The cosmic ray energy spectrum measured with the

The cosmic ray energy spectrum measured with the new Tibet hybrid experiment J. Huang for the Tibet ASγ Collaboration Institute of high energy physics, CAS, China, Beijing 100049 ISVHECRI 2018, Nagoya University, Nagoya, Japan, May 21 -25, (2018)

Contents • Knee of the spectrum. • Tibet hybrid experiment (YAC+Tibet-III). • Test of hadronic interaction models. • Primary proton and helium spectra obtained by (YAC-I + Tibet-III). • Explanations for the sharp knee. • Expected results by (YAC-II + Tibet-III). • Summary J. Huang (ISVHECRI 2018, Nagoya, Japan)

Tibet hybrid experiment (YAC+Tibet-III+MD) This hybrid experiment consists of low threshold Air shower core array (YAC) and Air Shower (AS ) array and Muon Detector ( MD ). Pb 7 r. l. Scint. Iron YAC 2 will measure the primary energy spectrum of 4 mass groups of P, He, 4<A<40, A>40 at 16 50 Te. V AS – 10 e. V range covering the knee. Tibet-III (37000 m 2) : Primary energy and incident direction. YAC 2 ( 500 m 2 ): High energy AS core within several x 10 m from the axis. MD Tibet-MD ( 5000 m 2 ) : Number of muon. J. Huang (ISVHECRI 2018, Nagoya, Japan) 16/ 31

Detector Calibration 1. PMT linearity, use of LED light source; 2. Linearity of PMT+scintillator, a. probe calibration; b. accelerator beam calibration. J. Huang (ISVHECRI 2018, Nagoya, Japan)

Probe Calibration Single muon calibration 1 MIP l Using a probe detector, we can obtain the single particle peak for each YAC detector. J. Huang (ISVHECRI 2018, Nagoya, Japan) 5

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. High gain PMT R 4125 106 1 MIP 106 MIPs Low gain PMT R 5325 Input light Nb_top PMT output charge [p. C× 0. 25] Dynamic range and linearity Proton Iron Primary Energy (Ge. V) 1017 e. V J. Huang (ISVHECRI 2018, Nagoya, Japan) 6

Electron beam calibration of YAC to get ADC count vs number of particles YAC 106 MIPs YAC Thick IC Thin IC The Beam BEPC: Beijing Electron-Positron Collider 17/ 28

Calibration using BEPC The experimental sketch Saturation of PMT & Saturation of scintillator Thin IC Thick IC Number of particles (Beam) l The accelerator-beam experiment shows a good linearity between the incident particle flux and YAC-ADC output below 5× 106 MIPs. l the saturation effect of the plastic scintillator satisfies YAC detector’s requirement. 18/ 28

Test of the hadronic interaction models J. Huang (ISVHECRI 2018, Nagoya, Japan)

- Full M. C. Simulation Hadronic interaction models • CORSIKA (Ver. 7. 5000 ) – EPOS LHC– 　　 = Air Shower simulation = CORSIKA 7. 5000 (EPOS –LHC, QGSJETII-04, SIBYLL 2. 3, SIBYLL 2. 1) – SIBYLL 2. 3 – ( 1 ) Primary energy: E 0 >1 Te. V ( 2 ) All secondary particles are traced until their energies become 1 Me. V in the atmosphere. ( 3 ) Observation Site : Yangbajing (606 g/cm 2 ) – SIBYLL 2. 1 – 　　 = Detector simulation = – QGSJETII-04– Primary composition models • l　Helium poor model [1] Simulated air-shower events are reconstructed with the same detector configuration and structure as the Tibet-III and YAC array. l　Helium rich model [1] l H 4 a model [1] Geant 4 ( Ver. 9. 5) Ref. [1]. Astropart. Phys 66 (2015) 18. J. Huang (ISVHECRI 2018, Nagoya, Japan) 10 20/ 28

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. Statistics of core events in MC simulation and experiment Live Time is 106. 05 days. Data set 1 (~ 80 Te. V) Data set 2 (~1 Pe. V) EPOS-LHC+H 4 a 231344 432819 EPOS-LHC+He-rich 155607 270962 EPOS-LHC-He-poor 5411 6368 QGSJETII-04+He-poor 5304 9351 SIBYLL 2. 3+He-poor 2616 4415 SIBYLL 2. 1+He-poor 4838 7777 Expt. data 921 1339 J. Huang (ISVHECRI 2018, Nagoya, Japan)

