Holt, Ewa M.

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Pierre Auger Collaboration

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© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).EPJ Web of Conferences 216, 02002 (2019) https://doi.org/10.1051/epjconf/201921602002ARENA 2018Estimating the mass of cosmic rays by combining radio andmuon measurementsEwa M. Holt1,2, for the Pierre Auger Collaboration3,1Karlsruhe Institute of Technology (KIT), Institut für Kernphysik, Karlsruhe, Germany2Instituto de Tecnologías en Detección y Astropartículas (CNEA, CONICET, UNSAM), CABA, Argentina3Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, ArgentinaAbstract. The Auger Engineering Radio Array (AERA) is a radio detector at the PierreAuger Observatory and it is dedicated to measure the radio emission of cosmic-ray airshowers. AERA is co-located with the underground muon detectors of the Auger Muonsand Infill for the Ground Array (AMIGA). This provides a perfect setup to experimentallytest the benefits of combining muons and radio emission for estimating the primary mass.We have investigated this combination using air-shower simulations. We compared theperformance for mass separation of this new method to alternative methods in which theelectrons and muons are measured with particle detectors at the surface. For showers withzenith angles below 50◦ the new method is of comparable performance, and for showersmore inclined than 50◦ it is clearly superior. Therefore, measuring the radio signal inaddition to the muons significantly improves the mass sensitivity compared to techniquesusing solely particle measurements.1 IntroductionTo reveal the origin of high- and ultra-high energy cosmic rays, mass dependent measurements ofthe energy spectrum and arrival directions are of prime importance. Cosmic rays at these energiesare measured via extensive air showers they induce in the atmosphere. The ratio of the muonicand electromagnetic component of these air showers is a well known mass estimator. The showercomponents can be measured with particle detectors at the ground or indirectly via the electromagneticradiation produced from the showers in the atmosphere, such as radio emission. We have developed anovel technique to estimate the mass of cosmic rays by combining measurements of muons and theradio emission to estimate the mass of the primary cosmic rays.At the Pierre Auger Observatory in Argentina [1], muons are measured separately by undergroundscintillators [2]. The radio emission is measured by AERA, consisting of a radio antenna array [3].Both detectors are co-located and measure cosmic ray air showers above an energy of 1017.5 eV.We investigated the power of the muon-radio combination to separate air showers induced by protonand iron nuclei, using CORSIKA simulations and the hadronic model QGSJETII-04. We studied theinfluence of the detector responses of AMIGA and AERA to the mass separation power and combinedmeasurements of AMIGA and AERA for the first time. In this work, we give an overview of the mostimportant results. For more details on the analyses, see Refs. [4] and [5].e-mail: ewa.holt@kit.eduFull author list at http://www.auger.org/archive/authors_2018_06.html2EPJ Web of Conferences 216, 02002 (2019) https://doi.org/10.1051/epjconf/201921602002ARENA 20180 10 20 30 40 50 60 70 80zenith angle in deg10−510−410−3ρ600µ/√Sρθ RDinm−2eV−1/2at1EeVproton iron(a)0 10 20 30 40 50 60 70 80zenith angle in deg0.00.51.01.52.02.5figureofmeritproton-ironseparationpowerFOM = |µP−µFe|√σ2P+σ2Feρ600µ /√SρθRD ρ600µ / Ne Xmax(b)Figure 1. (a) Muon-radio mass estimator versus zenith angle for showers with a primary energy of 1 EeV. The lines show the mean values µi and the shaded areas depict the standard deviations σi. (b) Mass separation power of different mass estimators, represented by the figure of merit (FOM). The indices signify proton (P) and iron nuclei (Fe). No uncertainties from detector effects are included.2 Mass separation powerThe ratio of the muons and the radio emission, represented by observables as measured in AMIGAand AERA, is shown in Fig. 1a. The size of the muonic component is proportional to the muondensity ρ600µ at a distance of 600 m to the shower axis, which is used here as the observable. The squareroot of the radiation energy S ρθRD , which is the energy contained in the radio emission, represents anobservable for the radio emission. Here, the observables are true values without any uncertaintiesarising from detector responses, background and reconstruction methods. The error band depicts thestandard deviation due to shower-to-shower fluctuations. The ratio ρ600µ /√(SρθRD) is clearly separated forproton and iron nuclei for all zenith angles. It is constant until a zenith angle of 55◦, which correspondsto the measurement range of AMIGA. It decreases for larger zenith angles, at which a fraction of themuons decay before reaching the ground.Fig. 1b shows the mass separation power of the novel mass estimator in form of the figureof merit (FOM). In addition, the figure of merit is shown for the ratio of muons and electrons,and the shower maximum Xmax, which are classical mass estimators. At large zenith angles, theelectromagnetic component is absorbed in the atmosphere, which diminished the mass separationpower when combining muons and electrons. Xmax is subject to shower-to-shower fluctuations, inparticular for proton showers. On the contrary, the atmosphere is transparent for the radio emission andhence the emission is approximately constant at the ground for all zenith angles. In addition, it suffersless shower-to-shower fluctuations. Therefore, considering true observables without measurementuncertainties, the muon-radio combination shows a superior mass separation power compared to someclassical estimators, in particular for large zenith angles.3 Application to AMIGA and AERA measurementsWe applied the novel mass estimator to coincident measurements of AMIGA and AERA. Therefore,we first studied the influence of the measurement uncertainties by adding the detector responses andmeasured radio background to the simulations. The reconstructed observables including the detectorsare shown in Fig. 2a. The figure of merit is reduced by about 0.5 compared