Primary Cosmic-Ray Energy Spectrum Around The Knee Energy Region Measured By The Tibet Hybrid Experiment Physics at the End of the Galactic Cosmic Ray Spectrum, Aspen, 26 April, 2005 Masato Takita ICRR, Univ. of Tokyo For The Tibet AS  Collaboration

The Tibet AS  Collaboration M. Amenomori(1), S. Ayabe(2), S.W. Cui(3), Danzengluobu(4), L.K. Ding(3), X.H. Ding(4), C.F. Feng(5), Z.Y. Feng(6), X.Y. Gao(7), Q.X. Geng(7), H.W. Guo(4), H.H. He(3), M. He(5), K. Hibino(8), N. Hotta(9), Haibing Hu(4), H.B. Hu(3), J. Huang(10), Q. Huang(6), H.Y. Jia(6), F. Kajino(11), K. Kasahara(12), Y. Katayose(13), C. Kato(14), K. Kawata(10), Labaciren(4), G.M. Le(15), J.Y. Li(5), H. Lu(3), S.L. Lu(3), X.R. Meng(4), K. Mizutani(2), J. Mu(7), K. Munakata(14), A. Nagai(16), H. Nanjo(1), M. Nishizawa(17), M. Ohnishi(10), I. Ohta(9), H. Onuma(2), T. Ouchi(8), S. Ozawa(10), J.R. Ren(3), T. Saito(18), M. Sakata(11), T. Sasaki(8), M. Shibata(13), A. Shiomi(10), T. Shirai(8), H. Sugimoto(19), M. Takita(10), Y.H. Tan(3), N. Tateyama(8), S. Torii(8), H. Tsuchiya(10), S. Udo(10), T. Utsugi(8), B.S. Wang(3), H. Wang(3), X. Wang(2), Y.G. Wang(5), H.R. Wu(3), L. Xue(5), Y. Yamamoto(11), C.T. Yan(3), X.C. Yang(7), S. Yasue(14), Z.H. Ye(15), G.C. Yu(6), A.F. Yuan(4), T. Yuda(10), H.M. Zhang(3), J.L. Zhang(3), N.J. Zhang(5), X.Y. Zhang(5), Y. Zhang(3), Zhaxisangzhu(4), X.X. Zhou(6) (1) Dept. of Phys., Hirosaki Univ., Hirosaki, Japan, (2) Dept. of Phys., Saitama Univ., Saitama, Japan, (3) IHEP, CAS, Beijing, China, (4) Dept. of Math. and Phys., Tibet Univ., Lhasa, China, (5) Dept. of Phys., Shandong Univ., Jinan, China, (6) Inst. of Modern Phys., SW Jiaotong Univ., Chengdu, China, (7) Dept. of Phys., Yunnan Univ., Kunming, China, (8) Faculty of Eng., Kanagawa Univ., Yokohama, Japan, (9) Faculty of Ed., Utsunomiya Univ., Utsunomiya, Japan, (10) ICRR, Univ. of Tokyo, Kashiwa, Japan, (11) Dept. of Phys., Konan Univ., Kobe, Japan, (12) Faculty of Systems Eng., Shibaura Inst. of Technology, Saitama, Japan, (13) Dept. of Phys., Yokohama Natl. Univ., Yokohama, Japan, (14) Dept. of Phys., Shinshu Univ., Matsumoto, Japan, (15) CSSAR, CAS, Beijing, China, (16) Adv. Media Network Center, Utsunomiya Univ., Utsunomiya, Japan, (17) NII, Tokyo, Japan, (18) Tokyo Metropolitan Coll. of Aeronautical Eng., Tokyo, Japan, (19) Shonan Inst. of Technology, Fujisawa, Japan

Outline i)Research purpose ii)Tibet hybrid experiment iii)Monte Carlo simulation iv)Selection of proton-induced events by ANN （ artificial neural network) v) Results and discussions iv) Summary

Research purpose Thus, measurements of the primary cosmic rays around the "knee" are very important and its composition is a fundamental input for understanding the particle acceleration mechanism that pushes cosmic rays to very high energies. According to the Fermi acceleration with supernova blast waves, the acceleration limit E max ≒ Z * 100 TeV. The position of "knee" must be dependent on electric charge Z

Features of the hybrid experiment 1) Protons penetrate more deeply into the atmosphere to generate  family events due to their smaller inelastic cross sections than other primary nuclei, so that the air shower size and lateral spread of the air shower core induced by protons are smaller than that by those nuclei. 2) Here, a  family event is a bundle of high energy particles observed in the air shower core and mostly composed of electromagnetic components generated by a high energy penetrating cosmic ray in the atmosphere. 3) From simulation, we found that among the selected events with (E  >= 4TeV, N  =4) at Tibet in case of the QGSJET + HD model (SIBYLL + HD), 57.3% (57.5%) are induced by protons, 16.6% (16%) by helium. That is, even if the primary is heavy-enriched, almost half of the observed events selected by the above criteria are induced by protons.

