on behalf of the GAMMA Collaboration Energy spectrum and mass composition of primary cosmic radiation in the region above the knee from the GAMMA experiment Romen Martirosov on behalf of the GAMMA Collaboration A.A. Alikhanyan National Laboratory, Armenia (Former Yerevan Physics Institute) Martirosov, 32nd ICRC, August 11 -18, 2011

GAMMA Collaboration Yerevan Physics Institute, ARMENIA Romen Martirosov Alexandr Garyaka Moscow Lebedev Institute, RUSSIA Anatoly Erlykin Natalya Nikolskaya University of Michigan, USA Lawrence Jones Southern University, Baton Rouge, USA Samvel Ter-Antonyan University of Montpellier II, FRANCE Yves Gallant Warsaw University of Technology, POLAND Janusz Kempa 2

All-Particle energy spectra 1 – 100 PeV GAMMA

! It is necessary an individual analysis and Sources of these uncertainties may be found: in the big fluctuations of showers deeper in the atmosphere; in different assumption concerning the primary interaction and cascade development models used in data analysis; and/or in energy normalization uncertainty. ! It is necessary an individual analysis and comparison of spectra !

!Primary spectrum above the knee is not smooth! Common resume (in near past): - Global characteristics of the all-particle spectrum agree within of about 20-30% of the systematic errors. - Changing of slope of the all-particle energy spectrum from about -2.7 below the “knee” to about -3.1 after the “knee” may be considered as an experimentally established fact; ? !Primary spectrum above the knee is not smooth! It is necessary to pay special attention to the energy region of 10 - 100 PeV, where experimental results are still very limited.

GAMMA experiment created by Yerevan Physics Institute – Lebedev Physical Institute ARAGATS scientific station (late autumn) Hill sides of the Mt. Aragats, Armenia, 65 km from Yerevan Elevation: 3200 m a.s.l. (700 g/cm2 of atmospheric depth) Geographical coordinates: Latitude = 40.470 N, Longitude = 44.180 E

(after several modifications) GAMMA 2011 (after several modifications) S = 3x104 sq.m Surface part - 41 stations – 116 1 sq.m scintillation detectors Muon carpet – 150 scintillation detectors (Eµ > 5 GeV) 33 fast-timing channels for estimation of the EAS angular characteristics

All-Particle Energy Spectrum (2003-2006) GAMMA05: R < 25m; Q < 300 GAMMA07: R < 50m; Q < 450 J.Phys. G: Nucl. Part. Phys. 35 (2008) 115201 9 9

Energy estimator (SYBILL interaction model) where Nch, Nµ, s, cos θ – ! experimentally measured parameters ! The best energy estimations as a result of 2min(E0,E1) were achieved for the 7-parametric fit:  where x = LnNch, y = LnN (R<50m), c = cos() 10

Recent results (2003 – 2009)

Recent results (2003 – 2009) - 2.7 - 3.1

Recent results (2003 – 2009) - 2.7 from ~20 PeV index - 3.1 also KASCADE-Grande and Tunka-133 – deviation from index – 3.1

Recent results (2003 – 2009) > 4σ at ~ 70 PeV - 2.7 from ~20 PeV - 3.1 > 4σ at ~ 70 PeV

It was shown in our previous works that rigidity-dependent primary energy spectra cannot describe such behavior of the energy spectrum and the presence of additional component in the primary flux of nuclei is necessary. The hypothesis of two-component origin of cosmic ray flux was used to explain the fine structure of the spectrum.

Age parameter dependence on E0 Also the s dependence on E0 becomes roughly constant in the same region. The age is dependent on the distance from the depth of shower maximum to the observation level. Constancy of the age approximately means the constancy of the depth of maximum. This behaviour is possible if the flux became heavier with energy. !This consideration is rather qualitative.!

As a rule in the case of constant mass composition this dependence ε ~ 0.13 As a rule in the case of constant mass composition this dependence looks like Ne ~ E01+ε in the range after the shower maximum. Just such behavior is seen before the knee where ε is about 0.13. After the knee ε decreases and becomes about 0 at E0 > 10 PeV. Such behavior is provided with the approximate constancy of the depth of shower maximum.

Position of ‘bump’

Conclusion The all particle primary energy spectrum was derived on the basis of GAMMA data for 2003-2009 years. We confirm our previous result. In the energy range of 10-100 PeV the spectrum shows statistically significant feature of deviation (flattening) from the power law with index of -3.1. The observed bump can be described by a two-component model of the primary cosmic ray origin. The constancy of the EAS age parameter s and the proportionality of the EAS size Ne to the primary energy E0 indicates the growth of the mean logarithm of the mass <lnA> of primary cosmic rays above the knee. The data 2010-2011 are still analysing

GAMMA experiment http://gamma-armenia.org Thank you

2010 ~80 PeV