Luca Cadamuro

This report comprises the outcome of five working groups that have studied the physics potential of the high-luminosity phase of the LHC (HL-LHC) and the perspectives for a possible future high-energy LHC (HE-LHC).The working groups covered a broad range of topics: Standard Model measurements, studies of the properties ofthe Higgs boson, searches for phenomena beyond the Standard Model, flavor physics of heavy quarks and leptonsand studies of QCD matter at high density and temperature.The work is prepared as an input to the ongoing process of updating the European Strategy for Particle Physics,a process that will be concluded in May 2020.

The High Granularity Timing Detector (HGTD) will be installed in the ATLAS experiment to mitigate pile-up effects during the High Luminosity (HL) phase of the Large Hadron Collider (LHC) at CERN. Low Gain Avalanche Detectors (LGADs) will provide high-precision measurements of the time of arrival of particles at the HGTD, improving the particle-vertex assignment. To cope with the high-radiation environment, LGADs have been optimized by adding carbon in the gain layer, thus reducing the acceptor removal rate after irradiation. Performances of several carbon-enriched LGAD sensors from different vendors, and irradiated with high fluences of 1.5 and 2.5 x 10^15 neq/cm2, have been measured in beam test campaigns during the years 2021 and 2022 at CERN SPS and DESY. This paper presents the results obtained with data recorded by an oscilloscope synchronized with a beam telescope which provides particle position information within a resolution of a few um. Collected charge, time resolution and hit efficiency measurements are presented. In addition, the efficiency uniformity is also studied as a function of the position of the incident particle inside the sensor pad.

The discovery of the Higgs boson in 2012, by the ATLAS and CMS experiments, was a success achieved with only a percent of the entire dataset foreseen for the LHC. It opened a landscape of possibilities in the study of Higgs boson properties, Electroweak Symmetry breaking and the Standard Model in general, as well as new avenues in probing new physics beyond the Standard Model. Six years after the discovery, with a conspicuously larger dataset collected during LHC Run 2 at a 13 TeV centre-of-mass energy, the theory and experimental particle physics communities have started a meticulous exploration of the potential for precision measurements of its properties. This includes studies of Higgs boson production and decays processes, the search for rare decays and production modes, high energy observables, and searches for an extended electroweak symmetry breaking sector. This report summarises the potential reach and opportunities in Higgs physics during the High Luminosity phase of the LHC, with an expected dataset of pp collisions at 14 TeV, corresponding to an integrated luminosity of 3 ab$^{-1}$. These studies are performed in light of the most recent analyses from LHC collaborations and the latest theoretical developments. The potential of an LHC upgrade, colliding protons at a centre-of-mass energy of 27 TeV and producing a dataset corresponding to an integrated luminosity of 15 ab$^{-1}$, is also discussed.

The charge asymmetry in the production of top quark and antiquark pairs is measured in proton-proton collisions at a center-of-mass energy of 8 TeV. The data, corresponding to an integrated luminosity of 19.6 inverse femtobarns, were collected by the CMS experiment at the LHC. Events with a single isolated electron or muon, and four or more jets, at least one of which is likely to have originated from hadronization of a bottom quark, are selected. A template technique is used to measure the asymmetry in the distribution of differences in the top quark and antiquark absolute rapidities. The measured asymmetry is A[c,y] = [0.33 +/- 0.26 (stat) +/- 0.33 (syst)]%, which is the most precise result to date. The results are compared to calculations based on the standard model and on several beyond-the-standard-model scenarios.

The aim of this paper is to fully characterize the new multi-anode photomultiplier tube R11265-103-M64, produced by Hamamatsu. Its high effective active area (77%), its pixel size, the low dark signal rate and the capability to detect single photon signals make this tube suitable for an application in high energy physics, such as for RICH detectors. Four tubes and two different bias voltage dividers have been tested. The results of a standard characterization of the gain and the anode uniformity, the dark signal rate, the cross-talk and the device behaviour as a function of temperature have been studied. The behaviour of the tube is studied in a longitudinal magnetic field up to 100 Gauss. Shields made of a high permeability material are also investigated. The deterioration of the device performance due to long time operation at intense light exposure is studied. A quantitative analysis of the variation of the gain and the dark signals rate due to the aging is described.

