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The Mercury EDM Experiment

Welcome to the home page of the mercury EDM experiment at the University of Washington! On this page, you'll find a brief description of our work and why (we think) it's important, beginning in layman's terms before moving to a more technical explanation.

General Introduction

We are interested in finding examples of time-reversal symmetry violation in atomic or nuclear physics. Although it is quite obvious from our daily experience that time only moves in one direction, on the quantum level the laws of physics are highly symmetric: almost any process that can occur between fundamental particles and fields is just as likely to happen in reverse, as if you were watching a movie played backwards. Examples of physical reactions that do not obey this symmetry are extremely difficult to find (and have led to one Nobel Prize to date), and those which have been observed all involve the decay of unstable, exotic particles created by smashing protons or electrons together in powerful accelerators.

By contrast, we are looking for symmetry violation in more ordinary, "everyday" objects-namely, atoms of mercury. In essence, we are trying to demonstrate time-reversal symmetry violation by building an atomic clock that is sensitive to the direction of time. In an ordinary clock, a stable oscillator (like a pendulum, a quartz crystal resonator, or a cloud of ultracold cesium atoms) is set up at some known frequency, and the clock keeps time by simply counting the number of oscillations, or "ticks". If the direction of time were to be reversed somehow, these clocks would not be able to tell you anything about it, as they would continue counting up the accumulated ticks at exactly the same rate. Ultimately, this is due to the fact that there is nothing in the underlying physics of regular clocks (the physics of gravity in the case of a pendulum clock, or the physics of the electron cloud in the case of cesium atoms) that is sensitive to the direction of time. However, we have reasons to suspect that there is something in the physics of the 'strong force' (i.e., the force that governs the physics of atomic nuclei) which is time-reversal-sensitive, although it is predicted to generate extremely subtle effects in neutral atoms-a clock that 'ticks' at a very slightly different frequency (about 0.00000001% at most) in a time-reversed universe. Therefore, we need to build an atomic clock with the highest possible degree of precision to have any chance of observing such an effect. Finally, to observe time-resersal asymmetry we also need some way of actually running the clock as if it were moving backwards in time. We can accomplish this by the arrangement of electric and magnetic fields  inside the apparatus (shown as E and B in the above diagram). Because these fields transform differently under a mathematical time-reversal operation, changing the relative orientation of E and B is physically equivalent to running the clock backwards in time. By using two sets of atoms (one in each field configuration) we can simultaneously measure the clock frequencies of atoms travelling both directions in time, and if the frequency is different in one direction, we know the nuclear physics underneath it all breaks time-reversal symmetry.

With all that out of the way, you are ready to understand the experiment! A diagram of our setup is shown above, with four glass cells containing a tiny amount of 199Hg vapor arranged in a stack, all inside of a magnet coil with ultraviolet laser beams going through each one. Each of these four cells acts as an independent clock, where the laser beams are used to measure the clock frequency determined by the interaction of the atoms with E and B. The magnetic field B is the same in all four cells, so our experimental signal is the difference of the clock frequency in the two cells with oppositely-directed electric fields E (the two outer cells have zero electric field inside and are used as experimental controls). Because the time-reversal-sensitive piece of the clock frequency is determined by the atoms interacting with the electric field, we quantify our measurement of time-reversal symmetry violation in terms of the electric dipole moment, or EDM.

Scientific Introduction

In order for an elementary particle, atom or molecule to possess a permanent electric dipole moment (EDM), time-reversal symmetry must be violated, and the combined charge and parity symmetry operation (CP) must be violated as well. The currently accepted Standard Model of Particle Physics provides limited room for time-reversal or CP-violating effects, so it predicts unobservably small dipole moments. EDM experiments are thus an ideal probe for 'new' physics beyond the Standard Model, such as supersymmetry, according to which the EDMs of neutrons, atoms and molecules should lie within a detectable range. Experiments to search for an EDM began many decades ago, and are expected to continue with increasing precision well in the future. However, to date no permanent EDM of any fundamental particle or atomic system has ever been found. A further introduction to the current state of EDM physics and a list of present and planned experiments may be found on the website of the neutron EDM collaboration at PSI.



