1. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 1 Measurement of the proton-air inelastic cross section with ARGO-YBJ A. Surdo Istituto Nazionale di Fisica Nucleare Lecce – Italy on behalf of ARGO-YBJ Collaboration 21° European Cosmic Ray Symposium 9-12 September 2008 – Kosice, Slovakia

2. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 2 Introduction Inelastic proton-air (and total p-p) cross section can be derived from cosmic ray measurements through several methods Direct method: measure the distribution of X 1, the 1st interaction point of p-air collisions, to directly measure the mean free path p-air  feasible (in principle) at relatively low energies only Indirect methods: (a) for fixed primary energy and zenith angle, measure the exponential tail of the shower maximum depth (X max ) distribution ARGO-YBJ approach (b) for fixed primary energies, measure the exponential zenith angle distribution of the shower intensity  slope parameter (  ) connected to p-air

3. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 3 Collaboration between:  Istituto Nazionale di Fisica Nucleare (INFN) – Italy  Chinese Academy of Science (CAS) Site: Cosmic Ray Observatory @ Yangbajing (Tibet), China High Altitude Cosmic Ray Laboratory @ YangBaJing Site Altitude: 4,300 m a.s.l., ~ 600 g/cm 2 Site Coordinates: longitude 90° 31’ 50” E, latitude 30° 06’ 38” N The ARGO-YBJ experiment

4. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 4 ARGO-YBJ is a full-coverage Extensive Air Shower detector with several main goals: ARGO-YBJ physics objects  VHE  -Ray Astronomy: search for point-like (and diffuse) galactic and extra-galactic sources at few hundreds GeV energy threshold  Poster by D. Martello  Search for GRB’s (full GeV / TeV energy range)  Talk by T. Di Girolamo  Sun and Heliosphere physics (E th  10 GeV)  Cosmic ray physics: anti-p / p ratio at TeV energy spectrum and composition (E th  few TeV) study of the shower space-time structure fundamental physics issues (p-air cross section,...)

5. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 5 ARGO-YBJ detector Central carpet: 130 Clusters 1,560 RPCs 124,800 Strips (  5,600m 2 active area) Strip = spatial pixel (6.5 x 62 cm 2 ) Pad = time pixel Time resolution ~1 ns 78 m 111 m 99 m74 m RPC + Analog RPC charge read-out + 0.5 cm lead converter (2009) 10 Pads = 1 RPC (2.80  1.25 m 2 ) 12 RPC =1 Cluster ( 5.7  7.6 m 2 ) 8 Strips = 1 Pad (56  62 cm 2 )

6. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 6 Currently completed and in data taking Inclusive trigger Npad > 20 (“shower mode” trigger) on the central carpet. Trigger rate  4 kHz and data flow  7 MB/s. ARGO-YBJ detector

7. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 7 Measurement of the Flux attenuation h0h0  Use the shower frequency vs (sec  -1) The absorption length  is connected to int through: The parameter k takes into accounts how the primary energy is dissipated in the shower k determined by simulations, depends on:  interaction model  shower fluctuations  actual set of experimental observables  ……..  p-Air (mb) = 2.41·10 4 / int (g/cm 2 ) for fixed energy and shower age (h 0 =vert. depth).  = k int  Warning Constrain X DO = X det – X 0 or better X DM = X det – X max Select deep showers (large X max, i.e. small X D0 or X DM )

8. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 8 Analysis general criteria Exploit peculiar detector features: space/time granularity, full-coverage technique, high altitude (h 0  600 g/cm 2 ) Select deep showers (large X Max, i.e. small X D0 or X DM ) in order to minimize the impact of shower development fluctuations detailed space-time pattern for unique EAS reconstruction 3-D view of a detected shower Top view of the same shower

9. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 9 Event selection Basic cuts: a.Shower fully reconstructed (0<  zen <40°) through conical fit b.Shower “detected size” (N strip ) > 400 c.Core reconstructed inside a fiducial area (64 x 64 m 2 ) + more specific cuts, based on extension of the detected shower, hit density near the reconstructed core and lateral profile, in order to: reduce the contamination of external core events put a constraint on the maximum X DM value Finally: strip multiplicity (N strip ) used to set primary energy intervals

10. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 10 CORSIKA showers, by p and He primaries Energy ranges: p (0.3-3000)TeV He (1-3000)TeV Zenith angle range: 0<  <45° QGSJET interaction model Use of information on the longitudinal shower profile (X max,…) Full detector response simulation based on GEANT package Proper choice of the sampling area including the detector Same analysis chain as for real data Monte Carlo simulation

11. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 11 Cuts in-dependence on the zenith angle No significant zenith angle dependence below 30 degrees. A slight shift might be seen above 40 degrees. In this analysis we stop at 40 degrees Energy X DM = X Det –X Max

12. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 12 The energy scale Use of the strip multiplicity (N strip ) for the estimation of shower size  up to  100 TeV primary energy > For  N strip  fold the MC energy distribution with parametrized  p-air  distribution of int. > Get the energy corresponding to (  E Log ). Average int also used to evaluate k factor: k =  (MC) obs / (MC) int Log(E/TeV) int (g/cm 2 ) (MC) int

13. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 13 NstripE log (TeV) (MC) int (g/cm 2 ) k 400 ÷ 10004.078.53 ± 2.12.01 ± 0.06 ± 0.05 1000 ÷ 20008.376.15 ± 1.81.53 ± 0.02 ± 0.04 3000 ÷ 400019.873.45 ± 1.51.59 ± 0.04 ± 0.03 6000 ÷ 800038.771.44 ± 1.31.68 ± 0.06 ± 0.03 8000 ÷ 1200053.570.51 ± 1.21.71 ± 0.07 ± 0.03 > 800076.769.50 ± 1.62.05 ± 0.06 ± 0.05 Analysis of MC data: sec  distributions (  (MC) obs =h 0 /|Slope|) and k factors k   MC  obs / (MC) int

14. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 14 Analysis of real data: sec  distributions  CR-air (mb) = 2.41·10 4 / (exp) int (g/cm 2 ) from MC

15. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 15 Heavy primaries contribution Hoerandel AP 19 (2003) 193 taken as reference. JACEE and RUNJOB for the evaluation of systematic error proton helium

16. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 16 NstripE log (TeV)Helium correction  pAir (mb) 400 ÷ 10004.01.00 ± 0.04 ± 0.01261 ± 13 ± 8 1000 ÷ 20008.31.00 ± 0.02 ± 0.01278 ± 7 ± 7 3000 ÷ 400019.81.00 ± 0.04 ± 0.01303 ± 15 ± 7 6000 ÷ 800038.70.96 ± 0.05 ± 0.03288 ± 19 ± 11 8000 ÷ 1200053.51.00 ± 0.05 ± 0.03289 ± 19 ± 10 > 800076.70.95 ± 0.04 ± 0.04322 ± 17 ± 16 Heavy primaries contribution Correction on reconstructed  at 80 TeV Above 1 TeV: primary Helium fraction ≈ 40% After analysis cuts: ≈ 15-20% Heavier primaries can be neglected

17. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 17 Inelastic p-air cross section (statistical errors only in the plot)

18. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 18 NstripE log (TeV)  pAir (mb)  pp (mb) 400 ÷ 10004.0261 ± 13 ± 838 ± 3 ± 3 1000 ÷ 20008.3278 ± 7 ± 742 ± 2 ± 3 3000 ÷ 400019.8303 ± 15 ± 749 ± 4 ± 3 6000 ÷ 800038.7288 ± 19 ± 1144 ± 5 ± 4 8000 ÷ 1200053.5289 ± 19 ± 1045 ± 5 ± 4 > 800076.7322 ± 17 ± 1655 ± 6 ± 5 Several available models to obtain  tot p-p from  inel p-air : Glauber – Matthiae theory Durand – Pi Wibig – Sobczynska …. Models agree within few % in our energy range  systematic error: 5% From p-air to p-p cross section

19. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 19 Systematic uncertainties Variations of the atmospheric depth (pressure) h 0 MC = 606.7 g/cm 2 (4300m a.s.l. standard atm.), h 0 MC / = 0.988 ± 0.007  impact on cross section analysis:  1% Uncertainty on (MC) int : RMS of MC distribution  2÷3 % Uncertainty on the contribution of heavy neclei: comparing slopes from different (p+He) fluxes (Hoerandel, Jacee, RunJob)  1÷4 % Uncertainty on “  p-air to  p-p ” : comparing different models  5 % Statistical and systematic errors independently propagated

20. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 20 Total p-p cross section

21. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 21 Total p-pbar cross section

22. ECRS 2008A.Surdo: Measurement of the p-air inelastic cross section with ARGO-YBJ 22 Summary and outlook The flux attenuation technique has been successfully used in ARGO-YBJ experiment, by exploiting the detector features and location. The inelastic proton-air (and the total p-p) cross section has been measured in a scarcely explored energy region and results are in agreement with previous ARGO results. More checks on systematics are in progress (shower fluctuations, interaction models, heavy primaries contribution, …). Shower age and energy determinations will be improved by the use of timing (rise time, front curvature,..) and topological information In the future, the analysis will be extended to higher energies (up to 1PeV), thus covering a region with few experimental data, by exploiting the analog RPC readout.