1. Temporal and spatial structure of the Extensive Air Shower front with the ARGO- YBJ experiment 1 - INFN-CNAF, Bologna, Italy 2 - Università del Salento and INFN Lecce, Italy 3 - INFN Lecce, Italy A.K Calabrese Melcarne 1, G.Marsella 2,3, D. Martello 2,3, L. Perrone 2,3, S. Sbano 2,3 for the ARGO-YBJ Collaboration XXXII International Cosmic Ray Conference, ICRC 2011 Beijing, China, August 10-18, 2011

2. High Altitude Cosmic Ray Laboratory @ YangBaJing,Tibet, China Site Altitude: 4,300 m a.s.l., ~ 600 g/cm 2 The ARGO-YBJ experiment ARGO-YBJ

3. Cosmic ray physics: spectrum and composition above few TeV (HE 224, 242,814) study of the shower space-time structure (HE 259, 402,755,1026) hadronic interaction studies (HE 754) anti-p / p ratio at TeV energies (HE 225, 226) CR anisotropy (HE 41, 507) VHE  -Ray Astronomy: search for point-like (and diffuse) galactic and extra-galactic (OG 241, 246, 256, 1112) sources at few hundreds GeV energy threshold (OG 530, 559, 1005, 1007, 1209) Search for GRB’s: full GeV / TeV energy range (OG 179,574) The Physics Case

4. Strip = space pixel Pad = time pixel Time resolution ~1.8 ns 10 Pads (56 x 62 cm 2 ) for each RPC 8 Strips (6.5 x 62 cm 2 ) for each Pad 78 m 111 m 99 m74 m (  43 m 2 ) 1 CLUSTER = 12 RPC RPC analog charge read-out dynamical range up to ~ 10 4 TeV (HE 774,1028)

5. Number of Fired Strips Full coverage, high time and space resolution provide a detailed view of shower front Data Selection - Quality cut on S 2 Reconstruction Time sequence and position of hit pads used to reconstruct the CR arrival direction and core position - Using a plane (α=0) - Using a conical correction (α  0) - Core reconstructed within the central carpet contamination of mis-reconstructed events less than 10% at low multiplicity, rapidly decreasing at higher multiplicity

6. Average Curvature: Average Curvature: the mean of time residuals Δt(R) with respect to a planar fit Time profile -larger than 10 ns for particles landing further than 60 m from the core. -no significant dependence on pad multiplicity observed. 3.5 10 8 events zenith < 15°

7. Data vs Simulations Very good agreement at the level of time profile curvature Shower generator Corsika 6.720 with SIBYLL+FLUKA as hadronic interaction models at high and low energies ~ 10 7 proton showers dN/dE  E -  (  =1) 300 GeV - 100 TeV Detector simulation Detector simulation GEANT3 zenith < 15°

8. Shower Morphology Δt  r 2 /H (r<< H) deep/shallow showers have large/small curvature For a given zenith angle, the measurement of the curvature provides hints on: - shower age - hadron/photon separation Shallow (“old”) Deep (“young”) Hadrons have more muons, they are mainly produced high in the atmosphere, flatter front expected compared to photon primaries.

9. Xmax: atmospheric depth at shower maximum (true value from Corsika) α: conicity coefficient derived from reconstruction Shower Age A third degree polynomial fit to the profile shown is used to parametrize the correlation between the observed α and the corresponding mean Xmax This method provides an estimate of the mean Xmax consistent with the true value within a ten g/cm 2. young old proton zenith < 15° Conicity coefficient α (rad)

10. Shower Age Impact on energy resolution (hit multiplicity for a given energy changes with shower age) - Spectrum - proton-air cross section analysis The sample has been divided in two categories: - “young” (α > 0,035 rad) - “old” (α < 0,035 rad) Experimentally challenging…

11. Large RMS Shower fronts Wide showers Double Front showers In order to exploit at maximum space-time information, we started a detailed study on the longitudinal time structures in data The idea is to study more in detail the shower structures in order to define selection criteria for particular analysis (gamma/hadron separation, composition, exotic physics) In particular we studied showers with large time residual with respect to the shower front Two Categories have been observed

12. Double Shower Fronts Reconstruction –Separation of subshowers –Planar Fit on subshowers –Quality cuts on reconstructed subshowers S 2 < 100 ns 2 Selection Hit Number >120 (10% of Ev.) RMS >15 ns (11% of Ev.) Fit on residual distribution Quality selection on global reconstruction: S 2 > 600 ns 2 Distance of two peaks > σ1+σ2

13. Double Shower Fronts What are these showers? accidental coincidences and possible multiple front showers How many events? How many are compatible with accidental coincidences? –Time delay –Nhit Distribution –Angular distribution –Observed vs Expected events S 2 < 600 ns 2 S 2 > 600 ns 2 Angular Distribution

14. Analysis Observed rates –3.1x10 8 events have been processed –1900 events selected as double coincidences Expected rates DAQ rate = 3.5±0.1 kHz (multiplicity of Events Nhit > 20 in a time window τ = 2 μs) Rate of observed showers with S 2 < 100 ns 2 : λ 1 = 2.7±0.1 kHz Expected double coincidences: λ exp = λ 1 x λ 1 x τ x η = η x 14±1 Hz (η is the event selection efficiency) η x 0.4% of the events 6.2x10 -4 % of the events (3 order of magnitude less than expected if η=1 )

15. Analysis Simulated double events Artificial double Events have been generated from real data Merged two consecutive events shifting the time of each hit of the second event by a randomly extracted ΔT compatible with the 2μs trigger window Verified the random double shower distributions (Angle, Multiplicity, relative time distribution) Tested the double shower selection and reconstruction efficiency η : The efficiency η is 0.14% (preliminary selection algorithm) This explains the difference between expected and observed events expected : η x 0.4% = 5.6x10 -4 % of the events observed : 6.2x10 -4 % of the events More efficient selection algorithm are on the way!

16. Conclusions In this work the very high space-time granularity of ARGO-YBJ has been exploited in detail to investigate deeply the shower front morpholgy. Various observables have been defined to better characterize EAS properties, such as shower age and primary composition. The possibility to select between ”old” and ”young” shower based on the conicity parameter has been shown. Longitudinal time structures have also been investigated selecting events with large time distribution around the shower front. The attention has been focused on double front showers and, in particular, to identify the random double coincidences expected in ARGO-YBJ experiment. This is a preliminary step to better investigate the possible “physics of multiple shower fronts”