Physics with e^+e^- Linear Colliders

TL;DR

<3-5 sentence high-level summary>Future e+e- linear colliders offer a comprehensive, high-precision program that probes the Standard Model to new levels and explores a broad landscape of beyond-Standard-Model physics. Through a tiered energy plan (≈500 GeV up to ≈2 TeV) and a variety of collision modes (e+e-, e-e-, γe, γγ), the approach enables precise top-quark property measurements, Higgs boson characterization including self-couplings, and detailed studies of gauge-boson interactions. The collider provides unique capabilities to discover and measure supersymmetric states, extended gauge sectors, and potential compositeness, with strong synergy with LHC results to illuminate fundamental questions about electroweak symmetry breaking, grand unification, and quantum gravity. The anticipated precision on masses, couplings, and mixing parameters promises to reconstruct underlying theories at high scales and to sharpen our understanding of physics beyond the Standard Model.</paper_summary>

Abstract

We describe the physics potential of $e^+e^-$ linear colliders in this report. These machines are planned to operate in the first phase at a center-of --mass energy of 500 GeV, before being scaled up to about 1 TeV. In the second phase of the operation, a final energy of about 2 TeV is expected. The machines will allow us to perform precision tests of the heavy particles in the Standard Model, the top quark and the electroweak bosons. They are ideal facilities for exploring the properties of Higgs particles, in particular in the intermediate mass range. New vector bosons and novel matter particles in extended gauge theories can be searched for and studied thoroughly. The machines provide unique opportunities for the discovery of particles in supersymmetric extensions of the Standard Model, the spectrum of Higgs particles, the supersymmetric partners of the electroweak gauge and Higgs bosons, and of the matter particles. High precision analyses of their properties and interactions will allow for extrapolations to energy scales close to the Planck scale where gravity becomes significant. In alternative scenarios, like compositeness models, novel matter particles and interactions can be discovered and investigated in the energy range above the existing colliders up to the TeV scale. Whatever scenario is realized in Nature, the discovery potential of $e^+e^-$ linear colliders and the high-precision with which the properties of particles and their interactions can be analysed, define an exciting physics programme complementary to hadron machines.

Figures (45)

• Figure 1: (a) The basic processes of the Standard Model: $e^+e^-$ annihilation to pairs of fermions and gauge bosons. The cross sections are given for polar angles between $10^0 < \theta < 170^0$ in the final state. (b) Elastic/inelastic Compton scattering and $\gamma \gamma$ reactions. $\sqrt{s}$ is the invariant $e \gamma$ and $\gamma \gamma$ energy. The polar angle of the final state particles is restricted as in (a); in addition, the invariant $\mu^+ \mu^-$ and $q \overline{q}$ masses in the inelastic Compton processes are restricted to $M_{inv} > 50$ GeV.
• Figure 2: The effect of beam polarization on the cross section for the production of $W^+ W^-$ pairs. $U$ denotes unpolarized electron and positron beams, R:80 denotes 80% right-handedly polarized electron beams, and L:60 denotes 60% left-handedly polarized positron beams.
• Figure 3: Left: The $\gamma$ energy spectrum in Compton back-scattering of laser light for three values of initial laser and electron beam helicities N11. Right: The distribution of the $\gamma \gamma$ invariant mass in Compton back-scattering of laser light with opposite laser/electron helicities. The dashed curves demonstrate how the monochromaticity can be sharpened by separating the conversion from the collision point; c.f. Ref.N10.
• Figure 4: Left: Branching ratio of top decays to charged Higgs bosons, in supersymmetric theories. Shown is also the range of charged Higgs masses as a function of the coupling $\tan \beta$ that can be detected experimentally for a given luminosity of 10, 20, and 50 fb$^{-1}$. Refs.N19N20. Right: The decay of top quarks to stop particles and the lightest neutralino in supersymmetric theories. The lower plots present the energy distributions in the two event hemispheres for SM decays and SUSY decays which are characterized by missing energy due to escaping neutralinos. Refs.N19N20.
• Figure 5: The cross section for the production of top-quark pairs in the continuum as a function of the total energy for three representative values of the top mass.