XXXI International Cosmic Ray Conference, ICRC 2009 Lodz, Poland, July 7-15, 2009 Time structure of the Extensive Air Shower front with the ARGO-YBJ experiment A.K Calabrese Melcarne1, L. Perrone2,3, A. Surdo3 for the ARGO-YBJ Collaboration 1 - INFN-CNAF, Bologna, Italy 2 - Università del Salento and INFN Lecce, Italy 3 - INFN Lecce, Italy

The ARGO-YBJ experiment High Altitude Cosmic Ray Laboratory @ YangBaJing,Tibet, China Site Altitude: 4,300 m a.s.l. , ~ 600 g/cm2

The Physics Case Cosmic ray physics: VHE g-Ray Astronomy: spectrum and composition above few TeV (HE 562, 407, 411) study of the shower space-time structure (HE 424, 682) p-Air cross section measurement (HE 319) anti-p / p ratio at TeV energies (HE 431) VHE g-Ray Astronomy: search for point-like (and diffuse) galactic and extra-galactic sources at few hundreds GeV energy threshold (OG 399, 398) Search for GRB’s: full GeV / TeV energy range (OG 391, 567, 683)

Strip = space pixel Pad = time pixel Time resolution ~1.8 ns (43 m2) (6.5 x 62 cm2) for each Pad 10 Pads (56 x 62 cm2) for each RPC 1 CLUSTER = 12 RPC (43 m2) 78 m 111 m RPC Strip = space pixel Pad = time pixel Time resolution ~1.8 ns analog charge read-out in progress dynamical range up to ~ 104 TeV (HE 1388)

Data Selection - Core reconstructed within the central carpet - Quality cut on S2 contamination of mis-reconstructed events less than 10% at low multiplicity, rapidly decreasing at higher multiplicity Number of Fired Strips 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) Full coverage, high time and space resolution provide a detailed view of shower front

Time profile Average Curvature: the mean of time residuals Δt(R) with respect to a planar fit 3.5 108 events larger than 10 ns for particles landing further than 50 m from the core. no significant dependence on pad multiplicity observed. zenith < 15°

Shower Thickness Average Thickness: the RMS of time residuals σ(R) with respect to a conical fit 3.5 108 events about 8 ns at 40 m from the core (intermediate multiplicity). thinner for increasing multiplicities. zenith < 15°

Data vs Simulations Shower generator curvature Corsika 6.720 with SIBYLL+FLUKA as hadronic interaction models at high and low energies ~ 107 proton showers dN/dE  E- (=1) 300 GeV - 100 TeV Detector simulation GEANT3 curvature zenith < 15° Very good agreement at the level of time profile Shower thickness systematically underestimated (about 1.5 ns, likely due to the missing contribution of heavier nuclei not yet included in simulation)

Shower Morphology Δt  r2/H (r<< H) deep/shallow showers have large/small curvature Shallow (“old”) For a given zenith angle, the measurement of the curvature provides hints on: shower age hadron/photon separation Deep (“young”) Hadrons have more muons, they are mainly produced high in the atmosphere, flatter front expected compared to photon primaries.

Shower Age old young α: conicity coefficient derived from reconstruction Xmax: atmospheric depth at shower maximum (true value from Corsika) proton zenith < 15° Experimentally challenging… Impact on energy resolution (hit multiplicity for a given energy changes with shower age) - Spectrum proton-air cross section analysis

Useful for /hadron separation? Different shower development Hadrons have more muons Protons exhibit a flatter front Larger RMS close to the core for proton

Conclusions The ARGO experiment offers a unique chance to observe the EAS image at ground on a full coverage active RPC array with high time resolution (< 2 ns) Average time profile and shower thickness measured as a function of core distance to axis measured in a large range of hit multiplicities (300 GeV < E < 100 TeV) Shower morphology studied with detailed simulations. The correlation of shower curvature with Xmax investigated as a hint for shower age and photon/hadron separation studies

Proton Shower Photon Shower

Corsika Xmax parameter from the fit of Gaisser-Hillas function fit 7 parameters of fit are stored in the event end block in Corsika : Nxmax,a,b,c,t0,tmax and dof Thinning used for our simulation:5g/cm2

TCut age_old="Alpha<0.0365"; TCut age_middle="Alpha>0.0365&&Alpha<0.04215"; TCut age_young="Alpha>=0.04215";

TCut age_old="Alpha<0.0365"; TCut age_middle="Alpha>0.0365&&Alpha<0.04215"; TCut age_young="Alpha>=0.04215";

Layer of Resistive Plate Chambers (RPC) 12 RPC = 1 cluster ( 5.7 ´ 7.6 m2 ) 8 Strips = 1 Pad (56 ´ 62 cm2) 99 m 74 m 10 Pads = 1 RPC (2.80 ´ 1.25 m2) 78 m 111 m Layer of Resistive Plate Chambers (RPC) Active area : central carpet  5600 m2 sampling guard-ring  1000 m2 Data taking : since July 2006 with the central carpet since November 2007 with the guard-ring Installation of analog charge read-out in progress ® dynamical range up to ~ 104 TeV 15-20 % aumento molteplicità per effetto piombo  95 % active surface Altri 1000 m2 attivi nel ring, 5200 m2 di area di sampling 124800 strip-pixel (space) 15600 pad-pixel (time) 3120 big pad-pixel (charge) Presently : 54 Clusters with Analog Read-Out in data taking Autumn 2007 : 110 Clusters with Analog Read-Out