By Vinca Lu
Why is there something rather than nothing? Why does the universe contain more matter than antimatter? Does the key to answering these questions lie in ultra-cold molecules? On October 7, the Amherst Physics and Astronomy Department hosted Professor Ben Augenbraun from the Williams College Chemistry department to deliver a colloquium talk titled “Searching for New Physics using Cold Molecules.” In general, Professor Augenbraun studies the properties and applications of molecules using laser spectroscopy. In his latest project, he used ultra-cold molecules to explore open questions in fundamental physics.
The Standard Model of particle physics explains all the normal matter we interact with on an everyday basis, like protons, neutrons, and electrons. It also includes other particles in nature, such as neutrinos, and particles such as photons that carry fundamental forces other than gravity. Finally, it fully describes how all these particles interact with each other. The Standard Model has also never been contradicted in experiments or observable evidence, and is thus one of our best models of fundamental physics.
However, there is still much left unexplained by the Standard Model, as it only currently accounts for around 5% of the universe’s energy content. It excludes dark matter, which has shown observable gravitational effects on distant matter and light, and dark energy, which governs the expansion of the universe. It also fails to explain a surprising fundamental asymmetry: why there’s so much more matter than antimatter in our universe. If we think about matter and antimatter as two sides of the same coin, the Big Bang should have produced equal amounts of each, annihilating into pure energy—which obviously did not happen. That imbalance, Augenbraun explained, implies a violation of time-reversal symmetry (T-violation), meaning that the laws of physics are not identical if you reverse the flow of time.
Presumably, this violation suggests that there are some new particles that we have yet to discover. Importantly, the introduction of new particles that violate time symmetry can give rise to electric dipole moments (EDMs) in electrons.
EDMs are a measure of how separated the positive and negative charges of a system are. Electrons typically have a related but distinct property called the magnetic dipole moment because they spin in a specific direction, generating a type of current. If the magnetic dipole moment and EDM of an electron are originally pointing in the same direction, then the moments will point in opposite directions under time reversal because the current reverses direction. This type of symmetry violation is measurable in a lab, and is precisely what Augenbraun’s lab aimed to detect. Specifically, by placing electrons with supposed dipole moments in strong electric fields, a rotational force will be applied to the electrons and lead to a detectable energy split, indicating the existence of EDMs and thus new particles that violate time symmetry.
There have been many attempts to detect electron EDMs using this effect. Although none have actually detected an electron EDM, these experiments have placed increasingly precise upper bounds on the value of the electron EDM—there was a time when Amherst College actually held the record for this, Augenbraun said. Augenbraun’s lab aimed to improve the precision of these experiments. Specifically, in these experiments, the molecules are beamed through a region with a small electric field in order to orient the bigger internal electric fields of the molecules, and then the molecules are detected. To increase the duration of this process, Augenbraun uses ultra-cold molecules, cooled to around a thousandth of a degree above absolute zero. In principle, extending the duration of the experiment could make EDM measurements 100 times more sensitive than current limits.
How are the molecules cooled? When an atom absorbs a photon, it will move from the ground state to a higher energy level, before re-emitting the photon after some time. The cycle where an atom does this over and over again is called optical cycling, which allows detection of and control over the atom’s state. This repeated absorption and emission of photons causes the atom to lose kinetic energy, effectively cooling it. But cooling and trapping molecules is notoriously difficult. Unlike atoms, which have simple electronic structures and can be laser-cooled with a single optical transition, molecules possess many vibrational and rotational degrees of freedom. When a molecule absorbs a photon and then re-emits it, it often decays into a different vibrational state, effectively “leaking” out of the cooling cycle. To achieve optical cycling, the molecule’s Franck–Condon factors (FCFs) must be nearly diagonal, meaning the bond lengths in the excited and ground states are almost identical. Augenbraun’s strategy is to identify new molecules that have favorable FCFs, produce large internal electric fields, and have low magnetic sensitivity.
To identify these molecules, Augenbraun’s team used quantum chemical simulations, in collaboration with theorist Lan Cheng at Johns Hopkins University, to explore a series of compounds containing heavy atoms (specifically combinations of copper, silver, and gold) bonded with elements like carbon, silicon, or lead. These molecules are promising because they produce very large internal electric fields while maintaining bond lengths that barely change between different states. This combination allows for more efficient optical cycling. One particularly promising candidate from these simulations was gold carbide (AuC). However, gold carbide had never actually been observed in a laboratory before.
To synthesize AuC, Augenbraun’s group hits a solid gold target with a high-powered pulsed laser inside a vacuum chamber. The intense laser pulse vaporizes a small amount of gold, which then reacts with a controlled flow of methane gas, forming AuC molecules. The team used laser-induced fluorescence (LIF) spectroscopy to detect the AuC. This technique uses a tunable dye laser that is scanned across a wide range of visible wavelengths, exciting any molecules whose electronic transitions match the laser frequency. If a molecule is excited, it re-emits light, which can be detected by a photomultiplier tube. The wavelengths where this fluorescence appears correspond to the molecule’s internal energy levels, revealing its electric and vibrational structure.
Though it took weeks, Augenbraun’s students finally observed a fluorescence line that matched theoretical predictions of AuC’s structure almost exactly. This was the first experimental observation of AuC, opening the door to a deeper study of AuC’s properties. With this first detection complete, Augenbraun’s group now plans to map AuC’s rotational and vibrational spectra to determine just how laser-coolable it is.
If the electron does have a nonzero EDM, it would indicate new sources of time symmetry violation, and thus new particles or interactions not accounted for by the Standard Model. The energy scale associated with such a discovery would reach beyond what current or even next-generation particle colliders can probe. Even if no EDM is found, continually tightening the upper limit provides significant constraints on speculative models of new physics. Beyond its implications for fundamental physics, Professor Augenbraun’s research is remarkable for another reason: it is being done at a liberal arts college with undergraduate students as the primary experimentalists. As new molecules are being discovered, new cooling schemes developed, and new experimental techniques refined, each advance brings physicists one step closer to answering the fundamental questions at the heart of our physical universe: why time has a direction, why matter even exists, and whether the laws of physics are truly symmetric.
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