This is the third out of five chapters of the final report [1] of the Workshop on Physics at HL-LHC, and perspectives on HE-LHC [2]. It is devoted to the study of the potential, in the search for Beyond the Standard Model (BSM) physics, of the High Luminosity (HL) phase of the LHC, defined as $3~\mathrm{ab}^{-1}$ of data taken at a centre-of-mass energy of $14~\mathrm{TeV}$, and of a possible future upgrade, the High Energy (HE) LHC, defined as $15~\mathrm{ab}^{-1}$ of data at a centre-of-mass energy of $27~\mathrm{TeV}$. We consider a large variety of new physics models, both in a simplified model fashion and in a more model-dependent one. A long list of contributions from the theory and experimental (ATLAS, CMS, LHCb) communities have been collected and merged together to give a complete, wide, and consistent view of future prospects for BSM physics at the considered colliders. On top of the usual standard candles, such as supersymmetric simplified models and resonances, considered for the evaluation of future collider potentials, this report contains results on dark matter and dark sectors, long lived particles, leptoquarks, sterile neutrinos, axion-like particles, heavy scalars, vector-like quarks, and more. Particular attention is placed, especially in the study of the HL-LHC prospects, to the detector upgrades, the assessment of the future systematic uncertainties, and new experimental techniques. The general conclusion is that the HL-LHC, on top of allowing to extend the present LHC mass and coupling reach by $20-50\%$ on most new physics scenarios, will also be able to constrain, and potentially discover, new physics that is presently unconstrained. Moreover, compared to the HL-LHC, the reach in most observables will generally more than double at the HE-LHC, which may represent a good candidate future facility for a final test of TeV-scale new physics.

The successful operation of the Large Hadron Collider (LHC) and the excellent performance of the ATLAS, CMS, LHCb and ALICE detectors in Run-1 and Run-2 with $pp$ collisions at center-of-mass energies of 7, 8 and 13 TeV as well as the giant leap in precision calculations and modeling of fundamental interactions at hadron colliders have allowed an extraordinary breadth of physics studies including precision measurements of a variety physics processes. The LHC results have so far confirmed the validity of the Standard Model of particle physics up to unprecedented energy scales and with great precision in the sectors of strong and electroweak interactions as well as flavour physics, for instance in top quark physics. The upgrade of the LHC to a High Luminosity phase (HL-LHC) at 14 TeV center-of-mass energy with 3 ab$^{-1}$ of integrated luminosity will probe the Standard Model with even greater precision and will extend the sensitivity to possible anomalies in the Standard Model, thanks to a ten-fold larger data set, upgraded detectors and expected improvements in the theoretical understanding. This document summarises the physics reach of the HL-LHC in the realm of strong and electroweak interactions and top quark physics, and provides a glimpse of the potential of a possible further upgrade of the LHC to a 27 TeV $pp$ collider, the High-Energy LHC (HE-LHC), assumed to accumulate an integrated luminosity of 15 ab$^{-1}$.

The Compact Muon Solenoid (CMS) experiment has implemented a sophisticated two-level online selection system that achieves a rejection factor of nearly 105. During Run II, the LHC will increase its centre-of-mass energy up to 13 TeV and progressively reach an instantaneous luminosity of 2 × 1034 cm−2 s−1. In order to guarantee a successful and ambitious physics programme under this intense environment, the CMS Trigger and Data acquisition (DAQ) system has been upgraded. A novel concept for the L1 calorimeter trigger is introduced: the Time Multiplexed Trigger (TMT) . In this design, nine main processors receive each all of the calorimeter data from an entire event provided by 18 preprocessors. This design is not different from that of the CMS DAQ and HLT systems. The advantage of the TMT architecture is that a global view and full granularity of the calorimeters can be exploited by sophisticated algorithms. The goal is to maintain the current thresholds for calorimeter objects and improve the performance for their selection. The performance of these algorithms will be demonstrated, both in terms of efficiency and rate reduction. The callenging aspects of the pile-up mitigation and firmware design will be presented.

