At the end of March, the LHC opened its doors to allow particles to travel around the ring for the first time since the year-end technical stop began in December 2015. Progress was good, and the phase of recommissioning with beam could rapidly start. The LHC team worked with low-intensity beam for a few weeks to re-commission all systems and to check all aspects of beam-based operation, to ensure that the LHC was fully safe before declaring "stable beams" – the signal that the experiments could start taking data.

Before the protons could circulate again, the machine underwent the final phase of preparation – known as the machine checkout. During this phase, all of the LHC’s systems are put through their paces without beam. A key part of the process is driving the magnetic circuits, radiofrequency accelerating cavities, collimators, transverse dampers, etc, repeatedly through the nominal LHC cycle.

A full programme of beam instrumentation checks took place, to ensure sure that active elements were working and that the complex acquisition chain was functioning properly. Detailed checks were performed on the collimation systems.

The radiofrequency system was re-commissioned and the LHC beam-dump system was subject to stringent operational checks. In parallel, a pilot beam extracted from the Super Proton Synchrotron (SPS) was sent down the two SPS-LHC transfer lines to the beam dumps just before the start of the LHC.

While the machine checkout was ongoing, the experiments were finishing their own last interventions before the closure of the caverns.

2015 saw the start of Run 2 for the LHC, during which the proton–proton collision energy reached 13 TeV. Beam intensity has increased, and by the end of the 2015 run, 2240 proton bunches per beam were being collided. This year, the aim is to increase the number of bunches even further, to the target of 2748. The goal is to reach an integrated luminosity of around 25 inverse femtobarns (fb–1), up from the 4 fb–1 reached by the end of last year. One fb–1 corresponds to around 80 million million collisions.