The physics program of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) is off to an historical start. The highlights have been the discovery and the measure of the properties of the Higgs boson, the missing building block of the Standard Model (SM) of particle physics. A discovery that resulted in the 2013 Nobel prize for physics. To continue the quest to answer fundamental questions in particle physics the LHC will need a major upgrade around 2025 to increase its luminosity (rate of collisions) by a factor of 10 beyond the original design value, the so-called LHC Phase-II or High-Luminosity LHC (HL-LHC).
One of the main motivations for the Phase-II program is the study of the Higgs boson self-coupling (or trilinear coupling), i.e. the way the Higgs boson interacts with itself. The Higgs trilinear coupling is a fundamental parameter of the SM and dictates several of the most fundamental properties of the mechanism by which the interaction with the Higgs field gives mass to the Standard Model particles, as well as regulating the behaviour of the most important properties of the Standard Model. The main way to access this parameter is the measurement of Higgs boson pair production, where the two H bosons in the final state come from the decay of a single H. This process is about 1000 times rarer than single H production and it is therefore out of reach at the LHC. Evidence of this process can be measured at the HL-LHC, but the successful completion of this research program will require significant optimisations and detector upgrades to overcome challenging HL-LHC conditions.
The main challenges for CMS during the Phase-II are the radiation damage to the detector from the very large number of particles being produced at the HL-LHC and the very high number of simultaneous collisions (pileup) originated from the high instantaneous luminosity. Under these challenging conditions to maintain its excellent performance the CMS experiment will perform an ambitious upgrade program. One of the main elements of this program is the replacement of the existing electromagnetic and hadronic endcap calorimeters with a High Granularity Calorimeter (HGCal) that uses a high transverse and longitudinal granularity.
The HGCal is a technological challenge and a true paradigm shift for a calorimeter at a hadron collider, showing the evolution of the calorimetry techniques in high-energy physics. The critical performance advantages of the HGCal with respect any another calorimeter is that, in addition to the conventional energy measurement, it is possible to track the growth and measure the angle of an electromagnetic shower and to apply the methods of Particle Flow to better identify and reconstruct individually each particles (leptons, neutral and charged hadrons, jets).
The measurement of double Higgs boson production in VBF is extremely ambitious but it is a key topic for HL-LHC. It aims both to access the self-coupling of the Higgs boson which as we said plays an essential role in the mechanism of spontaneous electroweak symmetry breaking and to produce extra information on SM interactions, with the measurement of the VVHH vertex that directly relates to the interaction between the Higgs boson and the weak force carriers (W, Z bosons) and is extremely sensitive to the presence of physics beyond the Standard Model. The FORT2 project planned to study and optimise the strategies to identify, select and collect these events as well as determine the minimal requirements HGCal must satisfy in order to be able to carry on this program during the Phase-II.
The three main objectives of FORT2 were hence:
(1) design and prototype the HGCal L1 trigger primitive generation;;
(2) develop innovative L1 jet algorithms;;
(3) perform the first complete prospective analysis of double Higgs boson production in Vector Boson Fusion (VBF) events at HL-LHC.
In order to fully appreciate the HL-LHC capabilities, it is important to compare these results with the best knowledge we can get with the current data available at the LHC. For this reason, I performed a full combination of all the LHC HH results and compared it with what can be achieved at the HL-LHC and at the future FCC collider, a bold project to establish a 100km collider at CERN, to be carried out after the HL-LHC.
