The next big problems in physics don't have their answers at massive billion-dollar colliders only, but also in how spherical an electron can be.
Particle physicists around the world are facing a unique problem. They are hoping that a theoretical tool they work with, called the Standard Model, is somehow wrong. This is because, while the Model has helped them solve many problems with astounding precision, it doesn’t seem to be able to explain many of the most pressing recent problems.
The Model doesn’t know what dark matter is. It is at a loss to say why the mass of the Higgs boson is what it is. It doesn’t know where to look to explain why the force of gravity is orders of magnitude weaker than the three other fundamental forces. It stays silent on why there is no natural antimatter in this universe. And so forth.
Using a torchlight with the Model for failing batteries, physicists have come a long way on their journey in the dark to explain why the universe is the way it is, but at a critical crossroads, the device is starting to flicker off. Fortunately, many physicists already have their eyes on one particular set of new batteries—these longer lasting—called Supersymmetry (SUSY). Unfortunately, these batteries have been hard to find.
Certificate of denial
The success of the Standard Model was established on the back of many experiments that attested to its accuracy. One important such result was announced in November 2012. Particle physicists working on the Large Hadron Collider (LHC) at CERN had found the rate of decay of a very rare particle, called the B_s meson, to be consistent with the Model’s predictions.
SUSY, however, has found such ‘certificates’ hard to come by. While it is theoretically ready to explain many problems where the Model has fallen short, it seems to have no experimental backing. The SUSY ‘batteries’ don’t seem practically reliable yet. In fact, most recent experimental results seem to deny SUSY’s feasibility.
On December 19, another such result was announced—and this one more curious than the rest because of what it tested. At the Advanced Cold Molecule Electron (ACME) experimental collaboration at Harvard University, scientists have found that the electron, that fundamental charged particle orbiting the nuclei of atoms, is almost perfectly spherical.
The shape that these physicists speak of is not exactly the shape of the electron itself. In quantum mechanics, all particles—fundamental or no—are surrounded by a swarm of other particles which are constantly popping into existence for fleeting instants of time and disappearing back into a vacuum. These are called virtual particles. They cloud around an electron in a specific shape determined by how some forces of the electron might push them out, and it is this shape that has been found to be very spherical.
“If you have a cloud of virtual stuff around a particle, but all the positively charged virtual stuff is pushed slightly off to one side, while all the negatively charged virtual stuff is pushed slightly to the other side, you end up with a dipole,” said Dr. Amar Vutha, a physicist and member of the ACME collaboration. His and his colleagues’ experiment has not found such a dipole, thus allowing for the surrounding swarm to be spherical, as if equidistant from the electron on all sides.
Their results were published on December 19 in Science Express.
The Standard Model allows for the electron to have a dipole, or an electric dipole moment (EDM), on paper—albeit a very small one. Some other theories do so, too, but experiments have not found it. And by not finding it, physicists haven’t as much disproved the Standard Model as have they left it unaffected, but they have dealt a blow to SUSY.
The ACME experiment measures electrons’ EDMs by looking for them to have extra wobbles in electric fields. “A good analogy is with a spinning top on a table. A top will precess, or wobble, as it spins because its shape allows for torques to be exerted on it by the Earth’s gravitational field,” explained Dr. Vutha. Similarly, a spinning electron will wobble about in an electric field if it has an uneven charge distribution—which in turn would have distorted the shape of the swarm of virtual charged particles.
“The extent to which we are sure it is zero is set by the uncertainty of the measurement,” Dr. Vutha added, “and our experiment happens to have the smallest uncertainty that has been realized so far.” In other words, the electron’s EDM has been found to be zero at the highest precision attained to date.
This was also made possible because the scientists had used for their observations molecules of thorium monoxide (ThO), a polar molecule. The outermost electrons in its atoms, the valence electrons, lent themselves to easier manipulation by lasers, exposing them to tremendously powerful electric fields due to the ions in the molecule.
“As you can imagine, the larger the electric field you can apply to an electron, the bigger the wobble, the easier it becomes to detect its EDM,” said Dr. Vutha.
Rules that won't bend
This is bad news for SUSY because it predicts a wobble. If SUSY were true, the virtual particles around the electron would have consisted of particles predicted by SUSY as well, and these particles would have coupled to the electrons in a way that would have violated two principles that the Standard Model assumes cannot be broken.
One principle is called the parity symmetry. If an electron had the anomalous EDM, it would be a dipole with distinct positive and negative charges. How would these charges be positioned around the electron? The moment you pick two arbitrary positions, you will have killed the particle’s parity symmetry: when its mirror-image will no longer behave like itself.
If someone saw a movie of you parting these charges but on rewind, they would think you pushed the negative charge to where the positive charge actually is and vice versa, because the electron is now spinning the opposite way. This is a violation of the second principle, time symmetry, where the laws of physics act the same forward and backward in time.
Thus, no wobble, no violation of parity and time symmetries, and so no SUSY.
The scientists are still at the crossroads, and as they try their SUSY batteries, the torchlight seems to work, but they can’t see the light. Perhaps they are looking the wrong way…