American Scientist

After two years of upgrades, the Large Hadron Collider (LHC)—the world’s most powerful particle accelerator—went back online earlier this year. The LHC can now create particle collisions at energies of 13 trillion electron volts, about twice what had been possible before. (An electron volt is a measure of motion energy; higher-energy collisions potentially can produce previously unknown particles, or greater numbers of known ones to study.) But before the LHC got restarted, it had to calibrate its detectors, including the 7,000-ton ATLAS, which was involved in the discovery of the Higgs boson. The event depicted below illustrates how ATLAS recorded a calibration test called a beam splash.

As Pauline Gagnon, a particle physicist who works at the LHC, explains, a wave of particles goes through a tracking detector (red) and provides simultaneous signals to all of ATLAS’s sensors; the length of the yellow and teal bars indicates the energy recorded by a set of sensors that measure heat exchange, the calorimeters (green). The blue lines show activity in detectors for subatomic particles called muons. After calibration, the detector recorded one of its first collisions of protons at full energy in May (below).

The higher-energy collisions have already yielded new information about poorly understood particle groupings called pentaquarks, which aggregate in patterns of five (unlike the more familiar three-quark structure of protons and neutrons). This long-sought state of matter may influence the physics of neutron stars, and may have been important just after the Big Bang. Ongoing studies at the upgraded LHC might even expose the true identity of dark matter or reveal the existence of extra dimensions.