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Fundamental Physics at Sussex

This article, written by Jim Grozier, also appears in the official history of the university, edited by Fred Gray and published in September 2011

Ken Smith and Roger Blin-Stoyle were the first professors of Experimental and Theoretical Physics, respectively, at Sussex. In the mid-1960s, Blin-Stoyle persuaded Smith to get involved in an experiment to determine the electric dipole moment (EDM) of the neutron. This initiative – to nail down one of the fundamental constants of nature – continues today, nearly 50 years later.

The EDM is a measure of asymmetry in the structure of the neutron. If the neutron consists, as we believe it does, of three charged constituents called quarks, then, even though it has no net charge, it is possible that the charge is not symmetrically distributed inside the particle. Quantitatively, such an asymmetry would be represented by a non-zero EDM, which can be represented by two equal and opposite charges separated by a small distance. If, however, the charge distribution is completely symmetrical, the EDM must be zero.

Blin-Stoyle’s interest in the EDM arose from an experiment performed in 1964 by a group of physicists at Princeton University in the USA that suggested that the universe is not quite as symmetrical as had been previously thought. What they had actually discovered was that in the case of certain sub-atomic particles called kaons, a symmetry known as CP symmetry, previously believed to be exhibited by all particles, was violated. Three years later, the Soviet physicist Andrei Sakharov showed that this result could be used to explain one of the most fundamental puzzles in physics: if the universe started with a big bang, equal amounts of matter and antimatter should have come into existence, but, since matter and antimatter annihilate each other on contact, and in equal amounts, there should by now be no matter or antimatter left, only energy: there would be no stars, planets or people. Luckily for us, there seems to have been some imbalance between the matter and antimatter; since no antimatter has ever been detected anywhere in the universe, one is forced to conclude that all the antimatter and nearly all the matter were annihilated, leaving enough matter to form all the stars and planets we observe – and us, of course. The violation of CP symmetry was a possible mechanism for such an imbalance to exist; but the effect seen by the Princeton group was far too small to explain the matter/antimatter imbalance. For Sakharov’s theory to work, CP violation had to be seen in other particles, such as neutrons.

To measure the EDM, you need a source of neutrons, and that usually means a nuclear reactor. Sussex didn’t have a nuclear reactor, but Smith and his colleague Mike Pendlebury were able to use neutrons from a reactor at the Atomic Weapons Establishment at Aldermaston. The experiment they built was based on a technique invented by US physicist Norman Ramsey, who later won the Nobel Prize in Physics for this and related work. Ramsey exploited the fact that neutrons have spin, and therefore behave like tiny magnets; if you put them into a uniform magnetic field with all their spins lined up, then by giving them a “kick” with a short burst of field at right angles to the spin direction you can make the neutrons precess, or “wobble”, around the field, in a similar way to the behaviour of a top when you set it spinning and then push it to one side. The rate at which it wobbles is dependent only on the applied magnetic field, and what’s more, it can be measured. If you now introduce an electric field, then if the neutrons have a non-zero EDM the rate of wobbling will change – and the change in the rate is proportional to the EDM.

Experiments to measure the EDM have been going on since 1950, but have never found a non-zero value for it. However, in any experiment there is always an experimental uncertainty, so that one cannot say it is exactly zero, only that it can be no bigger than zero plus or minus the uncertainty. In fact, various competing theories predict different values for the EDM; over the years, the experiment has been refined again and again, with the uncertainty being reduced each time, and on the way it has managed to disprove several theories which predicted a higher value. It is said in the literature that the neutron EDM experiment has disproved more theories than any other experiment in the history of physics.

Ramsey, who was involved in the very first neutron EDM experiment, teamed up with Smith and Pendlebury in the 1980s, and the group built a new experiment at the Institut Laue-Langevin, the world’s premier neutron source, located in Grenoble, France. This experiment, and its successors, used ultra-cold neutrons, which means that they had been slowed down to speeds of only a few metres per second. At such energies, it is possible to contain the neutrons in a suitably-designed “bottle”; instead of passing through the walls, as they would at higher energies, they just bounce off them, and hence stay inside the bottle – which means the duration of the experiment, and thus the accuracy, are greatly increased compared with previous methods relying on fast-moving beams of neutrons.

The current version of the experiment, run by a small collaboration in which Sussex is the leading group, employs cryogenic techniques to push the uncertainty even further down. Previous experiments have already narrowed this uncertainty down to such a small magnitude that, if the EDM is indeed non-zero, it is very very small indeed. To see how small, imagine the EDM as represented by an electron and a proton separated by a certain distance. This distance is then so tiny that, if it were scaled up to the size of a football, a football scaled up by the same factor would be the size of the visible universe! The experiment may eventually challenge the predictions of yet another theory: supersymmetry, which is a current favourite to explain such phenomena as dark matter.

Other Sussex contributions include measurements of the lifetime of the neutron, as well as the asymmetric nature of its decay, carried out by Jim Byrne and Peter Dawber until the late 1990s. The group is now also involved in some of the world's most precise measurements of properties of the neutrino – an ethereal particle that is extremely hard to detect as it can easily pass unhindered through the entire Earth – as well as the hunt for evidence of new kinds of physics at CERN's Large Hadron Collider.