LA-UR- ma Title: Author(s): Submitted to: Hyperon Particle Physics at HF R. E. Mischke International Workshop on HF Science KEK, Tsukuba, apan March 3-7,1998 L OAlamos ~ National Laboratory, an affirmative action/equal opportunity empldyer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, of to allow others to do so, for US. Government purposes. The Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Form No. 836 R5 ST 2629 10/91
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Hyperon Particle Physics at HF R.E. Mischke, Los Alamos National L a b o r a t o r y * This paper discusses the possibility of a program in hyperon decay physics at tfic) apan Hadron Facility. The study of hyperon decays has a history of over 10 vcars of experimental results. However, many of the earliest results are still the best availatile and there are many gaps in the data. The static properties of hyperons (masses. magnetic moments) are in relatively good shape. hiuch work remains to be done on the decays of hyperons and hyperon resonances. To illustrate the gaps. the available data on hyperon semi-leptonic decays are summarized in Table 1. The ( )indicates a measurement (with no threshold cut for qualit\-) and the indicates an upper limit. The semi-leptonic decays provide important information on the weak hadronic form factors, tests of time reversal, and measurements of' the F/D ratio. An understanding of the spectroscopy of the hyperons will make a major contribution to understanding the spectrum of baryons. The hadrons are the testing ground for descriptions of non-perturbative QCD. Much progress is being made in the meson sector with convincing evidence emerging for non qtj states (glueballs, hybrids). To make similar progress in the baryon sector will probably entail obtaining essentially a complete set of all observables, which can then be subjected to a complete global analysis. There are current experiments a t the major accelerator facilities that address various hyperon decays, but no comprehensive program. This presents an excellent opportunitjfor such a program to be considered at HF. The high-intensity beams of 7 r, E(. and p beams will allow experiments to be designed that optimize the properties of the hyperons being studied. It is possible to envision experiments with samples of lo9 hyperon decays in lo7 sec runs. Hyperon decays fall into four categories, three of which are weak transitions (semileptonic, non-leptonic, and radiative) and one that is electromagnetic (radiative). For the weak radiative decays, much attention has been paid to the asymmetry parameter. which for C + py is large and negative, contrary to naive assumptions. The experimental observation is not fully understood in spite of many theoretical papers addressing the subject.' The process has been considered a t both the hadron level and the quark level. The models discussed in various*views include the pole model, vector dominance, quark models (single quark and di-quark transitions), penguin diagrams. QCD sum rules, a n. chiral perturbation theory. Essentially all models agree with some of the data, but no model is consistent with all of the data. See Fig. 1. Table 2 shows that only one of these modes has been studied with high statistics. Even the -2-+ n*. mode, which ranks second for number orevents observed, has no information about the asymmetry.
Table 1. Experimental inforniation on hyperon decays d d d To investigate a specific example of a candidate for a HF experiment, a measurement of the asymmetry parameter for A -+ n y has been chosen. Although r?k. and p beams may be considered for 12 production, only the reaction K - p -+.Ino has been modeled thus far. Assuming advertised rates, the yield would be about 500/sec A -+ n-: events. This reaction features an all-neutral final state. However, using a T' for tagging requires sorting out y's, which isn't easy. The decay schemes that were evaluated are given in Table 3. A 47r photon detector is essential to reject modes with more than 3 gammas. Fortunately, much of the solid angle does not need good position and energy resolution for t h e photon because most of the ones of interest go forward. Also, it is necessary to detect the neutron angle, but not t o measure its energy. The separation of signal from the dominant background from 1 1 -+ nxo decay is shown in Fig. 2..\ssuming an overall detection efficiency of 10% and 10% polarization, the asymmetry parameter could be measured t o 0.4% in lo6 sec. The conclusion is that although much more work is required to develop a real experimental proposal, the first indications are that a successful program in hyperon decay physics is possible at HF.
Table 2. Status of radiative w a k hjrperon decays Decay 1Iode Branching Ratio (lo-') -1i n-, 1.73 i 0.15 1.20 i0.06 Eo -+ *I-, 1.06 h 0.12 3.56 k 0.42 0.12 5 0.02 0.46 Y -0 -+ pn I +Sr 3- - + c y R- -+ 2-7 -Asymmetry S o. Evts -0.72 & 0.09 0.13 f 0.44 0.2 f 0.3 1.0 f 1.3 lslg 31901 34754 116 85 21 1 RC ~ ~ P W I ~ ( T Lnrson. t't n l. :2 Tiriim c't a1. [:3 Foucher et ill..ames et al. '4,.. Teige et al. Y, Dubbs et al. [G! -4lbuquerque et al. [7: '-1 Table 3. Important all-neutral schemes for -2 -+ n? reaction xsect K - p -+ nko 6.1 mb K - p + AT' 2.5 2.5 K - p -+ AT' K - p -+ CTO 0.7 K - p -+ C7ro 0.7 mode, KO -+ TOTO -4 + nt0 11 -+ nni C -+ A?, A -+ TO C -+An/, 11 -+ ny fraction 0.5 *.31.358.0017 1. *.358 1. *.0017 gammas freq 4 4 3 3 21.5 4 '204 1 57 0.3 References 1. E. enkins et al., Nucl. Phys. B397 (1993) 84; P. Zenczykowski, Nucl.Phys.Proc.Supp1. 50 (1996) 211; Ya. Azimov, Z.Phys. A359 (1997) 75; P. Singer (hep-ph/9607429); M. Nielsen et al., Phys. Rev. D53 (1996) 3620;. W. Bos et al., Phys. Rev. D 54. 3321 (1996); T. Haberichter et al., Nucl.Phys. A615 (1997) 291; P. Zenczykowski (hep-ph/9710405) 2. K.D. Larson et al., Phys. Rev. D 47, 799 (1993) 3. S. Timm et al., Phys. Rev. D 51, 4638, (1995) - 4. C. ames et al.: Phys. Rev. Lett. 64, 843 (1990) 5. S. Teige et al., Phys. Rev. Lett. 63, 2717 (1989) 6. T. Dubbs et al., Phys. Rev. Lett. 72, 808 (1994) 7. I. Albuquerque et al., Phys. Rev. D 50, 18 (1994)
. e Figure 1: Predicted asymmetry for hyperon weak radiative decay modes 400 350 300 250 200 150 100 50 06 08 1 ' 2 0 1 4 n l angle vs mrn 2110 EANY I 300 p 250 200 150 100 50 0.6 0.8 1 1.2 n 1 angle Y S rnm 1.4 0 5 20 40 60 80 2 1 L O BAYY Figure 2: Separation of signal from background