1. Measurement of the CR light component primary spectrum B. Panico on behalf of ARGO-YBJ collaboration University Rome Tor Vergata INFN, Rome Tor Vergata RICAP 2011 – Rome May 25-27 2011

2. 2 1 particle m -2 s -1 1 particle m -2 year -1 1 particle km -2 year -1 The spectrum Still open questions…

3. Overlapping the direct measurements 3 BALLOONS and SATELLITES up to 100 TeV EAS-ARRAYS have an energy threshold of ~100 TeV ARGO-YBJ covers the energy range 1-10 4 TeV With digital and analog read-outs

4. Conflicting results at the knee 4 B. Panico - RICAP 2011 The mass of the knee is light: <2 STANDARD MODEL But experiment at high altitude find that is higher than the logarithmic mass number for carbon and the proton component is no more dominant at the knee

5. An unconventional EAS-array exploiting the full coverage approach at very high altitude to detect small air showers at an energy threshold of a few hundreds of GeV. The ARGO-YBJ experiment Longitude 90° 31’ 50” East Latitude 30° 06’ 38” North Altitude 4300 m a.s.l. Astrophysical Radiation with Ground-based Observatory at YangBaJing B. Panico - RICAP 2011 5

6. The ARGO-YBJ experiment 10 Pads = 1 RPC (2.80  1.25 m 2 ) 78 m 111 m 99 m74 m 12 RPC = 1 cluster ( 5.7  7.6 m 2 ) 8 Strips = 1 Pad (56  62 cm 2 ) Space pixel: single strip ( 7×62 cm 2 ) Time pixel: pad ( 56×62 cm 2 ) is the OR of 8 strips, with a resolution of ~ 1.8 ns Dynamical range for protons by means of pads, strips and big pads : ~ 1 - 10 4 TeV Real event B. Panico - RICAP 2011 6

7. Duty cycle B. Panico - RICAP 2011 7 In observation since July 2006 (commissioning phase) Stable data taking since November 2007 The average duty cycle ~ 85% Trigger rate ~3.5 kHz @ 20 pad threshold Dead time 4% 220 GB/day transferred to IHEP/CNAF data centers

8. N pad > 100, 71 s.d. Energy calibration Moon shadow analysis B. Panico - RICAP 2011 8 A natural tool to evaluate the performance of the detector  Pointing accuracy,  Angular resolution,  Absolute energy calibration The energy scale uncertainty is estimated to be smaller than 13% in the energy range 1 – 30 (TeV/Z).

9. Moon shadow analysis B. Panico - RICAP 2011 9 N pad >100: 10 s.d./month A tool to monitor the stability of the data and reconstruction Right figures: one point per month ! Position stable at a level of 0.1° Angular resolution stable at a level of 10%

10. CR spectrum: the method Selection of a “good” data period Pressure correction Criteria to select events with high efficiency (>85%) Rate calculation Effective area calculation for different spectra Comparison of experimental rate with expectations B. Panico - RICAP 2011 10

11. First step: selection 250 days ≈ 5 ∙10 11 events in 2009 Efficiency reconstruction >85% Contamination from external events <15% B. Panico - RICAP 2011 11 1.Select a “good” period requiring a small number of bad pads and small pressure variations 2. Zenith angle less than 15° 3.Shower core in a fiducial area A fid =40x40 m 2 4.Differential strip multiplicity classes Less than 500 “dead” pads (over 15600) on the central carpet Δp<4% in the YBJ-site

12. Second step: correction Correction for the barometric effect and for the “bad pad” effect Applied on the trigger rate B. Panico - RICAP 2011 λ 0 = (3.43 ± 0.02) kHz α = (0.0040 ± 0.0006)% pad -1 β = (0.70 ± 0.05)% mbar -1 N 0 c =425 p 0 =605 mbar

13. Third step: determination of experimental rate B. Panico - RICAP 2011 13 γ =-1.21±0.03 The statistical errors are negligible The systematic errors are <10%

14. MC simulation 1.Determination of the integral effective area 2.Determination of different integral contributes P 10 -3 – 4 ∙10 3 TeV He 4 ∙10 -2 – 4 ∙10 3 TeV CNO 4 ∙10 -2 – 4 ∙10 3 TeV MgSi 4 ∙10 -2 – 10 3 TeV Fe 4 ∙10 -2 – 10 3 TeV Core sampling up 3500 × 3500 m 2 B. Panico - RICAP 2011 14 Corsika v. 6.500 QGSJET-II.03 model + FLUKA

15. Effective Areas B. Panico - RICAP 2011 Strip multiplicity > 400

16. MC simulation B. Panico - RICAP 2011 16 Differential flux Determination of different integral contributes for each primary mass i with CREAM spectrum Obtained by fitting CREAM data The APJ, 728:122, 2011

17. The expected rate 17 [251-398] R P /R He /R CNO /R Heavy 67.6/28.2/2.7/0.7 % [6310-10000] R P /R He /R CNO /R Heavy 51.2/40.4/4.4/2.4 % Different contributes calculated with CREAM spectra Relative fractions (% of the total)

18. Light component B. Panico - RICAP 2011 18 The ligth component spectrum (p+He) for the data is obtained by subtracting the contribution of the heavy elements, calculated with the previous spectrum Data: γ=-1.25±0.03 Horandel: γ =-1.21±0.03 CREAM: γ=-1.15±0.03

19. Conclusions B. Panico - RICAP 2011 1.Data selected (250 days) according to small dead pad number and small pressure variation 2.The results of barometric correction on the trigger rate according with the ones found in literature 3.Simple selection criteria providing high reconstruction efficiency 4.The measured light component spectrum in agreement with other data 5.Unfolding of p and He spectra under way 6.Calculation of the Light component energy spectrum in progress 19

20. The Moon Shadow technique G. Di SciascioVulcano Workshop 2011 20 Geomagnetic Field: positively charged particles are deflected towards the West. Ion spectrometer The observation of the Moon shadow may provide a direct check of the relation between size and primary energy Cosmic rays are blocked by the Moon Deficit of cosmic rays in the direction of the Moon Moon diameter ~0.5 deg Size of the deficit Position of the deficit Angular Resolution Pointing Error Energy calibration West displacement