Threshold expansion of the three-particle quantization condition

We recently derived a quantization condition for the energy of three relativistic particles in a cubic box. Here we use this condition to study the energy level closest to the three-particle threshold when the total three-momentum vanishes. We expand this energy in powers of $1\mathrm{/}L$, where $L$ is the linear extent of the finite volume. The expansion begins at ${\cal O}(1/L^3)$,andwedeterminethecoefficientsofthetermsthrough${\cal O}(1/L^6)$\mathrm{.}Asisalsothecaseforthetwo-particlethresholdenergy,the$1/L^3$,$1/L^4$and$1/L^5$ coefficients depend only on the two-particle scattering length $a$. These can be compared to previous results obtained using nonrelativistic quantum mechanics and we find complete agreement. The $1\mathrm{/}{L}^6$ coefficients depend additionally on the two-particle effective range $r$ (just as in the two-particle case) and on a suitably defined threshold three-particle scattering amplitude (a new feature for three particles). A second new feature in the three-particle case is that logarithmic dependence on $L$ appears at $\mathcal{O}(1\mathrm{/}{L}^6)$. Relativistic effects enter at this order, and the only comparison possible with the nonrelativistic result is for the coefficient of the logarithm, where we again find agreement. For a more thorough check of the $1\mathrm{/}{L}^6$ result, and thus of the quantization condition, we also compare to a perturbative calculation of the threshold energy in relativistic $\lambda \phi^4$ theory, which we have recently presented. Here all terms can be compared and we find full agreement.