Houston Medium-Energy Physics Group
Brookhaven National Laboratory
E931 Experiment
"A Study of the
Δ
I=1/2 Rule in the Weak Decay of S-Shell Hypernuclei"
Experiment
Collaboration
Proposal
Drawings/Pictures
Instrumentation/Electronics
Physics
Calibration
Analysis & Software
Current Updates
Links
Contacts
(Site Under Construction)
The Experiment
B.N.L. E931 -
Study of the Δ
I=1/2 Rule
In The Weak Decay of S-Shell Hypernuclei
Spokesmen
D. Dehnhard, E. V. Hungerford, V. Zeps
Status
Data Taking Completed At AGS Beam C8,
Analysis Underway
This experiment was devised to address an unresolved fundamental question
of 'why' and 'when' to apply the Δ
I=1/2 rule to the weak decay of strange hadrons. After completion
of the experiment runs and data-taking phases, the calibration and analysis
is underway, to determine if this apparently universal rule applies to the
non-mesonic weak decay of a Λ
, by studying particle emission from the weak decay of lambda hypernucleus
Λ
4H. The experiment used NMS, Neutral Meson Spectrometer, and the
LESBII Beam line as its two central elements.
Arizona State University: J. R. Comfort, C. Gauland
Brookhaven National Laboratory: R. E. Chrein, R. Gill, M. May, P.
H. Pile, A. Rusek, R. Sutter
Carnegie-Mellon University: G. B. Franklin, B. Quinn, J. Parker
CEBAF: L. Tang
Christopher Newport College: J. Gerald
George Washington University:W. Briscoe
Los Alamos National Laboratory: J. Amann, D. Boudrie, C. Edwards, B. F.
Gibson, C. Morris,
J. O'Donnell, J-C. Peng, A. Thiessen
Louisiana Technical University: M. Barakat, K. Johnston
North Carolina A&T: R. Sawafta
R. Boskovic Institute: I. Supek
Tohoku University: O. Hashimoto
University of California at Los Angeles: B. Nefkens, W. B. Tippens
University of Colorado: G. A. Peterson
University of Houston: M. Ahmed, M. Bukhari, Y. Cui, A. Empl, E. V. Hungerford,
A. Lan, Y. Li,
B. Mayes, L. Pinsky, G. Xu
University of Kentucky: V. Zeps
University of Maryland: P. G. Roos
University of Minnesota: D. Dehnhard, H. Juengst, J. Liu
University of Texas at Austin: G. Glass, C. Fred Moore, H. Ward
University of Zagreb: D. Androic, I. Bertovic, M. Furic, T. Petkovic,
M. Planinic
Basic Concepts:
Hyperons are subatomic particles of the class known as baryons. Like all
baryons, they are composed of three quarks. The term hyperon is generally
used for a baryon containing one or more strange (s ) quarks, as opposed
(for example) to the proton and neutron, which contain only up (u) and down
(d) quarks. The strange quark being unstable, hyperons decay into lighter
baryons (such as protons or neutrons) plus mesons, with typical lifetimes
of approximately 1/10 of a nanosecond. At high energies, these lifetimes are
sufficient for a hyperon to travel several meters before decaying, since the
hyperon can be moving at very nearly the speed of light and thus experience
the time-dilation effect of Special Relativity. This long decay distance makes
hyperon experiments (such as the 931 experiment) feasible.
Hypernuclei are the nuclei with one or more hyperons embedded in them,
such as Λ 4H (1p, 1n,1 Λ
). These can be formed in stopped or in-flight reactions. Hypernuclei
are incredibly short-lived, surviving for less than a billionth of a second,
and typically decay into a pair of strange mesons, such as kaons. 35 varieties
of hypernuclei are already known from physics experiments, though it is expected
to see completely new hypernuclei, such as a hydrogen-7-lambda (Λ
7H), comprised of one proton, five neutrons, and one exotic lambda
particle, a hyperon that includes a strange quark.
Isospin is a quantum number, a vector quantity, and a property of particles
established after Heisenberg's idea of isospin which is in simple words different
projections of the same particle. For instance, proton and neutron are considered
two projections of the same nucleon, hence two different isospins or isospin
quantum numbers. Isospin law is one of the conservation laws in physics
which originates from the symmetries in nature.
Isospin invariance follows from the fact that the strong interactions
are independent of quark type, and so do not distinguish up quarks from
down quarks. Furthermore, the masses of the up and down quarks are small
compared to their energy in a proton or neutron, and thus protons and neutrons
have close to equal masses. As far as strong interactions are concerned,
protons and neutrons behave identically. Isospin is the invariance that relates
strong interaction processes or states that differ only by replacing
some number of protons by an equal number of neutrons.
Following chart depicts a relationship between Hypercharge (Y) and the
third component of Isospin (T3), which summarizes one of the symmetries involved
between baryons:
The E931 Experiment:
The E931 experiment was conceived to study and resolve the fundamental
question in hadronic physics that why and when to apply the
ΔI=1/2 rule to the weak decay of strange hadrons.
The basic underlying methodology was observing the neutron to proton stimulated
(non-mesonic) decay of the lambda hypernucleus, Λ
4H. Neutral Meson Spectormeter played a central role in the experiment
by virtue of its function to tag the formation of the hypernucleus. NMS by
detecting the pion from the He-Kaon-stopped reaction tags the formation of
a Hydrogen hypernucleus and therefore identifies when this hypernucleus is
the source of secondary particle emission from the target region. Thus the
confluence of the NMS with the high kaon flux of the AGS C8 beamline
provided a unique situation for this kind of hypernuclear physics studies.
