Sunday, July 21, 2013

QUARKS AND LEPTONS

By the early 1930’s we had a picture of three building blocks of an atom : electron proton and neutron ; a positively charged nucleus consisting of protons and neutrons at the centre surrounded by negatively charged electrons moving round the nucleus. Electrons and protons are subjected to an attractive long-range electromagnetic force, whereas the attraction between protons and neutrons is due to an entirely the different short-range force (range less than 10-15 metre ) called strong force. The simple picture of three building blocks of matter did not last long. From the 1930s to the beginning of 1950s experiments carried out with cosmic rays led to discoveries of several new particles such as pions, kaons, lambdas etc. From 1950’s onwards particle physics are on fast track due to the availability of man-made high-energy accelerators. The number of new particles increased to more than one hundred with large number of them very short-lived ones, like eta, rho, omega, phi, kstar, deltas etc. The proliferation of particles and the apparent regularities in them led to the idea of a substructure. In 1964, it is suggested that these particles are composed of three fundamental particles, named quarks: up (in short u), down (d) and strange (s). These are fractionally charged particles: if the electron is treated as having a minus one charge (-1), then the u-quarks has (+2/3) charge, d-quark (-1/3) and s-quark (-1/3) charge. In this simplistic picture quarks combine in two ways (i) in groups of three to form proton (p=uud), neutron (n=udd) and similar particles out of three quarks called baryons, and (ii) in groups of two to make particles known as mesons out of quark-antiquark pairs: pions, kaons etc.
                                      The direct confirmation of the inner structure of protons came from a series of experiments conducted during 1969 in which high-energy electrons were made to collide with hydrogen target. Most of the electrons passed through the hydrogen target with small or no deflection, whereas some of them experienced large angle scattering as if from very small hard objects inside protons. Detailed studies revealed these as point like, meaning structureless constituents of protons. These constituents were identified as quarks mentioned earlier.

Ranjna Bhardwaj

No comments:

Post a Comment