INTRODUCTION The Higgs boson with mass around 125 GeV recently discovered by the ATLAS and CMS experiments at the LHC is found to have properties compatible with the Standard Model predictions , as shown for example in Fig. [fig:ellis] . Coupled with the absence of any other indication so far for new physics at the LHC, be it either through precision measurements or via direct searches, this fundamental observation seems to push the energy scale of any physics beyond the Standard Model above several hundred GeV. The higher-energy LHC run, which is expected to start in 2015 at $ \sim 13$-14 TeV, will extend the sensitivity to new physics to 1 TeV or more. Fundamental discoveries may therefore be made in this energy range by 2017-2018. Independently of the outcome of this higher-energy run, however, there must be new phenomena, albeit at unknown energy scales, as shown by the evidence for non-baryonic dark matter, the cosmological baryon-antibaryon asymmetry and non-zero neutrino masses, which are all evidence for physics beyond the Standard Model. In addition to the high-luminosity upgrade of the LHC, new particle accelerators will be instrumental to understand the physics underlying these observations.
The first studies of the HL-LHC physics program and the performance of the upgraded LHC detectors were documented for the European strategy meeting in Cracow , the Snowmass workshop in the US , and the first ECFA HL-LHC workshop in 2013 . The second ECFA HL-LHC workshop was a significant step forward to develop understanding of the performance of the upgrade detectors in the harsh HL-LHC environment. The studies were organized to motivate a number of specific performance related detector upgrades and to address points raised during the first ECFA HL-LHC workshop. Below only some key aspects of the programme are discussed. We will examine progress and prospects regarding Higgs and BSM physics analyses at the general purpose detectors, continue with a discussion of detector aspects and close with heavy-flavour and heavy-ion prospects. An important component of the HL-LHC is to carry out studies involving the recently discovered Higgs boson at 125 GeV mass. One aspect is precision measurements of the properties of this scalar, in order to test the Standard Model pattern of couplings to elementary particles. Additionally, because the hierarchy problem, a quantum instability of the Higgs sector, many models of new physics affect precision Higgs observables, even in cases where such the corresponding new particles are hard to discover experimentally. The ATLAS and CMS experiments project comparable precision with an estimated uncertainty of 2-5% for many of the investigated Higgs boson coupling to elementary fermions and bosons, demonstrating that with an integrated luminosity of 3000 fb−1 the HL-LHC is a very capable precision Higgs physics machine. Figure [fig:Higgs1] shows the reduced coupling scale factors, yi, for weak bosons and fermions expected with 3000 fb−1 of data . To fully benefit from the higher experimental precision in the Higgs sector it will be necessary to reduce theory uncertainties relative to today’s state-of-the-art. Progress is being made on many fronts currently: for example steps towards a N³LO computation of the Higgs-boson cross section in gluon fusion or various NNLO computations for more exclusive final states (e.g. Higgs+jet ), both of which can affect Higgs parameter extraction. A reduction in the uncertainty of parton distribution functions (PDFs) will also be needed, and here too the advent of new calculations at NNLO may help, as may detailed studies of the origins of systematic different between different PDF sets (see e.g. Ref. ). Other progress that will be relevant includes higher order merging of parton showers and fixed-order calculations (e.g. Refs. ) as well improvements in methods to estimate higher-order uncertainties such as Ref. . As well improving the precision of Higgs-sector measurements, the substantial luminosity of HL-LHC will make it possible to probe important rare processes involving the Higgs boson. Some examples involve rare decays, such as the Zγ decay, or those involving second generation fermion couplings, which can open a novel window on the problem of flavour. Off-shell and high transverse momentum Higgs production to new physics near the TeV scale that may otherwise be hidden, in a way that overlaps only partially with precision Higgs measurements. Finally, the HL-LHC may have the potential to study di-Higgs production. In the Standard Model, with the Higgs boson mass and the Higgs-field (ϕ) vacuum expectation value now both known, the structure of the Higgs potential is fully predicted. This