Threshold expansion of the three-particle quantization condition

Abstract

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 $1/L$, where $L$ is the linear extent of the finite volume. The expansion begins at $\mathcal{O}(1/L^3)$, and we determine the coefficients of the terms through $\mathcal{O}(1/L^6)$. As is also the case for the two-particle threshold energy, 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 $1/L^6$ 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 $\mathcal{O}(1/L^6)$. 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 $1/L^6$ result, and thus of the quantization condition, we also compare to a perturbative calculation of the threshold energy in relativistic $\lambda {\varphi }^4$ theory, which we have recently presented. Here all terms can be compared and we find full agreement.

30 pages, 2 figures, v3: Journal version + erratum: A mistake was corrected that affects the value of the 1/L^6 term in the threshold expansion. The issue was discovered in a fourth-order λϕ^4 theory calculation, presented in arXiv:1707.04279, that was performed to further check the results of this work. Equations (126), (127), (135) and (136) have been modified to correct the mistake

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