There are many theories that try to explain the fundamental behaviour in nuclear and particle physics. One of the most promising is a theory called supersymmetry. This suggests that symmetry exists between fermions - subatomic particles with half-integer spin - and bosons - particles with integer spin. In supersymmetry, a fermion can be changed into a boson and back into a fermion through a series of space-time transformations. In a forthcoming paper in Physical Review Letters, an international team of physicists has found the first firm experimental evidence that supersymmetry exists in atomic nuclei.
Supersymmetry is an attractive theory to physicists as it provides a connection between all known elementary particles and can simplify many complex interactions. High energy supersymmetry suggests that instead of two types of basic elementary particles – bosons and fermions – there is just one. It also implies that high-energy physics experiments should detect twice as many particles than researchers have currently seen. For example quarks and electrons, which are fermions, should have bosonic equivalents called squarks and selectrons, while photons and gluons, which are bosons, should have fermionic equivalents called photinos and gluinos. Current research suggests that these particles are superheavy and hence only new accelerators, such as the powerful Large Hadron Collider at CERN – which is due to come on-line in 2005 – are likely to see them. Supersymmetry is also used in nuclear and solid state physics.
In 1990 Francesco Iachello of Yale University in the US proposed that it would be possible to see the effects of supersymmetry in the nucleus. He suggested that it should be possible to create an algebraic transformation that would allow a nucleus with an even number of protons and neutrons (even-even), two nuclei consisting of either an odd number of protons or neutrons (even-odd or odd-even), and a nucleus with an odd number of both neutrons and protons (odd-odd) to transform into each other via supersymmetry. These four types of nuclei can form what is known as a supersymmetric quartet. It was later realised that platinum-194 (even-even), gold-195 (odd-even), platinum-195 (even-odd) and gold-196 (odd-odd) formed the ideal quartet in which to search for nuclear symmetry .
Now Jan Jolie of the University of Fribourg in Switzerland and co-workers at the Ludwig-Maximilians University in Munich, the University of Bonn, and the University of Kentucky in the US have discovered experimental proof of nuclei supersymmetry. They used the Tandem accelerator in Munich to study the transfer reactions of very thin films of gold-196 nuclei.
In their experiments, they bombarded gold-197 with polarised deuterons. This causes a nucleon to be transferred from the gold to the deuteron to create triton – a tritium nucleus – leaving gold-196 behind. As the triton leaves the target, it is analysed by a magnetic spectrograph to determine its energy and spin. This, in turn, provides information on the excited energy states in the gold-196 nucleus.
Once they obtained the energy spectrum of gold-196, they applied their supersymmetry transformations to the results and found that they could produce the energy-level spectra of the other three nuclei in the quartet. “It’s a fascinating paper,” says Georgis Kraniotis from Royal Holloway and Bedford College in the UK, “and encouraging that they have used super-algebra in a non-accelerator experiment.”
“The fact that they claim to have found a realization of supersymmetry in nature is exciting,” adds Herbi Dreiner of the Rutherford Appleton Laboratory. “It would definitely further encourage us to look for supersymmetry in particle physics.”
“This finding is extremely important because firmly establishes the experimental occurrence of supersymmetry in physics,” says Iachello. “It represents, in my opinion, a major achievement in the study of symmetries in physical systems.”
