Are there any particles beyond the Higgs lurking where the LHC might discover them? A team of researchers that calls itself the ACME project has now produced a measurement that says the answer is "probably not." ACME looked for any imperfections in the shape of an electron's electric field and placed a limit on the measurements that is 12 times smaller than anyone had previously achieved. As far as they could tell, the electron has no imperfections, which rules out the possibility of finding many of the new particles predicted to be within the mass range that will be explored by the LHC.

The research relied on thorium atoms, which are mostly discussed as a potential nuclear fuel. But in this case, the authors were after the electrons. Thorium's high atomic number means that its innermost electrons orbit within an intense electric field that's internal to the atom, and they travel at relativistic speeds. These properties will serve to exaggerate even a minuscule imperfection in the electric field of the electron itself.

The imperfection the ACME team was after is called the electric dipole moment, and it's measured relative to the electron's spin. If the electron has an electric dipole moment, then its charge will be unevenly distributed along its spin. Nothing in the Standard Model can create an electron dipole moment, but most extensions to the Model, including Super Symmetry, posit heavier particles, the mere existence of which should alter the dipole moment.

To measure it, the ACME team built a device that could send a beam of cold thorium oxide molecules into a device. As the atoms entered, they were zapped with lasers that could create a specific transition in one of the thorium's inner electrons, affecting its spin. The atoms then passed between some plates that created an electric field. If the electron has an electric dipole moment, the resulting imperfection will interact with this electric field, changing how the orientation of the spin changes with time. The spin is read out with a second laser at the far end of the device, allowing the researchers to search for any signs of unexpected changes.

After building up a huge number of measurements, the authors searched through them for potential sources of error, identifying a few and correcting for them. They also added a constant value to their calculations that should throw them off, but the team kept themselves blind to it—this kept their own expectations from influencing how they conducted the experiment.

When all was said and done, no sign of an electric dipole moment was apparent. That means that, if one exists, it must be smaller than the experimental error of these experiments. And that experimental error is 12 times smaller than the one set by previous experiments.

That's a significant result, even though it was generated with equipment that just fills up a couple of bench tops. An electron dipole moment isn't predicted by the Standard Model, but it can arise from a new form of charge-parity violation. As the ACME team puts it, "Nearly every extension to the [Standard Model] introduces new CP violating phases." So, those extensions dictate that the electron should have an electric dipole moment. The smaller the dipole moment, the heavier the particle involved in its creation.

Thus, by lowering the size of the largest possible electric dipole moment, the new results rule out lighter forms of most of the particles people have predicted. In fact, they rule out finding them in the LHC entirely: "our more precise [electric dipole moment] limit constrains CP violation up to energy scales similar to or higher than those explored directly at the Large Hadron Collider." That doesn't mean we can't find any new particles there, but it does indicate that the search for the ones that are relatively easy to predict on theoretical grounds is likely to come up blank.

Science, 2013. DOI: 10.1126/science.1248213 (About DOIs).