Hyperon /?-decay

The above type of matrix element occurs in the analysis of hyperon / -decay, where, if SU 3)f invariance holds, each transition amplitude is expressed in the form... 

Lambda hyperons. (Other hyperons do not appear in the matter). Moreover, an almost equal percentage of nucleons and hyperons are present in the stellar core at high densities. A strong deleptonization of matter takes place, since it is energetically convenient to maintain charge neutrality through hyperon formation rather than /3-decay. This can have far reaching consequences for the onset of kaon condensation. 

Almost at the same time Rochester and Butler61 at Manchester observed in a cloud chamber triggered by counters two V events identified later as decays of a 0° (=K°)-meson and a A°-hyperon. 

Several decades ago the number of elementary particles known was limited, and the system of elementary particles seemed to be comprehensible. Electrons had been known since 1858 as cathode rays, although the name electron was not used until 1881. Protons had been known since 1886 in the form of channel rays and since 1914 as constituents of hydrogen atoms. The discovery of the neutron in 1932 by Chadwick initiated intensive development in the field of nuclear science. In the same year positrons were discovered, which have the same mass as electrons, but positive charge. All these particles are stable with the exception of the neutron, which decays in the free state with a half-life of 10.25 min into a proton and an electron. In the following years a series of very unstable particles were discovered the mesons, the muons, and the hyperons. Research in this field was stimulated by theoretical considerations, mainly by the theory of nuclear forces put forward by Yukawa in 1935. The half-lives of mesons and muons are in the range up to 10 s, the half-lives of hyperons in the order of up to 10 s. They are observed in reactions of high-energy particles. 

It is generally believed (Salam, 1959) that all matter and energy (except gravitation) consist of sixteen fundamental entities. They are the photon, the electron, the proton, the neutron, six species of hyperon (charged particles with masses greater than the proton), four species of meson (charged particles with masses between the mass of the proton and electron), the muon and the neutrino. With the exception of the mesons all the particles have spin. Only four, however, are stable, namely, the photon, electron, proton and neutrino. The free neutron (half-life 20 min) decays to a proton, an electron and a neutrino. 

This does not apply for axial currents. They are in contrast to the vector currents renormalised by the strong interaction and therefore only partially conserved. In order to obtain approximate axial coupling coefficients for the nucleons, Cahn and Kane [84] have related the constants for the nncleons to conpling parameters derived from observed decay processes, namely the /3 -decay of the nentron and the similar decay of the hyperon. The axial coupling coefficients of the nncleons read in this approximation as [81,84]... 

Baryons include the nucleons (n and p), hyperons (also called "strange baryons", with nig 1116 for A° to 1673 MeV for Q", and lifetimes 10" s), and all nuclei. and Tt" decay into corresponding anti-particles in a similar way. tt rest mass is 135.0 MeV. See also Table 10.2. 

Hyperons and nuclear hyperfragments. Rather recently, it has been found that unstable particles (hyperons ) exist which are slightly heavier than protons and neutrons and which decay in about 10 seconds. The new machines with energies several times 10 ev produce these particles. 

The principle of conservation of electric charge is illustrated by the decay reactions given in Table 20-4. For example, the lambda particle, which is a hyperon, with mass somewhat greater than that of a nucleon, can decompose either to form a proton and a negative pion or to form a neutron and a neutral pion. In the first case the lambda particle, which is neutral, forms a positively charged particle and a negatively charged particle in the second case it forms two neutral particles. 

In order to solve for the individual A/ we need one more relation. This can be obtained from a study of hyperon /3-decays, the data for which seem to be in good agreement with 5 7(3)ir invariance. If SU 3)f invariance holds, all decay amplitudes are expressed in the form [see (18.1.2) and (18.1.16)]... 

The CMU group is part of the CERN collaboration, PS 185, which studies hyperon-antihyperon (AA, AS + XA,XL) production at LEAR through the reaction pp- YY. The total cross sections, differential cross sections, final state polarizations, and spin correlation coefficients are studied near threshold where the number of partial waves contributing to the production mechanism is limited, simplifying calculations. In addition, from the spin correlation coefficients, Cij, it is possible to extract the singlet fraction, the probability that the two hyperons are produced with their spins coupled to zero, as S=(l+Cxx""Cyy+Czz)/4 (where Cij is a measure of the probability that one of the produced particles will have a positive spin component in direction f i if the second particle has a positive spin component in direction j). We have also extracted the decay asymmetry parameters of the A and A as a test of a possible technique for investigating CP violation in a system other than the neutral kaons. 

The A and IP hyperons are in the same spin-parity octet, and are related at the quark level by spin flips of two quarks. The production ratio of these two hyperons is predicted by spin-flavor SU(6) to be a(7P >K Z°) / o(7p—>K A) =1/3, while experiment yields about unity Since the iP has isospin 1, rather than isospin 0 like the A, the isospin 3/2 A resonances can play a role in the production process. The fact that additional terms are needed to describe Z photoproduction point out the desirability for more and better data on this reaction. Furthermore, no polarization information exists at all for Z° production. Because the IP decays 100% via an Ml transition to the A, any measurement of the decay A polarization also measures the polarization of the Z°. 

The third elementary hyperon photoproduction reaction possible on the proton is Y+p— K +Z". The study of this reaction is interesting to compare with the previous reaction since many of the same diagram contribute. This reaction has no t-channel exchange. The decay asymmetry for the Z" is very large (a=-0.98), so its decay into p7t° reveals information about the Z" polarization. To our knowledge, this reaction has never been measured. 

Another useful kinematic feature in these measurements is the fact that neutral strange hyperons have a decay length of several centimeters. This allows the possibility of using the neutral "V" to signal the production of strange particles. The vertex defined by the A —> + p... 

Barnes, P.D., "The Weak Decay of Hyperon Systems," XXIII Yamada Conference on Nuclear Weak Processes and Nuclear Structure, M. Morita, Ed., Osaka, Japan, June 1989, World Scientific Publishers. 

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