Principle of Measurement

A permenent EDM would alter the energy splitting of spin eigenstaes under time-reversal (T) or parity inversion (P)

Our research attempts to measure the permanent EDM of the 199Hg atom as a probe of nuclear physics. (Because the electronic ground state of Hg is a spin singlet filling the 6s subshell, the primary contribution to the atomic EDM is expected to come from the nucleus). The signature of an EDM can be found in the energy splitting between spin up and down states of the nucleus in parallel electric and magnetic fields. The EDM (if it exists) must lie along the axis of the nuclear spin magnetic moment, since this is the only vector available to characterize the system. The Hamiltonian and the relevant field configurations are shown in the diagram above. If the EDM is parallel to the spin, the energy splitting between the spin up and down eigenstates (defined by the holding magnetic field B) will be increased by applying an electric field E parallel to B. An ensemble of atoms polarized perpendicular to E and B will therefore precess faster if Eand B are parallel than if they are antiparallel. Because E is the only polar vector in the system, it transforms differently under a parity inversion (P) or time reversal (T) than the spin, EDM, and B (all of which are pseudovectors). The change in energy splitting due to d (which we would observe as a change in precession frequency) thus indicates T and P violation in the underlying physics. The CPT theorem dictates that the combined symmetry CPT must leave the system invariant, so T and P violation together imply CP violation as well.


Theoretical Motivation

In a general field theory, CP violation is caused by the presence of a complex phase between different fundamental fields. Thus CP violation in the Kaon system is accommodated within the Standard Model by allowing some elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix to be complex. For three generations of quarks exactly one complex phase is allowed, which turns out to have a value of order unity. The phase cannot be determined more accurately from measurements on the Kaon system because of the hadronic uncertainties.

Any extension of the Standard Model which introduces additional particles also allows for additional physically observable phases. The general classes of models that have been considered are supersymmetric models, left-right symmetric models, and multi-Higgs models. At present, the limits on the EDM from measurements in paramagnetic atoms, diamagnetic atoms and the neutron all place comparable constraints on the models, and these constraints are already becoming significant. For example, if supersymmetry were broken near the electroweak scale it would solve the gauge hierarchy problem, but then it would also very naturally generate EDMs close to currently observable sizes.

The role of CP symmetry in the theory of strong interactions is also not well understood. The QCD Lagrangian naturally contains a term which violates CP, but the limits on neutron and 199Hg EDM put an extremely small upper bound (of order 10-10) on the associated phase angle, which we would expect a priori to be on the order of 10-1. Several mechanisms have been postulated to explain the discrepancy between the expected and observed values. One of the most popular, Peccei-Quinn symmetry, predicts the existence of an additional pseudoscalar particle, the axion. Many searches for the axion have been conducted with negative results, and several more are ongoing, including the Axion Dark Matter Experiment (ADMX) experiment here at UW.

Some evidence for CP violation beyond the Standard Model comes from Cosmology. Astronomical observations indicate that our Universe is mostly made of matter and contains almost no anti-matter. In the Big Bang cosmology this asymmetry has to be generated dynamically during cooling of the universe, a process called baryogenesis. CP violation is one of the requirements for baryogenesis. One of the most attractive scenarios of baryogenesis involves the electroweak phase transition at the energy scale of 100 GeV. Because the interactions at this energy scale are well known, one can make relatively reliable estimates of the baryon asymmetry. These estimates indicate that if the only source of CP violation is in the CKM matrix, the baryon asymmetry is smaller than the observed value by many orders of magnitude. However, extensions of the Standard Model, such as supersymmetry or multi-Higgs theories, which involve additional sources of CP violation, can naturally produce the baryon asymmetry of the correct magnitude.