1. CR spectrum and composition measured by Tibet hybrid experiment (YAC+Tibet-III) J. Huang for the Tibet ASγ Collaboration Institute of high energy physics, Chinese Academy of Sciences China, Beijing 100049 Workshop Agenda, IHEP, CAS, Beijing, October 17 - 18, (2012)

2. Contents Knee of the spectrum. Two Possible explanations for the sharp knee. New hybrid experiment (YAC+Tibet-III). Primary proton, helium spectra obtained by (YAC-I + Tibet-III). Expected results by (YAC-II + Tibet-III). Summary J. Huang (Workshop Agenda, Beijing, China, (2012))

3. AS array at high altitude (4300m a.s.l.) Tibet-III array:37000m 2 with 789 scint. YAC array: 500m 2 with 124 scint. MD array: 5000m 2 with 5 pools of water Cherenkov muon D.s. Measure:energy spectrum around the knee and chemical composition using sensitivity of air showers to the primary nuclei through detection of high energy AS core. Tibet experiment 10 14 -10 17 eV Pb Iron Scint. Box 7 r.l. YAC Merit of high altitude J. Huang (Workshop Agenda, Beijing, China, (2012)) The merit of the Tibet experiment is that the atmospheric depth of the experimental site (4300 m above sea level) is close to the maximum development of the air showers, with energies around the knee almost independent of the masses of primary cosmic rays as shown in this figure.

4. Tibet-III: Energy and direction of air shower Cosmic ray(P,He,Fe…) Particle density & spread Separation of particles Tibet YAC array (Yangbajing Air shower Core arary) J. Huang (Workshop Agenda, Beijing, China, (2012))

5. 5 ModelKnee Position (PeV) Index of spectrum QGS.+ HD 4.0 ± 0.1R1= -2.67 ± 0.01 R2= -3.10 ± 0.01 QGS.+ PD 3.8 ± 0.1R1= -2.65 ± 0.01 R2= -3.08 ± 0.01 SIB.+ HD 4.0 ± 0.1R1= -2.67 ± 0.01 R2= -3.12 ± 0.01 5/ 31 All-particle spectrum measured by Tibet-III array from 10 14 ~10 17 eV (ApJ 678, 1165-1179 (2008)) J. Huang (Workshop Agenda, Beijing, China, (2012))

6. Tibet KASCADE HEGRA CASA/MIA BASJE Akeno DICE Energy spectrum around the knee measured by many experiments J. Huang (Workshop Agenda, Beijing, China, (2012))

7. Normalized spectrum J. Huang (Workshop Agenda, Beijing, China, (2012))

8. 8 (ApJ 678, 1165-1179 (2008)) A sharp knee is clearly seen (ApJ 678, 1165-1179 (2008)) 6/ 31 What is the origin of the sharp knee? There were many models: nearby source, new interaction threshold, etc. In the following, I would introduce our two analyses for the origin of the sharp knee. J. Huang (Workshop Agenda, Beijing, China, (2012))

9. 9 Two possible explanations for the sharp knee For explaining the sharp knee we proposed two composition models (called Model A and Model B) that are based on: 1) the up-to-now available experimental results; 2) some physics (or theoretical) assumptions. 7/ 31 ( M. Shibata, J. Huang et al. APJ 716 (2010) 1076 ) J. Huang (Workshop Agenda, Beijing, China, (2012))

10. 10 For ‘the up-to-now available experimental results’, we request: a) In the enery region lower than 100 TeV the directly measured p, He, …, iron spectra by CREAM, ATIC, JACEE, RUNJOB etc should be smoothly connected by the modeling spectra; b) In the energy region higher than 100 TeV the modeling p and He spectra should be consistent with our indirectly measured p and He spectra; c) The superposed spectrum of all elemental spectra in the modeling should be consistent with our measured all-particle spectrum. J. Huang (Workshop Agenda, Beijing, China, (2012))

11. 11 Diffusive shock acceleration in SNRs is assumed; Multiple galactic sources were considered. For each source there is an ‘acceleration limit’ ε (Z) which --- is proportional to the charge Z of accelerated nuclei, --- denotes the energy the accelerated particles start to deviate from the power law; ε max is introduced that denotes the ‘Maximum acceleration limit’ among multiple sources. 8/ 28 Some physics (or theoretical ) assumptions for both Model A and Model B J. Huang (Workshop Agenda, Beijing, China, (2012))

