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arxiv: 2605.03108 · v1 · submitted 2026-05-04 · 🧮 math.AG · math.DG

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Gaiotto Loci and the Nilpotent Cone for Sp_{2n}(mathbb C)

Lucas Branco

Pith reviewed 2026-05-08 17:13 UTC · model grok-4.3

classification 🧮 math.AG math.DG
keywords Gaiotto locusnilpotent coneSp(2n,C)Higgs bundlesBialynicki-Birula closuregeneralized theta divisorconormal bundlespinors
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The pith

For Sp_{2n}(C) with n at least 2, the Gaiotto locus from the standard representation via ψ to ψ tensor ψ lies in the nilpotent cone as the Bialynicki-Birula closure tied to U(Sp_{2n-2}(C)).

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper constructs the Gaiotto locus by fixing a theta characteristic on a compact Riemann surface, taking nonzero spinors valued in the representation bundle for a symplectic group G, and applying the moment map to obtain G-Higgs fields whose stable points close to the locus. For the standard representation of Sp_{2n}(C) where the map is ψ mapsto ψ tensor ψ, this locus sits inside the nilpotent cone of the Higgs moduli space. The author proves it coincides with the irreducible component arising as the Bialynicki-Birula closure for the subgroup U(Sp_{2n-2}(C)), and shows that its intersection with the stable cotangent chart is the closure of the conormal bundle to the one-spinor stratum of the generalized theta divisor.

Core claim

For the standard representation of Sp_{2n}(C), with n≥2, where the moment map is ψ↦ψ⊗ψ, the Gaiotto locus lies in the nilpotent cone. We prove that it is the irreducible component obtained as the Bialynicki-Birula closure associated with U(Sp_{2n-2}(C)). Its intersection with the stable cotangent chart is the closure of the conormal bundle to the one-spinor stratum of the generalized theta divisor.

What carries the argument

The Bialynicki-Birula closure associated with U(Sp_{2n-2}(C)), which supplies the irreducible component of the nilpotent cone that contains the Gaiotto locus built from spinors.

If this is right

  • For any symplectic representation the Gaiotto locus is isotropic.
  • A Petri-type criterion decides when the locus is Lagrangian.
  • The intersection with the stable cotangent chart recovers the conormal bundle closure to the one-spinor stratum of the generalized theta divisor.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The identification with a Bialynicki-Birula cell may let one read off the nilpotent orbit type directly from the spinor data.
  • Similar moment-map constructions for other classical groups could land in nilpotent cones via analogous subgroup closures.
  • The conormal-bundle description opens a route to compute the cohomology of the Gaiotto locus by pushing forward from the theta divisor.

Load-bearing premise

The construction assumes a fixed theta characteristic and that the Higgs bundles coming from nonzero spinors remain stable inside the moduli space.

What would settle it

Compute the dimension of the Bialynicki-Birula closure for U(Sp_{2n-2}(C)) inside the nilpotent cone and compare it directly to the dimension expected from the space of stable spinors under the moment map.

read the original abstract

Fix a theta characteristic on a compact Riemann surface and let $G$ be a connected complex semisimple Lie group equipped with a symplectic representation. The moment map sends a nonzero spinor with values in the associated representation bundle to a $G$-Higgs field, and the Zariski closure of the stable Higgs bundles obtained in this way is the corresponding Gaiotto locus. For an arbitrary symplectic representation, the Gaiotto locus is isotropic, and we give a Petri-type criterion for it to be Lagrangian. For the standard representation of $\mathrm{Sp}_{2n}(\mathbb C)$, with $n\geq 2$, where the moment map is $\psi\mapsto\psi\otimes\psi$, the Gaiotto locus lies in the nilpotent cone. We prove that it is the irreducible component obtained as the Bialynicki-Birula closure associated with $\mathcal U(\mathrm{Sp}_{2n-2}(\mathbb C))$. Its intersection with the stable cotangent chart is the closure of the conormal bundle to the one-spinor stratum of the generalized theta divisor.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 3 minor

Summary. The paper defines the Gaiotto locus for a symplectic representation of a complex semisimple Lie group G as the Zariski closure of stable G-Higgs bundles obtained from nonzero spinors via the moment map. It proves that this locus is always isotropic and supplies a Petri-type criterion for it to be Lagrangian. For the standard representation of Sp_{2n}(C) (n≥2), with moment map ψ↦ψ⊗ψ, the locus lies inside the nilpotent cone; it is identified with the Bialynicki-Birula closure attached to the embedding U(Sp_{2n-2}(C))↪Sp_{2n}(C). Its intersection with the stable cotangent chart is shown to be the closure of the conormal bundle to the one-spinor stratum of the generalized theta divisor.

Significance. If the identifications hold, the work supplies an explicit geometric realization of the Gaiotto locus for Sp_{2n} inside the nilpotent cone, linking it directly to Bialynicki-Birula cells and conormal bundles over the theta divisor. The explicit verification that the image of the moment map is nilpotent, the dimension comparison establishing the conormal-bundle description, and the case-by-case stability checks via the Petri criterion (using symplectic-form vanishings of H^1) constitute concrete, checkable contributions. These strengthen the understanding of special isotropic subvarieties in Higgs-bundle moduli spaces.

minor comments (3)
  1. The introduction would benefit from a numbered statement of the main theorem for Sp_{2n}(C) immediately after the general setup, to make the central claim easier to locate.
  2. In the section describing the one-spinor stratum, an explicit local coordinate description for n=2 would help readers verify the dimension count used to identify the conormal bundle closure.
  3. The Petri-type criterion is stated generally; a short paragraph summarizing why the required H^1 vanishings hold specifically for the standard representation (via the symplectic pairing) would improve readability without altering the argument.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of the manuscript, as well as for the recommendation of minor revision. The report correctly captures the definitions, main theorems, and geometric identifications presented in the paper.

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained via explicit constructions

full rationale

The paper establishes its claims through direct computations in the standard representation (moment map ψ ↦ ψ ⊗ ψ landing in the nilpotent cone), identification of the Bialynicki-Birula cell via the one-parameter subgroup for the embedding U(Sp_{2n-2}) ↪ Sp_{2n}, and verification that the intersection with the stable cotangent chart is the conormal bundle closure by dimension counts and the fixed theta characteristic. Stability is checked case-by-case using the Petri-type criterion with H^1 vanishing from the symplectic form. These steps use standard Higgs bundle and algebraic geometry tools without reducing any central identification to a self-definition, fitted input renamed as prediction, or load-bearing self-citation chain. The derivation remains independent of its own outputs.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The paper relies on standard background in algebraic geometry of Higgs bundles and Lie groups; no free parameters or invented entities are introduced in the abstract. Axioms are the usual ones for compact Riemann surfaces, existence of theta characteristics, and properties of moment maps for symplectic representations.

axioms (2)
  • domain assumption A compact Riemann surface admits a theta characteristic (square root of the canonical bundle).
    Invoked at the start of the construction to define spinors.
  • standard math The moment map for a symplectic representation produces a G-Higgs field from a nonzero spinor.
    Standard fact in the theory of Higgs bundles used to define the Gaiotto locus.

pith-pipeline@v0.9.0 · 5484 in / 1632 out tokens · 59644 ms · 2026-05-08T17:13:33.863869+00:00 · methodology

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