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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - FORT2 (Forward Trigger for the CMS phase-2)

Teaser

Summary

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;Í¾
(2) develop innovative L1 jet algorithms;Í¾
(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 th

Work performed

The work started with the simulation of the HGCal trigger primitives, i.e. the modelling of how the HGCal detector and its readout electronics where activated by passing particles, and how to separate genuine double Higgs events from other processes and background noise.
The implementation of the algorithms went hand-in-hand with studies and tests of the electronic used for the HGCal (SkyRoc2 chip), mostly carried out at FESB - Split (Croatia).
Knowing the kind of response to expect from taus and heavy jets in the HGCal, it was then possible to study trigger algorithms, i.e. quick selection strategies with the goal of removing from the data taking events that do not contain interesting physics. Retained events are mostly requested to contain several energetic jets, with at least a pair with an energy compatible with an Higgs boson.
Once a fast, efficient, and reliable algorithm has been identified, it was possible to study the performances of the HGCal, and the whole upgraded CMS experiment, under Phase-II conditions for the decay of pairs of Higgs boson in the bbtautau final state. This was performed for both the gluon fusion and VBF HH production.
These results of the project show that the HL-LHC has the potential to discover HH production and have been submitted for scrutiny to the CMS collaboration, and finally to the whole LHC community. They are finally available as LHC reports:
https://cds.cern.ch/record/2293646
A further extrapolation of these results to the higher energy Future Circular Collider FCC, based on schematic detectors design, has been prepared and presented at international conferences, such as the FCC-week in Amsterdam (https://indico.cern.ch/event/656491/), and dissemination seminars (Durham seminar series, Torino University). These results highlight the FCC capability to perform precision measurements of the HH coupling.
Finally, CMS lead the effort on HH production at the LHC, performing a combination of the whole CMS results on HH. The paper outlining these results is being submitted to PRL and will be available soon.

Final results

The FORT2 project showed the amazing sensitivities and the great potential for physics discoveries in the Higgs sector at the future accelerator facilities at CERN. The measurement of the Higgs trilinear coupling is crucial to complete the picture of the electroweak symmetry breaking, the most fundamental feature of the standard model. At the same time, given the rarity of the process, FORT2 showed just how important are the planned updates of the experiments and of CERN facilities in order to reach this milestone. Dedicated efforts at the LHC were carried out, helping to pin down the future strategies for the HH discovery.
These results are extremely important for planning the strategies of the European physics community after the end of the LHC data taking and into the period of precision physics at the HL-LHC and FCC