12. 12 (ApJ,716:1076-1083(2010)) In this physics picture the knee is caused by the‘minimum acceleration limit’ ε (Z), (see details in the paper). Taking different ε (Z) and ε max the obtained all particle spectrum shows a smooth structure (see the figure below). The sharp knee cannot be produced. To explain the sharp knee we proposed two approaches, called Model A and Model B. J. Huang (Workshop Agenda, Beijing, China, (2012))

13. 13 Model A: Sharp knee is due to nearby sources Substracting the smooth spectrum from the measured all particle spectrum, a power-law spectrum with index -2 is obtained (see the dotted line in the figure). This is very consistent with the assumption of CR particles coming from nearby source(s). (ApJ,716: 1076-1083 (2010)) Extra component can be approximated by: 9/ 28 J. Huang (Workshop Agenda, Beijing, China, (2012))

14. 14 Model B: Sharp knee is due to nonlinear effects in the defuse shock acceleration It was suggested (Malkov & Drury 2001; Ptuskin & Zirakashvili 2006) that: In the diffuse shock acceleration mechanism, the nonliner effect at supernova shock fronts is present that may produce a harder cosmic ray spectrum in the source. We included this effect by introducing an additional term in our formalism that showed to produce a dip below the ‘minimum acceleration limit’ of the spectrum of each element (see the figure). 10/ 28 J. Huang (Workshop Agenda, Beijing, China, (2012))

15. 15 Their superposition can well produce the all-particle spectrum including the sharp knee. (Model B) (ApJ,716: 1076-1083 (2010)) J. Huang (Workshop Agenda, Beijing, China, (2012))

16. Two possible explanations for the sharp knee 1) Model A: Sharp knee is due to nearby sources 2) Model B: Sharp knee is due to nonlinear effects in the diffusive shock acceleration (DSA) All-particle knee = CNO? All-particle knee = Fe knee? ( APJ 716 (2010) 1076 ) J. Huang (Workshop Agenda, Beijing, China, (2012))

17. Short summary Two scenarios (model A and model B) are proposed to explain the sharpness of the knee. In model A, an excess component is assumed to overlap the global component, and its spectrum shape suggests that it can be attributed to nearby source(s) because it is surprisingly close to the expected source spectrum of the diffuse shock acceleration. CNO dominant composition is predicted by this model at the knee. In model B, a hard observed energy spectrum of each element from a given source is assumed. The sharp knee can be explained by a rigidity-dependent acceleration limit and hard spectrum due to nonlinear effects. Iron-dominant composition is predicted by this model at the knee and beyond. 11/ 28 J. Huang (Workshop Agenda, Beijing, China, (2012))

18. In order to distinguish between Model A and Model B and many other models, measurements of the chemical composition around the knee, especially measurements of the spectra of individual component till their knee will be essentially important. Therefore, we planed a new experiment: 1) to lower down the energy measurement of individual component spectra to *10TeV and make connection with direct measurements; 2) to make a high precision measurement of primary p, He, …, Fe till 100 PeV region to see the rigidity cutoff effect. 15/ 31 These aims will be realized by our new experiments YAC (Yangbajing AS Core array) ! J. Huang (Workshop Agenda, Beijing, China, (2012))

19. YAC project (*10TeV -100 PeV) J. Huang (Workshop Agenda, Beijing, China, (2012))

20. New hybrid experiment (YAC+Tibet-III+MD ) Pb 7 r.l. Scint. Iron Tibet-III (37000 m 2 ) : Primary energy and incident direction. YAC2 ( 500 m 2 ): High energy AS core within several x 10m from the axis. Tibet-MD ( 5000 m 2 ) : Number of muon. This hybrid experiment consists of low threshold Air shower core array (YAC) and Air Shower (AS ) array and Muon Detector ( MD ). 16/ 31 MD AS YAC2 YAC2 will measure the primary energy spectrum of 4 mass groups of P, He, 4 40 at 50 TeV – 10 16 eV range covering the knee. J. Huang (Workshop Agenda, Beijing, China, (2012)) Proton Iron

21. YAC Detector Pb Iron Scint. Box 7 r.l. Observe shower electron size under lead plate (burst size N b ) induced by high energy E.M. particles at air-shower core. WLSF (wave length shifting fibers) is used to collect the scintillation light for the purpose of good uniformity. Two PMTs are used to cover wide dynamic range (1MIP is calibrated by single muon). For High gain PMT, N b: 1 – 3000 MIPs For Low gain PMT, N b : 1000- 10 6 MIPs 80cm 50cm WLSf Plastic scintillaors (4cm×50×1cm, 20pcs) High gain PMT R4125 Low gain PMT R5325 J. Huang (Workshop Agenda, Beijing, China, (2012))

