Protons in secrecy probe

A 14 foot-wide doughnut-shaped superconducting magnet is being tested at the University of Illinois to help unlock the innermost secrets of the proton.

Funded by the US National Science Foundation, the $2.75 million magnet has been designed for an upcoming experiment – dubbed the G-zero experiment – at the Thomas Jefferson National Accelerator Facility.

Over the next few months, researchers will inspect the magnet, cool it to liquid-helium temperatures and turn it on for the first time.

As the power is gradually increased, a robotic test rig will monitor the growing magnetic-field strength in three-dimensional space, and alert the researchers, co-ordinated by Steve Williamson, a UI physicist, to potential problems.

Later on in the year the magnet will be shipped to the Jefferson facility where it will serve as the centrepiece of the experiment — a major effort to closely examine the role that the quark plays in generating proton structure and nuclear magnetism.

‘We know that the proton’s structure — in particular, its magnetic moment — comes from the up, down and strange quarks inside the proton,’ said UI physicist Doug Beck, spokesman for the experiment. ‘But exactly how it is put together is what we are trying to find out.’

In the experiment, an intense beam of polarised electrons will scatter off liquid hydrogen and deuterium targets located in the magnets core. Detectors, mounted around the perimeter of the magnet, will record the number and position of the scattered particles.

According to a statement, the new magnet will provide a much broader view of the small-scale structure of the proton, compared to earlier ‘snapshots’ obtained with other experiments, such as the SAMPLE apparatus at the MIT/Bates Linear Accelerator Centre, said Beck.

In SAMPLE experiments, conducted during the summer of 1999, researchers used the weak magnetic force to deduce the presence of a surprisingly large parity-violating electromagnetic effect known as the protons anapole moment, a phenomenon that had long been predicted, but never measured

‘The new magnet should allow measurement of the anapole moment and other aspects of the proton structure with much greater precision over a wide range of momentum transfers,’ said Beck. ‘For example, instead of seeing the proton’s overall magnetic moment, we will be able to vary the size of our probe to study small structures within the proton.’