to the pure air-showersimulations due to measurement and reconstruction uncertainties. However, this is still sufficient formass composition studies.3EPJ Web of Conferences 216, 02002 (2019) https://doi.org/10.1051/epjconf/201921602002ARENA 20180 10 20 30 40 50 60 70 80zenith angle in deg10−510−410−3ρ600µ/√Sρθ RDinm−2eV−1/2at1EeVproton iron(a)0 10 20 30 40 50 60 70 80zenith angle in deg0.00.51.01.52.02.5figureofmeritproton-ironseparationpowerFOM = |µP−µFe|√σ2P+σ2Feρ600µ /√SρθRD ρ600µ / Ne Xmax(b)Figure 1. (a) Muon-radio mass estimator versus zenith angle for showers with a primary energy of 1 EeV. The lines show the mean values µi and the shaded areas depict the standard deviations σi. (b) Mass separation power of different mass estimators, represented by the figure of merit (FOM). The indices signify proton (P) and iron nuclei (Fe). No uncertainties from detector effects are included.2 Mass separation powerThe ratio of the muons and the radio emission, represented by observables as measured in AMIGAand AERA, is shown in Fig. 1a. The size of the muonic component is proportional to the muondensity ρ600µ at a distance of 600 m to the shower axis, which is used here as the observable. The squareroot of the radiation energy S ρθRD , which is the energy contained in the radio emission, represents anobservable for the radio emission. Here, the observables are true values without any uncertaintiesarising from detector responses, background and reconstruction methods. The error band depicts thestandard deviation due to shower-to-shower fluctuations. The ratio ρ600µ /√(SρθRD) is clearly separated forproton and iron nuclei for all zenith angles. It is constant until a zenith angle of 55◦, which correspondsto the measurement range of AMIGA. It decreases for larger zenith angles, at which a fraction of themuons decay before reaching the ground.Fig. 1b shows the mass separation power of the novel mass estimator in form of the figureof merit (FOM). In addition, the figure of merit is shown for the ratio of muons and electrons,and the shower maximum Xmax, which are classical mass estimators. At large zenith angles, theelectromagnetic component is absorbed in the atmosphere, which diminished the mass separationpower when combining muons and electrons. Xmax is subject to shower-to-shower fluctuations, inparticular for proton showers. On the contrary, the atmosphere is transparent for the radio emission andhence the emission is approximately constant at the ground for all zenith angles. In addition, it suffersless shower-to-shower fluctuations. Therefore, considering true observables without measurementuncertainties, the muon-radio combination shows a superior mass separation power compared to someclassical estimators, in particular for large zenith angles.3 Application to AMIGA and AERA measurementsWe applied the novel mass estimator to coincident measurements of AMIGA and AERA. Therefore,we first studied the influence of the measurement uncertainties by adding the detector responses andmeasured radio background to the simulations. The reconstructed observables including the detectorsare shown in Fig. 2a. The figure of merit is reduced by about 0.5 compared to the pure air-showersimulations due to measurement and reconstruction uncertainties. However, this is still sufficient formass composition studies.105 106 107 108 109SρθRD in eV10−1100101ρ600µinm−2proton QGSJETII-04iron QGSJETII-04proton fitiron fit(a)105 106 107 108 109SρθRD in eV10−1100101ρ600µinm−2data, # 104proton fit QGSJETII-04 × 1.61iron fit QGSJETII-04 × 1.61(b)Figure 2. Correlation of the muon density and the radiation energy for (a) simulations including detector responses of AMIGA and AERA and measured radio background, and (b) coincident measurements of AMIGA and AERA.We analyzed about one year of measurements of AMIGA and AERA and found 104 coincidentevents. The reconstructed muon density and radiation energy of these events are shown in Fig. 2b.For comparison, the fits for proton and iron from Fig. 2a are shown, shifted by a known discrepancybetween the muon number predicted from QGSJETII-04 and measurements of 61 % [6]. The averagedistribution of the events follows the simulated correlations and its magnitude lies within the mean ofthe simulations. This confirms the magnitude of the discrepancy of the muon number. More importantly,it demonstrates the potential of the novel mass estimator combining muon and radio measurements.4 ConclusionWe developed a novel technique to estimate the mass of cosmic rays by combining measurementsof muons and radio emission of air showers. Thereby, the muons are measured in form of the muondensity ρ600µ at 600 m distance to the shower axis and the radio emission in form of the radiationenergy S ρθRD . Air-shower simulations (without detector responses) show that the mass estimatorformed by these observables, ρ600µ /√(SρθRD) , has a superior mass separation power compared to someclassical mass estimators, in particular for inclined showers. We applied the novel technique tocoincidence measurements of AMIGA and AERA and confirmed experimentally the correlation of thetwo observables. The novel technique can further be applied to other experiments combining muonand radio detectors, as already aimed for in various activities, building the Auger Radio Upgrade [7] orequipping IceTop with radio antennas [8].References[1] A. Aab et al. [Pierre Auger Collaboration], Nucl. Instrum. Meth. A798, 172-213 (2015)[2] A. Aab et al. [Pierre Auger Collaboration], JINST 11, P02012 (2016)[3] E. M. Holt for the Pierre Auger Collaboration, Proceedings of Science ICRC2017, 492[4] E. M. Holt, PhD Thesis, KIT and UNSAM, 2018, DOI: 10.5445/IR/1000083318[5] E. M. Holt, F. G. Schröder, in preparation for Eur. Phys. J. C (2018)[6] A. Aab et al. [Pierre Auger Collaboration], Phys. Rev. Lett. 117, 192001 (2016)[7] Hörandel, J. R. for the Pierre Auger Collaboration, Eur. Phys. J. WoC, this Issue[8] A. Balagopal V. et al., Eur. Phys. J. C78.2, 111 (2018)

Estimating the mass of cosmic rays by combining radio and muon measurements

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Estimating the mass of cosmic rays by combining radio and muon measurements

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