22. YAC1 is well running now ( data taking started from 2009.04.01) YAC1 19/ 31 Total : 16 YAC detectors Effective area: 10 m 2 J. Huang (Workshop Agenda, Beijing, China, (2012))

23. Detector Calibration 1.PMT linearity, use of LED light source; 2. Linearity of PMT+scintillator, a. probe calibration; b. accelerator beam calibration. J. Huang (Workshop Agenda, Beijing, China, (2012))

24. 24 1 MIP Probe Calibration ( The determination of the burst size is calibrated using single muon peak ) Single muon calibration Using a probe detector, we can obtain the single particle peak for each YAC detector. J. Huang (Workshop Agenda, Beijing, China, (2012))

25. High gain PMT R4125 Low gain PMT R5325 1MIP 10 6 MIPs Input light 25 PMT output charge [pC×0.25] Proton Iron 10 17 eV Primary Energy (GeV) Nb_top Dynamic range and linearity 10 6 PMT linearity In order to record the electromagnetic showers of burst size from 1 to 10^6 particles, a wide dynamic range of PMT is required. For each PMT (high gain and low gain) used in YAC-I the linearity has been measured by using LED light source and optical filters. In the test we fixed the positions of LED, filter and PMT. By using different filters we can get light of different intensity, and then, we can check the Linearity of PMTs. J. Huang (Workshop Agenda, Beijing, China, (2012))

26. Thin IC Thick IC YAC The Beam YAC BEPC: Beijing Electron-Positron Collider Electron beam calibration of YAC to get ADC count vs number of particles 10 6 MIPs 17/ 28

27. Thin IC Thick IC Saturation of PMT & Saturation of scintillator Calibration using BEPC The experimental sketch 18/ 28 The accelerator-beam experiment shows a good linearity between the incident particle flux and YAC-ADC output below 5×10 6 MIPs. the saturation effect of the plastic scintillator satisfies YAC detector’s requirement. Number of particles (Beam)

28. Primary proton, helium spectra analysis J. Huang (Workshop Agenda, Beijing, China, (2012))

29. 29 = Air Shower simulation = CORSIKA 6.204 (QGSJET2, SIBYLL2.1) ( 1 ) Primary energy: E0 >1 TeV ( 2 ) All secondary particles are traced until their energies become 300 MeV in the atmosphere. ( 3 ) Observation Site : Yangbajing (606 g/cm 2 ) = Detector simulation = Simulated air-shower events are reconstructed with the same detector configuration and structure as the YAC array using Epics (uv8.64) - Full M.C. Simulation - Primary composition model NLA (above-mentioned Nonlinear effects model ). HD model (Heavy Dominant model: see M. Shibata, J. Huang et al. APJ 716 (2010) 1076 ) Hadronic interaction model CORSIKA (Ver. 6.204 ) – QGSJET2– – SIBYLL2.1– 20/ 28 J. Huang (Workshop Agenda, Beijing, China, (2012))

30. Primary cosmic-ray composition spectrum assumed in MC ( M. Shibata, J. Huang et al. APJ 716 (2010) 1076 ) J. Huang (Workshop Agenda, Beijing, China, (2012))

31. The difference between NLA and HD model 16/ 31  The proton spectrum of the two models is connected with the direct experiment in the low energy and consistent with the spectrum obtained from the Tibet (EC+AS) experiment in the high energy.  The He spectrum of HD model coincides with the results from RUNJOB and ATIC-I, we called ‘He poor’ model.  However, the He spectrum of NLA coincides with the results from JACEE, ATIC-II, CREAM, we called ‘ He rich’ model.  The sum of all single-component spectra can reproduce the sharp knee in all particle spectrum. NLA: ‘He rich’ model HD : ‘He poor’ model J. Huang (Workshop Agenda, Beijing, China, (2012))

32. Core event selection Event selection condition for AS core event was studied by MC and following criteria were adopted to reject non core events whose shower axis is far from the YAC array. Nb>200, N hit ≧ 4, Nbtop ≧ 1500, Ne>80000 | AS axis by LDF – burst center| < 5 m Statistics of core events in MC simulation and experiment Live Time is 106.05 days. Selected Events QGSJET+HD216942 SIBYLL+HD304785 QGSJET+NLA80861 SIBYLL+NLA64331 YAC15035 J. Huang (Workshop Agenda, Beijing, China, (2012))

33. Core event selection R< 5m Base on the above core event selection condition, we found the AS axis estimated by LDF is within 5 m from our YAC detector array. 10 5 <= Ne <= 10 6 R <= 5 m 3464/3483=99.5% J. Huang (Workshop Agenda, Beijing, China, (2012))

34. Interaction model dependence in (YAC1+Tibet-III) experiment Air shower size (Ne) spectra Total burst size (sum Nb) spectra Top burst size (Nb_top)spectraMean lateral spread (NbR)spectra These figures shows that QGSJET and SIBYLL, both models produce distribution shapes consistent with our experimental data.

