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 TakitaICRR, Univ. of TokyoFor 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 purposeii) Tibet hybrid experimentiii) Monte Carlo simulationiv) Selection of proton-induced events by ANN （ artificial neural network)

v) Results and discussionsiv) 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 Emax≒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 ExperimentFrom 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 : 36900 m2) was operated at Yangbajing (4300m a.s.l, 606 g/cm2) in Tibet. This experiment can detect a family accompanied by an air shower in the knee region.

EC and BD Total EC area : 80 m2

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 ind

uced 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. 6.030 )

　　 – QGSJET01–

　　 – SIBYLL2.1 –

Primary composition model•HD (Heavy Dominant)•PD (Proton Dominant)

HD model

1014eV 1015eV 1016eV

Proton 22.6 11.0 8.1

He 19.2 11.4 8.4

Iron 22.2 39.1 51.7

Other 35.6 38.2 31.7

PD model

1014eV 1015eV 1016eV

Proton 39.0 38.1 37.5

He 20.4 19.4 19.1

Iron 9.4 9.9 10.2

Other 30.4 31.7 33.0

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~1.3SIBYLL/QGSJET

～ 1.3

SIBYLL

QGSJET

SIBYLL

QGSJET

Model Depndence of Air Shower Size Accompanied by -family

Procedures to ObtainPrimary Proton Spectrum

( -family selection criteria : Emin=4TeV, Ng=4, sumE >=20TeV, Ne >=2x105 )

AS+ECfamily matching event ANN Proton identification(Correlations)(E,N,< R >,<ER>,sec(θ), Ne )

Int. models QGSJET Expt.(80m2)

(1996-1999)

(699days)

SIBYLL Expt.(80m2)

(1996-1999)

(699days)

Primary HD PD HD PD

Total sampling primary

2x108 1x108 2x108 1x108

Number of -family

5252 7303 177 6801 9655 177

Selected by ANN

(T <=0.4)

3308 4636 111 4312 6192 112

Event Matching between EC+BD+AS

AS+ECfamily matching event ANN Proton identification(Correlations)(E,N,< R >,<ER>,sec(θ), Ne )

Measurement Parameter

Location(x, y)

Time (t)

EC(family) AS BD

E,N,< R >,<ER>,sec(θ)

Direction(θ, )Y 　　　 　　　　　 NO 　　　　　　 Y

　 Y Y 　 NONO 　　　 Y 　 NO

Ne E0 Nb

AS&family matching bytime coincidence, Nburst>105 and test

cm)10(center burst andfamily between Distance:,

deg.) 0.2( AS andfamily between angle Opening:

)()()(

x

2222

y

yx

yx

yx

2

177 ev selected

192 + 14 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） < R > ( mean lateral spread ：　　　　　　　　 ( 　 < R > ～　 (<PT>×H) / <E>

EC)

　（４） <ER> （ mean energy flow spread EC ）　（５） sec(θ) ( Zenith angle of gamma family EC ）　（６） Ne （ Shower size of the tagged air shower

s AS ）

Selection of proton-induced events

with ANN

Parameters for training( sumE, Ng, < R >, <ER>, sec(θ), Ne )

Target value for protons=0

others=1

Define threshold value “Tth”

Selection efficiency of proton

events as a function of “Tth”

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/0312295)

All

ProtonKASCADE (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

PrimaryComposition

InteractionModel = 0.07 < stat.

&( dependence)

( 1 ) Possible steepening of the proton energy spectrum

in the knee region is observed.

power index= ~ -3.1 + ~ 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

END