The spin of a proton plays a critical role in a variety of Magnetic Resonance studies such as the MR Imaging (MRI) of body tissues. Like the spin of its neutron partner, it contributes to the spin of the many atomic nuclei suitable for MRI applications and to the study of chemical compounds using Nuclear Magnetic Resonance (NMR). The inert gases helium-3 and xenon-129, for example, show considerable promise for the rapid MR Imaging of the lung when the gases are suitably spin polarised.

A profound combination of quantum and relativity theories reveals that spin is an intrinsic property of a particle, taking values related to a particular quantum unit. The component quarks of a proton, for example, have intrinsic spin, as do the gluons which bind those quarks together. A recent surprise is that less than a third of a proton’s spin may be attributed to the spin of its charged component quarks. Electron accelerators at Stanford, Hamburg and CERN have deduced this result by probing the structure of the proton with photons which interact only with those quarks. The gluons in a proton are neutral and invisible to photons. Thus the major contribution to a proton’s spin seems to come from its gluons. To probe the gluon structure one must use a beam of ions. A Relativistic Heavy Ion Collider (RHIC) at Brookhaven near New York will accelerate polarised protons to unprecedented energy from the year 2000. One aim will be to measure the spin contributions from a proton’s gluon, quark and anti-quark components.

Calibrating the polarisation of a beam with energies higher than 30 times the rest energy of a proton poses a difficult challenge. One method involves scattering polarised particles at the small angles where strong and electromagnetic forces are of similar strength. This is equal to the challenge provided the spin dependence of the strong interaction is sufficiently well understood. Bounds on such spin dependence have been derived from mathematical principles based upon causality, probability and invariance under the reversal of matter with antimatter, of time, and of space. A collaborative project, part-funded by Enterprise Ireland, including researchers from Trinity, CERN and Brookhaven is successfully addressing these issues.

Measurement of the gluon polarisation and the quark and anti-quark polarisation by flavour – up, down and strange – will then be possible using the enormous detectors at RHIC with their considerable facility for data analysis using high performance computing. A calibrated proton beam whose polarisation is known to sufficient accuracy will assist in understanding the source of a proton’s spin.

Contact: Dr N.H. Buttimore,
School of Mathematics, TCD;
E-mail: nhb@maths.tcd.ie