Results from the completed Phase 1 Upgrade of the Compact Muon Solenoid (CMS) Level-1 Calorimeter Trigger are presented. The upgrade was performed in two stages, with the first running in 2015 for proton and heavy ion collisions and the final stage for 2016 data taking. The Level-1 trigger has been fully commissioned and has been used by CMS to collect over 43 fb−1 of data since the start of the Run II of the Large Hadron Collider (LHC). The new trigger has been designed to improve the performance at high luminosity and large number of simultaneous inelastic collisions per crossing (pile-up). For this purpose it uses a novel design, the Time Multiplexed Trigger (TMT), which enables the data from an event to be processed by a single trigger processor at full granularity over several bunch crossings. The TMT design is a modular design based on the μTCA standard. The trigger processors are instrumented with Xilinx Virtex-7 690 FPGAs and 10 Gbps optical links. The TMT architecture is flexible and the number of trigger processors can be expanded according to the physics needs of CMS. Sophisticated and innovative algorithms are now the core of the first decision layer of the experiment. The system has been able to adapt to the outstanding performance of the LHC, which ran with an instantaneous luminosity well above design. The performance of the system for single physics objects are presented along with the optimizations foreseen to maintain the thresholds for the harsher conditions expected during the LHC Run II and Run III periods.

The Compact Muon Solenoid (CMS) experiment implements a sophisticated two-level online selection system that achieves a rejection factor of nearly 105. During Run II, the LHC has increased the centre-of-mass energy of proton-proton collisions up to 13 TeV and may progressively reach an instantaneous luminosity of 2×1034 cm−2 s−1 or higher. In order to guarantee a successful and ambitious physics programme under this intense environment, the CMS Trigger and Data acquisition system has been upgraded. The upgraded CMS Level-1 (L1) trigger benefits from the recent μTCA technology and is designed to maintain the performance under high instantaneous luminosity conditions. More sophisticated, innovative algorithms are now the core of the first decision layer of CMS: this drastically reduces the trigger rate and improves the trigger efficiency for a wide variety of physics processes. In this document, we present the overall architecture of the upgraded Level-1 trigger system. The performance of single object triggers, measured on collision data recorded during the 2016 running period, are also summarised.

A search is performed for the production of a massive W′ boson decaying to a top and a bottom quark. The data analysed correspond to an integrated luminosity of 19.7 fb−1 collected with the CMS detector at the LHC in proton-proton collisions at $$ \sqrt{s}=8 $$ TeV. The hadronic decay products of the top quark with high Lorentz boost from the W′ boson decay are detected as a single top flavoured jet. The use of jet substructure algorithms allows the top quark jet to be distinguished from standard model QCD background. Limits on the production cross section of a right-handed W′ boson are obtained, together with constraints on the left-handed and right-handed couplings of the W′ boson to quarks. The production of a right-handed W′ boson with a mass below 2.02 TeV decaying to a hadronic final state is excluded at 95% confidence level. This mass limit increases to 2.15 TeV when both hadronic and leptonic decays are considered, and is the most stringent lower mass limit to date in the tb decay mode.

The CMS experiment relies on a two-level online selection system that achieves a rejection factor of nearly 10 5 .The first level (L1) is based on coarse information coming from the calorimeters and the muon detectors while the High Level Trigger combines fine-grain information from all subdetectors.During Run II, the centre-of-mass energy of the LHC collisions will be increased to 13 or 14 TeV, and progressively an instantaneous luminosity of 10 34 cm 2 s 1 will be reached.To guarantee a successful and ambitious physics program in this intense environment, the CMS Trigger and Data Acquisition system must be upgraded.The L1 Calorimeter Trigger hardware and architecture will be upgraded in particular, allowing sophisticated algorithms to be deployed.These algorithms will better exploit the calorimeter granularity and will open the possibility of making correlations between different parts of the detector.In this context, an optimised tau algorithm, implementing an innovative dynamic clustering technique, has been developed for the selection of hadronically decaying tau leptons, which represents a real challenge for an electronics trigger system.The performance of this tau trigger will be demonstrated, both in terms of efficiency and rate reduction.The different handles to control trigger rates in different pile-up scenarios will be described.

The Compact Muon Solenoid (CMS) employs a sophisticated two-level online triggering system that has a rejection factor of up to 105. Since the beginning of Run II of LHC, the conditions that CMS operates in have become increasingly challenging. The centre-of-mass energy is now 13 TeV and the instantaneous luminosity currently peaks at 1.5 ×1034 cm−2s−1. In order to keep low physics thresholds and to trigger efficiently in such conditions, the CMS trigger system has been upgraded. A new trigger architecture, the Time Multiplexed Trigger (TMT) has been introduced which allows the full granularity of the calorimeters to be exploited at the first level of the online trigger. The new trigger has also benefited immensely from technological improvements in hardware. Sophisticated algorithms, developed to fully exploit the advantages provided by the new hardware architecture, have been implemented. The new trigger system started taking physics data in 2016 following a commissioning period in 2015, and since then has performed extremely well. The hardware and firmware developments, electron and photon algorithms together with their performance in challenging 2016 conditions is presented.

