honored to summarize the work of so many brilliant colleagues pushing the boundaries of what we can (and can't yet) detect.

tl;dr: direct detection meets neutrino floor; indirect searches go truly multimessenger; non-WIMPs move to center; structural convergence of communities; where are we now & where are we headed---and more.

[14/]

that's where this story ends---for now. continued monitoring with Fermi and future gamma-ray missions (especially in the MeV band) could give us the full picture.

📖 In the meantime, take a look at this event with us:
arxiv.org/abs/2503.16606

🧪⚛️

[13/]

what are these two bright dots here, you ask?

we also checked two gamma ray sources nearby. one was too soft (wrong kind of spectrum), and the other (albeit harder spectrum) was the brightest 13 years *before* the neutrino showed up.

so neither is our likely culprit.

[12/]

here’s one takeaway: with no gamma rays, we can rule out a bunch of possible combinations of source distance (redshift z) and intergalactic magnetic field strength (B) that would produce a detectable cascade.

[Shaded regions = excluded by Fermi-LAT non-detection of gamma-ray cascade]

[11/]
maybe...

🤔 ...the source was far away---so far that the cascade light is too faint to detect, or
🤔 ...the space between was full of strong magnetic fields---which scattered the gamma rays, or
🤔 ...the source is "hidden"---buried in dust and matter that absorbed the light before it escaped.

[10/]

if there were gamma rays, and we didn’t see them, that tells us something about the universe between us and the source.

like...

[9/]

🚫 We find no cascade. No obvious counterpart in the gamma-ray sky.

But this is still interesting---I promise!

[here’s a sneak-peak into the year-by-year look at the gamma-ray sky around the neutrino.]

[8/]
Now is the time to welcome our favorite space-based gamma-ray observatory, Fermi! 🛰️

So, we dig into 17 years of Fermi-LAT data... searching really hard around the neutrino location in all kinds of time intervals, looking for steady signals or flares.
And…

[7/]

This just means we have to think a bit harder about the signal we are searching for, but fret not---Carlos' code to the rescue! We use a code called γ-Cascade to model how the gamma rays would travel, scatter, and arrive at Earth. 🧑‍💻

[6/]

If a neutrino this powerful was detected, where are the gamma rays that should follow it?

Well, we can look for them. But there is a catch: high-energy gamma rays can't travel very far through space---they crash into the cosmic background light and cascade down to lower energies (GeV--TeV).

[5/]

Charged pions decay into muons and -neutrinos-.
Neutral pions decay into -gamma rays-.

If neutrinos are being produced in these processes, gamma rays should be created right alongside them.

Also, lucky for us: gamma rays interact more readily with detectors!

[4/]

Let's put our physicist's hat on and ask: how was this high-energy neutrino produced?

We suspect it was produced in a hadronic interaction---where a high-energy cosmic ray---crashes into surrounding matter or light.

This kind of collision creates unstable particles: pions.

[3/]

Neutrinos barely interact with our usual surroundings. they zip through planets, stars... about 100 trillion neutrinos pass through our bodies every second.

Detecting a neutrino is hard. Detecting one with this energy?

Unprecedented.

[2/]

like...a lot.

30x more than the previous highest-energy neutrino.

or... think about the most powerful accelerator on Earth (Large Hadron Collider). LHC smashes particles at 13.6 TeV. This neutrino had ~220,000 TeV --- packed into *one tiny particle*.

[1/]
back in February 2023, the KM3NeT detector---sitting 3 km deep in the Mediterranean Sea---saw a neutrino so energetic, it shattered records.

Neutrino KM3-230213A carried ~220 PeV of energy.

that's a lot.