How Tabletop Experiments Could be the Future of Particle Physics

Just this week an article in Nature was published featuring the new activities that the Gabrielse lab (where I work) is starting up at the newly-found Center for Fundamental Physics at Northwestern. In particular, the article highlights the growing importance of relatively small-scale “tabletop” experiments like ACME to probe for new physics beyond the Standard Model. For the past few decades, large particle physics accelerator experiments like the Large Hadron Collider have been our main guide in investigating particle physics at a fundamental level. The principle is simple: you smash things together at a certain energy, and everything that can happen, will happen. The higher the energy, the more variety the things that can happen. So we’ve been steadily building more and more energetic accelerators, with the LHC currently operating at the ~10 TeV scale and costing several billion dollars to build. The fruits of these experiments have been clear: we have the Standard Model of particle physics, a theoretical framework that can explain 3 out of the 4 forces in nature.

But as my adviser Jerry Gabrielse highlights in the interview, the Standard Model doesn’t explain a few important things. In particular it doesn’t explain why matter exists right now at all – why didn’t matter and antimatter get produced equally during the Big Bang and just annihilate with each other? If that had happened then we would have no stars, planets, or humans, but instead just a bath of pure light.

So physicists have been looking for physics beyond the Standard Model. All sorts of experiments testing supposedly basic things. But the most direct way to look for new physics is to smash things together with colliders, so that’s part of why the LHC exists. Unfortunately, we haven’t really found any smoking gun evidence of new physics at the LHC. There have been all these theories – such as supersymmetry, which predicts a whole zoo of new particles – but no evidence has been found for them.

The Large Hadron Collider – 27 km of circular tunnel underground, with multiple detectors along the beam line where experiments are conducted. The most complex machine ever made by humankind. Image taken from https://i.ytimg.com/vi/bTHzB4h0po4/maxresdefault.jpg.

The next step, of course, is to build an even bigger particle accelerator. But the problem is that in the real world, the costs required go up as well. You have a 10 TeV collider which costs $10 billion. Now you want a 100 TeV collider (because going up by a factor of 2-3 isn’t interesting, you want to go an order of magnitude) – it might cost $100 billion, or even many times more that. Is it worth it to spend $100 billion on fundamental physics experiments in which we have no clear idea what we’re going to see? What if we see nothing? Would it be a waste? In any case, it’s become harder and harder to convince people to fund larger particle accelerators. It may be that we’re at the current economic limit for how much humanity is willing to invest for a particle physics experiment.

Tabletop Experiments to the Rescue

Enter the tabletop experiment. Like ACME, which I’m working on. Unlike the LHC, which involves thousands of scientists, engineers and technicians, ACME currently consists of three professors, a postdoc and 5 graduate students. Unlike the tens of billions needed to fund the LHC, ACME has “only” cost about $10 million over the last decade. Yet ACME is able to look for physics at the same scale of sensitivity as the LHC – looking for particles that would only show up at the TeV or tens of TeV scale. And the whole experiment fits inside a single room. (A large one, for sure, but not 27 km of tunnels!)

The ACME second generation experiment. The whole beam line is about 3 meters long. But we’re probing physics similar to that of the LHC!

What’s the secret? Why isn’t everyone doing it this way then? Did the accelerator particle physics just miss this?

The answer is that of course, there are advantages looking for new physics in a particle collider. The simplest one is that you can see exotic new particles (if they exist) directly, whereas in a tabletop experiment like ACME, if you “find” anything you probably have to go through a bunch of atomic and particle theory to interpret the result. You can also say that the ACME experiment is quite model-dependent: it only has implications for physics models that predict an asymmetric electron (in our language, a non-zero electron electric dipole moment). There are certain theories which do not, and we have nothing to say about that. So in a sense, experiments like ACME are complementary to the LHC, not really a competitor.

But maybe in the future we won’t have much choice. It would cost billions to build a new particle collider that boosts the reach of the LHC. A lot of planning and negotiation – if you’re a young particle physicist, perhaps decades before you could see the collider you planned actually accelerate its first particles. In contrast, experiments like ACME are still going strong. We published our first result four years ago, which was probing physics at the 1-10 TeV level – about the same level as the LHC. And only four years later (with only a few graduate students and “only” a few million dollars) we’ve upgraded our apparatus such that it’s about 10 times more sensitive. If it works, then we would be probing physics at the 10-100 TeV level! And we’re not done – there are still a few improvements that could be made over the next few years, further increasing our reach.

And how do we do it? How do we look for physics at the several TeV level with just a “tabletop” experiment? This is a topic that I’ve somewhat covered before – you can start here for a brief overview, or here for a list of all the articles I’ve written on ACME. But the gist is that we can only do this thanks to a confluence of amazing new technologies and methods of atomic and molecular physics developed in the past few decades. And we have not reach the absolute limit – there’s still stuff to be done. We’re also not the only experiment that does this: there are at least two other experiments, namely University of Colorado and Imperial College, which are also looking at the electron with different molecules. And there are experiments which look at mercury, xenon, radium, and others – all of which are probing physics at a very high energy level while being nowhere near as complex as a full-blown high-energy collider experiment like the LHC.

Probing Physics at the 1000 TeV level with Molecules

The ACME experiment uses molecules of thorium monoxide to look at the electron electric dipole moment. Thorium monoxide, or ThO, is a diatomic molecule, meaning that it consists of two atoms bonded together. Most molecules you see in daily life have more atoms – for example water is H_2 O – so three atoms. But ThO has several great features that allow our experiment to be so sensitive to new physics, but it also comes at a cost: molecules, even simple ones like ThO, have a much more complex structure and so are more difficult to control to our hearts’ desire.

But luckily in the last few decades advances in laser technology and optics have made it possible to do an experiment with such a molecule. And amazingly, the limits of this method are still nowhere in sight. Last May, two atomic physicists at Harvard, Ivan Kozyrev and Nick Hutzler (who used to work on ACME, and is now at Caltech), published a new paper, Precision Measurement of Time-Reversal Symmetry Violation with Laser-Cooled Polyatomic Molecules. Here they outline a path towards making experiments like ACME, but potentially tens to hundreds to thousands of times better – probing physics at the 100-1000 TeV level – far beyond the reach of any particle collider we can build in the next century. The key is to use polyatomic molecules like YbOH – molecules with three or more atoms. These molecules almost magically retain the relative simplicity of diatomics like ThO but contain other advantages that allow us to, for example, cool and trap it, making it possible to measure things with them more precisely.

While in the past, people have been skeptical of doing complex experiments with molecules, the technology available to us today – precise, stable lasers to address the energy transitions and advancements in atomic physics methods such as laser cooling and trapping – may make these sorts of experiments plausible in the next decade or two. And by that time, it may be the case that they would be our only window into physics at that scale, at least for the next century.

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