Measuring the roundness of the electron

We are making the most sensitive measurement ever of the shape of the electron. We are looking for a deviation from spherical would us 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. So, to measure the shape with sufficient accuracy we have to build a machine that can look at the electrons in molecules with unprecedented accuracy. 

The EDM experiment

Why measure it?

One of the biggest mysteries in physics today is the relationship between matter and antimatter.

The Standard Model predicts that antimatter should obey the same rules as their normal matter partner. And so far, every experiment we have performed confirms this- there is no difference between particles and anti-particles. However, the Standard Model also predicts that every antimatter particle is created with a normal partner, which means that we should observe an equal amount of matter and antimatter. Almost everything in the universe is made from matter: your computer, the Earth, and as far as we can tell, all of the stars and planets. Antimatter is only found in tiny quantities, in cosmic rays and radioactive decays. 

Now, here is the big mystery! What has happened to all the antimatter?

This experiment is trying to help answer this question by looking at the shape of the electron. Measuring the electron edm is a clean, sensitive probe for new physics. The Standard Model predicts almost vanishingly small electron EDMs, while many extensions predict measurably large values. This means any non-zero measurement of the electron EDM would be clear evidence for physics beyond the Standard Model. 

EDMsensitivity

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 solar system to better than the width of one human hair. And so far, we've seen no evidence of non-roundness. What we're planning on doing in the next few years is using some of the latest developments in atomic and molecular physics to make our experiment 1000 times more precise. With this increased precision we think we might be able to see a tiny deviation from perfect roundness and hopefully explain the mystery of the antimatter.

 

How do we measure it?

We perform our measurement with YbF (ytterbium fluoride). 

A heavy-metal-containing molecule can act as an amplifier for EDM, by enhancing the interaction with the electric field. If you choose your molecule carefully you can reach EDM enhancement factors up to 106, more than four orders of magntiude higher than the interaction with a bare electron and an electric field. Critically, molecules are also insensitive to some of the systematic effects that mimic the electron EDM that plagued previous experiments.

Even with this enormous enhancement, though, the interaction is still tiny, at most a few mHz (that's femto-wavenumber, or atto-electronVolts if you prefer!)

Fringes

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. 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. On the left is an example of interference from two molecular energy levels that have different energies in a tiny magnetic field.

We need to average a lot of data to reach the statistical sensitivity that we desire. An EDM measurement is around 3 months of continuous running; day and night, week-days and week-ends. Fortunately, the experiment runs autonomously, allowing us to take the required amount of data without killing the research team.

The amount of data generated, while small by high-energy physics standards, is enormous on the scale of a typical atomic physics experiment. [In fact, I'd be willing to wager that we have a higher data rate per person (Gb/day/person) than anyone else!- Jony]. We have developed a large library of analysis software to analyse this rich data set and look for unwanted trends and correlations.

This analysis sofware complements a number of models, both analytical and numerical that predict in detail what we'd expect to see in our data set. The agreement between the analysis and predictions give us the confidence that the machine is working as it should. This confidence is strong enough that we run the experiment completely blind to the EDM value.

Improving measurement sensitivity?

fountain

We are now trying to measure the electron's EDM with more sensitivity than ever before. Since our last measurement in 2011 we have developed several new optical pumping techniques that have dramatically increased the number of YbF molecules taking part in the experiment. This should allow us to measure zero better than any other experiment int he world.

As well as adding more molecules, we can also slow them down. This allows the EDM to interact for longer with the electric field, making us more sensitive to the superposition of the quantum states we saw earlier. We have developed a new molecular source where the YbF travels four times slower than before!

In the future, we are also planning to slow them down evern further with laser cooling techniques- similar to other experiments in our group with CaF. We can then perform the EDM measurement for several seconds instead of 1/1000th of a second. 

Other information?

Meet the team

The YbF electron EDM experiment is run by some very hard working PhD students, post docs and PIs. You can meet the team here.

Publications

A list of our current publications and theses are available here.

An Artist's perspective

In June 2012, the artist Geraldine Cox visited the experiment. You can see her work here: Artist's perspective

Other EDM experiments

  • The ACME collaboration (Harvard/Yale) are also trying to measure the electron EDM with cold molecules. You can see their work here.
  • Eric Cornell, based at JILA, is using trapped molecular ions to measure the electron EDM. You can see their work here.