Why should we care about the existence of the electron electric dipole moment (EDM)? In a previous post, we established that the existence of an electron EDM would imply a violation of a symmetry in physics called CP-symmetry, or CP violation (CPV). But it isn’t clear why CPV has any importance beyond being something interesting for physicists.
In reality, the existence of CP violation would have the potential to explain the existence of matter in the universe today.
Let’s get into why.1
The Big Bang and the Creation of Baryon Asymmetry
There are three facts we know about the Universe that result in a seeming contradiction, from which CP violation can rescue us.
The first fact is the Big Bang. The Big Bang is the current consensus among cosmologists as to how the universe began. First proposed by the priest-astronomer Georges Lemaitre in 1927, according to the theory, 13.8 billion years ago, all space and matter was condensed into one small point of singularity, which rapidly expanded into the Universe we know today. In my naive understanding as an atomic physicist, it makes sense that because the Universe is expanding, a long time ago everything must have started from a single point. The Big Bang establishes that the current condition of the Universe is not what it always was. There must have been physical processes by which everything – stars, galaxies, planets, and us – formed.
The second fact is the symmetry of matter and antimatter. All the things around are ultimately made of fundamental particles of matter – protons, neutrons, and electrons. However, we know that there exist other fundamental particles, some of whose properties have the inverse value of those of matter. So we know that there can be antiprotons, antineutrons, and positrons. Unlike the proton which has positive charge, the antiproton has negative charge, and similarly for the positron, which has positive charge.2 Otherwise, the antiproton has the same mass as the proton, and ditto for the antineutron with the neutron and the positron with the electron. Physicists have performed many experiments comparing other properties of matter and antimatter (many are still ongoing) – and so far they have found no evidence that the physics behaves differently. As far as we know, the Universe could have been made entirely of antimatter, and we wouldn’t notice a difference. The symmetry between the properties of matter and antimatter is called CPT symmetry, and it is one of the most fundamental tenets of theoretical physics.
However, an inconvenient fact is while an antiproton can behave towards a positron as a proton behaves towards an electron, if an antiproton ever touches a proton, both will annihilate into pure energy. This is the most efficient conversion of matter into energy (following Einstein’s famous equation, ), and is the basis of the bomb in Dan Brown’s book (and movie) Angels and Demons. So, while the Universe could have been made of matter or antimatter, it can’t be made of both and still have something solid left in it.
The third and most glaring fact is that…we exist. The symmetry of matter and antimatter is such that the physical processes that resulted in the formation of atoms and eventually stars and galaxies should have produced equal amounts of matter and antimatter. However, the fact is that there is a large imbalance in the amount of matter over antimatter. It’s very difficult to create and control antimatter. It currently exists only in specialized facilities like CERN’s Antiproton Decelerator. There, antimatter is created by smashing a beam of protons into a metal target. Due to the symmetry of physical processes, equal (but small) amounts of matter and antimatter are produced. The small number of antimatter particles have to be carefully controlled using electric and/or magnetic fields to be guided into the experiments which examine them. Any contact with matter – including the metal lining of the beam line – will result in its instant annihilation. Outside of this very specialized environment, there is almost no antimatter lurking around us.
The question is, why didn’t all of this annihilation also happen right after the Big Bang? Whatever physical processes happened, equal amounts of matter and antimatter should have been produced. Since there were no special electrodes built by humans to carefully keep the antimatter away from matter, the two would have eventually come into contact into each other and annihilated. The result would be an empty universe, filled only with photons – no stars or galaxies or even atoms. And certainly not us humans. But the fact is that we do exist. This is a fact called baryon asymmetry: that there is only one kind of matter predominant in the Universe (baryons instead of antibaryons – protons and neutrons are categorized as baryons). The issue of how to explain how baryon asymmetry came about is known as the issue of explaining baryogenesis.
Thus from our three facts, we have arrived at a large unsolved problem in physics: that our current understanding of the Universe cannot explain the imbalance between matter and antimatter.
