← The standard model, antimatter, and how we know

Read a track's curve for the charge sign

In brief

Here are two tracks recorded in the same detector, in the same magnetic field. The two particles had the same mass and the same speed. Yet one track bends left and the other bends right. The detector has caught a difference the energy record cannot show. What is it?

collision pointtrack Atrack Beach dot: the field points out of the page, toward you
Two tracks from the same detector: same length, same field — opposite bends.

You have already seen that a moving charged particle in a magnetic field feels a force. That is how an accelerator's magnets steer. A crosswind bends a thrown ball's flight a little at every point of its path. A detector sits in a magnetic field of its own, and that field bends every charged particle's track the same way: a little at every point.

Inside the detector's magnetic field, the magnetic force bends a charged particle's track point by point, all along its path. The deciding rule is short: oppositely charged particles curve in opposite directions.

You cannot read the sign from one track alone. You need a reference: a track from a particle whose charge is already known.

collision pointelectron (charge −1)each dot: the field points out of the page, toward you
The reference: in this detector, an electron's track curves to the left.

Now read the opening pair against the reference. One track curves to the right, opposite to the electron's. So its charge is the opposite sign: positive. The other track curves to the left, the same way as the electron's. So its charge is the same sign: negative.

Which sign of charge?collision pointsame way as the electron: negativeopposite to the electron: positiveeach dot: the field points out of the page, toward you
Read against the reference: the left-bending track matches the electron, so it is negative; the right-bending track is opposite, so it is positive.

A positron entering the same field curves to the right, because its charge is opposite to the electron's. And that explains the opening pair: same mass, same speed, opposite curves. The two particles carry opposite charge signs. The curve's direction reads out one thing: the sign of the charge, by comparison with the reference.

In a detector where electrons curve left, a new track curves right. What is the sign of the new particle's charge?

Oppositely charged particles curve in opposite directions.

The new track curves opposite to the electron reference track.

The electron's charge is negative, so the new particle's charge is the opposite sign.

The new particle's charge is positive.

In the same detector, a second track curves left, the same way as the electron reference. What is the sign of its charge?

Curving the same way as the reference means the same charge sign as the reference.

The electron's charge is negative.

The second particle's charge is negative.

Check what you know

In a detector, protons curve upward. A new track, from a particle moving the same way through the field, curves downward. What does this comparison show about the new particle's charge?

In a detector, protons curve counterclockwise. A new track curves clockwise. What is the sign of the new particle's charge?

In a detector, electrons curve clockwise. A new track curves counterclockwise. What is the sign of its charge?

In a detector, positrons curve toward the east wall. A new track, from a particle moving the same way through the field, curves toward the west wall. What is the sign of the new particle's charge?

In a detector, electrons curve to the right. Track A curves right; track B curves left. Which pair gives the signs of A and B?

A proton and an antiproton fly out of the same collision into the detector's magnetic field. How do their two tracks curve?