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Matter and antimatter spin alike.

Matter and antimatter spin alike

Physicists believe that matter and antimatter are precisely symmetric. That is, to every kind of subatomic particle there corresponds an antiparticle with exactly the same mass but with the opposite electric charge. Physicists frequently set up experiments to test this belief.

One way of doing it involves the spins of electrons and positrons. Electrons and positrons have opposite polarity of electric charge, the electron's being negative, the positron's positive. However, the amount of electric charge, the mass and the amount of spin ought to be exactly the same. The latest of a series of experiments done at novosibirsk in the USSR used the interaction of electron and positron spins with the magnetic field of the VEPP-2M storage ring to test the sameness of the spins.

Each electron or positron is a little magnet, producing a small magnetic field or, as physicists say, a magnetic moment. If the spins are exactly the same, so should be the strengths of the magnetic moments. As the magnetic moments of these particles interact with the magnetic field of the storage ring, their spins precess, or the spin axes describe a little circle.

The experiment started with electrons and positrons with their spins all in one direction. The researchers forced the spin axes into a horizontal alignment by imposing an electric field that alternates at radio frequencies. Then they turned off the electric field, allowing the spins to precess freely in the magnetic field. Any difference in the size of the spins would cause a phase difference that would gradually build up. After one-tenth of a second, which is time enough for 3 million spin revolutions, the alternating electric field was turned on again. It repolarized the spins, but would have preserved any accumulated phase difference. According to a report in the January/February CERN COURIER, the experiment found that electron and positron spins are the same to 1 part in 100 million, supporting the long-held belief in matter-antimatter symmetry.
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Publication:Science News
Date:Mar 12, 1988
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