Measurement of the asymmetry in the decay Antiomega+ ---> Antilambda K+ ---> anti-p pi+ K+ Page: 3 of 4
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-1.0 -0.8 -0.5 -0.4 -0.2 0.0 0.2 0.4 0.5 0.8
FIG. 2: The real (lines) and weighted HMC (points) cos 0
distributions of the p. The total number of HMC events has
been reduced by a factor of 10 to equal the number of real
tainty is statistical. The real and weighted HMC cos6
distributions of the 1 are shown in Fig. 2; differences be-
tween the real and HMC cos 0 distributions, unweighted
and weighted, are shown in Fig. 3. The nonisotropic na-
ture of the real cos 6 distribution in the top plot of Fig. 3
is unambiguous evidence of a nonzero a decay parameter.
0 .0 10 ..,, .. ...,.. .. ....... ..)
0.0 T 1 1r IT1+
-0.010 ' ..I.. I [...I.. . 1. H ]
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.5 0.8 1.0
FIG. 3: The differences between the real (Nr) and HMC
(Nmc) cosO distributions of the p, for unweighted (top) and
weighted (bottom) HMC events. The total number of HMC
events has been reduced by a factor of 10 to equal the number
of real events.
To extract dO A from the cos 6 slope of the p, the
background contribution to S was subtracted. To es-
timate the cos 0 slope from the background events the
same analysis procedure was performed on five sideband
regions, three below and two above the S2+ mass re-
gion. The average sideband cos 0 slope for the lower
region was Sb = (9.1 5.6)x 10-2, that of the upper re-
gion was S = (32.1 7.6)xl(-2. No statistically sig-
nificant dependence of the sideband cos 0 slopes on bin
- I I - I
width was found. The weighted mean of all five mea-
surements, Sb = 17.Ix10-2, was used as the estimate
of the contribution of the background events under the
mass peak to the cos 0 distribution of the p. Using it the
background-subtracted cos 6 slope was determined; and
at aoaA = [1.16t0.18(stat)]x10-2 it is only 4% (0.28 a)
less than the uncorrected slope.
The extracted value of aQ A was found to be inde-
pendent of the z location of the Q+ decay vertex- The
non-background subtracted slope, S, was measured on
a run-by-run basis for all 450 runs in the dataset. No
temporal dependence of S was evident.
Systematic uncertainties are listed in Table I. The ef-
fects of detector inefficiencies - wire chambers, trigger
hodoscopes, and hadronic calorimeter - on ancA were
found to be negligible: no statistically significant differ-
ence in S was found between using perfect and measured
detector efficiencies when simulating the HMC p and 7r+.
The effect of the uncertainties in the fields of the an-
alyzing magnets, 5.5G, was also negligible. A small
fraction of the daughter pions (0.7%) and kaons decayed
before exiting the apparatus. The effect of such decays
on S was studied using Monte Carlo events and data and
found to be negligible. To estimate the uncertainty due
to the background subtraction an uncertainty of 25% was
used for the background-to-signal ratio and the full size
of the background cos 0 slope, Sb = 17.1x10-2, was used
as the uncertainty in the background slope.
The largest source of systematic uncertainty was the
sensitivity of the measurement to the values of the cuts
used to define the data sample. The effect of small
changes in these cut values was 0.14x10-2. The total
systematic uncertainty, including the upper limit on the
uncertainty of the MC validation of the analysis program
(0.04x10-2), is estimated to be 0.17x10-2.
TABLE I: Systematic uncertainties.
Source Error (10~2)
Event selection cut variations 0.14
Validation of analysis code 0.04
Background subtraction uncertainty 0.06
Detector inefficiency uncertainties 0.06
Analyzing magnets field uncertainties 0.006
To conclude, from a sample of 1.890x106
+ A WK+ - -w+K+ decays, we find annA
[+1.160.18(stat) 0.17(syst)] x10-2. This is the first
evidence of a nonzero value for Zin, and hence of parity
violation in + -4 AK+ decays. The total uncertainty
in this measurement is a factor of twenty less than
that of the previous measurement , and the result
is 4.7 a from zero. This result is the most precise
measurement of the a decay parameter or product of a
decay parameters of any anti-hyperon.
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Lu, L.C.; Burnstein, R.A.; Chakravorty, A.; Chen, Y.C.; Choong, W.S.; Clark, K. et al. Measurement of the asymmetry in the decay Antiomega+ ---> Antilambda K+ ---> anti-p pi+ K+, article, January 1, 2006; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc884046/m1/3/: accessed June 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.