Fermi National Accelerator Laboratory, Batavia, Illinois 60510. P. M. Hod, M. J. Longo, and A. Nguyen. Department of Physics, University of Michigan, Ann Arbor, ...
e
Fermi National Accelerator
Laboratory
FERMILAB-Pub9l/165
Measurement
of the In- Magnetic
Moment
H. T. Diehl, S. Teige, G. B. Thomson, and Y. Zou Department of Physics and Astronomy Piscataway, New Jersey 08855-0849
C. James, K. B. Luk, and
R
Rameika
Fermi National Accelerator Laborafory P.O. Box 500, Batavia, Illinois 60510 P.
M. Ho, M. J. Longo, and A. Nguyen Department of Physics Ann Arbor, Michigan 48109
J. Duryea, G. Guglielmo, K. Johns, K. Heller, and K. Thorne School of Physics and Astronomy Minneapolis, Minnesota 55455
June 1991
l
4s
Submitted
Operated
to Physical Review Letters.
by Univsrsltlss
Research
Asroclstlon
Inc. under
contract
with
the United
States
Department
of Energy
Measurement
of the KY- Magnetic
Moment
H. T. Diehl”, S. Teigeb, G. B. Thomson, and Y. Zou Department of Physics and Astronomy, Rutgers - The State University of New Jersey, Piscataway, New Jersey 08855-0849 C. James, K. B. Luk’, and R. Rameika Fermi National Accelerator Laboratory, Batavia, Illinois
Department
60510
P. M. Hod, M. J. Longo, and A. Nguyen of Physics, University of Michigan, Ann Arbor, Michigan 48109
J. Duryea, G. Guglielmo, K. Johns’, K. Heller, and K. Thorne’ School of Physics and Astronomy, University of Minnesota Minneapolis, Minnesota 55455
Abstract
A sample of 24,700 R- hyperons was produced by a polarized neutral a spin-transfer
reaction.
The SY polarizations
beam in
are found to be -0.054 rt 0.019 and
-0.149 k 0.055 at mean Sl- momenta of 322 GeV/c and 398 GeV/c respectively. directions of these polarizations
The
give an W magnetic moment of -1.94 f 0.17 h 0.14
nuclear magnetons.
Nearly
thirty
years ago, when the first predictions
of the bazyon octet
were made by Coleman
baryons
the proton
other than
and neutron
for the static magnetic
and Glashow,l
experimental
was not imminent.
moments
verification
In the subsequent
for three
decades, as experimental ons have evolved, of baryon
techniques
so has our understanding
magnetic
moments
quark masses obtained
A magnetic
moments octet,
with parallel
The a-
magnetic
on its magnetic
moment,
of the experimentally
the moments
neutron
and
of other members
member,
which embellish
and
the R-.
the simple quark model
CL”-, holds particular
accessible baryons,
of pn- determines
At the
interest
(@N) for
because the
three strange valence quarks
the strange quark magnetic
moment
free of the effects of the light up and down quarks that are present in the
A. Whereas the P-wave mixing to their magnetic
calculations
The proton,
breaking
results at the level of about 0.2 nuclear magnetons
spins. A measurement
in an environment
to predict
of hyper-
Indeed the measurements of symmetry
of the long lived decuplet
present time, however, even theoretical
R- is the simplest
our understanding
have been used as inputs
disagree with the experimental
structure.
from the hadron mass spectra.
as well as that
some of the baryons.’
of hadronic
have confirmed
constituent
of the baryon
to produce, detect and measure the properties
moments,
in the spin-l/2
octet members can cause sizable corrections components
the D-wave (L=2)
moment. 94 For the a-
the theoretical
in the SV have negligible
predictions
effects
3-12 range from -1.3
to
-2.7 /LN. A standard of polarized
technique
hyperons,
production
The discovery
by observing
that hyperons
plane made possible precision
and C+ *M* magnetic
hyperon magnetic
precess the polarization
mine the final spin direction hyperons.
for measuring
moments.
