The shape of the electron. This is one of the most sensitive probes of the difference between matter and antimatter.We are looking for a deviation from spherical that would indicate a difference in the laws of physics that govern matter and antimatter. This would show that our current theory of particle physics, the Standard Model is incomplete. We know that any deviation from roundness is going to be tiny.
Detecting an electric dipole moment (EDM) would mean electrons, which buzz about atomic nuclei, have internal structure. So far we've checked the roundness of the electron to an incredible degree of precision: the equivalent would be measuring the diameter of the earth to better than the width of one human hair. And so far, we've seen no evidence of non-roundness. Measuring the inner shape of the famous particle could help solve a cosmic mystery, where's all the antimatter? By carefully measuring the shape of the electron, through a particular property known as the electric dipole moment, which would mean that the electron has some kind of internal structure, a bizarre concept for a particle that is supposed to buzz around the nuclear hearts of atoms and molecules with its mass concentrated into an essentially sizeless point. Although no one has yet measured the electron's electric dipole moment, researchers think it should exist and could be within reach of today's modern laboratory setups.
You can learn about the techniques we use, our latest results, what we're planning, and who we are. Trapped polar molecules may be used to measure the electric dipole moment of the electron, a search for physics beyond the standard model. By trapping chiral molecules, it should be possible to study the role of the weak interaction in the emergence of homochirality in biological molecules. Once we have trapped the molecules we hope to cool them further, bringing them into the ultracold regime. Ultracold polar molecules offer a new dimension in cold atom physics, primarily due to their long-range dipole-dipole interactions. By varying the switching timing, the length of the lenses can be varied, which affects the number of stable trajectories along the guide. The electrode geometry was chosen to provide a strongly harmonic field variation; transverse confinement in the high-field Stark limit can be treated as that of a harmonic potential.
"There are good theoretical reasons to think that it isn't too far away," says physicist Larry Hunter of Amherst College in Massachusetts, who has been hunting the electron's electric dipole moment since the 1980s. "What has made us all dedicate our lives to it is the real good chance that something might emerge soon." Within the next few months, scientists at Imperial College London are expected to report the latest limit on the size of the electron electric dipole moment, the first such improvement in a decade.Physicists suspect that electric dipole moments exist because they allow particles to violate what's known as time-reversal symmetry. Although symmetry sounds like a good thing, scientists know that processes involving other particles (such as B mesons) behave differently whether running forward or backward, a violation of time-reversal symmetry. In order for this to happen, the electron (and other fundamental particles) must have an internal structure, something an electric dipole moment can reveal.
Imagine that the electron has "a cloud of stuff following it and blow that electron cloud up to the size of Earth, and extra positive charge would appear as a tiny dent on the north pole while extra negative charge would be a tiny bulge on the south pole. Given current limits, the size of that dent or bulge would correspond to adding or subtracting no more than about one-thousandth the width of a human hair from either end of the planet.
Reversing time (by switching the direction of a particle's spin) changes the direction of the magnetic dipole (blue) but not the electric dipole (green). This means that possessing an electric dipole moment would give particles a way to violate time-reversal symmetry. Whether a property known as the particle's spin responds differently when the field is switched on in different orientations, which would mean the electron possesses an electric dipole moment. Seeing that difference is the hard part.
Our most recent published measurement of the electron edm is (-0.2 ± 3.2) 10-26 e.cm. [You can find the paper on our publications page]. The current limit has already ruled out the simplest version of a popular idea known as supersymmetry, which tries to explain the cosmic matter/antimatter imbalance by suggesting that every particle has an as-yet-unseen "superpartner." If researchers can push the limit to 10–29, that would rule out another extension to the standard model that tries to solve the matter problem by postulating multiple kinds of the particle known as the Higgs boson, which Europe's Large Hadron Collider was designed to detect. In 2002, Commins' team published the most stringent limit yet: 1.6 × 10–27. The standard model predicts that the electron's electric dipole moment is less than 10–38 in units of electron charge times centimeters.