is because the potential involves just two terms, proportional to ϕ² and ϕ⁴. An elementary field potential of this kind has never been seen before in nature and it is crucial to test whether it is indeed the potential associated with our vacuum. A study of the Higgs boson self-coupling provides one such test, because the self-coupling is related to the third derivative of the Higgs potential at its minimum, uniquely predicted in the Standard Model. One main avenue for studying the self coupling is through di-Higgs production, which is sensitive to the (off-shell) H* → HH process. One should be aware, however, that this interferes with other mechanisms for the production of two Higgs bosons, which complicates the determination of the self coupling. One should also keep in mind that new-physics can modify the relation between the Higgs potential and di-Higgs production: for example di-Higgs production can be greatly enhanced in cases where the Higgs is composite rather than elementary. Preliminary studies of the rare di-Higgs process, only accessible at the HL-LHC, have been performed by the ATLAS and CMS experiments, considering HH → bbγγ and bbWW final states. The findings of these analyses show the challenge that this physics process represents, where high performance on mass resolution, primary vertex, b-tagging and photon identification efficiencies, as well as mitigation of the event pileup effects, are crucial to the success of this measurement . As an example, Figure [fig:Higgs2] shows the relative uncertainty on the di-Higgs boson production cross section measurement as a function of the b-tagging efficiency. Studies of other di-Higgs final states, such as bbττ and bbbb, are needed to improve the accuracy of this analysis. As well as exploring the Higgs sector, with its corresponding scope for sensitivity to BSM physics, the HL-LHC also presents opportunities for direct discovery of new particles. The HL-HLC will extend the discovery reach of the LHC in a wide range of new phenomena from modified Higgs sectors to dark matter and new resonances. The sensitivity to BSM models is significantly enhanced at the HL-LHC compared to the corresponding sensitivities at lower integrated luminosities. One class of particles whose discovery potential benefits especially from the extra factor of ten in luminosity provided by the HL-LHC is those with poduction rates that are suppressed, for example by small couplings. The electroweak production of neutralino-chargino pairs provides such a striking example. Both ATLAS and CMS studied the production of χ₁+ χ₂⁰ with decay to WZ and WH final states in the context of a simplified SUSY model. In the case of the WZ final state, the mass reach increases by approximately 50% from Run 3 at the LHC to the HL-LHC, while the reach about doubles in the WH(bb) final state. This significant increase would not be achievable with the current detector due to degraded performance beyond 300 fb−1 from radiation damage. Other scenarios with suppressed rates to conventional final states include models such as supersymmetry and compositeness with a split spectrum (see recent search based on charm-tagging ) or models with compressed spectra and/or kinematic degeneracies in the decays. The ATLAS and CMS Collaborations have also shown that the mass reach for the discovery of new heavy states like gluinos/squarks in standard supersymmetric scenarios or new gauge bosons (W′ or Z′) typically increases by about 20% at the HL-LHC. In a detailed study of five full-spectrum SUSY models, CMS showed that a combination of nine different experimental signatures is able to establish discovery with differing amounts of integrated luminosity but only the HL-LHC is capable of discovering the physics nature of all five models. In the event of a discovery with the Run 2 or 3 datasets, it is likely to be difficult to distinguish between different new physics interpretations. For example, the CMS Collaboration has shown that a spin-1 dilepton resonance with a mass of 4 TeV could be discovered at the LHC but spin-0 or spin-2 interpretations could only be discriminated against at the 0.5 to 2 standard deviations level with 300 fb−1 of data using both angular and rapidity distributions of the dilepton system. The level of discrimination reaches 2 to 5 standard deviations with the data set of the HL-LHC. The mitigation of pileup at the HL-LHC is of fundamental importance to be able to deliver the physics goals of the LHC luminosity upgrade. An instantaneous luminosity of 5 × 10³⁴ cm−2s−1 is assumed to correspond to an average number of proton-proton interactions per bunch crossing (pileup), μ, of 140 events. The ATLAS performance has been evaluated using