Primary cosmic-ray composition spectrum assumed in MC ( M. Shibata, J. Huang et al. APJ 716 (2010) 1076 ) J. Huang (ISVHECRI 2018, Nagoya, Japan)

The difference between He-poor, He-rich and H 4 a model u The proton spectrum of the three models is connected with the direct experiment in 100 the. Te. V low energy and consistent with the spectrum obtained from the Tibet (EC+AS) KASCADE experiment in the high energy. u The He spectrum of He-poor model (HD) coincides with the results from Tibet-EC RUNJOB and ATIC-I. u However, the He spectrum of He-rich model coincides with the results from JACEE, ATIC-II, CREAM-III. u The H 4 a model (Gaisser’s fit model ) fits to the higher He model (the same as our He-rich model) at the low energy range and to the KASCADE-QGSJET data at high energy range in which light components (P and He) dominate the chemical composition. u The sum of all single-component spectra can reproduce the sharp knee in all particle spectrum. J. Huang (ISVHECRI 2018, Nagoya, Japan) 16/ 31

Tests of hadronic interaction models and composition models Some preliminary results from YAC-I data samples, primary energy: 100 Te. V -1 Pe. V l The experimental data are consistent with those MC distribution, which means that there exist no detection bias. l Note that the shape shows a (cascade) scaling behavior, while its intensity depends on the primary composition model.

Step 1: Check the transverse momentum below 100 Te. V Fig. 1 : Comparison of event mean lateral spreads (<Nb. R>) between Expt. and MC EPOS-LHC+ proton Expt. data ü Conclusion 1 : u The YAC experimental results well show that all interaction models produce no serious differences about the transverse momentum; u but EPOS-LHC is most consistent with our observation. In order to avoid the "ambiguous" interference of the cosmic ray components, we selected the pure proton events by ANN from experimental data and simulation data. Then, we can use lateral spread of <Nb. R> observed by YAC to check transverse momentum.

Step 2. Check primary composition model below 100 Te. V Fig. 2 : Comparison of event mean lateral spreads (<Nb. R>) between Expt. and MC EPOS-LHC+H 4 a Expt. data ü Conclusion 2 (Below 100 Te. V ): H 4 a model is most consistent with our observation below 100 Te. V.

Step 3. Check inelastic cross section below 100 Te. V Fig. 3. Integral ∑Nb (Sum. Nb) spectra obtained from five MC models, and the event absolute intensity ratios of ∑Nb to that Expt. data ü Conclusion 3 (Below 100 Te. V ): (1) The inelastic cross section description of EPOS-LHC, QGSJETII-04, SIBYLL 2. 3 and SIBYLL 2. 1 are consistent with YAC data within 20% systematic errors. (2) EPOS-LHC model is the most proper hadronic interaction model below 100 Te. V. EPOS-LHC+H 4 a Expt. data

Step 1. Check the transverse momentum around 1 Pe. V Fig. 4 : Comparison of event mean lateral spreads (<Nb. R>) between Expt. and MC EPOS-LHC+ proton Expt. data ü Conclusion 1 ( around 1 Pe. V): The YAC experimental results well show that all interaction models produce no serious differences among their results, but EPOS-LHC and QGSJETII-04 are most consistent with our observation.

Step 2. Check primary composition model around 1 Pe. V Fig. 5 : Comparison of event mean lateral spreads (<Nb. R>) between Expt. and MC Expt. data EPOS-LHC+ He-rich EPOS-LHC+H 4 a ü Conclusion 2 ( around 1 Pe. V): (1) He-rich composition model is most consistent with Expt. data around 1 Pe. V. (2) The H 4 a primary model gives smaller lateral spread than others, that is, there are too many primary light components in H 4 a model than our Expt. data. J. Huang (ISVHECRI 2018, Nagoya, Japan)

