactions p(e,e'K)L and
p(e,e'K)S, separated into its longitudinal and transverse components.
The U Md group developed the aerogel Cerenkov detector needed to
identify Kaons in the SOS spectrometer in
Hall C.
This experiment was the thesis project of Rick Mohring. A gzipped-PS
version of his thesis can be
found
here.
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SAMPLE@MIT-BATES
Like G0, the SAMPLE experiment at the MIT-Bates Lab
measured parity-violating electron scattering, although SAMPLE was
specifically focused on backward angle scattering at low energy, which
isolates the magnetic structure of the proton. The deuterium portion,
completed in 1999 provided the first experimental access to the
proton's anapole moment. The
combined measurements seem to indicate that strange quark effects are
small, but that the proton's anapole structure is more complicated than
previously thought. We also carried out a third measurement,
quasielastic
scattering from deuterium at half the beam energy of theprevious two
data sets, to verify our earlier results and provide the first
information on how the axial form factor might change with momentum
transfer.
The hydrogen portion of the experiment was completed in 1998 and the
results are in D.
Spayde et al., Phys. Rev. Lett 84,
1106 (2000). The 1998 SAMPLE hydrogen measurement was the
thesis project of Damon
Spayde. Damon's thesis can be
found
here.
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