Heptagon Symbols at Five Loops and All-Loop Sequences

TL;DR

This work establishes that the seven-point $MHV$ and $NMHV$ symbols in planar ${ m N}=4$ SYM, built from the $E_6$ cluster alphabet, are uniquely determined up to five loops by integrability, cluster adjacency, and first-/last-entry constraints, with collinear-limit consistency yielding a single solution in each sector and integer coefficients. The authors formalize the $E_6$ basic conditions, delineate the MHV and NMHV bootstrap, and show collinear limits alone suffice to fix the symbols to $L\le 5$, hinting at all-loop determinacy. They uncover remarkable all-loop coefficient sequences for special words, including explicit recursions and polynomial dependence on loop order, enabling predictions for coefficients without collinear input. These patterns, together with the observed integer coefficients and all-loop sequences, suggest a deep, universal structure underlying heptagon (and related) amplitudes and form factors, with potential extensions to higher loops and related kinematic quantities via antipodal dualities and cluster-algebraic constraints.

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.

Figures (2)

• Figure 1: Coefficient distribution of five-loop MHV symbol.
• Figure 2: Coefficient distribution of five-loop NMHV symbol.