Experimental Particle Physics Research Group

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Sussex celebrates two decades of world leadership in particle physics project

Experimental apparatus for measuring the electric dipole moment of the neutron

Professor Philip Harris

Friday 1 February 2019 marks the twentieth anniversary of the establishment of a Sussex-led world record in sensitivity in the measurement of the neutron electric dipole moment (EDM), a fundamental parameter in particle physics related to the question of why the Universe contains so much more matter than antimatter. Matter and antimatter were created in equal and opposite amounts in the Big Bang, and in a massive mutual annihilation they wiped each other out – almost but not quite completely; there was a tiny (part per billion) bit of matter left over at the end, which now constitutes all of the matter in the Universe. 

The question of why there was anything left over at all – which is fundamental to our existence - has been an outstanding mystery of cosmology for decades, but whatever the tiny underlying asymmetry is, it must also cause a small distortion in the shape of the neutron, a subatomic particle. Although it’s electrically neutral, one end will be a little bit positive and the other end a little bit negative. Different cosmological theories predict different amounts of distortion, so measuring it puts tighter and tighter constraints on new theories of physics beyond our current understanding. Indeed, it’s been said in the literature that, if taken as a whole, the neutron EDM experiment has probably ruled out more theories (of new physics) than any other experiment in history.

1 February 1999 saw the publication of the first results from an innovative measurement by a small Sussex-led collaboration, which established a new world record in the precision of the measurement. The experiment continued to run for several years, with further results published in 2006 and then finally in 2015. Each new result typically garners about 1000 citations from scientists looking at the implications.There are currently approximately half a dozen competing collaborations around the world (the largest being about 100 scientists in the USA), but throughout these two decades the Sussex-led experiment – which also involved the Rutherford Appleton Laboratory and the Institut Laue-Langevin, Grenoble, where the experiment took place - has managed to retain the world’s best limit.

The group’s present leader, Prof. Philip Harris, says: “This twentieth anniversary of our taking the world lead is a real milestone. The foundations were laid by the pioneering work of Mike Pendlebury and Ken Smith, and it’s been a real privilege to have been able to spend the majority of my research career on this beautiful and fundamentally important experiment. The techniques originally developed for the first EDM measurement in the 1950s led to world-changing developments such as atomic clocks and MRI scanners, and to this day it retains its huge and ongoing impact in the field of particle physics.”

The group is now part of a larger collaboration, based at the Paul Scherrer Institute in Switzerland, which is expected to publish a new world-best limit later this year and which is simultaneously building a next-generation experiment that will be an order of magnitude more sensitive still.    

Incidentally, as well as an electric dipole moment, the neutron also has a magnetic dipole moment – it’s a tiny magnet, with north and south poles – and 2019 also marks the 40th anniversary of what remains to this day the most accurate measurement of how strong a magnet it is. The same Sussex group, led by Pendlebury and Smith, also played a leading role in that experiment.

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By: Justine Charles
Last updated: Friday, 1 February 2019

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