The cosmic ray spectrum measured by the ARGO-YBJ experiment in the 1-1000 TeV energy range

Abstract

Despite a great experimental and theoretical e ort made in order to un- derstand the properties of the primary cosmic rays, several questions are still under discussion and investigation. Recent observations of the spectrum of the light component (protons and helium nuclei) performed by the CREAM experiment, a new{generation balloon{borne detector, present several dis- crepancies with data collected by past experiment. Moreover the origin of the knee is still an unresolved question in cosmic ray research. Several hy- potheses have been presented, actually is believed that the origin of the knee is related to a change of the composition of the primary cosmic rays. A measurement of the composition in the knee region is crucial for the un- derstanding of the origin of the knee. The peculiar characteristics of the ARGO{YBJ experiment like high segmen- tation coupled to a digital readout, full coverage and high altitude location, allow the detection of small showers produced by primaries with energies of the order of 1 TeV. Moreover the analog readout can extend the detector operating range up to 104 TeV allowing the detection of showers in the knee region with very high detail. The ARGO{YBJ observable is the shower multiplicity, namely the space{ time distribution of the particles in the shower front. The energy and mass of the primary that initiated the shower cannot be evaluated on an event{ by{event basis but must be determined by means of an unfolding proce- dure. The classical procedure is based on a tting procedure that requires to formulate hypothesis about the shape of the energy spectrum. In this work an unfolding procedure based on the Bayes theorem was used. The bayesian inference doesn't require any strong assumption about the shape of the spectrum and relies only on the knowledge of the conditioned probabil- ities that relate the observed shower multiplicity with the primary energy. These quantities can be evaluated by means of a Monte Carlo simulation of the development of the shower in the Earth's atmosphere and of the de- tector response. In this work a Monte Carlo data sample of EASs induced by protons, helium nuclei, CNO group and iron nuclei was produced and a full detector simulation was applied, including trigger and RPC's e ciency. Showers were generated in the energy range (0:1 104) TeV. A set of selec- tion criteria based on the shower multiplicity, the reconstructed zenith angle and on the particle density on the detector surface was applied. These cuts allow the selection of showers mainly induced by protons and helium nuclei with core localized inside a ducial area with radius 28 m. The cut on the particle density allows the selection of showers with well{shaped core, discarding the events produced by heavier primaries. The fraction of CNO induced showers is reduced of about ten times compared with the fraction of protons and helium nuclei. A rst selection of the data has been based on the reconstruction quality. A sample of 75 106 events was selected. The selection criteria described above have been adopted for both data and Monte Carlo events. The energy spectrum in the range (5 250) TeV was obtained by applying the bayesian unfolding procedure to the strip distri- bution obtained from the data sample. Statistical uncertainties are of the order of 1%. A study of the possible sources of systematical uncertainties has been performed. The sources of systematic e ects are essentially ef- fects related to the selection cuts used in data analysis, e ects due to the reliability of simulation of the detector response and e ects due to the vari- ation of the fraction of the helium component. All of these e ects have been taken into account and the overall uncertainty does not exceed 10%. The ARGO{YBJ data are fairly consistent with recent measurements of the light component spectrum performed by the CREAM collaboration, while they are in considerable disagreement with measurements reported by RUNJOB. The ARGO{YBJ measurements cover a wide energy range and bridges the energy gap between the low energy direct observations and the ground{based EAS experiments. Showers produced by di erent primaries present several di erence that are covered by uctuations. A mass composition discrimination can be based on the lateral particle density distribution in EASs. In order to achieve a dis- crimination of showers produced by primaries of di erent masses in the knee region a study of the lateral density distribution was performed on Monte Carlo events. A dedicated core reconstruction algorithm was developed. The study of the lateral particle density distribution allowed the determination of a set of discrimination parameters that can be used in order to discrim- inate showers produced by protons and light nuclei and showers produced by heavy nuclei. A preliminary measurement of the all{particle spectrum in the energy range (200 800) TeV has been performed by using the data taken with the analog readout system. Statistical uncertainties are of the order of 3%. The values of the spectrum are a ected by large uncertainties due to the variations of the values of the conditioned probabilities used in the bayesian unfolding technique that are related to large variations of the fraction of the light and heavy component used to build the Monte Carlo data sample.