The electron is an elementary particle with a negative electric charge equal to one elementary charge. Specifically, it is the first-generation lepton and, excluding neutrinos and massless particles, it is the lightest known particle by mass. It has Spin 1/2, which makes it a fermion.
The antiparticle of the electron is called the positron.
Discoveries#
The positron#
American physicist Carl Andersen organized and executed an experiment in 1932 using a cloud chamber to measure secondary cosmic rays. The cloud chamber, under a magnetic field, contained a 6 mm lead plate through which particles would pass through, acting as a "speed bump" that allows us to understand where the particle came from. The particle of course loses energy while passing through the lead (see Stopping power), so the side with the shorter trail must be destination, indicating the particle came from the other side.
PositronTrailAndersen.png|500
The photograph of the positron trail, from the original 1933 article.
There were a couple of surprise with this measurement:
- The range of the outgoing particle was way too high. The length of the ionization trail was just over 5 cm, whereas protons were known to generally not go longer than 5 mm, a tenfold decrease. Conveniently, the electron range was known to be around 6 cm or so, just right for this measurement.
- The trail was curving the wrong way. The direction of the curvature immediately suggested that the particle was positive. The assumption at the time was a proton (the only positive particle known at the time), but due to point 1. it couldn't be.
The combination of these two factors led Andersen to determine that this must be a particle with the mass of the electron, but opposite charge: a positron. Beyond being a new particle, this was a striking confirmation of the results from the Dirac equation and pointed to the Dirac sea being real (or at least a sound description).