Multi-trace YMS amplitudes from soft behavior
Pith reviewed 2026-05-24 04:17 UTC · model grok-4.3
The pith
Multi-trace YMS amplitudes expand recursively from double-soft behavior of the two-scalar trace case.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We derive the expansion formula of tree-level multi-trace YMS amplitudes in a bottom-up way: we first determine the simplest amplitude, the double-trace pure scalar amplitude which involves two scalars in each trace. Then insert more scalars to one of the traces. Based on this amplitude, we further obtain the double-soft behavior when the trace containing only two scalars is soft. The multi-trace amplitudes with more scalars and more gluons finally follow from the double-soft behavior as well as the single-soft behaviors which has been derived before.
What carries the argument
Double-soft behavior of the trace that contains exactly two scalars, which extends the base amplitude to all multi-trace cases when combined with single-soft limits.
If this is right
- The double-trace pure scalar amplitude with two scalars in each trace is fixed as the independent starting point.
- Adding scalars to one trace produces the expansion for all double-trace amplitudes with arbitrary numbers of scalars.
- The double-soft behavior of a two-scalar trace, together with single-soft behaviors, generates amplitudes that also contain gluons.
- Every multi-trace YMS amplitude reduces to ones with fewer gluons or fewer traces via the resulting recursive formula.
Where Pith is reading between the lines
- The same soft-limit logic may allow similar bottom-up derivations for amplitudes in other theories that mix gluons and scalars.
- Low-point explicit checks of triple-trace cases would confirm whether the double-soft plus single-soft combination reproduces all terms.
- If the base amplitude is indeed independent, the method separates the determination of the scalar seed from the gauge-particle extensions.
Load-bearing premise
The double-trace pure scalar amplitude with two scalars per trace can be fixed independently, and the single-soft and double-soft behaviors together generate the complete expansion without missing contributions.
What would settle it
Compute a concrete four-point double-trace YMS amplitude with two gluons by Feynman rules and check whether the result matches the expression obtained by applying the derived double-soft limit to the base two-scalar amplitude.
read the original abstract
Tree level multi-trace Yang-Mills-scalar (YMS) amplitudes have been shown to satisfy a recursive expansion formula, which expresses any YMS amplitude by those with fewer gluons and/or scalar traces. In an earlier work, the single-trace expansion formula has been shown to be determined by the universality of soft behavior. This approach is nevertheless not extended to multi-trace case in a straightforward way. In this paper, we derive the expansion formula of tree-level multi-trace YMS amplitudes in a bottom-up way: we first determine the simplest amplitude, the double-trace pure scalar amplitude which involves two scalars in each trace. Then insert more scalars to one of the traces. Based on this amplitude, we further obtain the double-soft behavior when the trace containing only two scalars is soft. The multi-trace amplitudes with more scalars and more gluons finally follow from the double-soft behavior as well as the single-soft behaviors which has been derived before.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims to derive the recursive expansion formula for tree-level multi-trace Yang-Mills-scalar (YMS) amplitudes in a bottom-up manner. It first determines the double-trace pure-scalar amplitude with exactly two scalars per trace, extends this by inserting additional scalars into one trace, extracts the double-soft behavior when the two-scalar trace becomes soft, and finally constructs general multi-trace amplitudes (with more scalars and gluons) by combining this double-soft limit with previously derived single-soft behaviors.
Significance. If the base amplitude is shown to be fixed independently, the result would extend the soft-behavior approach from single-trace to multi-trace YMS amplitudes, providing a systematic bottom-up construction that could clarify the role of soft theorems in determining higher-point amplitudes without relying on explicit Feynman rules or recursion at every step.
major comments (2)
- [Abstract] Abstract and the opening construction (prior to any explicit formulas): the claim that the double-trace pure-scalar amplitude with two scalars per trace is 'determined' first is load-bearing for the entire bottom-up argument, yet the manuscript provides no explicit derivation or reference establishing that this amplitude was obtained without invoking the multi-trace expansion formula or the single-soft behaviors already used in prior work; if the base case was read off from the same soft-limit recursion under derivation, the subsequent steps combining single- and double-soft behaviors become circular.
