1. Shoushan Zhang, ARGO-YBJ Collaboration and LHAASO Collaboration 4 th Workshop on Air Shower Detection at High Altitude Napoli 31/01-01/02 2013 IHEP (Institute of High Energy Physics), Beijing, China

2. Outline  Motivation  WFCTA and ARGO experiment introduction  Data analysis  Result and discussion  Conclusion

3.  Hybrid measurement  Aim: To build a bridge between balloon measurements and ground based experiment for cross- calibration of the experiments.  CREAM results: energy spectrum of single element up to 100TeV.  ARGO-YBJ （ proton+helium ）： 7TeV- 200TeV.  This work is to extend the ARGO-YBJ results to higher energy. Motivation -- ARGO-YBJ: lateral distribution In the core region  mass sensitive -- Cherenkov Telescope: longitudinal information Hillas parameter  mass sensitive Better energy resolution proton iron

4. ~ 80 m ARGO-YBJ array and Cherenkov telescope Full RPC carpet array Big pad Strips Wide Field of View Cherenkov Telescope (WFCTA)  5m 2 spherical mirror ；  16×16 PMT array  Pixel size 1° ；  FOV: 14°× 16°;  Elevation angle: 60°. One of Cherenkov event

5. Data selection  Data selection: From 2010.12 to 2012.02: 2,076,000 Coincidence events; After good weather selection: 1,740,000 events left;

6. Criteria of good weather selection Infrared detector result CloudyClear Temperature ( ℃ ) A star signal MJD Clear Cloudy Temperature ( ℃ ) correlation coefficient The correlation coefficient > 0.8 is considered as good weather. The correlation between the star flux in the FOV of Cherenkov telescope and FADC counts

7. Data selection  Cherenkov image cleaning Single channel threshold: S/N>3.5; Arrival time information: all triggered pixel should be within a time window Δt=240 ns; Rejection isolated pixel.  Period: From 2010.12 ~ 2012.02: 2,076,000 Coincidence events; After good weather selection: 1,740,000 events left;  Criteria for well measured events Core located in ARGO center carpet: (78m×74m): 388,000; Full Cherenkov image recorded by Cherenkov telescope （ Space angle = 6 : 32,700; After well measured events selection It is almost full trigger efficiency above 100 TeV.

8. Cherenkov Telescope Calibration Probe is calibrated by a HPD (calibrated by NIST) at HiRes lab in America; UV LED is calibrated by probe; PMT camera is calibrated by UV LED every day. Gain monitor results Probe Mirror UV light 375nm PMT camera UV LED  The systematic uncertainty of the calibration constant : ~ 7%. Probe

9. Simulation Cherenkov simulation : Ray tracing package ARGO simulation: G4argo  Detector simulation  Extensive air showers generation Tool: Corsika6735 + QGSJETII-03 + GHEISHA Primary particles: proton, helium, CNO, MgAlSi, iron Energy range: 10 TeV – 10PeV Geometry: the: 20 – 42, phi: 69-111, Core: +/- 130 m  Geometry reconstruction: From ARGO-YBJ Core resolution: < 2 m Angular resolution: < 0.4 o

10. Hottest big pad X Impact parameter Total Npe Zenith Hottest big pad Y MC and DATA comparison

11. Composition discrimination: light component selection Log10(RPC max) > 1.44×E’-0.912 => log10(RPC max) -1.44×E’>-0.912 E’=log10(total Npe)+0.0091×Rp-2.2

12.  Composition model, P:H:CNO:MgAlSi:Fe=1:1:1:1:1 @ Energy range: 100TeV – 1PeV  The contamination of heavy component < 5.1%.  P : H is change to 2.7:1 after light component selection. Therefore, the efficiency of light component (proton + helium) is sensitive to composition model.

13. J.R. Horandel (2003) CREAM Heavy Dominant Proton Dominant We should compare the even share composition model with other composition models.  Uncertainty due to composition model: ~ 14.0% on flux  The contamination of heavy component is between 2.6 % and 5.1% in the composition models.

14. Light component: proton + helium  The contamination of heavy component over energy: 4.9% @ 100 TeV, 9.7%@ 1PeV.  The aperture of light component is a constant over energy.

