Wednesday, March 11, 2015

Table-top particle physics

We had a great colloquium here today by Dave DeMille from Yale University.   He spoke about his group's collaborative measurements (working with John Doyle and Gerry Gabrielse at Harvard) trying to measure the electric dipole moment of the electron.  When we teach students, we explain that as far as we have been able to determine, an electron is a truly pointlike particle (infinitesimal in size) with charge -e and spin 1/2.  That is, it has no internal structure (though somehow it contains intrinsic angular momentum, but that is a story for another day), and that means that attempts to probe the charge distribution of the electron (e.g., scattering measurements) indicate that its charge is distributed in a spherically symmetric way.

We know, though, that from the standpoint of quantum field theory like quantum electrodynamics that we should actually think of the electron as being surrounded by a cloud of "virtual" particles of various sorts.   In Feynman-like language, when an electron goes from here to there, we need to consider not just the direct path, but also the quantum amplitudes for paths with intermediate states (that could be classically forbidden), like spitting out and reabsorbing a photon between here and there.   Those paths give rise to important, measurable consequences, like the Lamb shift, so we know that they're real.  Where things get very interesting is when you wonder about more complicated corrections involving particles that break time reversal symmetry (like B and K mesons).  If you throw in what we know from the Standard Model of particle physics, those corrections lead to the conclusion that there actually should be a non-zero electric dipole moment of the electron.  That is, along its axis of "spin", there should be a slight deficit of negative charge at the north pole and excess of negative charge at the south pole, corresponding to a shift of the charge of the electron by about \(10^{-40}) cm.  That is far too small to measure.

However, suppose that there are more funky particles out there (e.g., dark matter candidates like the supersymmetric particles that many people predict should be seen at the LHC or larger colliders).  If those particles have masses on the TeV scale (that'd be convenient), there is then an expectation that there should be a detectable electric dipole moment.  DeMille and collaborators have used extremely clever atomic physics techniques involving optical measurements on beams of ThO molecules in magnetic and electric fields to look, and they've pushed the bound on any such moment (pdf) to levels that already eliminate many candidate theories.

Two comments.  First, this talk confirmed for me once again that you really have to have a special kind of personality to do truly precision measurements.  The laundry list of systematic error sources that they considered is amazing, as are the control experiments.  Second, I love this kind of thing, using "table-top" experiments (for certain definitions of "table") to get at particle physics questions.   Note that the entire cost of the whole experiment over several years so far as been around $2M.  That's not even a rounding error on the LHC budget.  Sustained investing at a decent level in this kind of work may have enormous bang-for-the-buck compared with building ever-larger colliders.


Anonymous said...

It's Jerry Gabrielse!

Douglas Natelson said...

Fixed. Sorry about that. Not sure where my brain pulled "George" from.