The cold, dark secrets of the Universe in few-body physics

Understanding fundamental processes in physics, particularly physics beyond the Standard Model, is no easy task. Experiments and theories looking for new general theories to describe many of the phenomena that are missing in the Standard Model focus on particle physics experiments at places like CERN. Professor Jesús Pérez Ríos of the atomic, molecular, and optical (AMO) physics group at Stony Brook University, USA, takes a different approach – he uses a range of methods from atomic and molecular physics and ultracold chemistry to find new insights into the complexities of the world around us.

Atoms are the building blocks of all life. Every molecule is made of a collection of atoms; even large and complex biological species are simply arrangements of ever more complicated molecules.

Historically, atoms and molecules have proved an excellent playground for physicists aiming to understand many of the physical phenomena that govern the world around us. Atomic emission lines – the colours of light emitted by atoms when they absorb sufficient energy – have proved an important demonstration of the role quantum mechanics plays in describing atomic behaviour. It was also these spectral lines that led to the discovery of new types of electronic configurations for atoms and molecules, now known as Rydberg states.

Rydberg states are highly exotic atomic states in which the negatively charged electron orbits the positively charged nucleus but at much greater distances than normally seen by standard electronic states. Despite these distances, which are enormous on the atomic length scale, there is still some interaction between the electron and nucleus, but it is very weak compared to the strength of the interaction in a standard atom. As a result, Rydberg states in atoms and molecules are very easily perturbed by even small fluctuations in external electromagnetic fields or collisions with other molecular species.

The behaviour of unusual electronic configurations in atoms and molecules has been of great interest to Professor Jesús Pérez Ríos at Stony Brook University, USA. Ríos is part of the atomic, molecular, and optical (AMO) physics group, who have been using such systems to explore quantum mechanical effects. As part of his highly diverse toolkit for looking at fundamental problems in physics, Ríos has been using methods originally developed for atomic and molecular physics to re-examine some of the biggest challenges in physics.

Few-body physics

Few-body physics concerns the study of systems that are well-defined in terms of the number of particles involved – usually light atoms – and is one of the core topics in AMO science. Because the number of particles and collision conditions can be controlled so precisely, few-body systems are ideal for studying phenomena such as intermolecular interactions and how atoms and molecules collide and react.

Ríos has been making use of theoretical and numerical models to look at a number of different few-body processes, including what processes might be occurring in the atmosphere to form ozone. From a theoretical perspective, he has been investigating the mechanisms of how three-particle collisions can form ozone in conditions found in low- and high-temperature conditions in the stratosphere.

What his work has revealed is the identity of one of the main temporary complexes responsible for ozone formation at low temperatures and the role of special ozone states. Even for relatively small molecules like ozone, understanding the details of the reaction mechanism can be complex. This includes understanding whether the chemical bonds are formed and broken in a single step or over a series of sequential steps. With the high level of control over the reaction parameters, few-body physics studies are a unique opportunity to study the mechanisms underlying chemical reactivity.

Another reaction series that Ríos investigated is between charged atoms (ions) and two neutral atomic species. By colliding the three species together with varying amounts of energy, Ríos has been able to see how the reaction products are influenced by the dynamics involved in the collisional events.

Cold and ultracold chemistry

Temperature plays an important role in chemical reactivity. Thermal energy can be transferred to the reacting species that is necessary for the reaction to take place. However, studying reactions at incredibly low temperatures – near absolute zero – can be an incredibly useful tool for understanding exactly how the electrons and nuclei that make up atoms interact and behave in very well-controlled environments. In particular, there are a number of quantum mechanical effects involved in reactivity that can only be observed by looking at reactions happening at these temperatures.

Ríos made a number of theoretical predictions about the reactive behaviour of trapped ions with lithium dimers at very low temperatures. For some chemical species, it is possible to confine the atom in the optical field of a laser and prevent it moving. Such optical trapping methods can be used not just to study chemical reactions in detail, but they also form the basis of some designs of quantum computer and logic gates.

Following Ríos’s predictions, a team of experimentalists were able to observe reactions between lithium dimers and ions proceeding. As well as being an important validation of a theoretical prediction, these experimental results demonstrated a new of making molecular ions, the efficiency of which could be controlled by varying the dimer binding energy.

Machine learning

One of the challenges that scientists face when trying to develop models that describe phenomena as complex as those that take place in a chemical reaction is creating models that successfully include all the relevant physics. Many models are derived empirically – where experimental data is recorded, and a model is derived to fit the data and explain some of the observed phenomena. While not always considered as ‘elegant’ as theories derived from a pure mathematical and physics basis, empirical models have been highly successful in both explaining and predicting many phenomena in physics and AMO science.

Machine learning is one way of making use of extensive amounts of data to try to make predictions about how new species will also behave. Ríos has been integral in establishing the Diatomic Molecular Spectroscopy Database – an effort led by the Fritz Haber Institute to store information on the spectroscopy of two-atom molecular systems.

One of the purposes of the database is to identify which molecules might be excellent candidates for laser cooling experiments. However, this has also provided useful information for Ríos to develop a data-driven approach to the prediction of dipole moments for other molecular species, as high-quality databases are essential for the success of machine learning algorithms. The dipole moment is an important property of molecules that, among other things, determines how they will interact with laser fields.

Other areas of research where Ríos has applied machine learning include prediction of the second virial coefficient of gases. He has successfully created a highly efficient method for calculating these coefficients that are a measure of how much a gas deviates from ‘ideal’ behaviour – where each gas particle does not interact with any other particle.

Physics beyond the Standard Model

As atoms and molecules are quantum systems in themselves with a certain number of degrees of freedom in their motion, atoms and molecules can be exploited as sensors for certain physical phenomena. Ríos has been exploring how atoms and molecules can be used to look at phenomena, such as dark matter, that are not currently described by our Standard Model of physics.

As such high levels of energy resolution and control can be achieved in atomic and molecular physics measurements, Ríos and collaborators have proposed a way that mixtures of different diatomic gases could be used as highly sensitive detectors for ‘light dark matter particles’. Light dark matter particles are lighter than the proton and hard to detect as they rarely scatter off existing matter, so being able to detect photons emitted from such matter when scattering off nuclei of molecules may be a more effective approach. Alongside this, Ríos has been creating models to explain the Migdal effect in molecules – where nuclear scattering results in electronic excitation – which could be another approach to detect dark matter.

Atomic and molecular physics experiments allow for very high degrees of control over the reacting system and Ríos has found many areas of science where this high level of control can be applied to reveal new insights into the world around us.