[Paper Review] Higgs Physics at the HL-LHC and HE-LHC

M. Cepeda, Stefania Gori+375 more|arXiv (Cornell University)|Jan 1, 2019

Particle physics theoretical and experimental studies358 citations

TL;DR

This paper evaluates the potential of the High-Luminosity and High-Energy LHC to advance Higgs boson physics, focusing on precision measurements of couplings, exotic decays, and new physics via effective field theory and direct searches. It projects a 2% precision on the ttH coupling and a 10% measurement at 1 TeV energy equivalent to per-mille precision at the Higgs mass, enabling sensitivity to new physics scales up to 100 TeV.

ABSTRACT

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.

Motivation & Objective

• To assess the sensitivity of the HL-LHC and HE-LHC to precision measurements of Higgs boson couplings, particularly to top quarks and vector bosons.
• To evaluate the discovery potential for rare and exotic Higgs decays, including decays to light BSM particles such as dark photons and axion-like particles.
• To quantify the reach of the HE-LHC in probing new physics through high-energy scattering processes, including vector boson scattering and di-boson production.
• To examine the impact of increased center-of-mass energy and luminosity on the sensitivity to effective field theory operators and new physics scales.
• To project improvements in the measurement of Higgs coupling ratios and the extraction of polarized vector boson scattering components.

Proposed method

• Utilizes effective field theory (EFT) with higher-dimensional operators to systematically parametrize new physics effects on Higgs couplings and amplitudes.
• Performs global fits to Higgs, Drell-Yan, and di-boson observables at high energy to constrain Wilson coefficients and extract sensitivity to new physics scales.
• Employs advanced simulation and pile-up mitigation techniques to improve the precision of vector boson scattering (VBS) cross-section measurements in semi-leptonic final states.
• Applies kinematic reconstruction and c-tagging techniques to enhance sensitivity to H → c¯c and rare Higgs decays at HE-LHC energies.
• Uses theoretical predictions and experimental performance projections to estimate improvements in systematic uncertainties and coupling precision.
• Analyzes the reach for new Higgs bosons (e.g., S → hh) via gluon fusion and associated production, leveraging increased luminosity and energy.

Experimental results

Research questions

• RQ1 What is the projected precision on the ttH coupling at the HL-LHC and HE-LHC, and how does it improve with reduced theoretical uncertainties?
• RQ2 To what extent can the HE-LHC probe new physics scales beyond the SM through high-energy Higgs and vector boson scattering processes?
• RQ3 Can the HE-LHC observe H → c¯c or rare exotic Higgs decays such as h → XX → bbµµ or 4ℓ, and what branching ratios are reachable?
• RQ4 How does the HE-LHC improve the measurement of polarized vector boson scattering components and the significance of purely longitudinal final states?
• RQ5 What is the projected discovery reach for extended Higgs sectors, such as a heavy Higgs boson S in pp → S → hh, at the HE-LHC compared to the HL-LHC?

Key findings

• The precision on the ttH coupling is projected to reach approximately 2% at the HE-LHC, assuming a 2× improvement in theoretical uncertainties.
• The ratio of ttH to ttZ couplings can be measured at the percent level due to enhanced statistical power and reduced systematic uncertainties.
• The HE-LHC can achieve a 10% measurement at 1 TeV center-of-mass energy, equivalent to per-mille precision at the Higgs mass, enabling sensitivity to new physics scales of order 25 TeV.
• Sensitivity to dimension-eight operators describing anomalous quartic gauge couplings improves by a factor of 10–20 at the HE-LHC compared to the HL-LHC.
• The HE-LHC can probe exotic Higgs decays with branching ratios as low as ∼10⁻⁵ (e.g., h → XX → bbµµ) and ∼10⁻⁸ (e.g., h → 4ℓ), extending the HL-LHC reach by factors of ∼5 and ∼308, respectively.
• The HE-LHC extends the discovery reach for a heavy Higgs boson S in pp → S → hh to masses 1.5–2 times higher than those probed at the HL-LHC.