The most precise measurement ever of the fine-structure constant has placed new constraints on theories that predict the existence of “dark sector” particles. The new value, which researchers in France measured using clouds of cold rubidium atoms, provides a stringent test of the Standard Model of particle physics while also further limiting the properties of dark matter – the substance thought to make up more than 90% of the matter in our universe.
The fine-structure constant α is a composite of several physical quantities (including e, the charge on an electron, and c, the speed of light) that, together, characterize the strength of the electromagnetic interaction. This makes α ubiquitous throughout the universe. Because it is a dimensionless number, it is in some sense more fundamental than physical constants such as the strength of gravity or Planck’s constant ħ, which change depending on the units in which they are measured.
Electromagnetic interaction is weak
The relatively low value of α – it is approximately equal to 1/137 – implies that the electromagnetic interaction is weak. The main consequence of this is that electrons orbit some distance from their atoms, so they are free to form chemical bonds and build up molecules. This property made it possible for matter and energy to form stars and planets. Indeed, some physicists have argued that we owe our very existence to the exact value of α, because if it were slightly bigger or smaller, stars might not been have able to synthesize heavier elements like carbon, and life as we know it wouldn’t exist.
Precise measurements of α make it possible to rigorously test relationships between elementary particles. These relationships are described by the equations that make up the Standard Model of particle physics, and any discrepancy between the model’s predictions and experimental observations may provide evidence of new physics.
Determining the recoil velocity of atoms
Measurements of α generally begin by determining how strongly atoms recoil when they absorb photons. The kinetic energy of this recoil (or its velocity) reveals how massive the atoms are. Next, the electron’s mass is calculated using the precisely known ratio of the atom’s mass to that of an electron. Finally, α is calculated from the electron’s mass and the binding energy of a hydrogen atom, the value of which is likewise well known from spectroscopy measurements.
In the new work, researchers led by Saïda Guellati-Khélifa at the Kastler Brossel Laboratory in Paris cooled atoms of rubidium to a few degrees above absolute zero in a vacuum chamber. They then created a quantum superposition of two states of the atoms using laser pulses. The first state corresponds to atoms that recoil when they absorb photons and the second state to atoms that do not recoil.
The two possible versions of each of type of atom propagate through the experimental chamber along different paths. The researchers then applied a second set of laser pulses to “re-join” the two halves of the superposition.
The more an atom recoils after absorbing photons, the more out of phase it will be with the version of itself that does not recoil. By measuring this difference, Guellati-Khélifa and colleagues extracted the mass of the atoms, which they then used to determine the fine-structure constant. Their result shows that α has a value of 1/137.035999206(11) – a measurement that, with an accuracy of 81 parts per trillion, is 2.5 times more exact than the previous milestone, which was made in 2018 by Holger Müller and colleagues at the University of California at Berkeley, US.
Improved experimental setup
The Paris researchers say that improvements to their experimental setup were key to the new result. By controlling effects that can create perturbations in the measurement, they were able to reduce sources of inaccuracies. For example, the researchers considered, and compensated for, the gradient in the local strength of gravity across their experimental set-up and the Coriolis acceleration created as the Earth rotates. They also meticulously characterized properties such as laser beam alignment, frequency, wave-front curvature and the second-order Zeeman effect, which account for many errors in such experiments.
The only palpable evidence of physics beyond the Standard Model
The new measurement, which is reported in Nature, differs from the value obtained in the 2018 Berkeley experiment in its seventh digit. This result surprised the Paris researchers, since it implies that either one or both measurements has an error currently unaccounted for. However, the two groups’ measurements do agree closely with the value of α calculated from precise measurements of the electron’s so-called g-factor, which relates to its magnetic moment. In a related News and Views article, Müller notes that the Paris result “confirms that the electron has no substructure and is truly an elementary particle”.
The Paris researchers say they now plan to back up their results by measuring the recoil velocity of a different rubidium isotope. “We also plan to build an even more precise instrument,” Guellati-Khélifa tells Physics World.