Search for photons with energies above 10¹⁸eV using the hybrid detector of the Pierre Auger Observatory

Abstract

A search for ultra-high energy photons with energies above 1 EeV is performed using nine years of data collected by the Pierre Auger Observatory in hybrid operation mode. An unprecedented separation power between photon and hadron primaries is achieved by combining measurements of the longitudinal air-shower development with the particle content at ground measured by the fluorescence and surface detectors, respectively. Only three photon candidates at energies 1 − 2 EeV are found, which is compatible with the expected hadroninduced background. Upper limits on the integral flux of ultra-high energy photons of 0.027, 0.009, 0.008, 0.008 and 0.007 km −2 sr −1 yr −1 are derived at 95% C.L. for energy thresholds of 1, 2, 3, 5 and 10 EeV. These limits bound the fractions of photons in the all-particle integral flux below 0.1%, 0.15%, 0.33%, 0.85% and 2.7%. For the first time the photon fraction at EeV energies is constrained at the sub-percent level. The improved limits are below the flux of diffuse photons predicted by some astrophysical scenarios for cosmogenic photon production. The new results rule-out the early top-down models − in which ultra-high energy cosmic rays are produced by, e.g., the decay of super-massive particles − and challenge the most recent super-heavy dark matter models.

Ultra-high energy cosmic rays: Recent results and future plans of Auger

“…Recent updates of the diffuse flux limits of photons derived from Auger data are presented in Fig. 5 (left) together with expected diffuse photon fluxes originating from the GZK-process or from particle physics motivated top-down models [8]. These photon limits are the most stringent ones currently available above 1 EeV and they disfavour top-down models most strongly and also start to constrain the parameter space for cosmogenic photons in case of pure proton sources [35].…”

“…A strong flux suppression at the highest energies, similarly to the one expected from cosmic ray energy losses in the Cosmic Microwave Background (CMB) radiation (GZK-effect) [3,4], has been observed beyond any doubt [5,6]. Moreover, strong flux limits have been placed on the photon and neutrino components at EeV energies [7,8,9,10,11] disfavoring exotic particle physics models for the origin of the most energetic cosmic rays and starting to constrain the parameter space of cosmogenic neutrinos. Also, evidence is found for the presence of a large-scale anisotropy above the energy of the ankle [12,13], and for an anisotropy on smaller angular scales at E > 5.5 · 10 19 eV [14,15,16,17].…”

Probing heavy dark matter decays with multi-messenger astrophysical data

“…Observations Energy [GeV] Detected CL upper limits Gamma (γ) Fermi-LAT [20] 10 −2 -10 3 CASA-MIA [26] 10 5 -10 7 90% KASCADE [25] 10 5 -10 7 90% KASCADE-Grande [25] 10 7 -10 8 90% PAO [30,31] 10 9 -10 10 95% TA [34] 10 9 -10 11 95% Proton (p) PAO [37] 10 9 -10 11 84% Anti-proton (p) PAO [37] 10 9 -10 11 84% AMS-02 [21] 10 −1 -10 2 Positron (e + ) AMS-02 [22] 10 −1 -10 3 Neutrino (ν)…”

“…6. Upper bounds from the observations are given by CASA-MIA [26], KASCADE, KASCADE-Grande [25], PAO [30,31] and TA [34]. Fig.…”

Cosmic ray models

“…While this possibility is of importance for the search for particle physics beyond the standard model, it can give only a subdominant contribution to the observed CR flux: The main predictions of this scenario-a flat energy spectrum (∝ 1/E 1.9 ) of the decay products, a large photon fraction, equal matter and antimatter fluxes, and the (almost) absence of nuclei [49]-constrain this contribution e.g. in the energy range 10 18 -10 19 eV to be less than 0.1% of the total CR flux [50]. In the PeV range, it has been suggested that the diffuse neutrino flux observed by IceCube is generated by decays of dark matter particles [51].…”