the baseline Phase II central tracker in full detector simulation. CMS have shown the impact of aging on the detector after 1000 fb−1 data collected in 2019, and compared this to a Phase II detector performance. In both experiments the efficiency for finding the primary vertex in top-pair events with pileup is expected to be 96%. The b-tagging performance of the Phase II detectors with μ = 140 is close to that of the Phase I detectors with μ = 50. ATLAS showed that the performance continues to degrade with more pileup; 10 times more light jets are mistagged as b-jets with μ = 300 compared to μ = 140. Once the correct primary vertex is identified, the long-flat beam spot seemed to offer no further improvement for b-tagging. As anticipated, the CMS muon performance is strongly degraded by detector aging. The Phase II tracking detectors bring an improvement for both experiments in muon pT resolution compared to Run 1. The Phase II algorithms for e, γ, and τ-lepton identification need more development work by both collaborations. CMS showed that the Puppi algorithm to reject pileup is particularly effective for jet reconstruction, giving the best jet energy resolution of the algorithms studied, and eliminating jets from pileup. ATLAS have also used track and vertex matching algorithms to reject pileup jets. In addition, ATLAS have demonstrated that jet substructure algorithms continue to be effective at large pileup. A jet mass algorithm for large radius jets allows the identification of top-quark jets. The efficiency and resolution gradually degrades as μ increases from 140 to 300. Most of these studies are performed with Run 1 algorithms. A range of further methods is also being considered, some of which were discussed at a recent dedicated pileup workshop . The ATLAS and CMS collaborations are studying upgrades of forward tracking and calorimetry by understanding their piecewise impact on physics analysis, leading to an optimized set of upgrades for each detector. Primary upgrades towards exploiting the forward region are proposed extensions to the tracking detectors of each experiment. The ATLAS collaboration is studying multiple forward tracker upgrade options varying the gross detector geometry as well as sensor granularity. Moreover, the ATLAS tracking extension studies demonstrate a 90% selection efficiency of forward jets from the primary vertex with a background rejection factor of 500 until pseudorapidity 3.2, and 100 in the region 3.2<|η|<4.0, showing that jets can be purely selected by exploiting forward tracking. The impact of the forward tracking extensions on physics analysis is demonstrated in a vector boson fusion (VBF) Higgs →ττ study, with a reduction up to a factor of 3 on the expected signal strength uncertainty assuming a 90 % rejection of jets from pile-up collisions. ATLAS also showed preliminary results on the impact of extensions to the muon spectrometer, including the addition of a new warm toroid at large pseudorapidity. Improvements to the muon momentum resolution are typically dominated by the contribution of the extended inner tracker at small values of muon transverse momentum. However, when using the fully proposed set of upgrades there is significant improvement to the muon transverse momentum resolution at high energies from the increased lever arm. The impact of this upgrade is shown in an analysis studying the H → ZZ → 4ℓ final state, where the acceptance is increased by 35% by extending the tracking coverage to pseudorapidity |η|= 4.0. The CMS collaboration is pursuing a similar tracking extension in pseudorapidity up to |η|= 4.0. The extended tracker being proposed also provides a factor three improvement in the tracking fake rate and track momentum resolution. These improvements are critical to the performance of _particle flow_ reconstruction in the Phase-II upgrades, which attempts to reconstruct with best precision all charged and neutral particle content in the event. CMS is investigating two endcap calorimeter designs. At present, both calorimeters are being studied for their performance benefits on their own and within the context of a global event reconstruction. Updated results for the HL-LHC from both experiments will be presented after evaluating the physics impact of each of the proposed upgrades finalizing physics simulation studies whose preliminary results have been presented in the Workshops of this year and last year. The study of heavy flavour sector represents one of the most interesting domains for indirect searches for new physics. Substantial progress is expected in the next two decades, both from the experimental and from the theoretical side, with anticipated progress in lattice QCD