Researchers used an experiment that relied on the electrons (red dots) in Earth's mantle to look for new particles, possibly the unparticle, that are tied to a new fundamental force of nature called the long-range spin-spin interaction. Image: Marc Airhart (University of Texas at Austin) and Steve Jacobsen (Northwestern University).
It's a good time to be a particle physicist. The long-sought Higgs boson particle seems finally to have been found at an accelerator in Geneva, and scientists are now hot on the trail of another tiny piece of the universe, this one tied to a new fundamental force of nature.
An experiment using the Earth itself as a source of electrons has narrowed down the search for a new force-bearing particle, placing tighter limits on how big the force it carries can be.
As an added bonus, if the new particle is real, it will shed light on processes and structures inside Earth, say study researchers from Amherst College and the University of Texas at Austin. The experimental results appear in the Feb. 22 issue of the journal Science.
The new force of nature carries what is called long-range spin-spin interaction, said lead study author Larry Hunter, a physicist at Amherst. Short-range spin-spin interactions happen all the time: Magnets stick to the fridge because the electrons in the magnet and those in the fridge's steel exterior are all spinning around in the same direction. But longer-range spin-spin interactions are more mysterious. [Wacky Physics: The Coolest Little Particles in Nature]
The force would operate in addition to the four fundamental forces familiar to physicists: gravity, electromagnetism, and the strong and weak nuclear forces. Some physicists think this new force exists because extending the Standard Model of particle physics — a theory that defines the physics of the tiniest particles — actually predicts as-yet undiscovered particles that would carry it.
There are three possibilities for where this force comes from. The first is a particle called the unparticle, which behaves like photons (light particles) in some ways, and like particles of matter in others. The second is one called the Z' (pronounced "Z-prime"), a lighter cousin of the Z boson that carries the weak nuclear force. Both unparticles and Z's arise from extensions of current physical theories. And the third possibility is that there is no new particle at all, but the theory of relativity has some component that is affecting spin.
The unparticle was first proposed in 2007 by Harvard physicist Howard Georgi. Particles have a definite mass, unless they are photons, which are massless. An electron or proton's mass can't change no matter how much momentum it has — change the mass (and thus its energy) and you change the kind of particle it is. Unparticles would have a variable mass-energy.
Though scientists have not yet found a new particle tied to the force, they did see that the long-range spin-spin interaction had to be smaller by a factor of 1 million than earlier experiments showed. If the force exists, it is so tiny that the gravitational force between two particles such as an electron and a neutron is a million times stronger.
The normal, fridge magnet type of spin interactions, mediated by photons, operate only at very short distances. For example, magnetic forces drop as the inverse cube of distance — go twice as far away and the strength of the force drops by a factor of eight. Long range spin-spin forces don't seem to decrease by anywhere near as much. Physicists have been looking for the particles that carry this kind of interaction for years, but haven't seen them. The Amherst experiment puts tighter limits on how strong the force is, which gives physicists a better idea of where to look.