There have been several efforts to get out of this conundrum. First, one could propose that in the beginning, there was simply more matter than antimatter. A cold, hard fact without explanation. Certainly less satisfying. But even this is not enough. An initial excess of matter over antimatter would be quickly “washed-out” in the early moments after the Big Bang, when everything was close together and interacted with each other intensely.
It could also be that there are regions of space where large amounts of antimatter are currently hiding, and that really the baryon asymmetry isn’t that large. However, we have not found any astronomical evidence for this.
The more straightforward explanation is that there is some sort of hidden asymmetry in physics that is not covered by our current accepted theories. In this case, the accepted theory is the so-called Standard Model (SM) of particle physics. In a nutshell, the Standard Model is a theoretical framework explaining three of the four forces in nature: electromagnetism, strong and weak. It does not explain gravity. However, other than that it has been found to be experimentally very robust. The discovery of the Higgs boson in 2012 was its latest great triumph – a particle predicted 50 years ago was finally found to actually exist.
So what makes the SM unable to explain baryon asymmetry? To answer this we must turn to the Sakharov conditions, proposed by Soviet physicist Andrei Sakharov in 1967. Sakharov outlined the ingredients that must happen for the current baryon asymmetry to have came about:
- Baryon number violation
- C and CP violation
- Interactions outside of thermal equilibrium
The first condition, baryon number violation, simply means that there must be some physical process which results in violation of baryon number, or an imbalance in the number of matter particles produced versus antimatter particles. This is the starting point of our problem. In the SM, this is fulfilled, but only during the early epochs of the Universe after the Big Bang, when temperatures were hot enough.
The second condition, C and CP violation, describes forms of symmetry violation that must occur (a topic we covered in the last post, for those who are curious). The gist of why this condition is needed is because without such violations, any baryon-number-violating process which produced, say X amounts of matter over antimatter (condition 1), would be countered by a process which produced X amounts of antimatter over matter. So this condition ensures that the asymmetry in matter-producing processes is conserved. Now, the SM does in fact allow some CP violation (CPV) to occur – CPV was first discovered in the decay of kaons in 1964 by Cronin and Fitch, for which they won a Nobel Prize. However, not many more instances of CPV have been discovered since. Overall, the CPV in the Standard Model is not enough to explain the severity of the baryon asymmetry problem today.
The third condition, thermal inequilibrium, comes out of the fact that in a thermal equilibrium condition, nothing changes anymore – everything has settled into its final state. For change to occur, a thermal inequilibrium must occur before the system settles back into equilibrium. If we believe that the initial conditions of the Big Bang had equal amounts of matter and antimatter (baryon number of zero) and the current conditions of the Universe has more matter than antimatter (baryon number not equal to zero), at some point the system had to undergo change. Thus at some point it was in thermal inequilibrium. This, too, is fulfilled in the SM.
In conclusion, the SM technically fulfills all three of the Sakharov conditions to some extent. However, the second condition is not fulfilled sufficiently and so it cannot explain the current baryon asymmetry.
From CP Violation to the Electron EDM
As the SM was found to be wanting in fulfilling the Sakharov conditions, theoretical physicists turned to other models of physics which go beyond the Standard Model (BSM). Unlike the SM, these models would contain larger amounts of CP violation which would be sufficient to completely fulfill the Sakharov conditions and thus explain baryon asymmetry. There are many such models, and I have only limited understanding of them as I am not a theorist. In any case it would be too much space to try to explain them here. However, it’s an important point that when you introduce CP violation into a theory, it must have consequences on a number of different things. This is because physical theories are usually interconnected in intricate ways. If you propose some sort of hypothetical new, yet-undiscovered particle which would result in sufficient CPV for our purposes, such a particle might interact with other particles that we already know, resulting in measurable consequences. These consequences are what experimental physicists like the ACME collaboration focus on – we design, build and carry out experiments which investigate whether the theory is correct. Now, because the electron EDM (if it is not zero) would violate CP, many of these CP-violating theories predict that the EDM does have a non-zero value.3
In other words, an asymmetry responsible for the creation and preservation of matter over antimatter in the entire universe might express itself as a tiny bump in the shape of the electron! By running our humble molecular beam experiment, producing molecules and manipulating them with lasers, we are really peering into what might have happened in the earliest epochs of the universe. The contrast between the extremely minute nature of the asymmetry we’re looking for and its potentially cosmic consequences is what really draws me towards this experiment.
