Speaker
Description
We present a comprehensive theoretical investigation of the angular decorrelation of heavy quark pairs in electron-positron ($e^+e^-$) and electron-proton ($ep$) collisions. Emphasizing the back-to-back limit in $e^+e^-$ collisions, we highlight the necessity of a Transverse Momentum Dependent (TMD) factorization and resummation theorem for heavy quark pair production. Our study introduces a framework that extends beyond the traditional two-scale (hard scattering scale $Q$ and TMD scale $q_T$) approach of the Drell-Yan or Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes, by incorporating a third scale - the heavy quark mass $m_\mQ$. We analyze three distinct scale hierarchies: $Q \gg m_\mQ \sim q_T$, $Q \gg m_\mQ \gg q_T$, and $Q \sim m_\mQ \gg q_T$, applying soft-collinear effective theory (SCET), boosted Heavy Quark Effective Theory (bHQET) and Heavy Quark Effective Theory (HQET) as appropriate. The theoretical framework is adapted to each scenario, considering the varying degrees of heavy quark boosting. The results are verified through asymptotic expansions in different scale regimes, aligning with theoretical expectations.