calculations. The main experimental advances should come from LHC experiments, thanks to the planned luminosity increase with the HL-LHC upgrade, and from the BELLE-II experiment in Japan. Analyses from Run 1 data have firmly established the great impact of LHC experiments. LHCb has produced a plethora of results on a broad range of flavor observables in the charm and beauty sectors, and ATLAS and CMS have given significant contributions to the beauty sector, mainly using final states containing muon pairs due to constraints dictated by the trigger. One of the most striking results of Run 1 is the observation of the Bs decay, through the combined analysis of CMS and LHCb data. CP violation induced by Bs mixing has been measured with astonishing precision by LHCb, with relevant contributions also from ATLAS and CMS. The measurement of the γ angle performed by LHCb is now dominating the world average, and further improvements are expected in Run 2 and beyond. The aforementioned observables are particularly interesting at HL-LHC as they are sensitive to new physics while not being dominated by theoretical uncertainties. Selected observables associated to the B → K( * )l+l− decay channels, and observables sensitive to CP-violation (CPV) in the charm sector are also promising probes of new physics. In the context e.g. of supersymmetry with a non-trivial squark spectrum and a light, natural third generation, flavour physics plays a key role: effects of the order of 5 − 20% are expected in CPV in the Bs, d mixing, Bs → μ+μ−, and Bs → K*μ+μ− . The large number of top pairs expected at the HL-LHC further opens the possibility of new studies in B physics, especially in the case of CPV, using “top-tagged b decays" . By incorporating knowledge emerging from the analyses of Run 1 data sets, sensitivity projections for the HL-LHC phase have been updated. These show that improvements in sensitivity will come not only from increased data samples, but also from improved detection capabilities. This will lead to results of fundamental importance in the search for physics beyond the Standard Model (BSM). In case that BSM particles are seen to be produced in LHC collisions, results in the flavor sector will provide crucial information to determine the couplings and the flavor structure of new physics. If new particles are not observed, higher mass scales can be probed through quantum effects, giving a complementary approach to discover BSM physics. Finally, also the studies of heavy ion physics possible at the HL-LHC enriches and completes the physics program of this machine and detector upgrades. The goal of the experiments ALICE, ATLAS and CMS is to integrate, for Pb-Pb collisions at $} = 5.5$ TeV, a luminosity of more than 10 nb−1 during the LHC Run 3 and 4. This represents an increase by an order of magnitude with respect to the expectation for Run 2. In the case of the ALICE experiment, the upgrade of the detector read-out capabilities will allow for the recording of all interactions, increasing the minimum-bias statistics by two orders of magnitude. The experiments stress the importance of a proton-proton reference sample at the same energy as Pb-Pb collisions. The ALICE requirement, driven by low-pT measurements, is of 6 pb−1; the ATLAS and CMS requirement, driven by high-pT measurements, is of 300 pb−1. After LS2, the study of the Quark-Gluon Plasma formed in nucleus-nucleus collisions will focus on rare probes, on their coupling with the medium and hadronization processes. These include but are not limited to heavy-flavour particles, quarkonium states, real and virtual photons, jets and their correlations with other probes. New studies are now available, for example for low-pT heavy-flavour and charmonium measurements at forward rapidity in ALICE. High-luminosity p-Pb collisions will be an essential part of the heavy-ion program after LS2. The LHCb experiment will also contribute to this part of the program, as demonstrated by its excellent performance during the 2013 p-Pb run. Proton-nucleus collisions allow, on the one hand, to explore initial-state effects and low Bjorken-x gluon dynamics, on the other hand, to study the interplay between the initial conditions and the development of collective effects in the final state of high particle density collisions. In order to fully exploit this potential, the ALICE Collaboration is considering the technical feasibility and the physics case for a high-granularity calorimeter in the forward region (η ∼ 3 − 5), to be installed during LS3. This detector would give access to forward-rapidity photons and neutral pions, which are predicted to be very sensitive to small-x gluon dynamics and the possible onset of gluon saturation.