Tag Archives: positron

Credit: CERN.

There’s an abundance of antimatter in our atmosphere, and dark matter decay might be to blame

Credit: CERN.

Crab nebula. Credit: NASA.

In 2008, physicists found that the number of high-energy positrons — the antiparticle or the antimatter counterpart of the electron — hitting Earth was three times greater than the Standard Model predicted. Some scientists have suggested that nearby pulsars, which expel copious amounts of positrons as they spin erratically, could account for the extra positions. One new study, however, proposes that this antimatter may be the result of decaying particles of dark matter. 

Stranger than fiction

Dark matter is an elusive and hypothetical substance that is supposed to comprise roughly 5/6 of all matter in the universe. Scientists have never been able to directly observe dark matter, which seems to be invisible to current means of observation, but they do have good reasons to believe dark matter exists judging from the gravitational effects it exerts on ordinary matter.

Though not as weird as dark matter, antimatter is also another exotic physical quirk. You’ve likely heard a lot about antimatter in the science fiction universe. Star Trek’s starship Enterprise uses matter-antimatter annihilation propulsion for faster-than-light travel and in the famous book Angels and Demons, (spoiler alert) Professor Langdon tries to save Vatican City from an antimatter bomb. Unlike dark matter, though, we know for sure that antimatter is real.

Every particle of ordinary matter has an antimatter counterpart which is equal in mass but opposite in charge. When the two kinds of particles meet, they annihilate each other while releasing energetic photons called gamma rays.

There are many unsolved riddles surrounding antimatter. For instance, antimatter should have annihilated all ordinary matter during the Big Bang. Scientists think the reason why galaxies, stars, and eventually we exist in the first place is that there was one extra matter particle for every billion matter-antimatter pairs.

Antimatter is closer to us than most people think. Small amounts of antimatter — at a rate ranging from less than one per square meter to more than 100 per square meter — constantly rain down on the Earth in the form of cosmic rays, energetic particles from space. Antimatter can be found closer yet.  The average banana (rich in potassium) produces a positron roughly once every 75 minutes. That’s because potassium-40 will occasionally split out a positron in the process of radioactive decay.

When researchers working with the space-based instrument PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) detected an overabundance of positrons hitting Earth’s atmosphere, things got weird. For a long time, scientists have thought that all of this extra positrons are sourced from pulsars.

A pulsar is a rapidly rotating neutron star which emits electromagnetic signals. These objects are also very dense, concentrating the mass of the sun in a diameter comparable to that of a large city. Pulsars radiate two steady, narrow beams of light in opposite directions from their poles. Although the light from the beam is steady, pulsars appear to flicker because they also spin really fast, and so constantly spin these polar rays around. For this reason, pulsars have earned the nickname of ‘cosmic lighthouses’.

But now, writing in a recently published paper in the journal Science, a team of researchers claims these candidates —  a pair of pulsars less than a thousand light-years away — aren’t the likely culprit.

“When I started this work, I really believed it was pulsars,” lead author Rubén López Coto of the Max Planck Institute for Nuclear Physics told National Geographic. “But these two pulsars actually cannot provide enough positrons in order to account for this positron excess.”

A new suspect: dark matter

HAWC detector consisting of 300 large water tanks, each with four photodetectors

HAWC detector consisting of 300 large water tanks, each with four photodetectors. Credit: Jordan A. Goodman

Coto and colleagues refer to observations made at the High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC), a science laboratory nestled at the base of a volcano four hours away from Mexico City. When gamma rays from annihilated positrons interact with the atmosphere, they generate showers of particles that hit the planet’s surface. Inside HAWC, scientists have placed 300 corrugated steel tanks filled with water. When the extremely high-energetic particles strike the tanks, they generate flashes of light which can be used to distinguish the signature and origin of the gamma rays that triggered the particle cascade.

By back-tracking from the gamma-ray observations, the HAWC team found that the pulsars’ positrons were not moving fast enough to arrive on Earth. This implies that the interstellar medium between the two pulsars and Earth is likely murky, preventing the positrons from reaching Earth.

“HAWC scans about one third of the sky overhead, giving us the first wide-angle view of high-energy light from the sky,” says study co-author Jordan Goodman, a particle astrophysicist at the University of Maryland, College Park. “Before HAWC there were observatories that were highly sensitive to high-energy gamma rays, but they had relatively limited fields of view. With HAWC, we can see how gamma rays are diffusing from these pulsars across wide regions of sky.”

