The energy of this pair of electrons has a signature.
They leave a small edge or a little bump on the energy spectrum.
This edge or bump is the "ghost" that countless people like Shen Che have been staring at their own energy spectra for decades, day and night, desperate to catch.
And as the Wukong Satellite orbited and orbited, it actually saw something at around the 0.9 TeV mark.
The energy spectrum curve, which had been falling smoothly, suddenly snapped at this point, its slope abruptly steepening.
It was as if someone had waved from inside a window to the people outside.
But was this wave the "ghost" this group had been painstakingly searching for for decades?
They didn’t dare to claim it with certainty.
Why?
Because while this wave looked like a real physical structure, it could very well be nothing more than an Illusion Peak.
The Wukong Satellite team couldn’t tell the difference.
That’s why over the past five or six years, they went through generation after generation of unfolding frameworks.
First Tikhonov, then layering in Bayes, and later adding variational methods.
They could keep the low-energy section under control.
But once they passed the 0.9 TeV mark, their calculations became unclear.
Every so often, a peer or two from the international community would call out to the Wukong Satellite team from afar.
"What did you actually see? Do you dare to publish a number?"
Every time, the Wukong Satellite team could only grit their teeth and remain silent.
Until...
Li Dong’s "Comment" paper created an opening.
And what grew out of that opening was this very ruler.
The Li Criterion.
If this ruler could truly slice off that section at 0.9 TeV and prove that someone was really waving from the window...
...then the words this team had held back for five or six years could finally be spoken with confidence today.
And the moment they were spoken would mean a paper in Nature.
The final line of the author list would read—"DAMPE Collaboration" (the Wukong Satellite collaboration).
Although it was the name of an institution, it represented...
...the engineers who personally took this satellite from a blueprint on paper and launched it into the sky.
...the data analysts who kneaded the garbled mess of data returned by the satellite into a single energy spectrum.
...and the physicists who drew the first boundary around the "ghost."
All of their names would be bound to the Wukong Satellite.
And today, all of Shen Che’s team’s waiting over the past five or six years rested squarely on the shoulders of the math group, or more specifically, on Li Dong.
Li Dong listened to what Shen Che had to say.
He pushed his chair back slightly and stood up.
He walked to the whiteboard at the far end of the room.
He didn’t explain the Li Criterion from the beginning.
The professors present were all familiar with it.
So he just drew two curves on the whiteboard.
One was the trajectory of the unfolding operator’s condition number as it climbed along the energy bands.
The other was the trajectory of the Li Criterion’s coefficient as it climbed along the energy bands.
The two lines crossed at a certain point.
Above the intersection, in the small, faint, shaded area where the criterion curve entered, he wrote a single word.
"Anchor."
"The section where the criterion’s coefficient climbs into the anchor interval—treat that as a forbidden zone. Any spikes that appear within it are not to be used as physical conclusions."
"For the remaining section in the safe zone, just use the old method."
He placed the chalk on the table.
Just those two sentences.
The research assistants and postdocs in the room were stunned for a moment.
They all turned to look at Shen Che in unison.
Their gazes seemed to ask:
’Is that it? We’re not going to discuss it?’
Shen Che nodded at them.
"This is what we’ll do."
The room was quiet for about three seconds.
Then the research assistants and postdocs lowered their heads and began taking notes.
Old Zhou also gave Shen Che an "OK" sign.
"Old Shen, I can have a segment of the silicon micro-strip data ready this afternoon for the math group to run a first pass."
Shen Che nodded.
"Alright, let’s do that."
He nodded to everyone in the room.
"That’s all for today."
People in the room began to get up one by one.
A few research assistants, clutching their laptops, headed out in twos and threes, discussing how to implement the two curves from the whiteboard into code as they walked.
Li Dong stood before the whiteboard, about to erase the two curves.
A hand patted him lightly on the shoulder.
He turned around.
Academician Zhang Wenping was looking at him with a smile.
"Son."
"Your method is excellent."
Li Dong smiled.
"You’re too kind, Teacher Zhang."
Shen Che walked over at this moment.
He nodded toward Zhang Wenping.
"Old Zhang, you and Li Dong can just handle the oversight. Leave the specific, hands-on work to the people under you."
"If they’re unsure about anything, you two can help correct them."
He paused, then added,
"Otherwise, if there’s nothing particularly important, everyone can work freely."
"We’ll just stick to meeting once a week."
Zhang Wenping nodded.
"Alright. Old Shen, you keep an eye on things here for now."
He turned his head to look at Li Dong.
"I’m going to go over the details of the national mathematical modeling competition with Li Dong."
Shen Che waved his hand, gesturing for them to go ahead.
Zhang Wenping and Li Dong left the conference room side by side.
As they walked, Zhang Wenping began talking about the national competition.
"The national competition is starting soon."
Li Dong nodded.
Zhang Wenping smiled again.
"The number of people who registered this year is up quite a bit from last year."
The China Undergraduate Mathematical Contest in Modeling, or CUMCM for short.
Among university-level math competitions in the country, it was in the highest tier in terms of both scale and prestige.
This year, over a thousand universities and more than fifty thousand undergraduate teams from across the country had registered.
The numbers were up significantly from the previous year.

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