Joint Track Functions: Expanding the Space of Calculable Correlations at Colliders

The theoretical description of observables at collider experiments relies on factorization theorems separating perturbative dynamics from universal non-perturbative matrix elements. Despite significant recent progress in extending these factorization theorems to increasingly differential jet substructure observables, the focus has been primarily on infrared safe observables sensitive only to correlations in the energy of final state hadrons. However, significant information about the dynamics of the underlying collision is encoded in how energy is correlated between hadrons of different quantum numbers. In this paper we extend the class of calculable correlations by deriving factorization theorems for a broad class of correlations, $\langle \mathcal{E}_{R_1}(n_1) \cdots \mathcal{E}_{R_k}(n_k) \rangle$, between the energy flux carried by hadrons specified by quantum numbers, $R_1, \cdots, R_k$. We show that these factorization theorems involve moments of a new class of universal non-perturbative functions, the "joint track functions", which extend the track function formalism to describe the fraction of energy carried by hadrons of multiple quantum numbers arising from the fragmentation of quarks or gluons. We study the general properties of these functions, and then apply this to the specific case of joint track functions for positive and negative electromagnetic charges. We extract these from parton shower Monte Carlo programs and use them to calculate correlations in electromagnetically charged energy flux. We additionally propose and study a C-odd $\mathcal{E}_{\mathcal{Q}}$ detector, which results in a qualitatively distinct scaling behavior compared to the standard energy correlators. Our formalism significantly extends the class of observables that can be computed at hadron colliders, with a wide range of applications from particle to nuclear physics.