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Physicists led by Gerald Gabrielse report trapping and studying a single positron for a month using electromagnetic fields in an ultracold vacuum. The result sets up a planned comparison of the positron’s magnetic moment with the electron’s; that measurement has not yet been reported.
Physicists led by Gerald Gabrielse of Northwestern University report that they trapped and studied a single positron for a month, using electromagnetic fields to keep the antimatter particle from touching ordinary matter. The result gives researchers a way to isolate positrons for extended study and prepare a precision comparison with electrons, a test that could probe whether matter and antimatter obey the same basic symmetry.
The team reported the work in Physical Review Letters. A positron is the antimatter counterpart of an electron: the two have the same mass and opposite electric charges. When antimatter meets matter, the particles can annihilate, so the researchers had to slow the positrons and contain them without a physical wall.
To produce the positrons, the researchers used the radioactive isotope sodium-22, which emits a positron as it decays. They sent the particles through a thin piece of cold tungsten, where collisions with the material reduced their energy. Some positrons annihilated during this process; those that emerged were guided by electromagnetic fields into a vacuum held at three-tenths of a degree Celsius above absolute zero.
There, an electric field confined the particle above and below, while a magnetic field confined it at the sides. The setup let the researchers isolate and study one positron for a month. The report says the method could allow longer confinement, potentially indefinitely, but the demonstrated duration described in the source is one month.
A Route to Testing Matter–Antimatter Symmetry
The immediate value is experimental: a positron held in isolation for an extended period can be examined without the rapid loss that would occur if it contacted ordinary material. The team plans to use the trap for a precise measurement of the positron’s magnetic moment, a property related to the strength and orientation of its magnetic field.
That measurement could be compared with the electron’s magnetic moment. Gabrielse’s team has measured the electron’s value to a precision of one part in 10 trillion using a similar trap, according to the source report. The equivalent positron measurement has not yet been provided. If the two values differ, that would point to a difference between the particles and could guide research into why the observable universe contains far more matter than antimatter. It would not, by itself, explain that imbalance.
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From Positron Capture to Precision Tests
In the standard picture of the early universe, matter and antimatter were produced in roughly comparable amounts. Most would have annihilated, while a small surplus of matter remained and eventually formed stars, planets and other structures. The origin of that surplus remains an open problem in physics.
Researchers test for possible differences between matter and antimatter through several kinds of measurement. The symmetry at issue can be described by combining three operations: replacing particles with their antiparticles, reflecting left and right, and reversing the direction of time. The source report quotes theoretical physicist Roni Harnik of Fermilab, who was not involved in the experiment, describing this as a basic symmetry that current theories say should leave physics unchanged. Comparing the magnetic moments is one test, not a complete account of the cosmic matter surplus.
The team’s electron measurement provides a benchmark for the planned positron work. The new result is the ability to keep a lone positron available for study; it is not a reported discovery that positrons and electrons have different magnetic moments.
“The big challenge is to persuade these positrons to slow down and then capture them.”
— Gerald Gabrielse, Northwestern University physicist who oversaw the project
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The Magnetic-Moment Result Is Pending
The source report does not give a positron magnetic-moment measurement or state that the positron differs from the electron. The month-long confinement is the demonstrated result; the prospect of keeping a particle trapped indefinitely is a possibility described for the method, not a duration established in the report.
It also remains unknown whether the planned comparison will find a measurable difference. Even a difference would be a clue for investigating the matter–antimatter imbalance, rather than a standalone explanation for how the imbalance arose. The report does not specify when the team expects to complete or publish the positron measurement.
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The Team Plans a Positron Measurement
The researchers intend to use the trap to measure the positron’s magnetic moment. To do so, they need to determine the energy required to flip the positron’s spin and the energy needed to move it from its ground state to its first excited state. Those measurements can then be used to establish the magnetic moment and compare it with the electron’s value.
The next major milestone is the reported precision comparison. Until that result is available, the experiment’s significance rests on the new ability to hold a single positron for an extended period and make it accessible for detailed study.
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Key Questions
What antimatter particle did the researchers trap?
They trapped a positron, the antimatter counterpart of an electron. The particles have equal mass and opposite electric charges.
How long was the positron held?
The report describes a single positron isolated and studied for one month. It says the method could permit longer confinement, potentially indefinitely, but does not report a longer demonstrated hold.
How was the positron kept from touching matter?
The team used electric and magnetic fields to confine it in a vacuum. The fields acted as a container without material walls, which would risk contact with matter and annihilation.
Have researchers found that positrons and electrons have different magnetic moments?
No such finding is reported in the source material. The team plans to measure the positron’s magnetic moment and compare it with the electron’s.
Why does the comparison matter?
A difference could point to a departure from the expected matter–antimatter symmetry and provide a clue for research into why the universe contains more matter than antimatter. It would not alone explain the imbalance.
Source: hn
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