Cosmic Clash: LHAASO's Proton-Gamma Ray Mystery

In this episode, we dive into a fascinating new study that performs the **first direct consistency check** between two crucial measurements from the Large High Altitude Air Shower Observatory (LHAASO): the **cosmic-ray (CR) proton spectrum at the "knee"** and the **Galactic diffuse gamma-ray emission**.

The "knee" in the cosmic ray spectrum (around a few PeV) is thought to mark the maximum energy reached by Galactic CR accelerators. Diffuse gamma-ray emission, primarily from CR interactions with interstellar gas, provides a complementary view of the same underlying particle population.

The study reveals a **persistent mismatch**:

* The **predicted gamma-ray flux robustly overshoots the LHAASO data** in both inner and lateral Galactic regions.

* This discrepancy is evident in **both normalization and spectral shape**.

* This is particularly puzzling because while an excess of gamma-rays has been discussed before, **evidence of a deficit in observed emission represents a new and more puzzling feature**.

Key insights from the research:

* The disagreement **challenges conventional scenarios** linking the local cosmic-ray sea to Galactic gamma-ray emission.

* It **calls for a revision of current cosmic ray models** in the TeV-PeV sky.

* The mismatch is **not attributed to the hadronic interaction model** used for calculations; using alternative models would actually increase the tension.

* The findings suggest a **possible tension between the LHAASO gamma-ray observations and the CR proton flux measured by LHAASO itself**.

* One intriguing explanation is that the **CR spectrum measured locally might not be the same as the one responsible for the observed gamma-ray emission** throughout the Galaxy, possibly having a different "knee" location (e.g., around 300 TeV).

* Uncertainties also exist due to the **lack of helium flux measurements** between 100 TeV and a few PeV.

This research highlights the critical importance of evaluating the consistency between these two types of measurements and opens new avenues for understanding cosmic ray propagation in our Galaxy.

**Article Reference:**

Espinosa Castro, L. E., Villante, F. L., Vecchiotti, V., Evoli, C., & Pagliaroli, G. (2025). *LHAASO Protons versus LHAASO Diffuse Gamma Rays: A Consistency Check*. arXiv preprint arXiv:2506.06593.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: LHAASO