Any non-zero measurement of the electron edm would be clear evidence for physics beyond the Standard Model. Low-energy particle edm measurements can be viewed as complementary to high-energy accelerator experiments.
The minimal supersymmetric standard model, or MSSM (red), is a standard model extension that holds that every elementary particle has a “superpartner.” One of the simplest versions has been ruled out by the current limit. Another version (blue) of MSSM that sets a parameter dubbed phi to a different value is still a possibility, but it too may be ruled out when researchers lower the bar further. The multi-Higgs model and left-right symmetric models are also ruled out if the new limit is achieved.
What's more, the electric dipole interaction has a property that makes it particularly interesting: it violates time reversal symmetry. That's because the dipole must lie either parallel or anti-parallel to the spin, but the choice of one over the other violates time reversal symmetry. Direct observation of microscopic time-reversal asymmetry would be a profoundly interesting result.
Measurements of particle interactions at the smallest scales seem to indicate that matter and anitmatter are treated in a rather symmetric way. Andrei Sakharov showed that CP-violation is needed to explain the matter dominance of the universe. The degree of CP-violation known in particle physics falls well short of that which would be needed. So, one might reasonably expect that there must be extra CP-violating mechanisms at work - looking for them seems like a good idea. Our measurement of the electron edm could shed light on these mechanisms.
To measure this tiny interaction we use a neat trick. We choose a pair of energy levels that are shifted oppositely by the edm interaction. This gives us a powerful "common mode rejection" of many deleterious effects. We measure the energy difference between our two chosen levels directly using quantum interference. We prepare a coherent superposition of the two energy levels of interest and measure the phase evolution of this superposition interferometrically. We call this technique spin interferometry, and it is described in detail in many of our publications.
An example of interference from two molecular energy levels that have different energies in a tiny magnetic field.
See the cutting edge of research in molecule cooling, manipulation and trapping to build a foundation for the next generation of edm experiment.
From article comments: If the electron has no internal structure, and consists of a real point charge, its energy has to be infinite! And it is impossible to explain its 1/2 spin. (Just claiming that the spin is "intrinsic" is no explanation...)
There already exists a model of the electron that explains its charge and spin, based just on the particles structure/topology and else classical physics (google "neoclassical atom").
Watching An Atom's Electrons Move in Real Time
Physicists have used ultrashort flashes of laser light to directly observe the movement of an atom’s outer electrons for the first time - a process called attosecond absorption spectroscopy.
Researchers were able to time the oscillations between simultaneously produced quantum states of valence electrons with great precision. These oscillations drive electron motion. “This revealed details of a type of electronic motion – coherent superposition – that can control properties in many systems,” says Stephen Leone.
A classical diagram of a krypton atom (background) shows its 36 electrons arranged in shells. Researchers have measured oscillations of quantum states (foreground) in the outer orbitals of an ionized krypton atom, oscillations that drive electron motion.
The crucial role of coherent dynamics in photosynthesis as an example of its importance, (the Graham Fleming group) noting that “the method developed by our team for exploring coherent dynamics has never before been available to researchers. It’s truly general and can be applied to attosecond electronic dynamics problems in the physics and chemistry of liquids, solids, biological systems, everything.” - Leone. Theoretically, however, with longer ionization pulses the production of the ions gets out of phase with the period of the electron wave-packet motion. After just a few cycles of the pump pulse, the coherence is washed out. Thus, says Leone, “Without very short, attosecond-scale probe pulses, we could not have measured the degree of coherence that resulted from ionization.”
Electrons can also be used to create an hologram of the atom.
Scientists have created holograms of atoms using laser-driven electron motion, which could lead to a new type of ultra-fast photoelectron spectroscopy. In the future, this type of holography could enable scientists to study the structures of molecules in a more direct way than before. Ymkje Huismans from the FOM-Institute AMOLF in Amsterdam, The Netherlands, and an international research team have published their study in a recent issue of Science Express. They have experimentally demonstrated is that it is possible to make holograms by taking an electron out of a molecule and, using a laser field, redirect the electron toward the molecule.
