Heptagon Symbols at Five Loops and All-Loop Sequences

TL;DR

This paper advances the bootstrap approach for seven-particle amplitudes in planar $ ext{N}=4$ SYM, revealing unique solutions constrained by physical and mathematical conditions, and predicting all-loop sequences of coefficients.

Contribution

It introduces a highly restrictive bootstrap framework based on the $E_6$ cluster algebra, leading to unique all-loop solutions for seven-particle amplitudes without collinear input.

Findings

Unique solutions for MHV and NMHV amplitudes at up to five loops.

Recurrent patterns in $E_6$ symbol coefficients mirroring known form factors.

Sequences that determine the MHV symbol at all loops without additional input.

Abstract

We revisit the symbol bootstrap program for the seven-particle MHV and NMHV amplitudes in planar $\mathcal{N}=4$ super-Yang-Mills (SYM) based on the alphabet associated with the $E_6$ cluster algebra. After imposing integrability, cluster adjacency (or extended Steinmann), first- and last-entry conditions, the solution space is already highly restrictive: e.g. for MHV case there are exactly $1,1,2,3,4$ parity-invariant solutions for $L=1,2,\cdots, 5$, which automatically satisfy dihedral symmetry. Remarkably, after further requiring a well-defined collinear limit, we find a unique solution for both MHV and NMHV sectors where all coefficients (e.g. more than $3.1\times 10^{10}$ for MHV at $L=5$) turn out to be integers. Furthermore, we observe recurrent patterns for coefficients of special words in $E_6$ symbols mirroring those found for the $C_2$ symbol of three-point form factors, which lead to numerous predictions in the form of all-loop sequences. As an initial application, we show that these sequences uniquely fix the MHV symbol through five loops without input from collinear limits. Given the simplicity and surprisingly strong constraining power of both physical constraints and the newly observed sequences, we conjecture that there is a unique symbol satisfying these constraints at any loop order.