Understanding the Antimatter Excess

New results shed light on excess positrons.
21 November 2017


The HAWC detector array in Mexico


New data from nearby pulsars may have just solved a positron problem.

We’ve all heard of matter, but have you heard of antimatter..? 

Antimatter is the mirror partner of matter. Every matter particle has an antimatter partner – a particle with the same mass but opposite charge. When matter and antimatter collide they annihilate each other.

Back in 2008, astronomers at several space-based experiments observed some unexpected antimatter orbiting above the Earth’s atmosphere. The observations were puzzling: where could this excess of positrons (the antimatter partners of electrons) have come from?
There were two theories – could the positrons be related to Dark Matter, the invisible substance that has perplexed scientists for decades, or were they emitted by two nearby pulsars?

“Pulsars were first discovered when we looked up in the sky and saw pulsing objects, and what we know these are now are neutron stars – a neutron star is the result of a collapsed star that collapses, not to a black hole, but to an incredibly dense, small body. You have a giant nucleus spinning around and you’ve got a spinning magnetic field which produces a beam of light and particles. We call them pulsars because, if that beam along the magnetic field hits us, we often see pulses of light or x-rays or even gamma rays timed to the rotation of that neutron star” explained Prof. Jordan Goodman of the University of Maryland, who is a spokesperson for the HAWC collaboration.

“One of the possible backgrounds that could be producing this excess of positrons is local sources. The most likely local sources are middle-aged, nearby pulsars. The idea is a middle-aged pulsar has been around long enough to send its electrons and positrons to the vicinity of earth. These two objects that we’ve seen are two of the nearby pulsars that have been the prime suspects for this positron excess..."

When radiation from space enters our atmosphere it creates a shower of particles, with sometimes a million particles cascading to the ground. HAWC – the High Altitude Water-Cherenkov Gamma Ray Observatory in Mexico – detects these particles using an array of 300 tanks, each holding almost 200,000 litres of water. These tanks are known as water-Cherenkov detectors and are used to observe flashes of light – known as Cherenkov radiation – that are emitted by particles when moving through a medium, such as air or water, faster than light is able to travel in that medium.  HAWC uses measurements from each detector to build a picture of what’s happening.

“If you think about how we reconstruct the direction a particle came from, it’s like if you put your hand on the keyboard of your computer. If you touched all of the keys at the same time you’d know your hand was going straight down. If you came in from the PLM side you’d know it was coming in from the right or the AQZ side you’d know it was coming in from the left first - you can tell the arrival direction of your hand. That’s what we do with our showers, we point them back into space based on the arrival time of the light in the water across these 300 detectors.”

HAWC has been able to observe radiation from the two pulsars and use a simple model to show that it’s very unlikely the neutron stars could produce positrons able to reach Earth and account for the excess.  As Goodman puts it: "we haven’t ruled out that they’re not the cause, we’ve just given them a good alibi.” This could mean the excess positrons are related to dark matter, and may in the future help us to understand this mysterious part of our Universe.



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