| Summary: | Thesis (M.A.)--Boston University === An investigation of the detection of mesons and determination of properties from both cosmic ray and artificially produced mesons has been carried out. An introduction to the theory of mesons is also presented. It is hoped that the more important parts of the study of mesons which have heretofore been scattered about are now pieced together in one study so that a comprehensive knowledge of this phase of physics is embodied in one paper.
Mesons were first detected by Carl D. Anderson in 1937. They were detected in a cloud chamber by noticing their great penetration in dense absorbers. It was apparent that the energy of these particles was great to penetrate such thicknesses. If one tried to use Bethe's theory of loss of energy in absorbers on these penetrating particles, it was very evident that either Bethe's theory was wrong or these particles were not electrons. Theory can never hold sway over experiment, so investigations were performed to detect the possible energies, masses and charges possessed by these particles. By placing a photographic emulsion inside the cloud chamber, it was possible to determine the radius of curvature of the particle as it traveled through the emulsion. By this method it was possible to estimate the limits of the mass of this particle. Upon analyzing some of the tracks, it was observed that the ionization was too great for the particle to be an electron, and the radius of curvature was too small for the particle to be an alpha particle or a proton. A measurement of the change in radius of curvature together with the radius of curvature allowed an estimation of the mass. From this, a mass of 120 +/- 30 me was calculated, and this was certainly different than any particle discovered previously. Others made similar investigations and found correspondmg results which confirm the findings and conclusions put forth by Anderson. Because the particle's ionization, mass and energy differed from any other particle, it is safe to assume that a new particle has been discovered. The name meson was given to this particle.
As soon as it was determined that a new particle had been discovered, many physicists started investigations to find the properties of the meson. The property which has received the most attention, and about which most is known, is the mass. The method of finding the radius of curvature of the particle under a magnetic field, and finding the rate of change of radius of curvature to calculate the mass of the meson was only good enough to determine the mass within wide limits of error. To make the determination of mass more accurate, the charge of the meson was investigated very carefully, and this was found to be the same as that on the electron within good experimental error. Hughes presented a method of using nomographs which was not only more accurate, but convenient, and the mass of the meson was determined to be concentrated around 200 me. The mass was found to range from 60 me to almost that of the proton. Stable forms of mesons were observed at approximately 330 me and also at 700-900 me. The 200 me meson was called a Mu meson, the 330 me a Pi meson, and the 700-900 me a Tau meson. Most of the work in connection with cosmic rays was concerned with Mu mesons since they occur at the surface of the earth so much more than the other types of mesons. It is assumed, with some experimental verification, that the heavier mesons travel much faster and have shorter lifetimes so that they dissociate before reaching the earth. The Mu mesons and Pi mesons were observed to have both positive and negative charges, and when an investigation of the dissociation was made neutral mesons were predicted but not verified by experiment.
The decay of these Mu mesons was studied, and it was observed that among other particles, electrons were given off. The energies of these electrons were probed, and it was found that they had an energy varying from 10 MEV to 55 MEV. In this meson spectra the average energy of the decay electrons was 34 MEV, and the most probable energy was 40 MEV. This spectrum dropped off suddenly at the upper end, and indicates a concentration of the energy at the upper end. By following these mesons through the absorbers, a calculation of their lifetime was made. A difference in lifetime of the positive and negative mesons was predicted when it became noticeable that more positive mesons penetrated some absorbers than negative mesons. Because of this, absorbers with wide ranges of atomic numbers were experimented with, and it was found that with absorbers of low atomic numbers the ratio of positive to negative penetrating mesons was about unity, but as the atomic number was increased, the ratio increased. It was finally determined that the probability for capture of the negative mesons increased as the fourth power of the atomic number of the absorber, while for positive mesons it remained about the same. The mean life of the negative mesons also varied as the fourth power of the atomic number of the absorber. The mean life of the positive meson was determined as approximately 2 microseconds, and although the lifetime of the negative meson varied, it was found to be of this magnitude. As has been mentioned, one of the decay particles was identified as an electron, but nothing has been said of the other particles. From conservation of energy and momentum, a decay into three particles of very low mass was predicted, because no observed particles aside from the electron were found. If there were only one particle its energy would demand too great a mass for it to go undetected, and therefore two other particles were predicted. These were called neutrinos, and they fit into the picture very nicely in both theory and experiment.
Mesons have been produced artificially in a synchrocyclotron by bombarding a carbon target with 380 MEV alpha particles, and were found to be both Pi and Mu mesons. Both types of mesons were observed to carry positive or negative charges, and thus far all properties investigated agree very well with those observed in cosmic ray research. It is predicted that in the future, with more powerful accelerators, most all types of mesons will be produced in abundance, and thereby more accurate and detailed investigations can be carried out. X-Rays have been used as the bombarding particles against carbon and glass as targets, and have also produced Pi mesons of both charges.
The theory of mesons is very limited. Up to a certain extent, a successful analogy between electromagnetic phenomena and nuclear phenomena may be carried out, but this analogy is, very limited. In such an analogy the meson's characteristics in nuclear phenomena is compared with that of the photon in electronic phenomena, and it is seen that, at least classically, there is a good correspondence. However, a clear notion of electronic force fields is known, and such is not the case with self energy in meson interactions has to consider both translational and rotational motion since the interactions are spin I nuclear fields. Also, much work with electrons assumes self energy is composed of only translational motion, while the dependent. If a scalar field is used, this spin dependence can not be accounted for, and the calculations can not be carried out relativistically. The assumption of a pseudoscalar field allows the consideration of spin dependence, but reciprocal powers in the expression of the potential energy make the expression indefinable as the distances proceed to the zero limit. Methods of subtracting out these infinities have been tried by mixing fields, but as yet no satisfactory method has been forthcoming. If the nuclear distances are cut off the potential is defined, but relativistic requirements can not be met under these conditions. Both experimental evidence and relativistic quantum methods will have to be improved before any satisfactory theory can be attained.
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