35. Primary proton, He spectra analysis Identification of proton events ANN (a feed-forward artificial neural network) is used. Input event features: N e, ΣN b, N b top, N hit,,, θ Classification: proton/others Primary energy determination E 0 =f(N e,s) based on proton-like MC events J. Huang (Workshop Agenda, Beijing, China, (2012))

36. Purity – 95.2% Efficiency – 62% Purity – 94.5% Efficiency – 63% QGSJET SIBYLL Primary (P+He) separation by ANN P+HeOther Nuclei P+HeOther Nuclei J. Huang (Workshop Agenda, Beijing, China, (2012))

37. Purity – 79% Efficiency – 46% Purity – 78% Efficiency – 40% Primary proton separation by ANN for MC events QGSJET SIBYLL P roton Other P roton Other J. Huang (Workshop Agenda, Beijing, China, (2012))

38. Air shower size to primary energy The primary energy (E0 ) of each AS event is determined by the air-shower size (Ne) which is calculated by fitting the lateral particle density distribution to the modified NKG function. Modified NKG function J. Huang (Workshop Agenda, Beijing, China, (2012))

39. Size resolution (MC Data) (based on QGSJET+HD model ) (1.0 ≦ sec(Θ zenith ) < 1.1 ) Ne resoultion: ~7% (Ne>10 5 ) QGSJET ＋ HD QGSJET+HD

40. ( for T<=0.4 & sec(theta) <=1.1 ) (1.) Energy Resolution – Proton like events Energy Resolution : 22% @ 500TeV 1e+05 < Ne < 1e+06 Primary energy determination

41. Check the systematic errors by ANN J. Huang (ISVHECRI2012, Berlin, Germany) P+He Proton Helium The primary energy of (P+He)-like or P-like or Helium-like events is in a good agreement with the true primary energy spectrum.

42. Nb >= 200 Nhit >= 4 Nbtop >= 1500 Ne >= 80000 (SΩ) eff calculated by MC (1)

43. Primary (P+He) spectra obtained by (YAC1+Tibet-III) preliminary J. Huang (Workshop Agenda, Beijing, China, (2012))

44. Primary proton, helium spectra obtained by (YAC1+Tibet-III) preliminary J. Huang (Workshop Agenda, Beijing, China, (2012))

45. YAC2 is also well running now ( data taking started from 2011.8.1) 16/ 28 YAC-II Total : 124 YAC detectors Cover area: ~ 500 m 2 Pb 50cm 80cm J. Huang (Workshop Agenda, Beijing, China, (2012))

46. 46 Expected results by (YAC2+Tibet-III) Solid lines:input Symbols:reconstructed Expected primary energy spectra YAC2 will measure the primary energy spectrum of 4 mass groups of P, He, 4 40 at 10 14 – 10 16 eV range covering the knee. J. Huang (Workshop Agenda, Beijing, China, (2012))

47. Summary (1)YAC1 shows the ability and sensitivity in checking the hadronic interaction models. (2) The experimental distribution, sumNb has the shape very close to the MC predictions of QGSJET+NLA, QGSJET+HD, SIBYLL+NLA and SIBYLL+HD. Some other quantities, such as Ne, Nb_top, have the same behavior as well. (3) Some discrepancies in the absolute intensities are seen. Data normally shows a higher intensity than MC. Taking a more hard He spectrum as given by CREAM can improve this situation. A further study is going on. (4)(YAC1+Tibet-III ) could measure protons and heliums spectra at > 50 TeV which is shown to be smoothly connected with direct observation data at lower energies and also with our previously reported results at higher energies. J. Huang (Workshop Agenda, Beijing, China, (2012))

48. (5)We obtained the primary energy spectrum of proton, helium and (P+He) spectra between 50 TeV and 1 PeV, and found that the knee of the (P+He) spectra is located around 400 TeV. (6) The interaction model dependence in deriving the primary proton, helium and (P+He) spectra are found to be small (less than 25% in absolute intensity, 10% in position of the knee ), and the composition model dependence is less than 10% in absolute intensity, and various systematic errors are under study now ! (7) Next phase experiment YAC2 will measure the primary energy spectrum of 4 mass groups of P, He, 4 40 at 10 14 – 10 16 eV range covering the knee. J. Huang (Workshop Agenda, Beijing, China, (2012))

49. Thank you for your attention !!