Tibet Hybrid Experiment From 1996 to 1999, a hybrid experiment consisting of the Emulsion Chamber (EC) and Burst Detector (BD) and Tibet-II Air Shower (AS) array (total area : m 2 ) was operated at Yangbajing (4300m a.s.l, 606 g/cm 2 ) in Tibet. This experiment can detect a  family accompanied by an air shower in the knee region.

EC and BD Total EC area : 80 m 2

EC and BD 1)A structure of each EC used here is a multilayered sandwich of lead plate and photosensitive x-ray films, photosensitive layers are put every 2 (r.l.) (1 r.l.=0.5cm) of lead in EC. Total thickness of lead plates is 14 r.l. 2)  family is mostly cascade products induced by high energy  0 decay  - rays which are generated in the nuclear interactions at various depths. 3) It is worthwhile to note that the major behavior of hadronic interactions as well as the primary composition are fairly well reflected on the structure of the family observed with EC.

-M.C. Simulation - Hadronic int.model CORSIKA ( Ver ) – QGSJET01– – SIBYLL2.1 – Primary composition model HD (Heavy Dominant) PD (Proton Dominant) HD model 10 14eV 10 15eV 10 16eV Proton He Iron Other PD model 10 14eV 10 15eV 10 16eV Proton He Iron Other The experimental conditions for detecting  family (E  >= 4TeV, N   =4,  E  >=20 TeV) events with EC are adequately taken into account. For example, our EC has a roof, namely, the roof simulation and EC simulation are also treated.

Model Dependence of  -family (Generation+Selection) Efficiency in EC QGSJET SIBYLL SIBYLL/QGSJET ~1.3 SIBYLL/QGSJET ～ 1.3 SIBYLL QGSJET SIBYLL QGSJET

Model Depndence of Air Shower Size Accompanied by  -family

Procedures to Obtain Primary Proton Spectrum (  -family selection criteria : Emin=4TeV, Ng=4, sumE >=20TeV, Ne >=2x10 5 ) AS+ECfamily matching eventANN Proton identification (Correlations) (E ,N ,,,sec(θ), Ne ) Int. modelsQGSJETExpt.(80m 2 ) ( ) (699days) SIBYLLExpt.(80m 2 ) ( ) (699days) PrimaryHDPDHDPD Total sampling primary 2x10 8 1x10 8 2x10 8 1x10 8 Number of  -family Selected by ANN (T <=0.4)

Event Matching between EC+BD+AS AS+ECfamily matching event ANN Proton identification (Correlations) (E ,N ,,,sec(θ), Ne ) Measurement Parameter Location(x, y) Time (t) EC (  family) AS BD E ,N ,,,sec(θ) Direction( θ,  ) Y NO Y Y Y NO NO Y NO Ne E0 NbNb

AS&family matching by time coincidence, N burst >10 5 and test 177 ev selected ev expected

Selection of proton-induced events by Artificial Neural Network (ANN) （１） sumE （ Total energy EC ） （２） Ng （ number of ganma family EC ） （３） ( mean lateral spread ： ( ～ ( ×H) / EC) （４） （ mean energy flow spread EC ） （５） sec(θ) ( Zenith angle of gamma family EC ） （６） Ne （ Shower size of the tagged air showers AS ）

Selection of proton-induced events with ANN Parameters for training ( sumE, Ng,,, sec(θ), Ne ) Target value for protons=0 others=1 Define threshold value “T th ” Selection efficiency of proton events as a function of “T th ” Efficiency~75% Tth=0.4 Purity~85% Target Value (T)

Comparison of Target Value Distribution. between DATA and MC

Back check: Selection of proton-induced events by ANN

Primary energy estimation ( for proton like events )( 1.0 < sec(theta) <=1.1 )

Back check: Conversion factor for p-like EV ( by QGSJET + HD （ ANN out-put <= 0.4 ) )

Energy resolution

Air shower size spectrum of p-like events vs MC ( for proton like events (ANN out-put <=0.4))

Primary proton spectrum Preliminary (KASCADE data: astro-ph/ ) All Proton KASCADE (P) Present Results ( By QGSJET model) ( By SIBYLL model )

Primary helium spectrum (a) By QGSGET model(b) By SIBYLL model

Primary All - (P+He) component Tibet KASCADE (a) By QGSJET model (b) By SIBYLL model

Summary Primary Composition Interaction Model = 0.07 <  stat. & ( dependence ) ( 1 ) Possible steepening of the proton energy spectrum in the knee region is observed. power index= ~ ~ 0.15 above 500TeV cf. Gaisser line (-2.74) ( 2 ) The knee of all particle spectrum is composed of nuclei heavier than P + He. ( 3 ) The results : Insensitive to Tested Models

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