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Chapter 221: Chapter 213: Untainted by Standard Answers

The first page of the PDF was clean and crisp.

Title: [A Complete Derivation of the Chiral Anomaly for an SU(N) Gauge Field on a Four-Dimensional Compact Euclidean Manifold]

The paper began directly with a lemma.

”Lemma 1: Let D be the SU(N) gauge field-coupled Dirac operator acting on a four-dimensional compact Euclidean manifold M. Then its system of eigenfunctions {φn} forms a complete orthonormal basis for the space of sections of the Dirac spinor bundle.”

Zhou Qifeng nodded silently to himself.

’Standard. Nothing wrong with it.’

For the first question that followed, Li Dong’s entry point was the classic Fujikawa method.

He expanded the fermion field in terms of the Dirac operator’s eigenfunction system, then derived the Jacobian determinant by focusing on the change in the measure under a chiral transformation.

This approach itself wasn’t new.

But what Zhou Qifeng noticed was...

When Li Dong wrote, ”This sum diverges and requires regularization,” he used the heat kernel method to apply a Gaussian cutoff.

A(x) = lim(M→∞) tr[γ5 e^(-D²/M²)]

He handled this step with considerable finesse.

Then, he directly presented the heat kernel expansion of D², stating very cleanly that since tr(γ5) is zero in insufficiently high dimensions, only the a2(x) term could remain.

The entire process of finding the trace was seamless, and he even handled the sign issues in the intermediate steps—which could easily be confusing—with remarkable clarity.

In the end, he directly derived:

∂_μ j_5^μ = [g²/(16π²)] tr(F_μν F̃^μν)

Zhou Qifeng stared at the page for a full two minutes before letting out a soft ”Hm.”

’This kid’s heat kernel regularization is much cleaner than I imagined.’

He even discovered...

The substitution technique Li Dong used during the heat kernel expansion was somewhat similar to a method he himself had learned from an old professor during his postdoctoral research at Princeton.

But it wasn’t exactly the same.

Li Dong’s method was more direct, and more... natural.

It was as if someone completely untainted by traditional textbooks had walked this path from the very beginning on their own.

’A little interesting.’

Zhou Qifeng continued to the next page.

The second question was the most intimidating part of the entire problem.

It required linking the integral of the chiral anomaly to the Atiyah-Singer Index Theorem.

The Index Theorem was one of the most profound achievements in twentieth-century mathematics.

To use it proficiently, one first had to understand fiber bundles, Chen-Weil homomorphism, characteristic classes, and the spectral theory of elliptic operators.

Each of these was a topic that would take a graduate student a year or two to chew through.

Zhou Qifeng had expected Li Dong to dutifully follow the standard textbook approach: first writing about Chen-Weil, then the Chern character, then calculating the  genus, and finally aligning it all with the index formula.

But when he turned to the first page of the second question, he froze.

Li Dong hadn’t done that.

Instead, he started by constructing a ”cutoff flow” adapted to the principal connection of the gauge bundle, then extended the entire heat kernel expansion framework from the first question over to the topological side.

He then pulled the tr(F∧F) derived in the first question back into the heat kernel, and by following the kernel’s evolution over time, he reverse-engineered the topological invariant step-by-step from the analysis side.

Throughout the entire process, he never once mentioned the three words ”Chen-Weil homomorphism.”

But every step he took was, in fact, subtly recreating the spirit of the Chen-Weil homomorphism.

When Zhou Qifeng saw this, he felt a tightness in his chest and realized he had forgotten to breathe.

It wasn’t because it was so breathtaking.

Rather, he suddenly realized...

’This kid isn’t just solving the problem.’

’He’s ”rediscovering” the structure behind it.’

In other words, he had probably never systematically studied the standard proof of the Index Theorem, so he had no idea how textbooks handled this step.

He could only rely on the tools he had on hand and forge this path from scratch himself.

And the result...

The path he forged was shorter than the one in the textbooks.

Zhou Qifeng silently compared Li Dong’s approach with his own line of thinking.

His own way involved fiber bundles, connections, Chern classes... building it all up layer by layer. It was solid, yes, but its complexity was mind-numbing.

To put it bluntly, it was a bit clumsy...

And Li Dong’s approach?

Starting from the heat kernel, he effortlessly glided from the analysis side to the topology side, using only two auxiliary lemmas in between.

Which path was more beautiful?

Zhou Qifeng was an academic. He wouldn’t call a path beautiful just because he was familiar with it.

He would honestly admit that Li Dong’s path was more beautiful, and more inspired.

’This line of thinking is quite good...’

He subconsciously reached for a fresh cigarette, but his hand stopped halfway.

He didn’t want any distractions while reading this paper.

So, he continued to the next page.

...

The third question was his own home turf.

Renormalization.

He knew better than anyone just how deep the waters were in this area of Quantum Field Theory.

To explain the Adler-Bardeen theorem clearly, one had to explain ”why the coefficient of the chiral anomaly holds exactly at all orders of perturbation.”

You not only had to have an intuition for the one-loop exactness of this conclusion, but you also had to provide a convincing argument based on the structure of the renormalization group flow.

This was a question that tested one’s mathematics, physical intuition, and global understanding of the entire framework of Quantum Field Theory.

Zhou Qifeng had originally thought that even if Li Dong managed to complete the first two questions, his answer to the third would likely be rather superficial.

It would probably just be some conversational arguments cherry-picked from textbooks, padded with cure-all phrases like ”therefore, we can see” and ”obviously.”

But instead...

He started with a single sentence:

”The essence of the anomaly is the non-trivial Jacobian determinant generated by the path integral measure under a chiral transformation. Its origin is geometric, not dynamical.”

Starting from this sentence, he made what seemed to be a very bold entry...

He completely stripped the origin of the anomaly coefficient from the dynamical level and moved it to the geometric level of the measure.

All of his subsequent derivations revolved around this single sentence.

Since the anomaly coefficient is a product of geometry, it must be bound to the integer topological invariant protected by the Index Theorem.

And an integer topological invariant cannot be changed under continuous deformation.

Therefore...

Any perturbative correction, as long as it is continuous, cannot possibly alter this integer.

Thus, the anomaly coefficient must be one-loop exact.

When Zhou Qifeng read this, he fell silent for a full five minutes.

He had seen this angle of attack before.

But anyone who used it was typically a veteran who had been immersed in the path of Quantum Field Theory for ten or twenty years.

These were people who, having seen so much, could see through to both the dynamical and geometric aspects, and who knew that the anomaly fundamentally did not belong to the dynamical side.

But Li Dong was a first-year undergraduate.

A short while ago, he was still tearing up the mathematics world.

How could he possibly have such a precise grasp of this level of geometric intuition in Quantum Field Theory?

For a moment, Zhou Qifeng even had a ridiculous feeling...

’This doesn’t seem like the thinking of a nineteen-year-old undergraduate.’

’It’s more like the thoughts of a top-tier old professor in the field of Quantum Field Theory.’

Zhou Qifeng took a deep breath.

’Could he have come up with this line of reasoning himself?’

He admitted that, given enough time, he could probably have thought of one of these approaches.

But to have him connect all three lines of thought and write them up into a clean, seventeen-page derivation in just three days?

Absolutely impossible.

It wasn’t that he couldn’t do it; it was that he would never have even thought in that direction.

Zhou Qifeng sat dazedly in his chair...

Outside the study, his young daughter called out from the living room, ”Dad, dinner’s ready!”

He replied automatically, ”You guys eat first. I’ll be there in a bit.”