LHC/CMS Restart 2015
At about half past nine CET this morning, for the first time since the Large Hadron Collider (LHC) started up after two years of maintenance and repairs, the accelerator delivered proton-proton collisions to the LHC experiments ALICE, ATLAS, CMS and LHCb at an energy of 450 gigaelectronvolts (GeV). These collisions, which take place with each beam at the so-called injection energy, that is, the energy at which proton beams are injected into the LHC from the Super Proton Synchrotron, enable the LHC experiments to tune their detectors. This process is also an important step towards readying the accelerator to deliver beams at 6.5 teraelectronvolts (TeV) for collisions at 13 TeV. The picture shows a di-jet event produced from proton-proton collisions detected in the CMS detector. The green bars represent the energy deposited in the electromagnetic calorimeter and the blue ones represent the energy in the hadron calorimeter. The total energy is approximately 30 GeV in each jet. (Text/Image: CMS/Tom McCauley)