Higgs boson pair production is traditionally considered to be of particular interest for a measurement of the trilinear Higgs self-coupling. Yet it can offer insights into other couplings as well, since - in an effective field theory (EFT) parameterisation of potential new physics - both the production cross section and kinematical properties of the Higgs boson pair depend on various other Wilson coefficients of EFT operators. This note summarises the ongoing efforts related to the development of EFT tools for Higgs boson pair production in gluon fusion, and provides recommendations for the use of distinct EFT parameterisations in the Higgs boson pair production process. This document also outlines where further efforts are needed and provides a detailed analysis of theoretical uncertainties. Additionally, benchmark scenarios are updated. We also re-derive a parameterisation of the next-to-leading order (NLO) QCD corrections in terms of the EFT Wilson coefficients both for the total cross section and the distribution in the invariant mass of the Higgs boson pair, providing for the first time also the covariance matrix. A reweighting procedure making use of the newly derived coefficients is validated, which can be used to significantly speed up experimental analyses.

The ATLAS and CMS Collaborations are performing an extensive study of the Higgs boson properties and couplings thanks to the unprecedented possibilities offered by the LHC Run 2 dataset. The Higgs boson mass and width, its couplings, and the production cross section in several kinematic regions are studied with the combination of different production modes and decay channels. This document reviews the current status of these measurements and the prospects for the high-luminosity LHC.

The CMS Level-1 calorimeter trigger is being upgraded in two stages to maintain performance as the LHC increases pile-up and instantaneous luminosity in its second run. In the first stage, improved algorithms including event-by-event pile-up corrections are used. New algorithms for heavy ion running have also been developed. In the second stage, higher granularity inputs and a time-multiplexed approach allow for improved position and energy resolution. Data processing in both stages of the upgrade is performed with new, Xilinx Virtex-7 based AMC cards.

This thesis presents innovative contributions to the CMS experiment in the new trigger system for the restart of the LHC collisions in Run II, as well as original analysis methods and important results that led to official publications of the Collaboration.

A search for massive resonances decaying to a Z boson and a photon is performed in events with a hadronically decaying Z boson candidate, separately in light-quark and b quark decay modes, identified using jet substructure and advanced b tagging techniques. Results are based on samples of proton-proton collisions collected with the CMS detector at the LHC at center-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 19.7 and 2.7 inverse femtobarns, respectively. The results of the search are combined with those of a similar search in the leptonic decay modes of the Z boson, based on the same data sets. Spin-0 resonances with various widths and with masses in a range between 0.2 and 3.0 TeV are considered. No significant excess is observed either in the individual analyses or the combination. The results are presented in terms of upper limits on the production cross section of such resonances and constitute the most stringent limits to date for a wide range of masses.

The inelastic hadronic cross section in proton-lead collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV is measured with the CMS detector at the LHC. The data sample, corresponding to an integrated luminosity of 12.6 +/- 0.4 inverse nanobarns, has been collected with an unbiased trigger for inclusive particle production. The cross section is obtained from the measured number of proton-lead collisions with hadronic activity produced in the pseudorapidity ranges 3<abs(eta)<5 and/or -5<abs(eta)<-3, corrected for photon-induced contributions, experimental acceptance, and other instrumental effects. The inelastic cross section is measured to be sigma[inel,pPb]=2061 +/- 3 (stat) +/- 34 (syst) +/- 72 (lum) mb. Various Monte Carlo generators, commonly used in heavy ion and cosmic ray physics, are found to reproduce the data within uncertainties. The value of sigma[inel,pPb] is compatible with that expected from the proton-proton cross section at 5.02 TeV scaled up within a simple Glauber approach to account for multiple scatterings in the lead nucleus, indicating that further net nuclear corrections are small.

IT

The study of Higgs boson pair production at the LHC gives unique insight into the electroweak symmetry breaking mechanism. Both the resonant and non-resonant production modes need to be explored to search for extensions of the scalar sector and to probe the shape of scalar potential itself. The ATLAS and CMS Collaborations have searched for Higgs boson pair production in proton-proton collisions at $\sqrt{s} = 8\,\text{TeV}$ and $13\,\text{TeV}$, and the results of these searches as well as the prospects for future measurements are discussed here.

