Science: Sticky questions inside the proton: Brian Foster looks at emerging knowledge about subatomic particles that is exciting physicists
Monday 08 August 1994
But protons are the constituents of all atomic nuclei. The structure and internal dynamics of the proton, although it may seem rather recherche a topic, is thus one of the most fundamental issues in the study of the physical world.
And things are sticky inside the proton. Experiments at the Hera accelerator in Hamburg are throwing new light on the nature of subatomic particles. When the results were reported to the 26th International Conference on High Energy Physics in Glasgow last month they caused great excitement among physicists.
The Hera accelerator is the biggest and most advanced machine at Germany's laboratory for fundamental particle physics. It collides electrons and protons circulating in opposite directions inside an vacuum pipe of 6.3km (4 miles) circumference. In a sense, Hera is the world's biggest electron microscope.
The electron appears to have no measurable size or internal structure, and so it behaves in a very simple way when it collides with other particles. The proton is more complex. By measuring how electrons scatter after colliding with protons, physicists can assess the proton's structure in a manner analogous to the way our eyes perceive objects by recording light scattered from them. Hera is so much larger than conventional electron microscopes because its electrons need to be about a billion times more energetic than in the conventional electron microscope in order to penetrate sufficiently deeply inside the proton to resolve its many tiny constituents.
What does the scattering of electrons from the proton reveal? In our present understanding, the proton consists of three point-like constituents known as quarks. These so-called 'valence' quarks are for ever confined inside the proton by the 'strong' nuclear force. The carrier of this force is itself a particle, which is known as the gluon because it carries an attractive force which sticks or glues the quarks together inside the proton.
Occasionally, the gluon spontaneously converts itself momentarily into a pair of entities. One is a quark and the other its antimatter equivalent, an 'antiquark' with the same mass but all other properties opposite. This violates the fundamental principle that energy and momentum are always conserved, but is allowed by the aspect of quantum mechanics known as Heisenberg's uncertainty principle. Processes that violate energy conservation are allowed, provided they take place so quickly that the energy violation multiplied by the time taken is less than a fundamental quantity h, known as Planck's constant.
The effect of this is that, at a particular instant, the proton can appear to consist of a large number of transitory quarks and antiquarks spun out of gluons, but obeying the Heisenberg uncertainty principle. The violent collision of an electron with a proton in Hera is sensitive to these brief fluctuations. The two experiments operating at Hera, known as H1 and Zeus, can infer the density and time-structure of quarks and antiquarks inside the proton and hence discover information about the gluon and the strong nuclear force.
Over the past year, the experiments have revealed surprising details. The number of quarks and antiquarks, and hence the number of gluons which give rise to them, is much larger than expected.
The implications of this behaviour are unclear. The theory of the strong force, known as quantum chromodynamics, is extremely complex, and can only be solved by approximations and simplifications. The rapid increase in the number of gluons gives the first indication that the approximation may fail and that other theoretical approaches may be necessary.
Another important question raised by the experimental results is that the number of gluons cannot continue to rise at the presently observed rate. If many more are produced, the proton will not be able to contain them and they will interact so strongly that some will be absorbed.
In the Sixties, lower energy experiments scattering electrons from protons at Stanford, California gave the first evidence that they contained point-like constituents, later identified as quarks. This was crucial to our understanding of matter. It seems possible that the Hera experiments may be able to provide another big step forward in our understanding of the strong interaction by observing the number of gluons inside the proton. The interior of the proton seems to be a sticky place indeed]
Brian Foster is a Reader in Physics in Bristol University.
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