21 st ECRS , Košice, Slovakia

1. 21st ECRS, Košice, Slovakia Investigation of primary cosmic ray spectrum shape by means of EAS muon density technique I.I.Yashin, IIYashin@mephi.ru Moscow Engineering Physics Institute A.G. Bogdanov, D.V.Chernov, D.M. Gromushkin, R.P. Kokoulin, G. Mannocchi, A.A. Petrukhin, O. Saavedra, V.V. Shutenko, G. Trinchero, I.I. Yashin • Moscow Engineering Physics Institute, Russia • Istituto di Fisica dello Spazio Interplanetario, INAF, Torino, Italy • Universita di Torino, Italy

2. 21st ECRS, Košice, Slovakia Motivation • In EAS data interpretation, primary spectrum, composition, interaction characteristics, and their energy dependence are unknown. • To solve the problem involving several unknown functions, measurements of different EAS observables are necessary. • In this talk, a new approach to study of the features of primary CR spectrum shape based of a new EAS observable – local muon density distributions in a wide range of zenith angles – is considered.

3. 21st ECRS, Košice, Slovakia Local muon density phenomenology If the size of the detector is small compared to typical distances of substantial changes of muon LDF, the detector may be considered as a point-like probe. In a muon bundle event, the local muon density D (in a random point of the shower) is estimated: D = (number of muons) / (detector area); [D] = particles / m2. Collection area rapidly increases with zenith angle (~ 1 km2 at 80)

4. 21st ECRS, Košice, Slovakia New approach:Local density spectra of EAS muons Preliminary analysis of method: [ events / (s sr)] whereN (> E) - primary spectrum, E is defined by the equation: For a nearly scaling LDF around some energy E0 and power type primary spectrum

5. 21st ECRS, Košice, Slovakia New approach:Local density spectra of EAS muons Preliminary analysis of method: • Spectra exhibits a power type behavior : β = γ/κ ~ 2;κ ~ 0.9 (steeper than that of primary particles). • Zenith angle distributions are amazingly well described by power function of zenith angle cosine : • Frequency of detected bundles is determined by muon density in central part of shower. • LMDSsensitive to primary CR composition and to the features of VHE hadron-nucleus interaction, especially in the forward kinematical region. α ~ (4.5 - 4.8); β ~ (1.9 - 2.3)

6. 21st ECRS, Košice, Slovakia General view of NEVOD-DECOR complex Coordinate-tracking detector DECOR (~115 m2) Cherenkov water detector NEVOD (2000 m3) • SideSM: 8.4 m2 each • σx1 cm; σψ  1°

7. 21st ECRS, Košice, Slovakia A typical muon bundle event in Side DECOR Y-projection X-projection

8. 21st ECRS, Košice, Slovakia Muon bundle event (geometry reconstruction)

9. 21st ECRS, Košice, Slovakia DECOR data summary Muon bundle statistics 2002 - 2007 (*) For zenith angles < 75°, only events in limited intervals of azimuth angle (with DECOR shielded by the water tank) are selected.

10. 21st ECRS, Košice, Slovakia DECOR data summary Distribution in multiplicity

11. 21st ECRS, Košice, Slovakia DECOR data summaryDistribution in zenith angle

12. 21st ECRS, Košice, Slovakia Calculation details CORSIKA simulation of 2D muon LDF: • CORSIKA 6.200 – 6.600 QGSJET 01c + GHEISHA 2002;SIBYLL 2.1 + FLUKA 2003.1b • Set of fixed zenith angles, fixed primary energies • Primary protons and Iron nuclei • EMF Primary “all-particle” spectrum: • Power type spectrum with the knee at 4 PeV • Below the knee: dN/dE = 5.0 (E, GeV)  2.7 cm-2 s-1 sr-1 GeV-1 ; • After the knee: steepening to ( 1) = 3.1.

13. 21st ECRS, Košice, Slovakia Effective primary energy range Lower limit ~ 1015 eV (limited by DECOR area). Upper limit ~ 1019 eV (limited by statistics).

14. 21st ECRS, Košice, Slovakia Low angles: around the “knee”

15. 21st ECRS, Košice, Slovakia θ = 50º : 1016 – 1017 eV

16. 21st ECRS, Košice, Slovakia θ = 65º : 1016 – 1018 eV

17. 21st ECRS, Košice, Slovakia Combined estimator of primary energy Densities: (0.05 - 2.0 m-2); zenith angles: (40º - 80º); E ~ (1016 - 1018eV) For every event with some set of (m, θ, φ) it is possible to attribute a certain effective primary energy

18. 21st ECRS, Košice, Slovakia Distributions of experimental and expectedprimary energy estimates b = 2.13

19. 21st ECRS, Košice, Slovakia Combined estimator of primary energy

20. 21st ECRS, Košice, Slovakia Dependence of LMDS index βon primary energy

21. 21st ECRS, Košice, Slovakia Conclusion 1. In the frame of used assumptions about spectrum and hadron interaction model the analysis of DECOR data: • gives a possibility to study features of spectrum and composition of primary CR and characteristics of hadron interaction in wide energy range (from 1015 up to 1019 eV) on the basis of a single technique and by means of a single experimental setup. • revealed some important features of the shape: steepening of LMDS related with the knee of primary CR spectrum at PeV energies. 2. New method based on a combined estimatorof effective primary energy also reveal the steepening of spectrum around 108 GeV.

22. Thank you for the attention!

23. Large angles: around 1018 eV