ECFA PG1 Report

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 (Physics Briefing Book...), the Snowmass workshop in the US (Planning the Future o...), and the first ECFA HL-LHC workshop in 2013 (ECFA High Luminosity ...).

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 \ref{fig:Higgs1} shows the reduced coupling scale factors, \(y_i\), for weak bosons and fermions expected with 3000 fb\(^{-1}\) of data (Projected Performance..., Projections for measu...).

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 N\(^3\)LO computation of the Higgs-boson cross section in gluon fusion (Anastasiou 2014) or various NNLO computations for more exclusive final states (e.g. Higgs+jet (Boughezal 2013, Chen 2014)), 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 (Czakon 2013, Currie 2014) may help, as may detailed studies of the origins of systematic different between different PDF sets (see e.g. Ref. (Butterworth 2014)). Other progress that will be relevant includes higher order merging of parton showers and fixed-order calculations (e.g. Refs. (Hamilton 2013, Hoche 2014)) as well improvements in methods to estimate higher-order uncertainties such as Ref. (Bagnaschi 2014).

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 \(Z\gamma\) 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 (\(\phi\)) 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 \(\phi^2\) and \(\phi^4\). 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) \(H^*\to HH\) 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 \(HH \rightarrow bb\gamma\gamma\) 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 (Prospects for measuri..., CMS Upgrade and Physi...). As an example, Figure \ref{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 \(bb\tau\tau\) 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 \(\chi^+_1\) \(\chi^0_2\) 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  (Mahbubani 2013, Delaunay 2014) (see recent search based on charm-tagging (Search for Scalar-Cha...)) 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 \(5\times 10^{34}~cm^{-2}s^{-1}\) is assumed to correspond to an average number of proton-proton interactions per bunch crossing (pileup), \(\mu\), 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 \(\mu = 140\) is close to that of the Phase I detectors with \(\mu = 50\). ATLAS showed that the performance continues to degrade with more pileup; 10 times more light jets are mistagged as b-jets with \(\mu = 300\) compared to \(\mu = 140\). Once the correct primary vertex is identified, the