The scientists beamed an intense infrared laser light at an atom or molecule, which resulted in the atom or molecule becoming ionized and releasing an electron. The laser field causes the liberated electron to oscillate away from and toward the ion. Sometimes, an electron and ion collide, releasing a very short burst of radiation.
Because the electron motion is fully coherent, meaning it always has the same phase, the scientists realized that they could apply holographic techniques to record information about the ion and electron. The key to holographic electron imaging is to observe the interference between a reference wave (which is emitted by the electron and doesn’t interact with the ion) and a signal wave (which scatters off the ion and encodes its structure). When the reference wave and signal wave interfere on a detector, the encoded information about the electron and ion is stored and can be viewed in the future. As the scientists explained, the result is a hologram of an atom produced by its own electrons.The researchers also developed theoretical models to simulate their measurements, confirming that the hologram had stored spatial and temporal information about the electrons and ions. By using the holographic structures to develop a new kind of ultra-fast photoelectron spectroscopy, researchers could be able to directly measure electron and ion movements on the attosecond timescale.
Do electrons hold the key to matter/antimatter asymmetry?Sizing up the electron. Science News, Feb. 12, 2011 Sizing up the electron
Eleftherios Goulielmakis, Zhi-Heng Loh, Adrian Wirth, Robin Santra, Nina Rohringer, Vladislav Yakovlev, Sergey Zherebtsov, Thomas Pfeifer, Abdallah Azzeer, Matthias Kling, Stephen Leone, and Ferenc Krausz, “Real-time observation of valence electron motion,” Nature, 466, 739-743 (5 August 2010). Abstract.
Y. Huismans, et al. “Time-Resolved Holography with Photoelectrons.” Science Express. 16 December 2010. DOI:10.1126/science.1198450
Electron EDM. Centre for Cold Matter, Imperial College, London.
The Fleming Group 2010:
- Spectroscopic elucidation of uncoupled transition energies in the major photosynthetic light-harvesting complex, LHCII, G.S. Schlau-Cohen, T.R. Calhoun, N.S. Ginsberg, M. Ballottari, R. Bassi, G.R. Fleming, PNAS 107 (30), 13276-13281, JUL 2010.
- Quantum coherence and its interplay with protein dynamics in photosynthetic electronic energy transfer, A. Ishizaki, T.R. Calhoun, G.S. Schlau-Cohen, G.R. Fleming, PHYSICAL CHEMISTRY CHEMICAL PHYSICS 12 (27), 7319-7337, 2010.
- Quantum entanglement in photosynthetic light-harvesting complexes, M. Sarovar, A. Ishizaki, G.R. Fleming, K.B. Whaley, NATURE PHYSICS 6 (6), 462-467, JUN 2010.
- Branching Relaxation Pathways from the Hot S2 State of 8′-apo-β-caroten-8′-al, Y. Pang, G.R. Fleming, PHYSICAL CHEMISTRY CHEMICAL PHYSICS 12 (25), 6782-6788, 2010.
- Quantum superpositions in photosynthetic light harvesting: delocalization and entanglement, A. Ishizaki, G.R. Fleming, NEW JOURNAL OF PHYSICS 12 (5), 055004, May 2010.
- Ultrafast Spectroscopy of Midinfrared Internal Exciton Transitions in Separated Single-Walled Carbon Nanotubes, J. G. Wang, M. W. Graham,Y.-Z. Ma, G. R. Fleming, R. A. Kaindl, PHYSICAL REVIEW LETTERS 104 (17), APR 30 2010.
- Exciton annihilation and dephasing dynamics in semiconducting single-walled carbon nanotubes, M. W. Graham, Y.-Z. Ma, G. R. Fleming, A.A. Green, M. C. Hersam, Ultrafast Phenomena in Semiconductors and Nanostructure materials, Proceedings of SPIE, 7600-7613, 2010.
- Unusual Relaxation Pathway from the Two-Photon Excited First Singlet State of Carotenoids, Y. Pang, G. A. Jones, M. A. Prantil, G. R. Fleming, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 132 (7), 2264-2273, FEB 24 2010.