Galaxy cluster Abell 1689 seems to be held together by swaths of unseen dark matter; blue shows its theoretically inferred location. But could dark matter be an illusion? Credit: NASA

Exploring the nature of dark matter through the A’ particle

Per definizione, la materia scura è invisibile anche se la sua presenza viene “osservata” grazie ai suoi effetti gravitazionali. Nonostante ciò, secondo alcune teorie moderne che tentano di descrivere questa materia elusiva le sue proprietà potrebbero essere in definitiva rivelate. Infatti, alcuni scienziati del MIT hanno sviluppato un metodo che potrebbe permettere di verificare la validità di una teoria al fine di dare credito, o meno, alle sue previsioni.

The work is described in a paper in the journal Physical Review Letters co-authored by MIT physics professors Richard Milner and Peter Fisher and 19 other researchers. “We’re looking for a massive photon“, Milner explains. That may seem like a contradiction in terms: photons, or particles of light, are known to be massless. That’s why they travel at the speed of light, something that, according to Einstein’s theory of relativity, is impossible for anything that possesses mass. However, an exotic particle that resembles a photon, but with mass, has been proposed by some theorists to explain dark matter, whose nature is unknown but whose existence can be inferred from the gravitational attraction it exerts on ordinary matter, such as in the way galaxies rotate and clump together. “Now, an experiment known as DarkLight, developed by Fisher and Milner in collaboration with researchers at the Jefferson National Accelerator Laboratory in Virginia and others, will look for a massive photon with a specific energy postulated in one particular theory about dark matter“, Milner says. The idea is more than just a theoretical prediction, he adds: “There are hints of such a particle from other experiments, making it worthwhile to pursue a definitive answer“. But the previous hints, consisting of what Milner calls “anomalous moments of the muon”, do not rise to statistical significance.

The DarkLight experiment is designed to provide solid confirmation of the massive photon’s existence. If it does exist, that would represent a major discovery.

It’s totally beyond anything we understand about the physical world“, Milner says. “A massive photon would be totally different from anything allowed by the Standard Model, the bedrock of modern , he says. To prove the existence of the theorized particle, dubbed A’ (“A prime”), the new experiment will use a particle accelerator at the Jefferson Lab that has been tuned to produce a very narrow beam of electrons with a megawatt of power. That’s a lot of power, Milner says: “You could not put any material in that path without having it obliterated by the beam”. For comparison, he explains that a hot oven represents a kilowatt of power. “This is a thousand times that“, he says, “concentrated into mere millionths of a meter”.

The new paper confirms that the new facility’s beam meets the characteristics needed to definitively detect the hypothetical particle, or rather, to detect the two particles that it decays into, in precise proportions that would reveal its existence.

Doing so, however, will require up to two years of further preparations and testing of the equipment, followed by another two years to collect data on millions of electron collisions in the search for a tiny statistical anomaly. “It’s a tiny effect“, Milner says, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics“. While DarkLight’s main purpose is to search for the A’ particle, it also happens to be well suited to addressing other major puzzles in physics. It can probe the nature of a reaction, inside stars, in which carbon and helium fuse to form oxygen, a process that accounts for all of the oxygen that now exists in the Universe. “This is the stuff we’re all made of“, Milner says, “and the rate of this reaction determines how much oxygen exists”. While that reaction rate is very hard to measure, the DarkLight experiment could illuminate the process in a novel way: “The idea is to do the inverse. Instead of fusing atoms to form oxygen, the experiment would direct the powerful beam at an oxygen target, causing it to split into carbon and helium. That would provide an indirect way of determining the stellar production rateRoy Holt, a distinguished fellow in the physics division at Argonne National Laboratory in Illinois, says “This work is a novel and significant technical development that not only opens a new window to search for a new [particle], but also for new studies in nuclear physics“. “If the planned experiment detects the A’ particle”, he says, “it would signal that  could actually be studied in a laboratory setting“.

MIT: Seeing the dark: New MIT-led experiment could finally shed light on the mysteries of dark matter
arXiv: Transmission of Megawatt Relativistic Electron Beams Through Millimeter Apertures