Dark matter is aptly named. It emits no light and interacts with visible matter only via gravity. But dark matter might be only the tip of an invisible universe of unknown forces. This possibility has led to a hunt for “dark photons.” Such photons are analogous to ordinary photons, but they are exchanged among particles of dark matter, and according to some models, they may have mass.
Dark Matter Leads a Secret Life
Evidence for dark photons would be a game changer, says the journal Physics, revealing that dark matter leads a secret life that is much more complex than assumed by most theories. The latest search for dark photons at the Large Hadron Collider, however, has come up empty.
The discovery of the dark photon could explain many of the mysteries of the universe”
“The discovery of the dark photon could explain many of the mysteries of the universe, such as the nature of dark matter, while directly addressing experimental observations, such as recent tensions between the muon moment measurements and the Standard Model, and anomalies in cosmic ray observations,” physicist Cris Panda at UC Berkeley wrote in an email to The Daily Galaxy. “This breakthrough discovery could start a new era of Physics in which a myriad of new particles and physical laws are discovered in a short period of time. A wealth of possible experimental and theoretical results could find solutions to many of the currently unexplained phenomena in the Standard Model, such as the nature of gravity, dark matter and dark energy, or the matter-antimatter asymmetry in the Universe”.
Fine-Structure Constant Throws Doubt on Dark Photon Theory
In 2018 a team of researchers from the University of California and Lawrence Berkeley National Laboratory conducted an ultra-precise measurement of the fine-structure constant, and in so doing, have found evidence that casts doubts on dark photon theory. In their paper published in the journal Science, the group describes their measurement process and what they found by using it.
Dark matter has only been observed via gravitational interactions. Discovery of dark photons would change this, since dark photons can interact both with dark matter and everyday matter.
The fine-structure constant is a number that represents the force of electromagnetic interactions between charged particles, such as those that are involved in keeping electrons from traveling outside of their atoms. Up until now, it has been derived using the magnetic properties of electrons and calculations that are still considered to be theoretical. As the researchers note, more precise measurements allow for testing the Standard Model of particle physics. To that end, they sought to measure the constant through more direct means.
To accomplish this feat, they aimed a laser at cesium-133 atoms (matter-wave interferometry) to force them into quantum superposition and then took a close look at what happened between them as they relaxed back to their natural state. The interference that occurred, the team reports, revealed the speed at which the atoms traveled when they were struck by the laser—they used that number to calculate the fine-structure constant. They claim their work has allowed for calculating the fine-structure constant to better than one part per billion.
Still Room for Other Particle Theories to Explain the Discrepancy
The researchers report that the number they calculated closely matched the theory, which offers some confirmation of theories that suggest electrons are not made up of smaller, unknown particles. But it also casts doubt on theories surrounding the existence of dark photons. On the bright side, because the number they calculated was close to that theorized, but not exact, there is still room for other particle theories to explain the discrepancy.
The Last Word
“Currently the standard model of particle physics does not include gravity,” wrote CERN particle physicist Philip Ilten in an email to The Daily Galaxy. “However,” he notes, “dark matter has only been observed via gravitational interactions. Discovery of dark photons would change this, since dark photons can interact both with dark matter and everyday matter. Dark photons could provide a portal into the dark matter sector which could be just as complicated as everyday matter.”
The image at the top of the page shows a computer simulation of dwarf galaxy and dark matter forming when the Universe was half its current age. Galaxy formation occurs along dark matter filaments, and is a violent process of merging of gas clouds, spawning stars deep within their sheltered cores. (Credit: Bourke, Crain and Duffy).
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