Probing molecules in an effort toward understanding matter imbalance in the universe

Scientists measure the electric dipole moment of an electron by probing molecules, which could help us understand the matter-antimatter imbalance in the universe.
Tejasri Gururaj
Atomic Nucleus digital image
Atomic Nucleus


There is a matter-antimatter imbalance in the universe, and our current theory of particle physics, the standard model, doesn't explain this imbalance. 

Symmetry invariance plays a crucial role in understanding the properties and behavior of particles, fields, and the fundamental forces of nature. The charge parity symmetry, or CP symmetry, is one such symmetry fundamental to particle physics. 

According to the CP symmetry, the laws of physics are conserved, meaning that they remain the same if the charges of all particles are reversed, and their spatial position or coordinates are inverted (parity reversal). 

In other words, CP symmetry suggests that the behavior of particles and antiparticles should be identical under these transformations. However, the overwhelming amount of matter compared to antimatter in the universe suggests a violation of this.

Extensions to the standard model have been proposed to address this issue. And scientists have now reported making headway. 

Probing molecules in an effort toward understanding matter imbalance in the universe
TSV could explain the matter-antimatter imbalance in the universe, which the standard model fails to explain.

A team of scientists from the University of Colorado has used molecular spectroscopy to probe fundamental particles, focusing on measuring the electron's electric dipole moment (EDM).

Baryon asymmetry in the universe

The baryon asymmetry of the universe (BAU), which refers to the imbalance between matter and antimatter, requires three conditions to be fulfilled. These conditions are known as the Sakharov conditions, named after physicist Andrei Sakharov who proposed them in 1967.

One of these conditions is the violation of CPT symmetry, where T stands for time. Time symmetry refers to the assumption that if time were reversed, the laws of physics should stay the same. The violation of CP symmetry implies a time symmetry violation (TSV).

How does this tie into studying the electron's EDM? An EDM in an electric field causes an energy shift when its orientation is flipped with respect to the field. If TSV interactions beyond the standard model exist, they could induce an EDM in fundamental particles like the electron. 

Therefore, by measuring the electron's EDM, scientists can search for evidence of TSV, which implies a CP violation, and explore the existence of undiscovered particles and interactions.

Measuring the electron's EDM

The team, led by Tanya S. Roussy from the University of Colorado, measured the EDM using precision spectroscopy of hafnium fluoride (HfF+) molecules. The researchers used an ion trap to produce and trap the molecules. 

Probing molecules in an effort toward understanding matter imbalance in the universe
The experimental vacuum chamber containing the ion trap electrodes used for the experiment.

Following this, they applied electric and magnetic fields, lasers, radiofrequency, and microwaves to measure the energy of the molecules in two different configurations—when the electron's magnetic moment was aligned and anti-aligned with the molecule's electric field. 

By probing the energy shift in the molecule, they measured the electron's EDM to be no larger than 4.1 x 10^-30 centimeters. Here e is the charge of an electron, equal to 1.6 x 10^-19 Coulombs.

The use of molecules instead of atoms increased the sensitivity in the detection of the electron's EDM. The Standard Model predicts a very small electron EDM, significantly smaller than the current experimental limit.

Therefore, future experiments operating at higher sensitivities have the potential for discovery. Advances in TSV measurements are being pursued, including using exotic isotopes, polyatomic molecules, and solid-state systems, along with improvements in laser cooling, control of molecular ions, and spectroscopy.

These upgrades will improve our ability to study high-energy physics beyond the Standard Model.

The findings of the study are published in Science.

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