Study of VHE Cosmic Ray Spectrum by means of Muon Density Measurements at Ground Level

1. Vulcano 2008, 26th-31st May Study of VHE Cosmic Ray Spectrum by means of Muon Density Measurements at Ground Level I.I. Yashin Moscow Engineering Physics Institute, IIYashin@mephi.ru

2. Vulcano 2008, 26th-31st May A.G. Bogdanov, R.P. Kokoulin, G. Mannocchi, A.A. Petrukhin, O. Saavedra, V.V. Shutenko, G. Trinchero, I.I. Yashin • Moscow Engineering Physics Institute, Russia • Istituto Nazionale di Astrofisica, Sezione di Torino • Universita di Torino, Italy Contents 1. Motivation 2. Local muon density phenomenology 3. DECOR data VS simulation 4. Conclusions

3. Vulcano 2008, 26th-31st May Motivation • In EAS data interpretation, primary spectrum, composition, interaction characteristics, and their energy dependence are unknown. • To solve the problem involving several unknownfunctions, measurements of different EAS observables are necessary. • In this talk, phenomenology and recent data on a new EAS observable – local muon density distributions in a wide range of zenith angles – are considered.

4. Vulcano 2008, 26th-31st May 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)

5. Vulcano 2008, 26th-31st May Local muon density phenomenology Without considering fluctuations, spectrum of events in local density may be written as [Kokoulin at al., 2005] [ 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

6. Vulcano 2008, 26th-31st May 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. Vulcano 2008, 26th-31st May A typical muon bundle event in Side DECOR Y-projection X-projection

8. Vulcano 2008, 26th-31st May Muon bundle event (geometry reconstruction)

9. Vulcano 2008, 26th-31st May 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. Vulcano 2008, 26th-31st May DECOR data summary Distribution in multiplicity

11. Vulcano 2008, 26th-31st May DECOR data summaryDistribution in zenith angle α ~ (4.5 - 4.8); β ~ (1.9 - 2.3)

12. Vulcano 2008, 26th-31st May Procedure of the analysis Muon bundle experimental distributions Nev (m, θ, φ) Deconvolution accounting for Seff(θ,φ), Poisson fluctuations, trigger and selection conditions CORSIKA simulation of LDF for different A, E andθ Convolution with primary spectrum and composition model Local muon density spectrum dF (D, θ)/dD(detector independent)

13. Vulcano 2008, 26th-31st May 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.

14. Vulcano 2008, 26th-31st May Effective primary energy range Lower limit ~ 1015 eV (limited by DECOR area). Upper limit ~ 1019 eV (limited by statistics).

15. Vulcano 2008, 26th-31st May Low angles: around the “knee”

16. Vulcano 2008, 26th-31st May θ = 50º : 1016 – 1017 eV

17. Vulcano 2008, 26th-31st May θ = 65º : 1016 – 1018 eV

18. Vulcano 2008, 26th-31st May Combined estimator of primary energy Event-by-event analysis:for every event with some set of observables(m, θ, φ) it is possible to attribute a certain effective primary energy. Densities: (0.05 - 2.0 м-2); zenith angles: (40º - 80º); E ~ (1016 - 1018 эВ)

19. Vulcano 2008, 26th-31st May Combined estimator of primary energy

20. Vulcano 2008, 26th-31st May Combined estimator of primary energy

21. Vulcano 2008, 26th-31st May Large angles: around 1018 eV

22. Vulcano 2008, 26th-31st May Conclusions 1LMDS method • A new approach based on local muon density spectrum phenomenology provides possibility to study PCR in a very wide energy range (from 1015 to 1019 eV) by means of a single, not large detector. • Local muon density spectra are sensitive to primary spectrum and composition and forward region of hadronic interactions. • Analysis of local muon density spectra together with data of experiments on other EAS observables will allow putting new constraints on combinations of spectrum, composition and interaction models.

23. Vulcano 2008, 26th-31st May Conclusions 2DECOR data analysis Within the frame of the considered spectrum and interaction models, DECOR data: • are in a reasonable agreement with expectation in the knee energy region; • favor an increase of the effective primary mass at approaching energies 1017 eV; • indicate an increase of local muon density spectrum slope at effective primary energy around 1017 eV.

24. Vulcano 2008, 26th-31st May Thank you for the attention!