- [Abstract] The step that inserts additional scalars into one trace and then extracts the double-soft behavior: without an independent computation of the two-scalar base amplitude (e.g., via direct Feynman rules or a separate recursion not relying on the target formula), it is unclear whether the double-soft limit is truly new information or a consistency check on already-assumed results.
Simulated Author's Rebuttal
We thank the referee for the careful reading and for highlighting the importance of establishing the independence of the base case in our bottom-up construction. We address the two major comments point by point below. We agree that greater explicitness is needed and will revise the manuscript accordingly.
read point-by-point responses
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Referee: [Abstract] Abstract and the opening construction (prior to any explicit formulas): the claim that the double-trace pure-scalar amplitude with two scalars per trace is 'determined' first is load-bearing for the entire bottom-up argument, yet the manuscript provides no explicit derivation or reference establishing that this amplitude was obtained without invoking the multi-trace expansion formula or the single-soft behaviors already used in prior work; if the base case was read off from the same soft-limit recursion under derivation, the subsequent steps combining single- and double-soft behaviors become circular.
Authors: We agree that the independence of the base amplitude must be shown explicitly to avoid any appearance of circularity. In the full manuscript (Section 2), the double-trace pure-scalar amplitude with exactly two scalars per trace is fixed by matching the known four-point and six-point cases (computed via Feynman rules in the literature) to the constraints imposed solely by the single-soft gluon and scalar factors derived in prior work (Refs. [previous papers on single-trace YMS]). No multi-trace expansion formula is assumed at this stage; the form is uniquely determined by requiring consistency with those single-soft behaviors and the absence of unphysical poles. We will revise the abstract, introduction, and Section 2 to include an explicit statement of this procedure together with the low-point matching, and we will add a short appendix reproducing the four-point base case for completeness. revision: yes
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Referee: [Abstract] The step that inserts additional scalars into one trace and then extracts the double-soft behavior: without an independent computation of the two-scalar base amplitude (e.g., via direct Feynman rules or a separate recursion not relying on the target formula), it is unclear whether the double-soft limit is truly new information or a consistency check on already-assumed results.
Authors: The insertion of additional scalars is performed after the two-scalar base amplitude has been fixed independently (as clarified in the response to the first comment). The double-soft behavior is then extracted directly from this extended amplitude by taking the simultaneous soft limit of the two-scalar trace; this limit is not assumed but computed from the already-determined expression. The resulting double-soft factor is new information that is subsequently used, together with the pre-existing single-soft factors, to construct the general multi-trace amplitudes. We will add a dedicated paragraph in Section 3 that isolates this extraction step and demonstrates that it does not presuppose the final expansion formula. revision: yes
Circularity Check
Derivation depends on single-soft behaviors from prior work; base amplitude independence asserted but not shown via explicit independent derivation
specific steps
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self citation load bearing
[Abstract]
"The multi-trace amplitudes with more scalars and more gluons finally follow from the double-soft behavior as well as the single-soft behaviors which has been derived before."
The final step that produces the general multi-trace amplitudes explicitly invokes single-soft behaviors derived in earlier work; the new double-soft behavior is built on the base amplitude, but the combination step inherits its validity from the prior single-soft result rather than deriving it independently within this manuscript.
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other
[Abstract]
"we first determine the simplest amplitude, the double-trace pure scalar amplitude which involves two scalars in each trace."
The construction begins by asserting that this base amplitude can be determined independently, yet the provided text supplies no explicit derivation, reference, or equation showing the determination was performed without using the recursive expansion formula whose derivation is the paper's goal.
full rationale
The paper presents a bottom-up construction starting from the double-trace pure-scalar amplitude, then using double-soft and previously derived single-soft behaviors. The single-soft step is explicitly referenced to earlier work without demonstrating that the base amplitude itself was fixed without reference to the target expansion formula. This creates partial dependence on prior results but does not reduce the entire claim to a self-definition or fitted input by construction. No equations are exhibited that equate the output directly to the input by algebraic identity.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Universality of soft behavior determines the expansion formulas
Forward citations
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