Proton-H2 Experiment Reveals Atomic Scattering Mystery

Results from an experiment exploring how atoms interact on a very fundamental level show that scientists may understand less about what’s going on in some atomic scattering experiments than previously thought. Published recently in the American Physical Society’s journal Physical Review Letters, the international team of researchers hopes that this work will spark follow-up studies that could help us better address one of the most fundamental unsolved problems in physics.

Just as bits of light, called photons, exhibit properties of both waves and particles, so do bits of matter. The wave properties of everyday objects are too insignificant to be measured with current technology, but scientists can observe the wave-like behavior of microscopic particles like electrons and hydrogen in precise experiments. For example, if you fire a beam of protons at an H2 molecule (the natural ground state of hydrogen gas), proton waves can diffract from the molecule and lead to interference patterns that can be observed.

These types of experiments provide insight on how systems of interacting particles behave over time in situations where quantum mechanical effects are important. According to the lead researcher on this new experiment, Michael Schulz from Missouri University of Science & Technology, how three or more mutually interacting particles behave over time is “one of the most fundamentally important yet unsolved problems in physics.” This is often referred to as the “few body problem” and it comes up again and again in atomic, molecular, nuclear, and particle physics.

The essence of the few body problem is that for a system with more than two mutually interacting particles, you can’t simply solve an equation and get the answer even if you know the underlying forces. Instead, scientists have to make approximations and test them with detailed data from experiments. Often these experiments involve colliding atoms and looking at the results—such as whether and how atoms are fragmented during collisions. Scientists need a solid understanding of the results of such experiments and what they mean in order to apply them to the few body problem.

In this research, the team sent a beam of protons into a target made of cooled H2 molecules. The protons interact with the molecules in such a way that they create interference patterns. The scientists study these interference patterns by looking at how the protons and H2 molecules fragment on impact, and where the projectiles and the molecular fragments ended up after scattering.

Team members (left to right) Michael Shulz, Krishna Koirala, Trevor Voss, Madhav Dhital, and Basu Lamichhane at work in their lab.
Image Credit: Jan Gargus.

In their experiments, the team identified and studied two types of interference structures. One structure revealed something particularly unexpected. At the time of scattering, the interference pattern underwent an immediate phase shift of 180°. In other words, the maxima became minima and vice versa.

Researchers have reported similar phase shift findings in two other fragmentation experiments. In those experiments, the molecules fragmented due to electronic transitions and the shift was explained by symmetry considerations related to this process. The problem is, this explanation doesn’t work for the new experiment. In the new experiment, the researchers were looking only at cases when the molecule was fragmented by nuclear interactions, not electronic transitions.

According to the researchers, this result suggests that instantaneous phase shifts in atomic scattering experiments are not as well understood as previously thought—they can occur even when they aren’t predicted by symmetry considerations. Studies are needed, they say, that explore the cause of this shift starting from basic physics principles.

Designing and implementing an experiment like this is a comprehensive process. It involves developing equipment, setting it up, and then a lot of waiting. “The actual data taking is a long and in some sense boring process because once the experiment is running it is just about collecting enough statistics. So, you just sit there and watch the data coming in for several months. Then comes the exciting part: the data analysis,” says Schulz. “That’s when you find out that the experiment actually did what you wanted it to do and it forces nature to reveal some of its secrets (like e.g. an unexplained phase shift),” he says.

Many Physics Buzz articles close by highlighting potential real-life applications of the featured research. With this article, we wanted to close by reminding you that research for the sake of understanding the secrets of nature is equally important. Doing applied research but neglecting fundamental research is like building a house without a foundation, Schulz says, and that is a dangerous road to go down.

Please consider voicing your support for fundamental research and other science policy issues by contacting your representatives. For assistance, check out the American Physical Society’s Advocacy Dashboard.

Kendra Redmond

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