The Forces between Nucleons. Strong Interactions

In 1932, when the neutron was discovered, it was recognized that the heavier atomic nuclei can be described as being built of protons and neutrons, with the electric charge equal to the number of protons and the mass number equal to the sum of the number of protons and the number 

It was evident that the force of attraction between nucleons must be a strong force at small distances, stronger than the repulsion due to the positive electric charges on the protons, and a weak force at large distances, weaker than the electrostatic repulsion. Careful studies of the 

An event recorded in the 72-inch liquid-hydrogen bubble chamber of the University of California (L. W. Alvarez and coworkers). The incident particle is a negative kaon. in a beam of these particles. By collision with a proton it forms a positive kaon and a negative xion. 

The negative xion then decomposes to form a lambda particle and a negative pion. The lambda particle, which is neutral, produces no track. It is shown as decomposing to form a proton and a negative pion. 

however, the two particles are farther apart than 1.4 fm, as in B, the emitted messenger particle is not able to traverse the distance between the particles, but instead turns back and disappears. There is accordingly no interaction between the nucleons at the larger distance. The reason that the range of the messenger particles is restricted can be understood by consideration of the uncertainty principle. 

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