Recently,
moments is to produce a beam
vector in a magnetic
the asymmetry produced
the E+ magnetic
2
in the decay distributions
by protons
measurements
field, and then deter-
were polarized
of the A13, Zolcls,
normal
of the to the
2-1617, C-161s,
moment was also measured using
this method.”
This experiment
determination
of pn-,
employed
but a different
the same precession technique
approach
was used to produce
to make the first
a sample of polarized
0-k.
The W’s polarized
were produced
in a spin-transfer
A’s and 2”‘s was produced
process. First a neutral
by a Fermilab
800 GeV/c proton
reaction p + Cu -+ (A, 2’) + X at f2.0 mrad. The polarized at 0 mrad to produce
R-‘s
constituted
less than
10% of the neutral
production
cross-sections,
times more a-‘s
by the reaction
Z”s
(A,=‘)
meter long magnet,
Ml,
channel with a defining
respectively.
and selected negative was operated For W
beam was then targeted + X. Although
to produce
*3 The polarization
hyperons
strange
which
was fitted
with
of 24,700 W’s
produced
in
pn-.
a brass and tungsten
of 5 x 5mm r. Using a right-handed
aperture
particle
at least 20 times and 5
target (Cu, 5 x 5 x 152 mm3) was located just upstream
which 6 is up and i is along the neutral The channel curvature
a-
beam in the inclusive
beam, based on the measured
this manner was measured and used to determine The R- production
+ Cu +
and A’s were estimated
than neutrons
neutral
beam containing
of a 7.3
momentum
selecting
coordinate
system in
beam axis, the field in Ml was in the -y
direction.
gave the central ray an effective bend of 14.7 mrad in the + - I plane particles
in a momentum
range of 240 to 500 GeV/c when the magnet
at a field of 1.98 T. hyperons
ter ‘r-r3 recorded
which
exited
Ml,
a multiwire
the charged decay products
Signals from scintillation
counters
proportional
of the G- +
AK-,
chamber A +
px-
spectromedecay chain.
and wire chambers were used to form a data acquisition
trigger for selecting three track events with at least one positively
3
charged and one negatively
charged track. All
three-track
triggers
searched for the three-track, ometric and kinematic topological
was required
target.
two vertex
criteria.
hypothesis.
mass not to fall between
1297 and 1350 MeV/cr.
on the angle of the A in the A-n-
a 3% background of the a-
than
sample without
The vector polarization
The reconstructed typically
in the ratio
the A-xof ?‘s
+ Axinvariant
was achieved
to the cuts as described in
mass under the A-K-
hypothesis
was required
events were predominately
R- ---t AK-
+
Zen-
decays.
The invariant
the final mass selection is shown in Figure P;, is related to the polarization
2(5+
pn=
events
under both the 2-
elimination
were identical
due to W
of the n-,
mass
center of mass system and on the angles of
Th e resulting
mainly
invariant
to
vector of the reconstructed
1.4 mm.
Further
rest frame. a’ Th ese requirements
to be between 1657 and 1687 MeV/cr. but contained
The tracks belonging
Most of the Z’-‘s were rejected by requiring
the invariant
x2 for the
6.3 mm of the center of the Cl- production
All events were reconstructed
details in reference 24. Finally,
which
was based on both ge-
which were recorded along with R- -t AK-
of 70 to 1 after reconstruction.
distribution
The momentum
of this process was better
Z:- -+ Ax-
K- in the A-K-
program
to have a geometric
30 degrees of freedom.
to trace back to within
The resolution
and W -+ AK-
selection
Selected events were required
to be from 1108 to 1124 MeV/cr.
were primarily
Event
reconstruction
vertex were assigned to be the proton and pion, and the p-x-
was required
by cutting
topology.
fit not larger than 70 for typically
the downstream
hyperon
were passed to an off-line
1$7”(2J$l)
4
1)
@*,
mass
1.
of the daughter
(1)
where 5=3/2
is taken as the spin of the W.
