The role of polarized positrons and electrons in revealing fundamental interactions at the Linear Collider

TL;DR

The paper argues that the ILC’s physics reach is greatly amplified by polarized e− and e+ beams, with positron polarization providing especially large gains in precision and discovery reach. It develops a comprehensive formalism for polarized cross sections, demonstrates how dual-beam polarization sharpens SM tests (notably in top-quark couplings, Higgs properties, and TGCs), and enables powerful model-independent searches for new physics via both direct production and indirect observables. Through detailed studies of Higgs separation, GigaZ precision measurements, and extensive SUSY scenarios (including extended sectors and R-parity-violating cases), the work shows that simultaneous electron and positron polarization can dramatically improve parameter determinations, CP-violating observables, and the sensitivity to new interactions. These results underpin the case for designing ILC facilities capable of delivering highly polarized beams and robust polarization measurements, such as the Blondel scheme, to maximize scientific return across SM and beyond.

Abstract

The proposed International Linear Collider (ILC) is well-suited for discovering physics beyond the Standard Model and for precisely unraveling the structure of the underlying physics. The physics return can be maximized by the use of polarized beams. This report shows the paramount role of polarized beams and summarizes the benefits obtained from polarizing the positron beam, as well as the electron beam. The physics case for this option is illustrated explicitly by analyzing reference reactions in different physics scenarios. The results show that positron polarization, combined with the clean experimental environment provided by the linear collider, allows to improve strongly the potential of searches for new particles and the identification of their dynamics, which opens the road to resolve shortcomings of the Standard Model. The report also presents an overview of possible designs for polarizing both beams at the ILC, as well as for measuring their polarization.

Figures (35)

• Figure 1.1: The various longitudinal spin configurations in $e^+e^-$ collisions. The thick arrow represents the direction of motion of the particle and the double arrow its spin direction. The first column indicates the corresponding cross section, the fourth column the fraction of this configuration and the last column the total spin projection onto the $e^+e^-$ direction.
• Figure 1.2: Possible configurations in $s$-channel diagrams: the helicities of the incoming $e^+e^-$ beams are directly coupled. Within the Standard Model (SM) only the recombination into a vector particle with $J=1$ is possible, which is given by the LR and RL configurations. New physics (NP) models might contribute to $J=1$ but also to $J=0$, hence the LL or RR configurations.
• Figure 1.3: Possible configurations in $t$- and $u$-channel diagrams: the helicity of the incoming beam is directly coupled to the helicity of the final particle and is completely independent of the helicity of the second incoming particle.
• Figure 1.4: Single $W^+$ production: the vertex $e^+ W^+ \bar{\nu}$ depends only on $P_{e^+}$.
• Figure 1.5: Effective polarization vs. positron beam polarization.