Chapter IV: A Periodic Table of Effective Field Theories
4.1 Introduction
While much of the progress in S-matrix has centered on gauge theory and gravity, another important class of theories—effective field theories (EFTs)—
have received substantially less attention, even though they play an important and ubiquitous role in many branches of physics. At the very minimum, the EFT approach provides a general parameterization of dynamics in a particular regime of validity, usually taken to be low energies. If the EFT has many free parameters then its predictive value is limited. However, in many examples the interactions of the EFT are dictated by symmetries, e.g as is the case for the Nambu-Goldstone bosons (NGBs) of spontaneous symmetry breaking. At the level of scattering amplitudes, these rigid constraints are manifested by special infrared properties. The archetype for this phenomenon is the Adler zero [73],
p→0limA(p) =0, (4.1)
which dictates the vanishing of amplitudes when the momentum of an NGB is taken to be soft. This imprint of symmetry on the S-matrix is reminiscent of gravity, which is also an EFT with a limited regime of validity.
At the same time, the longstanding aim of the modern amplitudes program is to construct the S-matrix without the aid of a Lagrangian, thus relinquishing both the benefits and pitfalls of this standard approach. But without a La- grangian, it is far from obvious how to incorporate the symmetries of an EFT directly into the S-matrix. However, recent progress in this direction [51] has shown that the symmetries of many EFTs can be understood as the conse- quence of a “generalized Adler zero” characterizing a non-trivial vanishing of scattering amplitudes in the soft limit. Here an amplitude is defined to have a “non-trivial” soft limit if it vanishes in the soft limit faster than one would naively expect given the number of derivatives per field.
By directly imposing a particular soft behavior at the level of the S-matrix, one can thenderive EFTs and their symmetries from non-trivial soft behavior.
From this “soft bootstrap” one can rediscover a subclass of so-called “excep-
⇢
0 1 2 3
1 2 3
0
P(X) DBI
NLSM
Gal sGal
trivial soft behavior
forbidden
4 WZW
Figure 4.1: Plot summarizing the allowed parameter space of EFTs. The blue region denotes EFTs whose soft behavior is trivial due to the number of derivatives per interaction. The red region is forbidden by consistency of the S-matrix, as discussed in Sec. 4.5. The white region denotes EFTs with non-trivial soft behavior, with solid black circles representing known standalone theories. The d-dimensional WZW term theory corresponds to (ρ, σ) = (d−2d−1,1). The exceptional EFTs all lie on the boundary of allowed theory space and (ρ, σ) = (3,3) is forbidden.
tional” EFTs [51] whose leading interactions are uniquely fixed by a single coupling constant. These exceptional theories include the non-linear sigma model (NLSM) [52–54], the Dirac-Born-Infeld (DBI) theory, and the so-called special Galileon [51, 64].
In [74], it was shown that the space of exceptional EFTs coincides precisely with the space of on-shell constructible theories via a new set of soft recursion relations. These very same EFTs also appeared in a completely different con- text from the CHY scattering equations [14], which are simple constructions for building the S-matrices for certain theories of massless particles. Alto- gether, these developments suggest that the exceptional theories are the EFT analogs of gauge theory and gravity. In particular, they are all simple one- parameter theories whose interactions are fully fixed by simple properties of the S-matrix.
In this chapter, we systematically carve out the theory space of all possible Lorentz invariant and local scalar EFTs by imposing physical consistency con-
ditions on their on-shell scattering amplitudes. Our classification hinges on a set of physical parameters (ρ, σ, v, d) which label a given hypothetical EFT.
Here ρ characterizes the number of derivatives per interaction, with a corre- sponding Lagrangian of the schematic form
L =∂2φ2F(∂ρφ), (4.2)
for some functionF. This power counting structure is required for destructive interference between tree diagrams of different topologies [51]. Meanwhile, the parameter σ is the soft degree characterizing the power at which amplitudes vanish in the soft limit,
p→0limA(p) =O(pσ). (4.3)
Obviously, for sufficiently largeρ, a large of valueσis trivial because a theory with many derivatives per field will automatically have a higher degree soft limit. As shown in [51] the soft limit becomes non-trivial when
σ ≥ρ for ρ >1,
σ > ρ for ρ≤1. (4.4)
The other parameters in our classification are v, the valency of the leading interaction, and d, the space-time dimension.
Taking a bottom up approach, we assume a set of values for (ρ, σ, v, d) to bootstrap scattering amplitudes which we then analyze for self-consistency.
Remarkably, by fixing these parameters—without the aid of a specific La- grangian or set of symmetries—it is possible to rule out whole swaths of EFT space using only properties of the S-matrix. Since our analysis sidesteps top down considerations coming from symmetries and Lagrangians, we obtain a robust system for classifying and excluding EFTs. This approach yields an overarching organizing principle for EFTs, depicted pictorially in Fig. 4.1 as a sort of “periodic table” for these structures. See Appendix A.3 for a brief summary of the EFTs discussed in this chapter. Our main results are as fol- lows:
• The soft degree of all EFTs is bounded by the number of derivatives per interaction, so in particular,σ ≤ρ+1. The exceptional EFTs—the NLSM, DBI, and the special Galileon—all saturate this bound.
• The soft degree of every non-trivial EFT is strictly bounded by σ ≤ 3, so arbitrarily enhanced soft limits are forbidden.
• Non-trivial soft limits require the valency of the leading interaction be bounded by the spacetime dimension, so v ≤d+1. For 4< v ≤ d+1, this is saturated by the Galileon [57, 75] and the Wess-Zumino-Witten (WZW) term for the NLSM [76, 77].
• The above constraints permit a theory space of single scalar EFTs and multiple scalar EFTs with flavor-ordering in general d populated by known theories: NLSM, DBI, the Galileon, and WZW. In principle this allows for new theories at the these same values of (ρ, σ, d, v) but we exclude this possibility in d= 3,4,5 by direct enumeration.
The core results of this chapter focus on the soft behavior of EFTs of a single scalar, or multiple scalars where there is a notion of flavor-ordering. However, we also briefly discuss the space of general EFTs with multiple scalars, as well as alternative kinematical regimes like the double soft or collinear limits.
This chapter is organized as follows. In Sec. 4.2, we define the parameters of the EFT theory space and outline our strategy for classification. We then derive soft theorems from general symmetry considerations in Sec. 4.3. The tools for classification—soft momentum shifts and recursion relations—are summarized in Sec. 4.4, and then applied to carve out the space of allowed EFTs in Sec. 4.5.
In the permitted region, we search and enumerate EFTs numerically in Sec. 4.6.
Other kinematics limits and more general classes of theories are considered in Sec. 4.7. Finally we conclude in Sec. 4.8.