Measurement of the 0 Magnetic Moment 12 AUOUST 1991

Nearly thirty years ago, when the first predictions for the static magnetic moments of the baryon octet were made by Coleman and Glashow [1], experimental verification for baryons other than the proton and neutron was not imminent. In the subsequent three decades, as experimental techniques to produce, detect, and measure the properties of hyperons have evolved, so has our understanding of hadronic structure. Indeed the measurements of baryon magnetic moments have confirmed our understanding of symmetry breaking and constituent quark masses obtained from the hadron mass spectra. The proton, neutron, and A magnetic moments have been used as inputs to predict the moments of other members of the baryon octet, as we11 as that of the long-lived decuplet member, the 0 . At the present time, ho~ever, even theoretical calculations which embellish the simple quark model disagree with the experimental results at the level of about 0.2 nuclear magneton (pjv) for some of the baryons [2]. The 0 magnetic moment p„holds particular interest because the 0 is the simplest of the experimentally accessible baryons, three strange valence quarks with parallel spins. A measurement of p „determines the strange-quark magnetic moment in an environment free of the eff'ects of the light up and down quarks that are present in the A. Whereas the P-wave mixing in the spin& octet members can cause sizable corrections to their magnetic moments, the D-wave (L, =2) components in the 0 have negligible eltects on its magnetic moment [3,4]. For the 0 the theoretical predictions [3-121 range from —1.3ptv to —2.7p~. A standard technique for measuring hyperon magnetic moments is to produce a beam of polarized hyperons, precess the polarization vector in a magnetic field, and then determine the final spin direction by observing the asymmetry in the decay distributions of the hyperons. The discovery that hyperons produced by protons were polarized normal to the production plane made possible precision measurements of the A [13], :[14,15], [16,17], Z [18,19], and Z+ [20,21] magnetic moments. Recently, the =+ magnetic moment was also measured using this method [22]. The present experiment employed the same precession technique to make the first determination of p~-, but a diAerent approach was used to produce a sample of polarized 0 's. The 0 's were produced in a spin-transfer process. First a neutral beam containing polarized A's and:" 's was produced by a Fermilab 800-GeV/c proton beam in the inclusive reaction p+Cu (A, :)+X at ~2.0 mrad. The polarized neutral beam was then targeted at 0 mrad to produce 0 's by the reaction (A, :) +Cu 0 +X. Although hyperons constituted less than 10% of the neutral beam, based on the measured strange-particle-production cross sections, = 's and A' s were estimated to produce at least 20 times and 5 times more 0 's than neutrons, respectively [23]. The polarization of 24700 0 's produced in this manner was measured and used to determine @„-. The A production target (Cu, 5X5X152 mm ) was located just upstream of a 7.3-m-long magnet M1, which was fitted with a brass and tungsten momentum-selecting channel with a defining aperture of 5x5 mm . Using a right-handed coordinate system in which y is up and z is along the neutral beam axis, the field in M1 was in the —y direction. The channel curvature gave the central ray an effective bend of 14.7 mrad in the x-z plane and selected negative particles in a momentum range of 240500 GeV/c when the magnet was operated at a field of 1.98 T. For 0 hyperons which exited M1, a multiwire proportional chamber spectrometer [22,23] recorded the charged decay products of the 0 AK, A pz decay chain. Signals from scintillation counters and wire