Discovery of Meson

Yukawa predicted that it is due to the exchange of a massive particle between the nucleons leading to a short range force.

A result of much calculation is that the Range of a force is of same order of magnitude as compton wave length of exchanged particle. By analogy the nuclear force has a Range of about 1.4 x 10⁻¹³ cm.

A particle for which ℏ/mc = 1.4 x 10⁻¹³ cm will have its rest mass energy equal to 150 MeV or about 275 times the mass of electron.

The name Mesotron was given to this exchanged particle whose mass is intermediate between that   of electron and Proton. The modern name is Meson.

In 1937, a particle believed to be of the type was discovered by "S H Neddermeyer" and "C D Anderson" and independently by "J C Street" and "E C Stevenson" in cloud chamber studies of cosmic rays.

Estimates of the mass of this Meson were made from measurements of curvature of its track in a magnetic field which yielded values for mass of Meson in neighbourhood of 200 electron masses. Both positive and negative particles were observed.

WB Fretter (1946) made some very careful measurements of masses of mu particles, using two cloud chambers, one above the other. They were expanded simultaneously when ever a penetrating particle passed through them. This was accomplished by placing the Geiger Counters above each chamber, the two sets of actuating the expansion mechanism whenever an ionizing particle passed through  them as shown in below Fig.

The upper cloud chamber was placed in a magnetic induction of 5300 Gauss so that momentum of particle could be measured. The lower cloud chamber had a set of lead plates 0.5 inch thick and placed 1.5 inch apart so that Range in lead of particles could be measured. Out of 2100 tracks observed, 26 were suitable for measurement, their mass determination is yielded a value of 202Me.

The present accepted value is 207mₑ.

Later Occhialini and Powell and D M Perkins using a special nuclear emulsion photographic plates exposed at high altitudes, observed that some of Mesons stopped in photographic emulsions and produced so called stars - that is, nuclear disintegration with the emission of slow protons or alpha particles.

The photographs showed the curved track of heavy Meson which is named '𝚷' Meson; when captured by a nucleus in the emulsion, the resulting nuclear disintegration produces a star in which 3 charged particles are emitted.

The kinetic energy of muon emitted in the decay of a Pi Meson is always same and is equal to about 4 MeV.

𝚷⁺  ------------>  𝛍⁺ + 𝝂
𝚷⁻  ------------>  𝛍⁻ + 𝝂'    ;   ð‚' is anti neutrino     

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