1. EAS Time Structures with ARGO-YBJ experiment 1 - INFN-CNAF, Bologna, Italy 2 - Università del Salento and INFN Lecce, Italy A.K Calabrese Melcarne 1, G.Marsella 2 for the ARGO-YBJ Collaboration XXIII European Cosmic Ray Symposium, ECRS 2012 Moscow, Russia, July 3-7, 2012 1

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 2

3. 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 3

4. Number of Fired Strips Full coverage, high time and space resolution provide a detailed view of shower front Data Analysis - 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 4

5. Average Curvature: Average Curvature: the mean of time residuals Δt(R) with respect to a plane fit Time profile -larger than 10 ns for particles landing further than 60 m from the core. -no significant dependence on pad multiplicity observed. 7.9 10 8 events zenith < 15° 5

6. 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° 6

7. 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 7

8. Double Shower Fronts Reconstruction –Separation of subshowers –Planar Fit on subshowers –Quality cuts on reconstructed subshowers S 2 < 100 ns 2 New Selection Hit Number >100 (10% of Ev.) Fit on time distribution Used TSpectrum class of root Distance of two peaks > σ1+σ2 8

9. Double Shower Fronts What are these showers? accidental coincidences and possible delayed showers How many events? How many are compatible with accidental coincidences? –Angular difference distribution –Nhit Distribution –Time delay –Observed vs Expected events 9

10. Angular difference Distribution 10 Angular difference between the reconstructed subshower directions In agreement with CR distribution of consecutive events

11. Multiplicity Distribution 11 Multiplicity distribution of the reconstructed subshowers Shower 1 is the one which triggers the detector! We expect lower multiplicity distribution from shower 2 (trigger not required)

12. Time Distance 12 How to define time distance between two subshowers? At least 2 variables: Peak mean value T0 from the shower plane Fit

13. Time Distance 13 Both variables can be used as indicator

14. Reconstucted Angles 14 Quality requirements on subshowers reconstruction are applied (S 2 <100ns 2 ) Applying a planar fit, the direction of both subshowers is reconstructed

15. Analysis Observed rates –7.9 x10 8 events have been processed –700 x10 3 events selected as double coincidences –370 x10 3 events selected as double coincidences with quality cuts on subshowers 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 = 2 λ 1 λ 1 τ η = η (28±1) Hz (η is the event selection efficiency) η x 0.8% of the events 4.5x10 -2 % of the events (1 order of magnitude less than expected if η=1 ) 15

16. 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 5% (preliminary selection algorithm) This explains the difference between expected and observed events expected : η x 0.8% = 4.0x10 -2 % of the events observed : 4.5x10 -2 % of the events 16

17. In this work the shower time profile and distribution have been presented. The curvature of the showers around the core have been studied, illustrating the reconstruction effect on ARGO-YBJ data. 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 expected random double coincidences. A new algorithm have been introduced, increasing the detection efficiency. More efforts are on the way. The detailed study of the angular and time distribution of the selected events will be useful in studying possible shower anomalies. In particular narrow Angular difference events will be analyzed on large statistical sample in order detect anomalies in spatial, time and multiplicity distributions as a flag of possible ‘exotic’ physic events in CR. Conclusions 17 This is a preliminary step to better investigate the possible “physics of multiple shower fronts”