Cette these presente une recherche de la production de paires de bosons de Higgs en utilisant les donnees des collision proton-proton a sqrt(s) = 13 TeV enregistrees avec les detecteur CMS aupres du LHC du CERN. Les evenements avec les deux boson de Higgs se desintegrant en une paire de quark b et de leptons tau (HH -> bb tau+tau-) sont utilises pour l’exploration des mecanismes de production resonante et non-resonante.La mesure de la production de HH est experimentalement difficile a cause de la petite section efficace prevue par le modele standard de la physique des particules. Sa recherche est neanmoins motivee par les informations qu’elle peut reveler sur la nature de la brisure de la symetrie electrofaible. La production de HH donne acces a l’auto-couplage trilineaire du boson de Higgs et, par consequent, a la forme du potentiel scalaire. En plus, elle est sensible a la presence de physique au-dela du modele standard. La presence de nouvelles resonances se desintegrant en HH et de couplages anormales du boson de Higgs sont etudiees dans ce travail.Les leptons tau ont un role de premier plan dans cette recherche et un effort important a ete consacre a l’amelioration de leur efficacite de selection par le systeme de declenchement de l’experience CMS. En particulier, le declenchement de premier niveau (L1) a ete ameliore pour faire face aux nouvelles conditions des collisions du Run II du LHC, marque par une augmentation de l’energie dans le centre de masse et de la luminosite instantanee. Ce nouveau systeme de declenchement permet de developper un algorithme specifique pour la reconstruction des leptons tau se desintegrant en hadrons (tauh) et un neutrino.Cet algorithme se fonde une technique avancee de regroupement dynamique de l’energie et utilise des criteres dedies pour la rejection du bruit de fond. Le developpement, l’implementation, et la verification de son fonctionnement pour le redemarrage du LHC sont presentes ici. La performance de l’algorithme est initialement evaluee grâce a une simulation et puis mesuree avec les donnees de l’experience CMS. Son excellente performance est un element essentiel dans la recherche de la production de HH.L’investigation du processus HH -> bb tau+tau- utilise les trois canaux de desintegration du systeme tau+tau- avec au moins un tauh dans l’etat final. La selections et categorisation des evenements sont concues pour optimiser la sensibilite de la recherche, et des techniques d’analyse multivaries sont mises en place pour distinguer le signal du bruit de fond.Les resultats sont presentes en utilisant une luminosite integree de 35.9 fb-1. Ils sont compatibles, dans les incertitudes experimentales, avec les predictions du modele standard pour les bruits de fond. Des limites superieures a la production resonante de HH sont evaluees et interpretees dans le contexte du modele standard supersymmetrique minimal. Les limites superieures a la production non-resonante permettent de contraindre l’espace des parametres des couplages anormales du boson de Higgs. Les limites superieures observees et attendues correspondent respectivement a environ 30 et 25 fois la prediction du modele standard. Ils representent l’un des resultats les plus sensibles a la production de HH atteint jusqu'a present au LHC.Les perspectives pour l‘observation de la production de HH au LHC sont enfin discutees. Les resultats precedemment obtenus sont extrapoles a une luminosite integree de 3000 fb-1 sous des differentes assomptions pour la performance du detecteur et de l’analyse.

A measurement of the ttbar production cross section at sqrt(s)=13 TeV is presented using proton-proton collisions, corresponding to an integrated luminosity of 2.3 inverse femtobarns, collected with the CMS detector at the LHC. Final states with one isolated charged lepton (electron or muon) and at least one jet are selected and categorized according to the accompanying jet multiplicity. From a likelihood fit to the invariant mass distribution of the isolated lepton and a jet identified as coming from the hadronization of a bottom quark, the cross section is measured to be sigma(ttbar)= 835 +/- 3 (stat) +/- 23 (syst) +/- 23 (lum) pb, in agreement with the standard model prediction. Using the expected dependence of the cross section on the pole mass of the top quark (m[t]), the value of m[t] is found to be 172.7+2.4-2.7 GeV.

