Azimuthal asymmetries in signals of non vertical showers have been observed in ground arrays of water Cherenkov detectors, like Haverah Park and the Pierre Auger Observatory. The asymmetry in time distributions of arriving particles offers a new possibility for the determination of the mass composition. The dependence of this asymmetry on atmospheric depth shows a clear maximum at a position that is correlated with the primary species. In this work a novel method to determine mass composition based on these features of the ground signals is presented and a Monte Carlo study of its sensitivity is carried out.
Deep Dive into Time asymmetries in extensive air showers: a novel method to identify UHECR species.
Azimuthal asymmetries in signals of non vertical showers have been observed in ground arrays of water Cherenkov detectors, like Haverah Park and the Pierre Auger Observatory. The asymmetry in time distributions of arriving particles offers a new possibility for the determination of the mass composition. The dependence of this asymmetry on atmospheric depth shows a clear maximum at a position that is correlated with the primary species. In this work a novel method to determine mass composition based on these features of the ground signals is presented and a Monte Carlo study of its sensitivity is carried out.
The determination of the nature of ultra high energy cosmic rays (UHECR) is a crucial point to help understanding their origin, acceleration mechanisms and propagation from the sources to the Earth. At energies below 10 15 eV, both charge and mass can be measured directly using space detectors, however, the properties of cosmic rays of the highest energies have to be inferred from the features of the shower induced in the atmosphere. Air shower experiments are either ground arrays of detectors that trigger in coincidence when a shower passes through them, or optical detectors that observe the longitudinal development of the extensive air shower (EAS) [1,2,3,4,5,6].
The measurement of the primary mass in EAS experiments is known to be very difficult due to the large fluctuations resulting from the statistical nature of the shower development, in particular those associated to the depth and the number of particles produced in the first interactions. Furthermore, the interpretation of data to determine mass composition has to be obtained by comparison with Monte Carlo predictions dependent on high energy hadronic models. With increasing primary energy, this task becomes more difficult as the gap to the energy range studied in accelerator experiments increases and the hadronic interaction properties have to be extrapolated over a wide range. One of the main sources of uncertainties in any analysis to determine mass composition comes from the different predictions for different hadronic interaction models. 2 ) before the shower core impact point and the opposite “late” region. Note the different amount of atmosphere traversed by the particles reaching the detectors in each region.
The distribution of shower maximum, X max , that is the atmospheric depth at which the number of charged particles in the EAS is maximum, is sensitive to the composition of cosmic rays. Protons produce deeper showers with fluctuations larger than those of heavier nuclei. Therefore, for a given primary energy, the < X max > value and its fluctuations decrease with heavier primary mass. This is the principle in which separation methods using < X max >, its fluctuations and the elongation rate, d(X max )/d(logE) [7,8], as measured by fluorescence detectors, are based.
In ground array experiments the analysis is usually performed by projecting the signals registered by the detectors into the shower plane (see Figure 1) and thus, neglecting the further shower evolution of the late regions. As a consequence, for inclined showers, the circular symmetry in the signals of surface detectors is broken. This results in a dependence of the signal features on the azimuth angle in the shower plane [9], mainly due to the different amount of atmosphere traversed by the shower particles [10].
Evidences of azimuthal asymmetries in the signal size were first observed at Haverah Park [11]. Recently the Pierre Auger Observatory has found in addition, the expected asymmetry in the particle arrival time distributions [9]. The observation of these asymmetries for incoming directions with zenith angle smaller than 60 • , has been possible at the Pierre Auger Observatory due to the large size of the array and the high time resolution electronic of the surface detector stations [12]. The design of the observatory allows measuring this feature of EAS which, as demonstrated below, carries very valuable information on to the chemical composition of cosmic rays. First results showing the sensitivity to primary species at the Pierre Auger Observatory have been presented in [13] and [14].
In this work it is shown that the asymmetry in risetime, t 1/2 , defined as the time to reach from 10% to 50% of the total integrated signal in each station, is related to the shower stage of development [14,15,16]. Thus, for a given primary energy E, the asymmetry depends on zenith angle θ of the primary cosmic ray in such a way that its behavior versus sec θ is reminiscent of the longitudinal development of the shower. This “longitudinal development of the asymmetry” is strongly dependent on the nature of the primary particle. The method presented here is quite general and, in principle, might be applied to other timing parameters describing the time signal structure as well as other shower observables like signal size which was observed to be less sensitive to mass composition.
The analysis described in this work is based on Monte Carlo simulations carried out with the code aires [17] using the hadronic interaction models qgsjetii(03) [18] and sibyll 2.1 [19]. The aires generated showers were subsequently used as input in the detector simulation code and finally reconstructed using for both tasks the official Offline reconstruction framework of the Pierre Auger Observatory [20].
The paper is organised as follows. In section 2 the relationship between asymmetry in the time structure and shower evolution is discussed in detail. A brief description of the Pierre Auger Observato
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