Step 3. Check inelastic cross section around 1 Pe. V Fig. 3. Integral ∑Nb (Sum. Nb) spectra obtained from five MC models, and the event absolute intensity ratios of ∑Nb to that Expt. data ü Conclusion 3 ( around 1 Pe. V): (1) The inelastic cross section description of EPOS-LHC, QGSJETII-04, SIBYLL 2. 3 and SIBYLL 2. 1 is consistent with YAC data within 20% systematic errors. EPOS-LHC and QGAJETII-04 models are the most proper hadronic interaction models. He-rich composition model is most consistent with Expt. data around 1 Pe. V. H 4 a model always gives a higher flux by a factor about 1. 4 than the experimental result which suggests that there are too many primary light components (P, He) around 1 Pe. V region. EPOS-LHC+H 4 a (2) (3) (4) Expt. data Poster Id: 14 “Test of the hadronic interaction models SIBYLL 2. 3, EPOS-LHC and. EPOS-LHC+He-rich QGSJETII-04 with Tibet EAS core data” by Ying Zhang. J. Huang (ISVHECRI 2018, Nagoya, Japan)

Primary proton, helium spectra analysis J. Huang (ISVHECRI 2018, Nagoya, Japan)

Primary proton, He spectra analysis Identification of proton+helium events ANN (a feed-forward artificial neural network) is used. Input event features: Ne, ΣNb, Nbtop, Nhit, <Rb>, < Nb. Rb>, θ Classification: proton/others Primary proton+helium energy determination E 0=f(Ne, s) based on (P+He)-like MC events J. Huang (ISVHECRI 2018, Nagoya, Japan)

Core event selection Statistics of core events in MC simulation and experiment Live Time is 106. 05 days. Selected core events EPOS-LHC+He-rich 680989 EPOS-LHC-He-poor 21726 QGSJETII-04+He-poor 21856 SIBYLL 2. 3+He-poor 10152 SIBYLL 2. 1+He-poor 19176 Expt. data 3416 J. Huang (ISVHECRI 2018, Nagoya, Japan)

Interaction model dependence in (YAC 1+Tibet-III) experiment l These figures shows that all interaction models produce no serious differences among their results. l The experimental data are consistent with those, which mean that there exist no detection bias.

Primary (P+He) separation by ANN for MC events Purity – 93. 5% Efficiency – 80. 4% Purity – 93. 7% Efficiency – 83. 0% EPOS-LHC+He-poor Purity – 94. 3% Efficiency – 82. 3% SIBYLL 2. 3+He-poor QGSJETII-04+He-poor Purity – 93. 0% Efficiency – 80. 0% SIBYLL 2. 1+He-poor

Comparison of the air-shower size (Ne) between MC and Expt. data (From this two figures, we can see that, the air-shower size (Ne) has the shape very close to the MC prediction before and after ANN selection. Some other quantities have the same behavior as well. ) l Before ANN ( all events) Air shower size spectrum J. Huang (ISVHECRI 2018, Nagoya, Japan) l After ANN (Tcut<=0. 3) (Proton+He)-like events Air shower size spectrum

Air shower size to primary energy The primary energy (E 0 ) 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 (ISVHECRI 2018, Nagoya, Japan)

Size resolution (MC Data) (based on QGSJET+He-poor model ) (1. 0 ≦ sec(Θzenith) < 1. 1 ) QGSJET＋HD Ne resoultion: ~7% (Ne>105 ) J. Huang (ISVHECRI 2018, Nagoya, Japan) QGSJET+HD

Air shower size to primary energy 25%@200 Te. V Ne-E 0 J. Huang (ISVHECRI 2018, Nagoya, Japan) 17%@1 Pe. V

Check the systematic errors by ANN P+He 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. J. Huang (ISVHECRI 2018, Nagoya, Japan) Proton Helium

(SΩ)eff calculated by MC J. Huang (ISVHECRI 2018, Nagoya, Japan)

Primary (P+He) spectra obtained by (YAC 1+Tibet-III) P+He J. Huang (ISVHECRI 2018, Nagoya, Japan) preliminary

The phenomenological model ( Please see J. R, Horandel, Astroparticle Physics 19 (2003) 193 -220 ) J. Huang (ISVHECRI 2018, Nagoya, Japan)

Primary (P+He) spectra obtained by (YAC 1+Tibet-III) The spectral index of primary (P+He) spectra is about – 2. 66 below 100 Te. V, while it is about -3. 07 above 400 Te. V among all hardronic interaction models and composition models. preliminary J. Huang (ISVHECRI 2018, Nagoya, Japan)

