Leveraging intermediate resonances to probe CP violation at colliders

TL;DR

This paper proposes a novel collider search strategy using asymmetries to detect CP violation through interference effects in three-body final states, demonstrating potential for significant sensitivity improvements over traditional methods.

Contribution

It introduces an innovative asymmetry-based approach to probe CP violation in $2\to 3$ processes, utilizing interference effects in intermediate energy regimes, and shows machine learning can enhance detection sensitivity.

Findings

Percent-level asymmetry at parton level demonstrates sensitivity to BSM CP phases.

Machine learning classifiers outperform standard methods in detecting interference effects.

Potential to achieve 2σ significance with current LHC data and 8σ at HL-LHC.

Abstract

We explore the phenomenological impact of interference in tree-level contributions to three-body final states in $2\to 3$ scattering processes. This work introduces a novel search strategy leveraging asymmetries to enable sensitivity to CP-violating effects in less well-explored regions of phase space. Analytically, we demonstrate the effectiveness of this observable in probing interference between Standard Model charged-current decays and effective left-handed vector interactions, illustrated in a toy model featuring a scalar leptoquark, $S_1 \sim (3, 1, -1/3)$. Numerically, we apply this framework to studying the process $pp\to b \tau\nu$; unlike traditional high-$p_T$ searches or ``bump hunts", this approach utilizes an intermediate energy regime -- where new physics is neither light enough to be produced on shell or heavy enough to justify an effective field theory treatment. A proof-of-principle analysis at parton level demonstrates a percent-level asymmetry, with sensitivity also to BSM weak-CP phase. While the specific phase sensitivity is diminished at particle level due to showering and detector effects, a machine learning classifier can recover sensitively to the presence of SM-BSM interference, significantly outperforming standard analysis methods. Notably discrimination between BSM signal and SM background could be achieved at the 2$\sigma$ level for the current LHC dataset and 8$\sigma$ at the High-Luminosity LHC. Moreover, this asymmetry observable as defined can also be more broadly applied to other searches for CP-violation in $2\to 3$ processes in present and future collider environments.