We perform precise measurements in atomic/molecular/optical systems to search for new particles and forces.  These systems are especially sensitive to the types of new particles and forces that are required to explain the imbalance between matter and anti-matter in the universe.  This allows us to study high energy physics in a low energy, table-top setting.

Click to see our poster for the Caltech graduate program 2017 open house:
Open house poster, 2017

Motivation: Approximately 100% of the Universe is a Complete Mystery

Consider the composition of the universe, as revealed most recently by the Planck Mission:
The composition of the universe
Figure: The composition of the universe
  • About 68% of the universe is Dark Energy.  Nobody has any idea what it is.
  • About 27% of the universe is Dark Matter.  Lots of people are looking for it, and we have some clues about what it is, but we still don't know what it is.
  • The remaining 5% of the universe is everything that we know - galaxies, planets, semiconductors, biochemistry, etc.
Before taking solace in the fact that we (at least partially) understand 5% of the universe, we should point out that nobody knows how the known 5% of the universe was created.

In particular, that 5% of the known universe is made of matter, not anti-matter.  This is a problem because no known physical processes could have created such a huge abundance of matter yet essentially no anti-matter.  This matter/anti-matter asymmetry, also called the Baryon Asymmetry of the Universe (BAU), is one of the biggest mysteries facing physics today.

If no known particle or force lead to the creation of the matter asymmetry, then the prime suspects are new particles and forces.  One way we can search for these particles is to try to directly produce them in an accelerator.  However, that is not the only way.

Atoms and Molecules

Our approach is to look for the influence of these new particles and forces on the behavior of atomic/molecular/optical (AMO) systems, in particular the interaction of molecules with electromagnetic fields.  You may think that the effect of "high energy particle physics" on laboratory, table-top spectroscopy is vanishingly small... and you would be right.  However, the technology to make precision frequency measurements is extremely advanced, and atoms/molecules are ideally suited for this purpose, for a few reasons:
  1. AMO systems can be coherently controlled.  This means that if we want to measure some interaction, we can let it build up for long periods of time before measuring.  The Higgs boson decays in ~10-22 seconds, but AMO systems can have uninterrupted, coherent interactions for seconds... and more.
  2. Every atom or molecule of the same species is exactly, perfectly, quantum-mechanically identical.  This means that we can acquire tremendous amounts of statistics and perform very precise measurements.
  3. Atoms and molecules are intrinsically sensitive to a lot of "high energy" physics - after all, they contain electrons, nucleons, quarks, pions, QED/QCD/weak interactions, relativistic effects, and so on.  If we look close enough, we really can see these things, and possibly even more.
On the other hand, the structure of atoms and molecules is extremely complicated, and it can be hard to distinguish these tiny interactions from all of the other physical effects that give rise to spectral features.  An extremely powerful trick that we can play is to look for spectral features that should be exactly zero because they violate some symmetry, because the physics we want violates symmetry as well.

Searching for Fundamental Symmetry Violations

In the 1960's, Andrei Sakharov made a critical observation that connects the BAU to atoms and molecules.  In order to explain the BAU, not only must there be processes that favor matter over anti-matter, but there must also be unknown processes that violate CP (charge-parity) symmetry, the simultaneous inversion of spatial coordinates and exchange of positive/negative charge.  The connection to atoms and molecules is that any object with a permanent electric dipole moment (EDM) violates the same symmetry.  In fact, the CP-violating physics that is required to explain the BAU generically results in permanent EDMs of particles. Therefore, we can search for the signature of permanent electric dipole moments from electrons or nucleons in atoms as a probe of the physics that caused the BAU.