Results and discussions P+He preliminary (YAC 1+Tibet-III ) could measure protons and heliums spectra at > 50 Te. V which is shown to be smoothly connected with direct observation data at lower energies and also with our previously reported results at higher energies. We also found that the knee of the (P+He) spectra is located around 400 Te. V. J. Huang (ISVHECRI 2018, Nagoya, Japan)

On the primary model to explain the relation between a rigidity-dependent spectral hardening of proton and helium spectra and a sharp knee of the all-particle spectrum J. Huang (ISVHECRI 2018, Nagoya, Japan)

Comparing with DSA model Based on the experimental results from 1) directly measured spectra of p, He, … at energies below 100 Te. V by AMS 02 and CREAM etc; 2) indirectly measured all-particle spectrum by Tibet; 3) indirectly measured spectra of (P+He) by YAC-I+Tibet. we’ll analyse whether they could be described by theoretical picture of SNR acceleration and rigidity dependent cutoff of CRs at the knee energy region with DSA model. Jing Huang (ISVH 2008 -PARIS) 15/ 33

Hardening spectrum observed by direct experiment 　 (1) Tabel 1: The adopted values in function(1) for Ek , γ 1 and ∆γ 1 are listed in Table 1, another two parameters: Ec and ∆γ 2 are obtained by fitting the function (2) J. Huang (ISVHECRI 2018, Nagoya, Japan)

Indirect experiment　 (2) l Here, we assume that the cut-off energy of proton Ec is about 200 Te. V. The cut-off energy for each individual element depends on its charge Z, namely Ec(z) = Z * Ec (p) is assumed. l The energy spectra of proton, helium, carbon, silicon and iron can be fitted well by the function (2). We then sum up individual nuclei to obtain the all-particle spectrum called a global component (black solid line) , but the sharpness of the knee is more prominent in the experimental data. J. Huang (ISVHECRI 2018, Nagoya, Japan)

Excess Component at the knee Subtracting the global component from all-particle spectrum we can see hard spectrum of power index close to -2, which is just the expected value of source spectrum before the modulation by the propagation, and it also shows a cut off feature indicating that the excess component is due to the contribution from nearby source(s). J. Huang (ISVHECRI 2018, Nagoya, Japan)

YAC 2 is also well running now ( data taking started from 2014. 3) YAC-II 50 cm Pb 80 cm Total : 124 YAC detectors Cover area: ~ 500 m 2 J. Huang (ISVHECRI 2018, Nagoya, Japan) 16/ 28

Expected results by (YAC 2+Tibet-III) YAC 2 will measure the primary energy spectrum of 4 mass l Solid lines: input 16 e. V range groups of P, He, 4<A<40, A>40 at 50 l Te. V – 10 l Expected primary energy spectra Symbols: reconstructed covering the knee. J. Huang (ISVHECRI 2018, Nagoya, Japan) 42

Summary 1. 2. 3. 4. YAC-I shows the ability and sensitivity in checking the hadronic interaction models. Based on the “He-poor” primary model, we estimate that the difference of EPOS-LHC, QGSJETII-04, SIBYLL 2. 3 and SIBYLL 2. 1 is within 20 % in our concerned energy region. High core events are very sensitive to the light components in CRs and the core parameters of sum Nb, Nb_top, <R> and <Nb*R> are very useful to separate the light components from all the observed events using a ANN technique. The flux of high energy core events are sensitive to the light components, since the interaction model seems to depend strongly on the production of high energy core events, we can obtain the energy spectrum of light components in primary CRs with sufficient accuracy as discussed in the YAC-I experiment. (YAC 1+Tibet-III ) could measure protons and heliums spectra at > 50 Te. V which is shown to be smoothly connected with direct observation data at lower energies and also with our previously reported results at higher energies. We also found that the knee of the (P+He) spectra is located around 400 Te. V. The interaction models dependence in deriving the (P+He) spectra are found to be small (less than 20% 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 !

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Core event selection Base on the above core event selection condition, we found the AS axis estimated by LDF is within 6 m from our YAC detector array. N(R 0<6 m)/N( all) R< 6 m J. Huang (ISVHECRI 2012, Berlin, Germany) E 0<500 Te. V 99. 5% 500 Te. V<E 0< 1 Pe. V 97. 6%