The decay parameter
measured, but the value of the decay parameter sign of 7” is predicted the asymmetry
an- (-0.026
to be positive. ‘szs The A polarization
in the distribution
7” has not been directly
f 0.026)r5 implies 7n1 N 1. The was determined
by measuring
of the decay proton in the A rest frame, which is given by
1 dNP 2(l+anP;I.~cosep) 4, d(cos0,) where Nt,, is the total
number
of events, the fi are unit
axes, cos 0, is iL . j, and j is a unit vector in the direction
vectors parallel of the proton
to the laboratory
momentum
in the A
rest frame. The measured distribution both the apparatus 28
a modification
of protons is affected by the acceptance
and the reconstruction
of the hybrid
events were generated
with
Monte
software.
event but with zero A polarization.
Each simulated
30
For each real event, Monte
and momentum
Carlo
for the A as the real
event passed the same reconstruction
and event selection as the data. The accepted Monte Carlo events were then weighted polarization
so that the cosine distributions
those of the data. simulation
But any difference
program
does a polarization, the polarization A for opposite
of the proton
for property
can be determined production
shows the components
of the real apparatus
or bias.
of the real apparatus,
when the sign of the polarization
it does not reverse its sign, as
of the neutral
angles, and the bias can be determined and bias measured 5
and the
Since such a bias arises
beam changes.
from the difference of the measured asymmetries
of the R- polarization
by a
in the A rest frame agreed with
between the behavior
can give rise to a false asymmetry
from some unaccounted
of
To correct for these effects, we used
Carlo technique.
the same decay vertex
and resolution
from the sum.
Thus of the
Table 1
for the two different
Ml
fields.
The polarizations
&i-squares
were primarily
of the A asymmetry
in the --+ direction.
determination
The bias was also measured with W’s at the -14.77
T-m field integral.
as required
The polarization field perpendicular
by an unpolarized
neutral
beam with
Ml
The results were also small, in agreement with the 2 mrad
of the polarization
by conservation
The
were less than 35 for 19 degrees of freedom.
produced
data, as shown in Table 1. An additional by the y components
The biases were small.
of parity
of a negatively
test of the validity
of the asymmetry
is provided
for the two fields which were consistent
with zero
in strong interactions. charged hyperon,
to both the momentum
Y, with spin Jmoving
and the spin wiU precess through
in a magnetic
an angle, relative
to its momentum,
9= w e (zs P where e is the magnitude
of the electron
is the mass of the hyperon, Tesla-meters
+ l)jBdl
charge, p= v/c, m, is the mass of the proton,
JBdl is the field integral
of the precession magnet,
Ml,
my
given in
and ,~y is the magnetic moment given in Ed. The expected a and L components
of the polarization
downstream
of Ml are given by P. = Pt,t cos 0 and P, = Ptnt sin 8, where asymmetries
at each Ml
field
value gave JL~- results of -1.90 ck 0.29 and -1.96 f 0.20 PN for the -14.77
and -19.53
T-m
P tg( is the polarization
field integrals, polarization /I”-.
respectively.
polarizations
Although
and the field integral
Constraining
The measured
at the second target.
the -19.53
T-m sample was about 8 times smaller, its
were larger, leading to a smaller
both data samples to have the same magnetic
statistical moment
uncertainty determined
of the R- at the second target, the zz and z biases, and the magnetic
The polarizations
the
moment.r3
at the target were -0.054 f 0.019 and -0.149 f 0.055 for the -14.77
6
in
T-m
and the -19.53
T-m field integrals
Since the direction known a prior,
respectively.
of the polarization
at the target and the sense of the precession were not
the spin precession
angle for a given field would have a four-fold
when only angles less than 2~ were considered. Tabel 2. The preferred
solution
gave pn-
moment due to the addition
The four lowest order solutions
ambiguity are shown in
as -1.94 +c 0.17 PN. Because this result depends
on the ratio of P, to P,, it is independent magnetic
31 Figure 2 shows P. and P, for the two fields.
of the sign of -yn. Alternative
of integral
multiples
solutions
for the
of r to the precession angles were
also eliminated. The systematic and by measuring
uncertainties
were investigated
the asymmetries
without
sizable change to the polarization was comparable
to the statistical
uncertainty.