If the two pulsars, called Geminga and Monogem, aren’t the source of positron overabundance in Earth’s atmosphere, then “other pulsars, other types of cosmic accelerators such as microquasars and supernova remnants, or the annihilation or decay of dark matter particles” must be considered.

Not everyone is convinced that dark matter could be the source, though. Since dark matter is literally everywhere, it follows that the signature generated by dark matter particles annihilating one another should be ubiquitous — but they’re not. It could be that the positrons are sourced from even more unconventional sources such as high-energy particles produced by black holes.

Dark matter discovered for the first time? Significant discovery coming in 2 weeks

A new, probably significant discovery will be published in two weeks, the leader of a space-based particle physics experiment said Sunday at the annual meeting of the American Association for the Advancement of Science.

Why this is important

dark matter

Dark matter is every bit as mysterious and cool as it sounds; from what we know so far, our Universe’s energy comprises of 73% dark energy, 23% dark matter, and “regular” matter, like stars, planets, and everything else only accounts for the 4% left. When you think about matter, dark matter takes the big chunk, at 84%.

Some physics theories suggest that dark matter is made of WIMPS (weakly interacting massive particles), a class of particles that are their own antimatter partner particles. When matter and antimatter partners meet, they annihilate each other with a blast, so if two WIMPS would collide, they would be destroyed, releasing a pair of daughter particles — an electron and its antimatter counterpart, the positron (same mass, opposite charge).

A significant discovery

In May 2011, a $2 billion machine called the Alpha Magnetic Spectrometer-2 was mounted on the side of the International Space Station. The device may have just detected dark matter, for the first time; until now, everything we know about it comes from indirect observations and effects. According to MIT’s Samuel Ting, a physicist and the Alpha Magnetic Spectrometer’s principle investigator, the first results will be published in two weeks, and “it won’t be a minor discovery”; the paper has so far passed 30 revisions and is finally set for publishing.


So far, AMS has detected 25 billion particle events, including about 8 billion electrons and positrons. This first science paper will report how many of each were found, and what their energies are, Ting said. The thing is, if there is an abundance of detected positrons peaking at a certain energy, that could indicate a detection of dark matter, because while electrons are abundant in the universe around us, we know of much fewer processes that lead to the creation of positrons. Another interesting question is if these positrons signatures come from preferential directions or from all around.

“The smoking gun signature is a rise and then a dramatic fall” in the number of positrons with respect to energy, because the positrons produced by dark matter annihilation would have a very specific energy, depending on the mass of the WIMPs making up dark matter, said Michael Turner, a cosmologist at the University of Chicago who is not involved in the AMS project. “That’s the key signature that would arise.”

But regardless of whether AMS has found dark matter yet, scientists said they expected the question of dark matter’s origin to become clearer soon.

“We believe we’re on the threshold of discovery,” Turner said. “We believe this will be the decade of the wimp.”

Via Space

Antihydrogen and antimatter

Coldest antimatter yet might help scientists probe its secrets

A novel technique for cooling antimatter down to the point where it might become almost stationary might provide scientists with a better basis for studying one of the greatest modern mysteries today.

Antimatter, as it name implies, is the total opposite of matter and when the two meet they cancel each other out. For instance the opposite of an electron is a positron. It’s because of this fascinating behavior, however, that studying antimatter is extremely difficult, since we’re ubiquitously surrounded by matter. A great puzzling dilemma for physicists today is why is there so little antimatter present in the Universe compared to matter.

Antihydrogen and antimatter

It’s theorized that after the first Big Bang sparks, equal amounts of both matter and antimatter were spewed through out the early Universe, but for some yet unknown reason matter won out. Currently, the only instances antimatter has been observed naturally are those following radioactive decay or cosmic ray collisions, and though the existence of antimatter has been proven since the 1930s only recently could it be artificially produced, and in small quantities to top it over.

With this in mind, scientists have been looking to devise ways to ease antimatter research. A recent technique that has garnered quite a bit of attention was developed by Makoto Fujiwara, a research scientist at Canada’s particle physics lab TRIUMF and an adjunct professor at the University of Calgary, along with colleagues, and is called Doppler cooling. It implies chilling anti-hydrogen (with one positron and one anti-proton) to just a tad over absolute zero Kelvin – 25 times colder than ever attempted.

