Still elusive

Tuesday 15 June 1999

Some recent findings question the little we know about sub-atomic particles

for an entity known for almost 90 years, the structure of the sub-nuclear particle proton remains remarkably mysterious. Physicists thought they understood the internal structure of the proton fairly well on the basis of their theories. But recent results from two particle accelerators have again proven that nature is more complicated than we had hitherto imagined. (An accelerator is an apparatus for speeding up charged particles to high energies by means of electric or electromagnetic fields.)

Research groups at the Hadron-Electron Ring Accelerator ( hera ) in Hamburg, Germany, and the Tevatron accelerator in Fermilab, usa , have reported that they see an excess of virtual down antiquarks over up antiquarks in the proton. This contradicts the theoretical expectation of having the same number of these constituents inside the proton ( Physical Review Letters , Vol 81, p5519).

Protons, together with electrically neutral particles called neutrons, make up all atomic nuclei, except for the hydrogen nucleus, which comprises a single proton. In ionised hydrogen (atoms which have been stripped of electrons), protons are given high velocities in particle accelerators and are commonly used as projectiles to produce and study nuclear reactions. Protons are the chief constituents of primary cosmic rays and are among the products of some types of artificial nuclear reactions. They are made up of smaller units of matter known as quarks.

In much the same way that protons and neutrons make up atomic nuclei, these particles themselves are believed to consist of quarks. The proton and the neutron are made up of up and down quarks and their antiparticles. Quarks seem to occur in combination with other quarks or antiquarks, never on their own. The explanation of the interactions of quarks is in the theory of quantum chromodynamics ( qcd ).

The way physicists come to know about the structure of the sub-nuclear particles is through collisions with other particles. Protons and other particles are hurled at each other and made to collide in particle accelerators. The resulting debris is then analysed and one can infer about the internal structure of the colliding particles. These collisions are somewhat similar to collision of billiard balls. If two of them collide, they bounce off each other, and this is known as elastic collision. On the other hand, if a ball is hit by a bullet, it is smashed and this kind of a collision is an inelastic collision.

In the early 1970s, a series of remarkable experiments were conducted in accelerators in which electrons were shot at protons. These experiments revealed for the first time that nucleons (a particle of which protons and neutrons are regarded as different states) are themselves made up of quarks. For instance, the proton is made up of two up quarks and a down quark. These quarks are called constituent quarks. The theory also predicts that apart from the constituent quarks, any sub-nuclear particle, including the proton, has a whole "sea" of virtual quarks.

Virtual particles are those which have a fleeting existence and cannot be detected by experiments. Particle and antiparticle pairs of virtual particles are created and destroyed before we can detect them. Their existence is predicted by the famous uncertainty principle of quantum physics. In the case of the proton, qcd predicts that the number of down type antiquarks should be equal to the number of up antiquarks in this sea of virtual particles. If we believe in qcd , as most physicists seem to now, then this is one of the tests we have of the theory. It needs to be stressed that in most other experiments, the predictions of qcd have been validated.

However, the two recent experiments have reported data which contradicts this claim. At hera , positrons (the antiparticles of electrons) were scattered off protons and neutrons, while the Tevatron experiment used protons on hydrogen and heavy hydrogen targets. Both the experiments have found that there is an excess of down antiquarks in the sea of virtual quarks in the proton, contrary to the predictions of qcd .

qcd as a theory of the interactions of quarks is the best theory we have for the phenomenon. However, as the theory itself is notoriously difficult to calculate, it will take some more experiments to confirm these results. It may also turn out that we need to improve our calculation techniques in qcd to be able to calculate all the subtle properties of a composite object like a proton. It is a measure of the mysteries of nature that a particle known for almost a century is still not completely understood.

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