15. Impact parameter (Rp): 5m/bin Log(total Npe) bin: 0.1/bin Rp bin ： linear interpolation Total Npe bin ： quadratic curve interpolation Energy resolution ： ~25% is constant over energy Bias: < 2%  table: light component table Energy reconstruction: look- up table log10(Energy/TeV )

16. WFCTA-ARGO Light component energy spectrum: proton + helium Result and discussion  Light component energy spectrum of 100-800TeV is measured;  Spectra index: γ=-2.69 ± 0.06 （ ARGO ： -2.61 ± 0.04 ） ;  This work agree with the ARGO previous result and extent the previous measurement to higher energy.  No significant structure in the spectrum from 5 TeV to 800 TeV has been observed after combine both results.  The result connects to the CREAM light component result well too;

17. Uncertainty  Energy determination uncertainty: ~9.7 % (~ 17% on flux) calibration: 5.6%; Weather condition （ include mirror reflection and glass window transmittance ）： 7.6%; Method of energy reconstruction ： < 1.2% High energy hadronic interaction model （ QGSJET II-03 vs. SIBYLL2.1 ）： <1.0% Low energy hadronic interaction model (GHEISHA vs. fluka) ： <2.0%  Composition model: ~ 14.0% on flux  RPC max: ~3.7 % on flux -- QGSJET II-03 and SIBYLL2.1: <1.0% -- GHEISHA and fluka ： <3.5 %  The total system uncertainty is about 22.5% on flux.

18.  It is important to set a bridge between the balloon measurements and ground based experiment for cross-calibration of the experiments to help us to learn the more details of system uncertainties in the ground based experiments.  The future project : LHASSO will extend the energy to EeV.

19. Conclusion  Light component of 100-800TeV is measured ， which is consistent with the ARGO previous result and extent the result to higher energy.  The contamination of heavy component < 5.1% ；  Energy resolution: ~25% with bias <2% ；  Uncertainty : ~22.5% on flux  Spectra index: γ =-2.69±0.06  No significant structure in the spectrum from 5 TeV to 800 TeV has been observed after combine with the ARGO previous result and this work.

20. Thanks ！

22. Weather selection Star light Infrared detector result Clear CloudyClear Cloudy Temperature ( ℃ ) Transparency Galactic Plane The correlation coefficient is defined as the transparency of the atmosphere Mean total Npe of every day

24. 1. 5m 2 spherical mirror ； 2. Camera: 16×16 PMT array 3. Pixel size 1° ； 4. FOV: 14°× 16°. 5. Elevation angle: 60° Wide Field of View Cerenkov Telescope (WFCTA) ARGO-YBJ experiment 2. Detector introduction Start from July 2006 Rate ： 3.5kHz Threshold ： ~ 300 GeV Duty cycle: >86% FOV: 2 sr One of Cherenkov event Hybrid observation: WFCTA & ARGO ARGO Cherenkov telescope 78.9 m Big pad Strips

25. The proton spectrum obtained from this experiment can be represented by the power-law fit as shown in Fig. 15. The power indexes are estimated to be -2.97 +/- 0.06 and -2.99 +/-0.06 for the spectra obtained using the ANN trained by the CORSIKA1HD and CORSIKA1PD events, respectively, where errors quoted are statistical ones. Direct measurements of the proton spectrum in the energy region up to about 100 TeV @17,29,30#, while statistics is still scanty, may suggest a slightly flat spectrum with the slope of -2.5–-2.7. When both results are combined, we may say that the proton spectrum changes its slope at energy around 100 TeV. This may be in favor of shock acceleration at SNR’s and when we compared this with the all-particle spectrum obtained by the Tibet air-shower array @17#, the primary composition becomes heavy dominant at energies around the knee. M. Amenomori, PHYSICAL REVIEW D, VOLUME 62 (2000), 112002 Primary proton spectrum between 200 TeV and 1000 TeV observed with the Tibet burst detector and air shower array

26. protonheliumCNOMgAlSiironsum Primary20% 100% After cut 69.1%25.8%3.8%1.1%0.2%100%  P:H:CNO:MgAlSi:Fe=1:1:1:1:1, Energy range: 100TeV – 1PeV  The contamination of heavy component < 5.1%. Efficiency Light component: proton + helium

27. 100-1PeVProtonheliumCNOMgAlSiIron Primary31.1%29.1%13.8%10.8%15.2% J.R. Horandel (2003)  CREAM: P:H:CNO:MgAlSi:Iron=29.6%:37.0%:16.7%:6.7%:10.0%

31. M. Amenomori etc., Advances in Space Research 37 (2006) 1938–1943