Roughly a year ago, a paper came out describing some very strange atomic nuclei that had been produced at the Relativistic Heavy Ion Collider, based at Brookhaven National Lab on Long Island. These atomic nuclei were not only comprised of antimatter, but some of their components incorporated strange quarks, instead of the usual up and down versions. The RHIC is back in the news today, as one of its detectors has found evidence of the production of anti-helium-4 nuclei. The rates at which these particles were produced, however, suggests that we won't be seeing anti-nuclei of any greater complexity anytime soon.
4He, also known as an alpha particle, is comprised of two protons and two neutrons, or four baryons in total. Antimatter versions of 3He were first detected decades ago, but the anti-alpha has been harder to identify. They're hard to spot in a particle collider simply because of the process by which they're formed. "A light nucleus emerging from a relativistic heavy-ion collision is produced during the last stage of the collision process," the authors state. "The quantum wavefunctions of the constituent nucleons, if close enough in momentum and coordinate space, will overlap to produce the nucleus."
It's that proximity of both location and energy that makes forming an antimatter version so unlikely. You have to have the right number of antimatter baryons of the right types traveling near each other for a nucleus to condense. The more baryons involved, the lower the probability. RHIC ups the odds of this happening by colliding two gold atoms, which bring a lot of baryons together in a very compact space.
To spot anti-alphas, RHIC's STAR detector team used a filter that selected only head-on collisions (which rejected about 90 percent of the data), leaving 109 collisions from 2010 to work with. Those were filtered for particles that traveled as if they had the appropriate charge and a heavy mass—this required real-time reconstructions of particle trajectories from these collisions, which is an impressive computational feat.
Plotting the particles according to their behavior revealed heavy bands corresponding to 3H and 3He, and a thin band beyond them that corresponded to (depending on the charge) regular and antimatter alpha particles. The production rate apparently agreed nicely with theoretical values.
Both the RHIC and the Large Hadron Collider (which spends part of its time colliding heavy lead atoms), are sometimes referred to as "mini Big Bang machines," because the soup of quarks and gluons they create were last seen during the Big Bang. But the colliders are actually better at producing complex antiparticles than the Big Bang was, since the collisions evaporate off into empty space—the Universe's density immediately after the Big Bang was so high, the antiparticles wouldn't go far without running into something else.
Unfortunately, the theories that the RHIC data support indicate that the LHC's collisions won't be sufficient to produce any anti-nuclei with higher numbers of baryons, like an anti-lithium. The authors do note that, should the LHC produce some, then we'd be spotting some really unexpected physics: "A deviation from the usual rate reduction with increasing mass would be an indication of a radically new production mechanism."
But the results could help provide a new perspective on astronomy. We're just now planning to put hardware into space that could detect an anti-alpha. Given the RHIC results and a clear indication of how much anti-4He is out there, we should get a measure of the amount of antimatter present in the cosmos.
Nature, 2011. DOI: 10.1038/nature10079 (About DOIs).
http://arstechnica.com/science/news/2011/04/anti-helium-4-detected-heaviest-anti-nucleus-yet.ars
4He, also known as an alpha particle, is comprised of two protons and two neutrons, or four baryons in total. Antimatter versions of 3He were first detected decades ago, but the anti-alpha has been harder to identify. They're hard to spot in a particle collider simply because of the process by which they're formed. "A light nucleus emerging from a relativistic heavy-ion collision is produced during the last stage of the collision process," the authors state. "The quantum wavefunctions of the constituent nucleons, if close enough in momentum and coordinate space, will overlap to produce the nucleus."
It's that proximity of both location and energy that makes forming an antimatter version so unlikely. You have to have the right number of antimatter baryons of the right types traveling near each other for a nucleus to condense. The more baryons involved, the lower the probability. RHIC ups the odds of this happening by colliding two gold atoms, which bring a lot of baryons together in a very compact space.
To spot anti-alphas, RHIC's STAR detector team used a filter that selected only head-on collisions (which rejected about 90 percent of the data), leaving 109 collisions from 2010 to work with. Those were filtered for particles that traveled as if they had the appropriate charge and a heavy mass—this required real-time reconstructions of particle trajectories from these collisions, which is an impressive computational feat.
Plotting the particles according to their behavior revealed heavy bands corresponding to 3H and 3He, and a thin band beyond them that corresponded to (depending on the charge) regular and antimatter alpha particles. The production rate apparently agreed nicely with theoretical values.
Both the RHIC and the Large Hadron Collider (which spends part of its time colliding heavy lead atoms), are sometimes referred to as "mini Big Bang machines," because the soup of quarks and gluons they create were last seen during the Big Bang. But the colliders are actually better at producing complex antiparticles than the Big Bang was, since the collisions evaporate off into empty space—the Universe's density immediately after the Big Bang was so high, the antiparticles wouldn't go far without running into something else.
Unfortunately, the theories that the RHIC data support indicate that the LHC's collisions won't be sufficient to produce any anti-nuclei with higher numbers of baryons, like an anti-lithium. The authors do note that, should the LHC produce some, then we'd be spotting some really unexpected physics: "A deviation from the usual rate reduction with increasing mass would be an indication of a radically new production mechanism."
But the results could help provide a new perspective on astronomy. We're just now planning to put hardware into space that could detect an anti-alpha. Given the RHIC results and a clear indication of how much anti-4He is out there, we should get a measure of the amount of antimatter present in the cosmos.
Nature, 2011. DOI: 10.1038/nature10079 (About DOIs).
http://arstechnica.com/science/news/2011/04/anti-helium-4-detected-heaviest-anti-nucleus-yet.ars
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