The Antihydrogen trap (ATRAP) is an experiment to compare hydrogen atoms with their antimatter equivalents – antihydrogen atoms. In 2002, ATRAP provided the first glimpse inside antihydrogen atoms after researchers successfully created and measured a large number of them.

An atom of antihydrogen consists of an antiproton and a positron (an antielectron). One of the difficulties in making antimatter is the energy the antiprotons possess when they are first made, shooting out of the apparatus at close to the speed of light. The researchers use a process called "cooling" to slow the antiprotons down so that they can be studied.

ATRAP was the first experiment to use cold positrons to cool antiprotons. The two ingredients were confined in the same trap and when they had both reached a similar temperature, some combined to form atoms of antihydrogen (a positron orbiting an antiproton). This technique was developed from another experiment at CERN called TRAP, the predecessor of ATRAP.

The current experiment was set up in the late 1990s at the same time as the ATHENA experiment. Both experiments had the same goals and used similar methods to produce antihydrogen atoms, but had different detectors.

While the ATHENA experiment came to an end in 2004, ATRAP is still in operation. It continues to create antihydrogen cold enough and trapped for long enough to enable precise measurements and comparisons with ordinary hydrogen.