Searching for New Physics with the Large Hadron Collider

TL;DR

This chapter surveys how collider phenomenology translates theoretical models into LHC measurements by detailing the modeling chain (PDFs, hard matrix elements, parton showers, hadronisation), detector reconstruction, jet physics, and robust statistical inference. It emphasizes the design of kinematic observables and object tagging to reveal quantum numbers, and showcases end-to-end analyses through Higgs diphoton discovery, high-mass resonance searches, and SMEFT constraints using vector boson production. The work highlights tools, open data practices, and reinterpretation strategies that enable global fits and cross-experiment comparisons, while outlining future collider directions and the continued role of machine learning and theory developments. Overall, it presents a cohesive framework for turning fundamental Lagrangian physics into quantitative statements about discovery, exclusion, and parameter constraints at the energy frontier.

Abstract

This chapter provides an introduction to collider phenomenology, explaining how theoretical concepts are translated into experimental analyses at the Large Hadron Collider (LHC). Beginning with the principles of collider operation and detector design, it outlines how collisions of protons are modelled through parton distribution functions, hard matrix elements, parton showers, and hadronisation. The discussion then turns to the reconstruction of physical objects and the definition of kinematic observables that expose the quantum numbers and dynamics of the underlying interactions. Special emphasis is placed on jet physics, including infrared- and collinear-safe algorithms, grooming and tagging techniques, and modern reconstruction approaches to jet substructure. The chapter introduces event selection strategies, object identification, and multivariate classification methods, before presenting the statistical framework underpinning modern collider analyses, from likelihood construction to hypothesis testing and uncertainty treatment. Three representative case studies, the Higgs discovery in the diphoton channel, high-mass dilepton resonance searches, and constraints on new physics through the Standard Model Effective Field Theory, demonstrate how these ingredients combine in end-to-end analyses. The chapter concludes with a perspective on future colliders and the growing role of open data and simplified likelihoods in enabling reinterpretation and global fits.

Figures (5)

• Figure 1: Collider phenomenology at a glance. Left: proton--proton collisions produce final-state objects reconstructed as isolated photons and leptons, jets (including large-radius jets with internal substructure), and missing transverse momentum. Middle: observables and tagging methods, such as the groomed invariant jet mass, expose the quantum numbers and dynamics of the underlying interactions. Right: likelihood-based statistical inference turns measured spectra into discoveries, exclusions, and parameter constraints.
• Figure 2: Schematic view of a collider detector. The green arrows correspond to charged particles, like electrons and muons, the blue arrows to neutral particles, like photons and neutrons. The orange arrow shows the calculated missing transverse energy of the event.
• Figure 3: Lego plane plot of a multi-jet event at the LHC. The height of the towers represents the transverse momentum $p_T$, while the rapidity and azimuthal angle coordinates $(y, \phi)$ correspond to the particle directions.
• Figure 4: Theory picture overlaying a $pp \to t\bar{t}h$ collision event. The event is initiated by the hard scattering of two protons, represented by the blue blobs. The PDFs describe the momentum fractions $x_1$ and $x_2$ of the incoming partons. The hard matrix element $\hat{\sigma}_{ij \to X}$ is calculated for the partonic subprocess, which in this case is $gg \to t\bar{t}h$. The parton shower simulates the emission of additional quarks and gluons, leading to hadrons that subsequently decay into long-lived final objects that can be detected.
• Figure 5: Generic interaction sequence for the search of a very BSM resonance that decays into electroweak-scale particles that subsequently decay hadronically. The interaction produces a boosted resonance, which decays into two jets with highly collimated substructure. The kinematics of the jets are crucial for reconstructing the resonance mass and distinguishing it from background QCD jets.