The technique has yet to be proven, though an advanced prototype experimental setup is in the works, however a computer simulation showed extremely promising results.

“The ultimate goal of antihydrogen experiments is to compare its properties to those of hydrogen,” physicist Francis Robicheaux of Auburn University in Alabama said in a statement. “Colder antihydrogen will be an important step for achieving this.”

“By reducing the antihydrogen energy, it should be possible to perform more precise measurements of all of its parameters,” Robicheaux said. “Our proposed method could reduce the average energy of trapped antihydrogen by a factor of more than 10.”

Fujiwara led in 2011 a team of scientists at CERN that made the first direct measurement of antimatter’s energy and also held particles of anti-hydrogen stable for as long as 15 minutes, still a record. Comparing the properties of hydrogen and anti-hydrogen might allow scientists to explain why there is such a great quantitative gap between the two. If the two are indeed proven to exhibit totally opposite properties then a sounds basis for further study might be built.

The present technique uses precisely targeted lasers on antihydrogen in order to loose energy and chill it down. Remember, however, that antimatter and matter annihilate each other, so the key to their research is trapping antihydrogen – the scientists hope to achieve this through a system of magnetic fields.

“We want anti-hydrogen atoms as cold as possible in our trap, and by cold I mean not moving. In particular, to measure the gravitational properties, antihydrogen in our trap is still moving way too fast. So this paper has shown that the technique called laser cooling can be applied in our experimental set-up,” Mr. Fujiwara said.

The first immediate goals for Fujiwara and colleagues is to study basic properties of antihydrogen like colour, weight, how it reacts to light or gravity and so on. The laser cooling technique was described in a paper published in the Journal of Physics B.


Astronomers capture light from first stars using bright galaxies

I gotta say, sometimes it absolutely baffles me to see the kind of complex studies astrophysicists do, and this is definitely one of them. The light from the first stars in the Universe is still lingering around in the cosmos, and researchers have found a new way to capture it: using ultra-bright galaxies that act as cosmic beacons, capturing relict photons in a blaze of gamma rays.


But it’s not just these early wandering photos that are captured – every light particle can be ensnared.

“We now have constraints on the total number of stars that ever formed,” Volker Bromm, an astronomer at the University of Texas at Austin, says of the new way to see old light, described online November 1 in Science. “It provides us with a review of the entire history of cosmic star formation, including the very first epochs of star formation in the very early universe.”

The biggest interest is finding out more about the early days of the universe and its first stellar inhabitants, which are currently too far from us to provide any information directly. Now roughly 13.7 billion years old, the universe is believed to have spawned the first stars some 4-500 million years after its birth. Studying these first stars would fill in some major gaps regarding what we know of the universe.

“Detecting these stars is very important but currently impossible,” says astrophysicist Marco Ajello of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and a coauthor of a the new study. “In this way, we are already able to set constraints on the amount and role of these stars in the early universe.”

Typically, it’s quite difficult to separate this relict starlight from every other object that’s shining; black holes, dust, other stars – all these and many more are just lurking around there, hiding the photon signature of primordial stars, and since we’re well inside the galaxy, there’s not much we can do about it.

“If we were located outside the Milky Way, then we could have measured the background light more easily,” says Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “We are embedded too deep inside the galaxy.”

To get around this problem, Ajello and his colleagues used the orbiting Fermi Large Area Telescope to study distant blazars, a type of bright, active galaxy. Like virtually all galaxies, blazars have supermassive black holes at their centers, but their black holes shoot out enormous jets of energy toward Earth. The jets include gamma rays, a high-energy form of radiation that interacts with the photons sent out by early stars. The photons interact with the gamma rays, and are converted into electrons and their antimatter equivalents – positrons. The transition produces a fade dimming effect, but one that the Fermi telescope can actually observe, and correlate it with the amount of photons between the source and the Earth.

Artist representation of a blazar


Since blazars are relatively common and distributed throughout the entire Universe, astronomers use them to measure the photon fog at different ages, and calculate the contributions from early stars. So far, the initial results are quite promising; they show that early stars took more time to form than previously believed, and unlike today’s stars, which also hold heavier elements, early stars were made entirely of hydrogen.

“The first stars were in general more massive — up to hundreds of times as massive as the sun — hotter, brighter, and more short-lived.”