mated to be 0.14 /x~. As an additional events taken under the same conditions and -0.137
which occurred
in the -19.53
The systematic
check for systematic were analyzed.
uncertainty
to be -0.688
of -14.77
and -19.53
The polarizations
zb 0.024 p,v, consistent
with previously
We have measured the polarization beam in a spin-transfer -0.149 ho.055
reaction.
at mean momenta the R- magnetic
T-m, respectively.
in p,-,- was esti-
of 318 and 394 GeV/c, moment was found
measurements.
produced
The average nt- polarizations
agreement with the naive quark model prediction
7
ISI,
by a polarized are -0.054
of 322 GeV/c and 398 GeV/c respectively.
moment is determined
T-m sample,
were -0.108f0.013
The magnetic
reported
of G- hyperons
a2 The most
errors, a sample of 64,000 2-
zlc0.025 for the samples recorded with average momenta
and field integrals
polarizations,
the event selection criteria
on Monte Carlo simulation.
relying
components,
by varying
neutral
f 0.019 and Based on these
to be -1.94 + 0.17 zt 0.14 p,v, in good
of -1.84
p,v.
This work was supported tional
Science Foundation.
Investigator
K. B. Luk was also supported
award and an Alfred P. Sloan Fellowship.
for lending Fermilab
in part by the U. S. Department
wire chambers to this experiment. Research Division
and neutral
for their expedient
of Energy
by a DOE Outstanding
We would like to thank
We are extremely
grateful
help in installing
beam channel, which made this measurement
and the Na-
possible.
Junior
L. Pondrom
to the staff of the
the new targeting
station
References [a]
Present address: Fermi National
Accelerator
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[b]
Present address: Physics Department,
[c]
Present address: Department
[d]
Present address: Lawrence Berkeley Laboratory,
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11
should be multiplied
(P)
Ml Field
Production
(T-m 1
rmrad) at Ml
:GeV/c)
2.0
322
-14.77
L
0.0
:omponent
I
-0.033 f 0.012
I
-0.001
Bias + 0.012
to.003 f 0.013
$0.003 f 0.013
-0.012 * 0.017
-0.029
f 0.017
-0.012
f 0.028
332
+0.007 f 0.021
-19.53
2.0
a function
of targeting
f 0.02:
+ 0.032
f0.031
f 0.03:
-0.059
* 0.033
+0.017 f 0.03:
-0.050
+ 0.041
f0.005
-0.077
398
Table 1: The average momentum,
-0.006
the components
1
of the polarization
angle for the two values of the Ml field integral.
12
f 0.041
and bias are shown as
Initial
Precession angles
iirection
(degrees) at
of p;,
-14.77
T-m
k-~- (n.m.1
-19.53
T-m
w
+23 zk15
+31+20
-1.94
Lk 0.17
0.03
w
-373
-493 f18
+2.44
f 0.15
1.63
fz
+171*14
+226+
-3.58
f 0.16
1.31
fx
-134ztl6
-177*21
-0.20
f 0.18
2.50
dc16
19
Table 2: Under the column for the initial
direction
and the bottom
For the precession
clockwise
Xz/d.o.f.
sign is for 7”
= -I.
rotation.
13
of Pi, the top sign is for the case 7” = 1 angles, positive
sign indicates
LIST
Fig. 1 A - K- invariant
OF FIGURES
mass of the final data sample.
The mass selection
criterion
is
shown by the arrows.
Fig. 2 z component m field integrals.
vs. z component The polarization
of the polarization
for the -14.77
at the target is in the --z direction.
angle is the angle between the --t axis and the final polarization field integral.
14
and -19.53
T-
The precession
vector for a particular
8
z ~/~ali\lg’o
8
.md sJuaiq
0
JO ‘ON
,
4 -0.10 -0.15 I . . * I
-0.05
.C 1.00 C- -%
-0.05
-0.10
’ -0.15
-19.53 T-m
Figure 2
., ,. .,.
