The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) is the highest-energy particle collider in the world. After successful operations at a center-of-mass energy (√s) of 7 and 8TeV, the two general-purpose experiments of the LHC, ATLAS and CMS, have provided the breakthrough discovery of the Higgs boson. In the first data-taking period of its operation, known as Run-I, the collider delivered an integrated luminosity of 25fb-1 to its experiments. In early 2013 the LHC concluded Run-I when the CERN accelerator complex shut down for 2 years of planned maintenance and consolidation, known as Long Shutdown (LS) 1. This work has prepared the LHC accelerators and experiments for p-p collisions at √s=13 TeV and at design luminosity of 1034 cm-2s-1 and 25ns bunch spacing. The first beams re-circulated the LHC in March 2015, kick-starting Run-II and delivering more than 4.22fb-1 (November 2015) of additional collision data to the experiments. To date, several published results from ATLAS and CMS confirm the existence of the Higgs boson at the mass region of 125 GeVc-2.
In order to build on the Higgs boson discovery and extend the discovery potential, CERN is planning for a major upgrade to the LHC known as High-Luminosity LHC (HL-LHC), which will be implemented over the next decade. It is expected that in Run-II CMS will experience an average of about 25 inelastic interactions per bunch-crossing, referred to as event pileup (PU) and the luminosity will exceed the original design before LS2, which is planned for mid-2018. During LS2, the injector chain will be further improved and upgraded to increase the peak luminosity and help deliver an integrated luminosity of 300 fb-1 for Run-III by the end of 2022. The LS3 is scheduled for 2023, and the main task will be to replace the quadrupoles that focus the beams at the ATLAS and CMS collision regions. In addition, crab-cavities will be added to optimize the bunch overlap at the interaction region. These changes will produce a significant increase in the luminosity by a factor of 10 beyond the LHC’s design value, and will bring us to HL-LHC or Phase-II of the LHC physics program.
The HL-LHC, which should be operational by 2025, will allow precise studies of the new particles observed at the LHC and the observation of rare processes that are inaccessible at the LHC’s current sensitivity level. The central mission of the HL-LHC Physics program includes measurements of the Higgs boson properties (such as spin-parity, production cross-section, couplings to particles) with high precision and the continuation of searches for new Beyond the Standard Model (BSM) Physics. The higher luminosity capabilities of HL-LHC will extend the discovery mass reach, thus allowing more sensitive searches for signatures of new Physics or precision studies in case new particles are found. There are many ways that Supersymmetry (SUSY) might be eluding observation, and while the LHC Experiments are slowly closing these loopholes, a significant amount of integrated luminosity is required to exclude all possible scenarios, and some of these scenarios will not be fully excluded before the HL-LHC. There are at the moment very strong indications for new physics from Run-II of LHC, with the presence of a di-photon resonance at the mass region of 750 GeVc-2. Both ATLAS and CMS experiments have reported an excess of events in the mass region of 750 GeVc-2 with a global statistical significance that exceeds the 3σ-level, increasing confidence that new Physics is indeed just around the corner.
The HL-LHC project was announced as the top priority of the European Strategy for Particle Physics in 2013 and its funding is included in CERN's Medium-Term Plan. Its development depends on several technological innovations. The design study came to a close on 31 October 2015 with the publication of a technical design report, marking the start of the construction phase for the project at CERN and in industry. CERN will devote 950 million CHF of its budget over a period of 10 years to the development of the HL-LHC.