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Research narrows down search for possible fifth fundamental force

By Mo-Yu Zhou | March 7, 2013

The Standard Model of physics is currently associated with four fundamental forces: gravity, electromagnetism and the strong and weak nuclear forces. But some particle physicists have proposed a fifth force that would extend the Standard Model, which describes matter and the way it interacts by breaking down matter into a number of particles. In a study published in the Feb. 22 issue of the journal Science, Larry Hunter of Amherst College and his colleagues at Amherst and the University of Texas at Austin have narrowed down the search for what scientists call “long-range spin-spin interactions” that would be tied to such a fifth force. This force would involve electrons, protons and neutrons interacting over long distances.

The study, besides being a potential breakthrough in particle physics, is also unique for its interdisciplinary nature. It combines particle physics, atomic physics, solid-state physics, nuclear physics, mineral physics and geophysics.

“This has been exciting. I really did not know much about geophysics before starting this project.  I had never even taken a course in geophysics,” Hunter said in an email to The News-Letter. Hunter explained that his interest in geophysics was piqued when he came up with the question, “Can we use the electrons in the Earth as a spin source?”

Every proton, neutron, and electron has an atomic property of “spin,” which can be thought of as a vector or an arrow that points in a certain direction. Some of the electrons in the Earth’s mantle are slightly spin-polarized due to Earth’s magnetic field. This means that the directions of their spin are not random, but collectively have a preferred orientation.

To shed light on long-range spin-spin interactions, the researchers used the Earth itself as a source of electrons. The team created a comprehensive computer model of Earth’s interior, mapping expected densities and spin directions of electrons within the Earth. Hunter described developing the model of the spin-polarization of the Earth as the most challenging and the most fun aspect of the work.

This model was based in part on the lab experiments of co-author Jung-Fu “Afu” Lin of the University of Texas at Austin, which measured electron spins in minerals of Earth’s interior.

Then, the researchers examined whether the spins of protons, neutrons and electrons in various laboratories might interact with the spins of electrons within the Earth. In other words, they tried to find out whether the spins of laboratory subatomic particles have a different energy depending on their direction with respect to the Earth.

“We know […] that a magnet has a lower energy when it is oriented parallel to the geomagnetic field and it lines up with this particular direction ­—that is how a compass works,” Hunter said.

Hunter and his colleagues removed the magnetic interaction and tried to see if there were any other interactions that were taking place. In particular, one such interaction might be the long-range spin-spin interaction, as there could be an interaction between the spins of the subatomic particles in the labs and the spins of the electrons within the Earth.

Although their apparatus did not detect such interactions, the team of researchers was able to infer that long-range spin-spin interactions, if

they exist, are incredibly weak, about one million times weaker than the gravitational force between particles. This provides a useful path for more precise experiments using the same method.

“With more sensitivity, it is possible we will uncover a new force of nature,” Hunter said.

The experiment also has exciting implications for our understanding of the characteristics and composition of the deep Earth.

“[It] might eventually be possible to turn the arguments of our paper around and use the force to probe the composition of the lower mantle of the Earth,” Hunter said. More generally, this study has revealed the potential and power of particle physics in studying geophysics.

While the results of the study are not conclusive, they hint at the possibility of exciting discoveries in the future. It is precisely this sort of process that draws Hunter to the study of particle physics.

“Particle physics establishes the basic components and interactions of nature,” he said. “What we are able to do it to explore some of the possible choices that nature might make. We shine our light where we can and hope that nature is kind enough to reveal a new secret.”

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