Truncated cluster algebras and Feynman integrals with algebraic letters

TL;DR

This paper links the symbol alphabet of certain Feynman integrals to truncated cluster algebras derived from kinematic boundaries, revealing algebraic letters and enabling the construction of integrable symbols for complex integrals.

Contribution

It introduces a novel approach to derive symbol alphabets for Feynman integrals from truncated cluster algebras based on kinematic boundaries, including algebraic letters and integrable symbols.

Findings

Finite cluster algebras correspond to specific kinematic configurations.

Constructed the space of integrable symbols with algebraic and rational letters.

Identified patterns in algebraic letters for double-pentagon integrals.

Abstract

We propose that the symbol alphabet for classes of planar, dual-conformal-invariant Feynman integrals can be obtained as truncated cluster algebras purely from their kinematics, which correspond to boundaries of (compactifications of) $G_+(4,n)/T$ for the $n$-particle massless kinematics. For one-, two-, three-mass-easy hexagon kinematics with $n=7,8,9$, we find finite cluster algebras $D_4$, $D_5$ and $D_6$ respectively, in accordance with previous result on alphabets of these integrals. As the main example, we consider hexagon kinematics with two massive corners on opposite sides and find a truncated affine $D_4$ cluster algebra whose polytopal realization is a co-dimension 4 boundary of that of $G_+(4,8)/T$ with 39 facets; the normal vectors for 38 of them correspond to g-vectors and the remaining one gives a limit ray, which yields an alphabet of $38$ rational letters and $5$ algebraic ones with the unique four-mass-box square root. We construct the space of integrable symbols with this alphabet and physical first-entry conditions, whose dimension can be reduced using conditions from a truncated version of cluster adjacency. Already at weight $4$, by imposing last-entry conditions inspired by the $n=8$ double-pentagon integral, we are able to uniquely determine an integrable symbol that gives the algebraic part of the most generic double-pentagon integral. Finally, we locate in the space the $n=8$ double-pentagon ladder integrals up to four loops using differential equations derived from Wilson-loop $d\log$ forms, and we find a remarkable pattern about the appearance of algebraic letters.