Jethalal S. Zaveri
Prof. Muni Mahendra Kumar
The continual change and constant movement in the subatomic world is not arbitrary and chaotic but is symmetrical and rhythmic. Firstly all particles of a given kind have exactly the same mass, electric charges, and other characteristic properties. The electric charges carried by them are exactly equal (or opposite) to that of electron or exactly twice that amount. The definite and clear patterns in the structure of particles follow the same regularity observed in the world of atoms. For example, all hadrons which are the strongly interacting particles and which include protons and neutrons, seem to fall into sequences whose members have identical properties except for their masses and the masses also increase in a well-defined way within each sequence. These regularities suggest an analogy to the excited states of atoms. The similarities between the atoms and hadrons suggest that the latter too are composite structures capable of absorbing energy to form a variety of patterns. Now, it should be clarified that the classical notion of "composite objects" consisting of "constituent parts" cannot be applied to subatomic particles. For example, two protons when they collide with high velocities, can break up into 'fragments', but there will never be fractions of a proton among them. The fragments will always be entire hadrons, which are created out of the energies and masses of the colliding protons. The decomposition of a hadron into its "constituents" depends on the energy involved in the breaking-up process. In the sixties, therefore, when most of the presently known particles were discovered, most physicists concentrated their efforts on mapping out the emerging regularities rather than finding out the constituents.
[1] There are twelve Conservation Laws:
Strong Interactions are restrained by all the twelve. Electromagnetic interactions by all except the last. Weak interactions lose (xi) Strangeness (x) Charge conjugation and (ix) Space inversion but the Combination PC remains valid. |
| [2] Something is symmetrical if certain aspects of it remain the same under varying conditions. A mirror reflection is the most common case and one half of a circle always mirrors the other half. Regardless of how we turn a circle the right half always mirrors the left half. The position changes but the symmetry remains. |
The other laws correspond to such quantum numbers (The basic quantum numbers are spin, isotopic spin, charge, strangeness, charm, baryon number and lepton number.) as isospio and hypercharge, which are too complicated for our discussion. Suffice is to say that these are used to arrange particles into families forming neat symmetrical patterns called 'meson sextet' and 'baryon octet" etc.
Now, if one assumes that all hadrons are composed of smaller elementary entities, most of these regularities can be represented in a very simple way. Murray Gell Mann who postulated their existence in 1964 as stated earlier has called such entities „quarks“ (The fanciful name caught the fancy of its postulator from a line in James Joyce's book "Finnegan's Wake") Strong nuclear force binds and holds the quarks together in the proton and neutron and holds the protons and neutrons tightly together in the nuclei of atoms. It is believed that this force is carried by a 'spin-1' particle, called the 'gluon', which interacts only with itself and with quarks. A curious property of the strong nuclear force called 'confinement' prevents one from observing an isolated quark or 'gluon' and might seem to make the whole notion of quarks and gluons as particles somewhat metaphysical, and a few years ago, it was believed that quarks are permanently 'confined' within hadrons and will never be detected. However, there is another property of the strong nuclear force called 'asymptotic freedom' that makes the concept of quarks and gluons well defined. At normal energies, the strong nuclear force is indeed strong and it binds the quarks tightly together. However, at high energies, it becomes much weaker, and the quarks and gluons behave almost like free particles. By collision between a high-energy proton and anti-proton, several almost free quarks have been produced.
On the theoretical side, the quark model is very successful in accounting for the regularities found in particle-world. From three kinds of quarks in the original model of Gell Mann, the number has increased to at least eighteen quarks plus eight gluons to account for the observed patterns in the hadron spectrum. The terms 'colours' and 'flavours’ have been introduced to distinguish different kinds of quarks and so there arc quarks of different 'colours' and 'flavours'. (This is comparable to the laws of combination of paramanus which will be discussed in 2.)
Property of confinement always binds particles together into combinations that have no colour, e.g., one cannot have a single quark because it would have a single colour (red, green or blue). Instead a red quark has to be joined to a green and a blue quark by a string of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron.
Another possibility of combination is a pair consisting of a quark and an anti-quark (red + anti - red and so on = white). Such combinations make up the particles known as mesons, which are unstable. Similarly, confinement prevents one having a single gluon on its own, because gluons also have colour. Instead, one has to have a collection of gluons whose colours add up to white. Such a collection forms an unstable particle called a 'glue ball'.
In spite of severe theoretical difficulties for accepting the existence of physical quarks, the fact cannot be denied that hadrons do often behave exactly as if they consisted of point like elementary constituents.
In spite of all these difficulties, many physicists still hang on to the idea of classical 'building blocks' of matter, which is so deeply ingrained in western scientific tradition.