The discovery of the Higgs boson confirms the existence of a scalar sector of the standard model, responsible for electroweak symmetry breaking, but the nature and properties of the potential at the origin of this mechanism are still unknown. By studying the production of pairs of Higgs bosons (HH), physicists can directly measure the coupling of the Higgs boson to itself and thus determine the shape of this potential, which has far-reaching implications on the origin and evolution of our Universe. Because of this deep connection to the foundations of electroweak symmetry breaking, HH production is also an ideal place to search for manifestations of yet-unknown physics, such as modifications of the strength of the self-coupling and of the interaction between pairs of vector bosons and Higgs bosons. In this review article, we summarize the current searches for HH production at ATLAS and CMS, using the LHC Run 2 dataset, discuss the implications of our current constraints on physics beyond the standard model, and briefly review prospect for future HH searches.

This thesis describes a search for Higgs boson pair ($\text{HH}$) production using proton-proton collision data collected at $\sqrt{s} = 13~\text{TeV}$ with the CMS experiment at the CERN LHC. Events with one Higgs boson decaying into two $\text{b}$ quarks and the other decaying into two $\tau$ leptons ($\text{HH}\to \text{b}\bar{\text{b}}\tau^+\tau^-$) are explored to investigate both resonant and nonresonant production mechanisms. $\text{HH}$ production gives access to the Higgs boson trilinear self-coupling and is sensitive to the presence of physics beyond the standard model. A considerable effort has been devoted to the development of an algorithm for the reconstruction of $\tau$ leptons decays to hadrons ($\tau_\text{h}$) and a neutrino for the Level-1 calorimeter trigger of the experiment, that has been upgraded to face the increase in the centre-of-mass energy and instantaneous luminosity conditions expected for the LHC Run II operations. The algorithm implements a sophisticated dynamic energy clustering technique and dedicated background rejection criteria. Its structure, optimisation and implementation, its commissioning for the LHC restart at $13~\text{TeV}$, and the measurement of its performance are presented. The algorithm is an essential element in the search for $\text{HH}$ production. The investigation of the $\text{HH}\to \text{b}\bar{\text{b}}\tau^+\tau^-$ process explores the three decay modes of the $\tau^+\tau^-$ system with one or two $\tau_\text{h}$ in the final state. A dedicated event selection and categorisation is developed and optimised to enhance the sensitivity, and multivariate techniques are applied for the first time to these final states to separate the signal from the background. Results are derived using an integrated luminosity of $35.9~\text{fb}^{-1}$. They are found to be consistent, within uncertainties, with the standard model background predictions. Upper limits are set on resonant and nonresonant $\text{HH}$ production and constrain the parameter space of the minimal supersymmetric standard model and anomalous Higgs boson couplings. The observed and expected upper limits are about 30 and 25 times the standard model prediction respectively, corresponding to one of the most stringent limits set so far at the LHC. Finally, prospects for future measurements of $\text{HH}$ production at the LHC are evaluated by extrapolating the current results to an integrated luminosity of $3000~\text{fb}^{-1}$ under different detector and analysis performance scenarios.

The chapter presents the motivations for the search of Higgs boson pair production at the CERN LHC and discusses the theoretical foundations of this work. After an overview of the standard model of particle physics (SM), with a special focus on the scalar sector and its role in the electroweak symmetry breaking, the importance of Higgs boson pair (HH) production in the SM exploration is discussed. The main HH production modes at the LHC are presented and the properties of the HH signal desribed. The chapter then discusses HH production in scenarios of physics beyond the SM, that predict either the presence of new scalars decaying to HH (resonant production), or the existence of anomalous Higgs boson couplings that modify the HH cross section and the signal kinematic properties (nonresonant production). Finally, the HH decay channels used in the experimental study of this process are described and the current status of searches for HH production at the LHC presented.

An accurate modelling of the signal and background processes is crucial to explore the selected data events. Both simulations performed with the Monte Carlo technique and estimations in signal-free data regions (usually referred to as “data-driven” methods) are adopted to model the processes involved in the search as described in this chapter. Residual discrepancies between the modelling and the observed data, as well as statistical uncertainties affecting the data-driven methods, are taken into account as systematic uncertainties, that are part of the statistical interpretation of the data. A description of their sources and of their effects on the sensitivity of the search is presented.

