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A model in dispute

A model in dispute THE Standard Model of particle physics, which explains the behaviour of matter at the smallest levels, is 'by now a well- established theory. These levels of matter have been under scrutiny at the Illinois-based Fermi National Accelerator Laboratery (Fermilab) in us (Down To Earth, Vol 2, No 2).

The crowing moment came about a year ago when two collaborations at Fermilab announced the discovery of the top quark. However, another recently released report from there brings forth results which could spell trouble for the Standard Model. The pre-print (No 96/020-E) from the CDF (collider detector at Fermilab) contains hints of some results which are hard to explain within the framework of the. accepted theory. The results are the outcome of a year-long run on the most powerful accelerator in the world, the Tevatron, which smashes protons and antiprotons with extremely high energies of about 1.8 trillion electron volts.

To gain insight into the structure of matter, these violent collisions are analysed using very sophisticated detectors. According to quantum mechanics, the higher the energy with which one probes matter, the smaller is the distance perceived. Thus - by employing these huge amounts of energy - the physicists are probing the structure of matter from very close quarters and in this case, from within the elementary particles themselves.

It is a well-known fact that there exists a hierarchy in structure at the smallest levels. The atom has a nucleus which comprises of protons and neutrons. Each of these nucleons contain quarks and antiquarks. These quarks are held together by a strong nuclear force provided by particles called gluons. It is the behaviour of the quarks and the gluons which Quantum Chromodynamics (QCD) describes.

The CDF has observed an unexpectedly large number of hard and violent collisions. When protons and antiprotons confront each other in the Tevatron, most of the encounters are glancing collisions. But occasionally, there could occur a hard head-on collision which would disperse debris in the form of jets of particles at sharp angles to the beams. QCD, predicts the frequency of such occurrences the given energy of the beams of protons and antiprotons. CDF discovered that QCD was. good for lower energies, but deviated sharply as the levels of energy increased. The deviation in some cases was upto 50 per cent higher.

The announcement has created an upheaval in the particle physics community. Many possible explanations are being offered for the phenomenon. One group of scientists argues that much more data is required and that the result need not be taken very seriously. On the other hand, there is also the possibility that what has been observed is indeed the substructure inside the quarks. If the quarks - which are the building blocks of matter - actually turn out to be conglomerations of even more elementary particles, it would mean a radical change in our picture of the structure of matter at the most basic level. The Standard Model would, in this case, require a major revision.

However, this possibility, which has been suggested in the paper released by the CDF collaboration, has not found favour with physicists as of now. Scientists are advocating a much more conservative approach. It is being pointed out that there are several uncertainties in the calculations worked out by the CDF. The most important and vital question disputes the CDF projections of the proton energy being shared by the constituent quarks. It might be possible that QCD calculations are mistaken in this crucial input. What remains to be seen is whether these results invalidate the Standard Model, or the theory' escapes with minor modifications.

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