1. 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

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

3. 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)

5. 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)

6. 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

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

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

9. Data vs Simulations Shower generator curvature
Corsika with
SIBYLL+FLUKA as hadronic interaction models at high and low energies
~ 107 proton showers dN/dE  E- (=1) GeV 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)

10. 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.

11. 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

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

13. 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

14. Proton Shower
Photon Shower

15. 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

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

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

19. 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 with the central carpet
since November 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
strip-pixel (space)
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