Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory

TL;DR

This work reports the first search for a diffuse flux of photons with energies between $50$ PeV and $200$ PeV using the Pierre Auger Observatory in the Southern Hemisphere. The analysis combines a dense surface-detector array with buried muon detectors, employs Monte Carlo–driven photon energy calibration, and uses a muon-based observable $M_b$ to discriminate photon-initiated from proton-initiated showers under a unified energy scale. No photon candidates were found in 14,299 events after blinding, leading to upper limits on the integral photon flux of $[13.8,13.5,13.3,13.6]$ km$^{-2}$ sr$^{-1}$ yr$^{-1}$ above $[50,80,120,200]$ PeV at 95 extpercent CL, with fractional limits on the photon fraction at the $10^{-2}$–$10^{-3}$ level. The results extend Auger’s photon searches toward lower energies, demonstrate the power of combining SD and UMD measurements, and set the stage for future sensitivity gains (factor of ~20) over the next decade via continued data taking and the AugerPrime upgrade, enabling constraints on SHDM models and diffuse Galactic gamma-ray flux predictions.

Abstract

Diffuse photons of energy above 0.1 PeV, produced through the interactions between cosmic rays and either interstellar matter or background radiation fields, are powerful tracers of the distribution of cosmic rays in the Galaxy. Furthermore, the measurement of a diffuse photon flux would be an important probe to test models of super-heavy dark matter decaying into gamma-rays. In this work, we search for a diffuse photon flux in the energy range between 50 PeV and 200 PeV using data from the Pierre Auger Observatory. For the first time, we combine the air-shower measurements from a 2 km$^2$ surface array consisting of 19 water-Cherenkov surface detectors, spaced at 433 m, with the muon measurements from an array of buried scintillators placed in the same area. Using 15 months of data, collected while the array was still under construction, we derive upper limits to the integral photon flux ranging from 13.3 to 13.8 km$^{-2}$ sr$^{-1}$ yr$^{-1}$ above tens of PeV. We extend the Pierre Auger Observatory photon search program towards lower energies, covering more than three decades of cosmic-ray energy. This work lays the foundation for future diffuse photon searches: with the data from the next 10 years of operation of the Observatory, this limit is expected to improve by a factor of $\sim$20.

Figures (16)

• Figure 1: Left: A schematic view of the SD-433 array. Solid black positions are equipped with buried scintillators. Events acquired by the three highlighted hexagons are considered for the photon search. Right: One of the events acquired on February 27, 2021, part of the selected data set. The reconstructed energy is [$(4.1\pm0.1)\times10^{17}$]eV, obtained with a dedicated data energy scale BrichettoICRC2023, with a zenith angle of [$(26.3\pm0.3)$]$^\circ$. Dashed lines indicate the fitted lateral distribution functions (see \ref{['sec:MCRec']}).
• Figure 2: The trigger efficiency as a function of the simulated energy for photon and proton primaries, depicted by empty and filled markers respectively, and different zenith angle intervals. Lines correspond to sigmoid functions fitted via a maximum likelihood method.
• Figure 3: Left: The power-law index from \ref{['eq:S250E']} as a function of zenith angle. The solid line represents the model of \ref{['eq:alphaCos2']}. Right: The average ratio between shower size and primary energy raised to the power of $\alpha$, in terms of the zenith angle. The fitted model of \ref{['eq:GH']} is represented as the solid line.
• Figure 4: The bias and resolution of the energy calibration for photon events defined by \ref{['eq:S250E']}.
• Figure 5: The bias (top) and resolution (bottom) of the energy calibration for proton events defined by \ref{['eq:S250E_pr']}.