Physicists at Northwestern University have successfully trapped a single positron for an entire month, allowing researchers to study its quantum properties with unprecedented precision. "Further investigations using the trap might reveal the secret of why the world around us is made of matter and not antimatter, when things could have presumably gone either way," reports Scientific American. From the report: Every normal particle of matter has an antimatter counterpart with equal mass but opposite charge. To build their trap, physicists started with atoms of sodium 22, a radioactive isotope with a half-life of about two and a half years. When one of these atoms decays, it emits a positron -- the antimatter counterpart of the electron. "The big challenge is to persuade these positrons to slow down and then capture them," says Gerald Gabrielse, a physicist at Northwestern University, who oversaw the project. The team reported its achievement on Wednesday in Physical Review Letters (PDF).
The first step is to send the positrons through the crystal lattice of a sliver of cold tungsten. As the particles go pinballing through the material, they slow down and lose energy. "Every time they bang into something, there's a chance of annihilation, but it doesn't always happen," Gabrielse says. "If they make it to the end, they get popped out." Next, the physicists steer the positrons along a channel using electromagnetic fields. Eventually the particles end up in a vacuum, just three tenths of a degree Celsius above absolute zero, trapped by an electric field on top and bottom and a magnetic field on the sides. "If you want to hold on to antimatter, you have to hold it in a container without walls," Gabrielse says. This method allowed the researchers to isolate and study a solitary, lone positron for a monthâ"and could let them do so indefinitely.
The researchers intend to use their trap to make precise measurements of the positron's magnetic moment -- basically the strength and orientation of its magnetic field. To do this, they must determine two things: the energy needed to flip the direction of the positron's spin and the energy needed to make it jump from its ground state to its first excited state. Using a similar trap setup, Gabrielse and his team have already measured the magnetic moment of an electron to a precision of one part in 10 trillion. Taking the equivalent measurement for a positron will allow them to make a crucial comparison. "We're looking to see if they have an exactly opposite magnetic moment," Gabrielse says. That's the prediction of the Standard Model of particle physics. If the two have different values, however, this could help explain why our universe is made of matter instead of antimatter.
The first step is to send the positrons through the crystal lattice of a sliver of cold tungsten. As the particles go pinballing through the material, they slow down and lose energy. "Every time they bang into something, there's a chance of annihilation, but it doesn't always happen," Gabrielse says. "If they make it to the end, they get popped out." Next, the physicists steer the positrons along a channel using electromagnetic fields. Eventually the particles end up in a vacuum, just three tenths of a degree Celsius above absolute zero, trapped by an electric field on top and bottom and a magnetic field on the sides. "If you want to hold on to antimatter, you have to hold it in a container without walls," Gabrielse says. This method allowed the researchers to isolate and study a solitary, lone positron for a monthâ"and could let them do so indefinitely.
The researchers intend to use their trap to make precise measurements of the positron's magnetic moment -- basically the strength and orientation of its magnetic field. To do this, they must determine two things: the energy needed to flip the direction of the positron's spin and the energy needed to make it jump from its ground state to its first excited state. Using a similar trap setup, Gabrielse and his team have already measured the magnetic moment of an electron to a precision of one part in 10 trillion. Taking the equivalent measurement for a positron will allow them to make a crucial comparison. "We're looking to see if they have an exactly opposite magnetic moment," Gabrielse says. That's the prediction of the Standard Model of particle physics. If the two have different values, however, this could help explain why our universe is made of matter instead of antimatter.
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