Probing LHC-scale Physics in a Basement
Just as amazing is the fact that due to the interconnected nature of these physical theories, when we look for the electron EDM, the results of our investigations might have large implications for beyond-SM theories in general. For example, you might have heard of supersymmetry, the notion that each particle has a supersymmetric partner – with weird names like the photino (partner of the photon) and squark (partner of a quark). Supersymmetry is necessary for string theory (a hypothetical ultimate Theory of Everything which would unify all four forces in the universe), and many theories use it to explain other problems with the Standard Model. Looking for the existence of supersymmetric partner particles is a huge reason for why giant particle physics experiments like the Large Hadron Collider were built. But many models of supersymmetry also predict a significant amount of CP violation which would show up as a non-zero electron EDM. (This would be an added bonus to the theory, as it would thus be able to explain baryon asymmetry in addition to rectifying other problems in the SM which we have not covered in this post.) Thus by measuring the electron EDM, one is probing the same kind of physics as the LHC.
The question is, are we doing the same kind of measurements as well as the LHC? The LHC is designed to collide particles with energies up to 13 TeV. Whether our ACME EDM experiment is able to look for physics at a similar energy scale depends on its precision. The first generation ACME experiment (published in 2014) had an uncertainty of . There are different possibilities of how the electron EDM can interact with new physics in general, so the conversion to energy units is not straightforward. However, under reasonable assumptions4 the ACME I experiment probed energies of 1 TeV, or one-tenth of the scale of the LHC.
Considering that the ACME I experiment had about a thousandth of the funds of the LHC (a few million dollars instead of a few billion), about 5-10 scientists working on it at any one time instead of several thousand, and the experiment can be contained in a single room, this is a magnificent achievement indeed! In fact, the current generation of the experiment, ACME II, has a projected precision about 10 times better than ACME I – thus we would be probing energies of 10 TeV, or at the same scale as the LHC. The implications are drastic: if we don’t find an electron EDM, then it becomes less likely that experiments like the LHC will find anything, because physics tends to show up at similar energy scales.5
Conclusion: the Power of EDM Measurements
Overall, we’ve seen how electron EDM measurements such as the one performed by the ACME experiment can have monumental impact in our quest to find an explanation for why there is much matter in the universe as opposed to antimatter. We’ve also discussed the efficiency of the experiment – how it is able to look for new, beyond-Standard-Model physics with a precision similar to large collider experiments like the LHC. These reasons point to the amazing reach of EDM measurements. This is true of the electron EDM that the ACME experiment is investigating, and also other experiments around the world which are looking for other EDMs – for example in mercury.
- This blog post was possible through the great explanations of the connection between the ACME experiment and baryon asymmetry in various dissertations by past ACME graduate students. I am also indebted to explanation of the Sakharov conditions and their relation to the Standard Model in this paper by Canetti et al. ↩
- The neutron is neutral, so the antineutron has the same charge, but the antineutron is still different from the neutron in that the quarks making it have the opposite composition – called a difference in baryon number. ↩
- The SM also predicts that the electron EDM has a non-zero value, but it is many orders of magnitude smaller than our current experimental sensitivity. ↩
- In this case, crudely estimating a new physical process which produces an EDM through a particle interacting in a 1-loop Feynman diagram. ↩
- Or so we think. This is the principle of naturalness, which underpins all of theoretical physics. ↩