Production of <em>ηπ</em>⁻ and <em>η</em>′<em>π</em>⁻ final states in high-energy <em>π</em>⁻<em>p</em> scattering at COMPASS

Abstract

Understanding the mechanisms of hadron production at high energies provides crucial insight into the strong interaction and the non-perturbative regime of Quantum Chromodynamics (QCD). This thesis focuses on the production of the <em>η</em>^(′)<em>π⁻</em> system in high-energy <em>π</em>⁻<em>p</em> scattering at the COMPASS experiment, with particular emphasis on the regime dominated by double-Regge exchange. <br/>

We analyze the full dataset collected during two dedicated years of data taking at COMPASS, selecting the exclusive <em>η</em>^(′)<em>π⁻</em> + <em>p</em> final state and reconstructing the <em>η</em>^(′)<em>π⁻</em> system via the decay chain <em>η</em>^(′) → <em>π</em>⁻<em>π</em>⁺<em>π</em>⁰(<em>η</em>), <em>π</em>⁰(<em>η</em>) → <em>γγ</em>. Of key importance for the description of the data is an accurate modeling of the detector acceptance. We describe the implementation of previously missing detector components in the Monte Carlo simulation frame work of COMPASS. Furthermore, we have significantly improved the calorimeter description. Previously, an effective shower simulation in the COMPASS calorimeters was needed to process events. We have enhanced the implementation such that this effective simulation is no longer required, resulting in a more detailed and precise shower simulation. <br/>

For the first time, the double-Regge formalism is applied to fit the data in the high-mass region, with resonance and double-Regge contributions separated using kinematic cuts. Six double-Regge amplitudes are considered: two for forward <em>η</em>^(′) production and four for backward <em>η</em>^(′) production. The fit, performed with only 13 free parameters, determines the relative strengths of the contributing amplitudes and accurately models the <em>t</em>-dependence at each vertex. The forward region is dominated by <em>a</em>₂ℙ exchange, with a minor <em>a</em>₂<em>f</em>₂ contribution, while the backward region is dominated by ℙℙ exchange, with significant interference effects involving <em>f</em>₂ and Pomeron exchange. <br/>

This thesis demonstrates the first event-based extraction of double-Regge amplitudes for the <em>η</em>^(′)<em>π⁻</em> system. The methods and results establish a foundation for future studies, including the extraction of coupling strengths at individual vertices and the connection of the double-Regge and resonance regions using theoretical approaches such as finite energy sum rules.