The high luminosity phase of the LHC (HL-LHC) is planned to follow a LHC Run III where $${300}{{\,\text {fb}^{-1}}}$$ of collisions are expected to be collected, and to start after the completion of the LHC upgrades during the long shutdown in 2024–2025. The goal is to increase the integrated luminosity further by a factor of 10, collecting a dataset of about $${3000}{{\,\text {fb}^{-1}}}$$ in a decade of collisions at $$\sqrt{s} = {14}\,\mathrm{TeV}$$ . It is an unprecedented opportunity to study very rare phenomena, where production represents a top priority subject of research. An early understanding of the sensitivity to observe production and measure the Higgs boson trilinear coupling is thus important to organize future searches. This chapter presents the prospects for HH measurements at the HL-LHC from an extrapolation of the current results. Results are derived under various scenarios of detector performance and analysis improvements.

The exploration of Higgs boson pair production in the decay channel requires the experimental capability to identify and reconstruct several different types of final state objects and to use them for the selection of signal-like events. This in turn requires the reconstruction of the and decays and the usage of their properties to identify the specific signature of signal events and to reject background ones. These selections and techniques globally constitute the “analysis strategy”, the backbone of the search to which this chapter is devoted. After introducing the experimental challenges of the decay channel, the discussion focuses on the experimental signature of the signal processes under study and on the main background sources, and on the analysis strategy developed. The analysis strategy description is structured in four main parts, corresponding to trigger requirements, object preselections, event categorization, and definition of the signal regions.

The identification of the decays of a $$\tau $$ lepton to final state hadrons ( $$\tau _{{\text{ h }}}$$ ) and a neutrino, representing about 65% of the total $$\tau $$ decays, is a challenging task at a hadron collider, especially for the hardware L1 trigger where a high optimization of the sequence of algorithm operations to be performed is required. The upgrade of the L1 trigger represents an opportunity to develop and implement a sophisticated $$\tau $$ algorithm as described in this chapter.The discussion is introduced by a general overview of the experimental challenges of $$\tau _{{\text{ h }}} $$ identification at the hardware level, followed by a short description of the algorithm used in the Run I and of its limitations. The algorithm used for the 2016 data taking is then described in detail. Finally, the commissioning of the algorithm with 2015 data is presented, and its performance measured with the data collected in 2016.

Exploring production with the CMS data requires the definition of sensitive observables and of a statistical method to evaluate the presence of a signal, that are the object of the first part of this chapter. The results obtained on the data collected in 2016 are then described and compared to other searches performed at the LHC Run II.

The exploration of the scalar potential and of its role in the spontaneous breaking of electroweak symmetry is one of the main goals of the physics program of the Large Hadron Collider. The study of Higgs boson pair production is particularly important in this context, as it gives a direct experimental access to the shape of the scalar potential itself. Because of the small cross section, this process is particularly challenging and its experimental study will greatly benefit of the large datasets collected at the High-Luminosity LHC (HL-LHC). This document describes the main challenges for the CMS experiment, the upgrade program to address them, and the prospects for Higgs boson pair production studies at the HL-LHC.

The experimental study of Higgs boson pair production is carried out at the CERN Large Hadron Collider (LHC), and the work described in this thesis is based on the proton-proton collision data collected with the Compact Muon Solenoid (CMS) experiment. This chapter introduces the reader to the properties and operations of the LHC and to the structure of the CMS detector, and presents the algorithms used to reconstruct from the raw detector data the particles produced in the collisions. A special focus is given to the description of the trigger system, designed to perform an online selection of collision events, and to its recent Level-1 hardware system upgrade.

This preamble introduces the reader to the scientific and experimental contexts in which the thesis work has been developed, presenting the motivations for the experimental study of Higgs boson pair production and its role in the exploration of fundamental particle physics at the CERN LHC. The two main topics of this works are the development of an innovative tau algorithm for the CMS experiment trigger upgrade, and the search for Higgs boson pair production in the decay channel with the data collected with the CMS detector. A brief overview of both topics is given in the preamble with a focus on their preparation and evolution, the personal contribution by the author, and the main results achieved.

With the mass of the Higgs boson precisely known and its properties in good agreement with the theoretical predictions, production is becoming increasingly important as a probe of the scalar sector of the SM. The presence of new resonances, anomalous couplings, and eventually the Higgs boson self-coupling can be simultaneously studied in searches. A summary of the results presented in this work, of their impact on the CMS experimental programme, and a view of the present and future role of studies in the exploration of high energy physics are given in these conclusions.