The non-leptonic strangeness-changing weak decays,
ΔS=1, of kaons and hyperons are enhanced when
the change in isospin is by half. This observation is generalized into the
ΔI=1/2 rule, which states
that the non-leptonic decays of all strange hadrons proceed through
Δ I=1/2 amplitudes. However there is no universal
explanation for this apparently universal rule and most likely the effect
is due to complicated dynamics in the decay process. In fact the rule may
only be associated with pion decay, as these are the non-leptonic decay
processes which have been recently studied in detail. However the rule is
applied to all non-leptonic decays.
Non-mesonic hyperon decays occur because a Lambda (Λ
) embedded in a nucleus finds that its mesonic decay channel is Pauli
blocked, as the nucleon recoil in its decay channel to N and pion has momentum
much lower that the Fermi momentum of bound nucleons in the nucleus. This
interaction, which proceeds through a four-fermion weak vertex can only
be studied within hypernuclei and the high momentum transfer involved in
the process probably enhances sub-nucleon degrees of freedom. In any event,
the applicability of the rule to this process is experimentally undetermined
and theoretically questionable.
The operation of the NMS can be briefly summarized in short as, that
it determines the energy of the emitted pion by measuring the opening angle
of the two decay photons under the conditions that they almost equally share
the reaction energy. Thus geometry rather than calorimetry determines
the energy resolution to the first order.
The pion is detected by measuring the energy and position of each of the
two decay photon showers in two out-of-beam crystal arrays of the Neutral
Meson Spectrometer (NMS). Using the conversion points of the photons and
the position of the reaction vertex, the opening angle of the two photon decay
and thus the energy of the pion was determined to higher accuracy than is
possible by measurement of the energies deposited in the crystal arrays.
The resolution of the opening angle measurement dictates the energy resolution
of the pion detection. The opening angle in turn depends on the ATC vertex
and the NMS conversion point measurements.
Instrumentation/Electronics
Principal Components:
- Neutral Meson Spectrometer (NMS)
- Wire Straw Chambers (WSC's)
- Neutron Detectors (ND's)
- Liquid Helium/Copper Target
- Degrader
Following schematic illustrates an overview of the detectors and
target in the experiment (PDF Format):
Detector Layout
The pictures below illustrate a realistic view of some of the components
being installed during the initial setup. The NMS, target chamber and Neutron
Detectors are prominent:
The schematic diagram below illustrates an overview of the NMS Calibration
setup utilizing a K+ beam and a copper target at the BNL C8 beamline. This
setup has been incorporated in Houston and CMU calibration involving the
run numbers 5146-5152.
As shown in the figure, the Neutral Meson Spectrometer (NMS) constitutes
two detector arms (Forward and Backward), with two layers of Wire Strip Chambers
and BGO's on each arm. Each arm has sixty 10.6 x 10.6 cm Cesium Iodide
(CsI) crystals (plus the CsI light-guides) connected optically to PMT's,
which convert the scintillations into signals which are conveyed to the read-out
electronics and archived by DAQ.
Technical Schematics
Engineering Drawings
Technical drawings and engineering drawings of some components of the
experiment, especially NMS, shall be posted here soon.
Neutral Meson Spectrometer (NMS) calibration has been one of the main
tasks in 931 analysis.
Here is a very detailed paper written by 907 collaboration on NMS calibration. This paper gives a detailed account
on the NMS calibration carried out in the initial stages of the experiment
E907.
The results and details of calibration carried out by the CMU can be downloaded
from the Joe Parker's Webpage at CMU website.
Houston team has also been involved in the calibration of NMS and analysis
of data. The results will be posted soon.
At the initial stage of analysis, we identified the bad crystals in Cesium
Iodide arrays, which was really helpful in identifying and isolating the
crystals which were giving systematic errors and a constant output. Plot below
illustrates an inverse plot of crystals. The bad crystals, such as 1,1,1
;1,1,6; and 2,1,6, stand out in the histogram whereas the good crystals
give an expected output.
In addition, data from individual crystals, after conditioning and appropriate
clustering algorithm cuts, was written and analyzed, in an attempt to understand
the behavior and performance of each crystal. For instance, here the data
from channel 2 (crystal number 1,1,2) is plotted in a histogram (binned in
a number of 512 bins).
At this stage, we at Houston are concerned with the energy reconstruction
of the detected pion by means of the In-target stopped Kaon decay process
via the hadronic channel (B.R. 21.13%), by utilizing the k+ runs and the
calibration of CsI crystals and PMT's in the NMS. The aim of this reconstruction
is to achieve an energy resolution better than a factor of 5%. By measuring
the energy deposited by this pair in CsI crystals (and later adding the energy
deposited in BGO's), we can estimate the energy deposition in NMS and reconstruct
the energy of original pion.
Software for analysis, mainly IDA (Interactive Data Analyzer), can be
downloaded from the Greg
Franklin's IDA & IDA DAQ pages at the CMU website . The site contains
helpful details on this package, installation, usage and the SYNOP database.
931 K+ Calibration Data:
931 K+ Data files can be downloaded here. This is a selection of
comparatively more reliable calibration runs:
Analysis updates by CMU can be downloaded from the Joe Parker's Webpage
here.
Analysis updates by UHMEP group can be downloaded here (password protected).
Brookhaven National Laboratory
Ed Hungerford's Webpages
at Houston MEP
Carnegie Mellon University Physics
Joe Parker's 931 Webpage
at CMU
Henry Juengst's "HyperHall"
Webpages at JLab
Zeps' University of Kentucky
E931 Update Webpage
Riedel's
University of Montana 931 Web Update
Jefferson National Laboratory
Prof. Ed. V. Hungerford III (Principal
Investigator)
Masroor Bukhari, Research Assistant
(PhD III)
