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New horizons

  • 14/06/1995

The bedrock of our present day understanding of particle physics is the Standard Model. This model seeks to understand the interactions of elementary particles by building up on the 2 pillars of 20th century physics: quantum mechanics and relativity; and abstract concepts of internal symmetries.

Electromagnetic interaction has been well understood since the '50s when the theory of quantum electrodynamics (QED) was propounded. QED looks at the electromagnetic interaction between charged particles as originating in the exchange of particles called photons. Though the weak nuclear force has also been well understood since the '60s, the real breakthrough came with the development of the electroweak theory by S Weinberg, S Glashow and A Salam. This theory views the weak and the electromagnetic interaction in a unified way in terms of a more fundamental electroweak force.

Striking confirmation for this theory came from the discovery of the W and Z bosons, the carriers of the electroweak force. Particles that experience the weak force are called leptons. In the late '60s, quantum chromodynamics (QCD) was developed to explain the strong nuclear force. In QCD, the nuclear particles like the proton are not considered elementary but conceived as conglomerations of quarks. These particles, proposed by Murray Gell-Mann in the late '50s to explain the proliferation of sub-nuclear particles, have strange properties.

For one, quarks are not integrally charged, that is, unlike any other particle we know, their electrical charge is not a multiple of the electron charge. Instead, they carry charges like -1/3 or 2/3 (in units of the electron charge). Each quark has an associated antiquark with equal mass but opposite electric charge.

When the quark model was proposed, the number of nuclear particles known were limited and thus only 3 kinds of quarks were needed to explain the former's existence. These were called up, down, and strange quarks. Using these (and their antiparticles) one could build up any known hadron, that is, particles that experience the strong nuclear force. For instance, the proton is a composite formed by 2 up quarks and a down quark while a neutron is formed by 2 down quarks and an up quark.

But when more hadrons were observed in particle accelerators, more quarks were added to the model to accommodate the new discoveries. The ingenuity of the physicists bestowed upon these quarks a new quality, arbitrarily called colour (hence the prefix chromo in QCD), thereby increasing the number of members in the quark family. Colour is like charge except that it comes in 3 kinds (called red, blue and green). It is this property which is responsible for the quarks experiencing the strong nuclear force, much like the property of mass makes an object experience gravity.

Another strange property of the quarks is that they are never seen by themselves but only in combination with other quarks, when they form hadrons. Several searches have been carried out to look for isolated quarks (they will have a distinctive signature because they would carry non-integral charges like 1/3 or 2/3), but they have all been unsuccessful. Though W Fairbanks at the Stanford University has reported some evidence of seeing fractional charge on niobium coated tungsten balls, the results have not been replicated.

The Standard Model, which allows us to understand the interactions of the quarks and other elementary particles, had postulated the existence of the top quark. Though this model has been validated in several experiments, there are a lot of unanswered questions which have motivated physicists to look beyond the Standard Model.

One of the primary constituents of the electroweak theory is the Higgs particle. This particle is responsible for the electroweak interaction separating itself into the electromagnetic and the weak force. It is also responsible for the mass of particles like quarks and electrons. Unfortunately, the search for the Higgs has so far proved elusive.

Other puzzles of the Standard Model include the seemingly arbitrary masses of the fundamental particles, the arrangement of these particles into families that are replicated and the mass of the Higgs particle itself. It is these conundrums of the Standard Model which physicists seek to sort out with bigger and bigger accelerators and more refined theories like the Grand Unified Theories and Supersymmetry.

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