New physics leads to new electromagnetic moments
Figure: New physics leads to symmetry-violating electromagnetic moments, and vice-versa

We can get an intuitive understanding of the connection between EDMs and symmetry violation from the figure below.  Say that a particle has a quantum mechanical spin s, which we can picture as the particle spinning around an axis, and a permanent electric dipole moment d, which can we can picture as a charge separation along some axis.  Because the spin is the only internal vector that describes the particle, the dipole moment must be parallel to it: \parallel  d.  Now, consider looking at a mirror image of that particle.  In the mirror particle, the dipole moment d will flip but the spin s will not (see figure), resulting in an opposite orientation between s and d.  This results in a different particle that could be distinguished in the lab with suitable application of electromagnetic fields, i.e. a different particle.  Since we know that there is only one type of electron/neutron/proton/etc., the existence of a permanent EDM in a particle would mean that the universe is not symmetric under mirror reflection.  Of course we care about CP-violation not P-violation, and it turns out that that weak force is known to violate P (a lot), but we can make similar arguments to show that permanent EDMs violate not only P but CP and T (time-reversal) symmetries as well.

A particle confronting its mirror image
Figure: A particle with a permanent EDM would violate P symmetry

In order to measure an EDM d, we can place the particle of interest in an electric field E and look for energy shifts of the form U = ­­– d·E.  We can see that larger electric fields will lead to larger effects, and it turns out that the electric field inside polar molecules can be extremely large - as much as 1,000,000 times larger than the largest electric fields that can be reasonably created in the laboratory.  The spectral signature of an EDM is an energy shift whose sign depends on the relative orientation of the particle's spin (and therefore dipole moment) and the molecule orientation (and therefore internal electric field), as shown in the figure below.  Since permanent EDMs are disallowed by symmetry in normal quantum mechanics and electromagnetism, energy shifts that mimic an EDM are heavily suppressed, but not entirely eliminated, because combinations of various effects can look like an EDM.  We must therefore be extremely careful to manage our experimental systematics.

Measuring an EDM in a molecule
Figure: Using the electric field inside a molecule to look for an electron EDM.  These two configurations would have equal but opposite energy shifts from d·E.

Permanent EDMs are the "classic" CP-violating electromagnetic moments, but there are others: nuclear Schiff moments, nuclear magnetic quadrupole moments, and more.  Each of these moments is sensitive to different sectors of new physics, and atoms and molecules have enhanced sensitivity to all of them.

Controlling Atoms and Molecules

Molecules have a number of advantages over atoms for precision measurements, one of which is that they can be polarized, leading to very large internal electric fields.  However, they suffer from a number of serious technical disadvantages, stemming mainly from the fact that they have many internal states not present in atoms, in particular rotation and vibration.  First, these levels can be easily thermally populated (especially rotational levels), so the usual method for creating gas phase species - vaporizing in an oven - would spread the population out over hundreds or thousands of internal states, all with different properties.  These same internal states make molecules difficult to laser-cool, a standard technique for cooling atoms; it is somewhat easy to accidentally excite rotational and vibrational modes in a molecule, which would again lead to population spreading out over many levels.  These problems are compounded by the fact that many molecules with enhanced sensitivity to new physics are heavy, refractory, and chemically unstable.

We overcome these challenges by using cryogenic buffer gas cooling to cool our molecules down to the few Kelvin temperature regime, where population is concentrated in a few levels.  This technique works by introducing a hot sample of the molecules, for example created by laser ablation, into a cryogenic cell containing an inert (buffer) gas.  The hot molecules thermalize with the buffer gas and reach an equilibrium temperature of a few Kelvin.  If we poke a hole in the cell (see figure below), we can form a beam of these molecules in a vacuum chamber and create a well-controlled environment for their study.  This method is conceptually simple but extremely powerful, and can be used to cool just about anything, from atoms to complex polyatomic molecules with carbon rings, etc.

Cryogenic buffer gas beam (CBGB)

Figure: A cryogenic buffer gas beam.  A continuous flow of inert gas into a cryogenic copper cell maintains a density of cold buffer gas (filled blue circles.)  Top: a high-energy, pulsed, green laser hits a solid target to create hot, gas phase molecules (open red circles).  Bottom: the hot molecules are cooled by the buffer gas, and the cold molecules (open blue circles) exit the cell through a hole into a vacuum chamber.

These cryogenic buffer gas beams (CBGBs) can be used directly for precision measurement, or as a starting point for further cooling and trapping.

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