Primitive asymptotics in $\phi^4$ vector theory

Balduf, P; Thürigen, J

Abstract

A longstanding conjecture in $\phi^4_4$ theory is that primitive graphs dominate the beta function asymptotically at large loop order in the minimal-subtraction scheme. Here we investigate this issue by exploiting additional combinatorial structure coming from an extension to vectors with $\O(N)$ symmetry. For the 0-dimensional case, we calculate the $N$-dependent generating function of primitive graphs and its asymptotics, including arbitrarily many subleading corrections. We find that the leading  asymptotic growth rate becomes visible only above $\approx 25$ loops, while data at lower order is suggestive of a wrong asymptotics. Our results also yield the exact asymptotics of Martin invariants. In 4D, each graph comes with a  nontrivial Feynman integral, its period. We give bounds on the degree in $N$ for  primitive and non-primitive graphs, and  construct the primitive graphs of highest degree explicitly using a bijection method. We calculate the 4D primitive beta function numerically up to 17 loops. The qualitative behaviour turns out to be similar to the 0D series, with a small but systematic tendency for the 4D data to grow faster with $N$, indicating a correlation between periods and $O(N)$-symmetry factors. The zeros of the 4D primitive beta function approach their asymptotic locations quickly, but, like in the 0-dimensional case, the growth rate of the 4D primitive beta function does not match its asymptotics even at 17 loops. Our results improve on the knowledge of asymptotics in QFT by putting individual observables into a broader context of $\phi^4_4$ theory. We provide concrete analytic and numerical values to demonstrate that both in the 0D and 4D theory,  the reliability of  asymptotic expansions  greatly depends on the quantity in question. Even if certain quantities are in excellent agreement with the asymptotics already below 10 loops, this must not be mistaken as evidence that overall an asymptotic regime has been reached.