& \tau <
1 \end{array} \right. \, ,
\label{eq:ftau}\end{aligned}$$ where $m_t$ denotes the top mass, while the more involved analytical expressions for $F_\Box$ and $G_\Box$ can be found in Ref. [@Plehn:1996wb]. In the HTL, the LO form factors approach the values $$F_\triangle \to \frac{2}{3}, \qquad F_\Box \to -\frac{2}{3}, \qquad
G_\Box \to 0 \, .
\label{eq:ffhtl}$$ There are two tensor structures contributing which correspond to the total angular-momentum states with $S_z=0$ and $2$, $$\begin{aligned}
T_1^{\mu\nu} & = & g^{\mu\nu}-\frac{q_1^\nu q_2^\mu}{(q_1q_2)}\, , \nonumber \\
T_2^{\mu\nu} & = & g^{\mu\nu}+\frac{M_H^2 q_1^\nu q_2^\mu}{p_T^2 (q_1q_2)}
-2\frac{(q_2 p_1) q_1^\nu p_1^\mu}{p_T^2 (q_1q_2)}
-2\frac{(q_1 p_1) p_1^\nu q_2^\mu}{p_T^2 (q_1q_2)}
+2\frac{p_1^\nu p_1^\mu}{p_T^2} \nonumber \\
\mbox{with} \quad p_T^2 & = & 2 \frac{(q_1 p_1)(q_2 p_1)}{(q_1 q_2)} -
M_H^2 \, ,\end{aligned}$$ where $p_T$ is the transverse momentum of each of the final-state Higgs bosons. Working in $n=4-2\epsilon$ dimensions, the following projectors on the two form factors can be constructed, $$P_1^{\mu\nu} = \frac{(1-\epsilon) T_1^{\mu\nu} + \epsilon
T_2^{\mu\nu}}{2(1-2\epsilon)}\, , \qquad \qquad
P_2^{\mu\nu} = \frac{\epsilon T_1^{\mu\nu} + (1-\epsilon)
T_2^{\mu\nu}}{2(1-2\epsilon)} \, ,$$ such that $$P_1^{\mu\nu} {\cal A}_{\mu\nu} = F_1 \, , \qquad \qquad
P_2^{\mu\nu} {\cal A}_{\mu\nu} = F_2 \, .$$ Using these projectors, the explicit results of the two form factors $F_{1,2}$ can be obtained in 