The LHC machine and experiments

The Large Hadron Collider is a 26.7 km circular accelerator based on a twin aperture superconducting magnet design with a design proton beam energy of 7 TeV. The four particle physics experiments ALICE, ATLAS, CMS, and LHCb are located around the ring. The LHC was first operated with beams for short periods in 2008 and 2009. In 2010, a first experience with the machine was gained at a beam energy of 3.5 TeV, with moderate beam intensities of 1.1 × 10$^{11}$ protons per bunch (ppb) and up to ∼200 bunches. In 2011, the beam intensity was increased to ∼1400 bunches of 1.4 × 10$^{11}$ ppb, while 2012 was dedicated to luminosity production with higher bunch intensities (1.6 × 10$^{11}$ ppb) and a beam energy of 4 TeV. The running years 2010–2013 are commonly referred to as Run 1. In early 2013 beam operation was stopped for a 2 year long shutdown (LS1) to complete work on the magnets in view of reaching the design beam energy. Beam operation resumed in 2015 with beam energies of 6.5 TeV following a dipole training campaign that took place at the end of LS1. The LHC experiments had expressed a strong preference for beams with 25 ns bunch spacing, as opposed to the 50 ns spacing used in 2011–2012, as this would result in too many inelastic collisions per crossing (pile-up). On the machine side, this posed additional challenges, so 2015 became a learning year dedicated to preparing the machine for full luminosity production in 2016–2018 (Run 2). In addition to the proton beams, one month per year is dedicated to running with heavy ions, providing either Pb–Pb or p–Pb collisions. The first two years of Run 1 provided Pb–Pb collisions to the experiments, and the final year was dedicated to p–Pb. Run 2 (2015–2018) again saw a mix of Pb–Pb and p–Pb set-ups, except in 2017- when Xe–Xe collisions were provided for the first time to the experiments.