Prof. Larry Hunter and colleagues at Amherst College in Massachusetts, together with Jung-Fu Lin of the University of Texas, Austin, have used the Earth to put bounds on long-range spin-spin forces associated with the virtual exchange unparticles [2].

Background
Recall that intrinsic angular momentum, or spin, of a particle of what gives rise to a particle’s magnetic moment and it is the interaction between spins that generates a magnetic field. In quantum electrodynamics the standard understanding of the interactions between the spins is in terms of the exchange of virtual photons.

In 2007, Howard Georgi of Harvard University proposed the existence of unparticles, which represents an as of yet undiscovered scale invariant sector to the standard model [1]. The exchange of unparticles would lead to a new type of spin–spin interaction. It is this new interaction that Larry Hunter and colleagues have been looking for.

Unparticles and the crust?
Prof. Hunter and colleagues created a map of the Earth’s polarized electron spin density. Then they calculated the potentials associated the possible spin interactions and integrated this across the whole of the Earth. The effect of these potentials on two detectors that can probe long-range interaction between spins in the Earth’s interior was worked out.

Using this they were able to place new upper bounds on the forces due to the exchange of unparticles. Improvements in sensitivity as compared to existing laboratory experiences is by a factor of a million.

In our obscure business of precision measurements it might take a decade to improve the sensitivity of an experiment by an order of magnitude, so using just laboratory sources it might have taken 60 years to get to the limits we did. Prof. Larry R. Hunter

References

[1]Howard Georgi (2007). “Unparticle Physics”. Physical Review Letters 98 (22): 221601

[2] Larry Hunter Joel Gordon, Stephen Peck, Daniel Ang and Jung-Fu Lin, Using the Earth as a Polarized Electron Source to Search for Long-Range Spin-Spin Interactions, Science 22 February 2013: Vol. 339 no. 6122 pp. 928-932

Links
Search for ‘unparticles’ focuses on Earth’s crust (Physics World)