Time structure of the Extensive Air Shower Time structure of the Extensive Air Shower

front with the ARGO-YBJ experimentfront 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 MelcarneA.K Calabrese Melcarne11, L. Perrone, L. Perrone2,32,3, A. Surdo, A. Surdo33

for the ARGO-YBJ Collaborationfor the ARGO-YBJ Collaboration

XXXI International Cosmic Ray Conference, XXXI International Cosmic Ray Conference, ICRC 2009 Lodz, Poland, July 7-15, 2009ICRC 2009 Lodz, Poland, July 7-15, 2009

High Altitude Cosmic Ray Laboratory @ YangBaJing,Tibet, ChinaSite Altitude: 4,300 m a.s.l. , ~ 600 g/cm2

The ARGO-YBJ experimentThe ARGO-YBJ experiment

ARGO-YBJ

Cosmic ray physics: 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 -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)

The Physics CaseThe Physics Case

Strip = space pixel

Pad = time pixel

Time resolution ~1.8 ns

10 Pads (56 x 62 cm2)for each RPC

8 Strips (6.5 x 62 cm2) for each Pad

78 m

111 m

99 m

74 m

(43 m2)1 CLUSTER = 12 RPC

RPC

analog charge read-out in progressdynamical range up to ~ 104 TeV (HE 1388)

Num

ber

of

Fire

d S

trip

s

Full coverage, high time and space resolution provide a

detailed view of shower front

Data SelectionData Selection

- Quality cut on S2

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 (α0)

- Core reconstructed within the central carpet

contamination of mis-reconstructed events less than 10% at low multiplicity, rapidly decreasing at higher multiplicity

Average Curvature:Average Curvature: the mean of time residuals Δt(R) with respect to a

planar fit

Time profileTime profile

- larger than 10 ns for particles landing further than 50 m from the core.

- no significant dependence on pad multiplicity observed.

3.5 108 events

zenith < 15°

Shower ThicknessShower Thickness

Average Thickness: the RMS of time residuals σ(R) with respect to a

conical fit

- about 8 ns at 40 m from the core (intermediate multiplicity).

- thinner for increasing multiplicities.

3.5 108 events

zenith < 15°

Data vs SimulationsData vs Simulations

Very good agreement at the level of time profile

curvaturecurvature

Shower thickness systematically underestimated (about 1.5 ns, likely due to the missing contribution of heavier nuclei not yet included in simulation)

Shower generatorShower generator

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 simulationDetector simulation GEANT3

zenith < 15°

Shower MorphologyShower Morphology

Δt r2/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.

youngold

α: conicity coefficient derived from reconstruction

Xmax: atmospheric depth at shower maximum (true value from Corsika)

Experimentally challenging…

Impact on energy resolution (hit multiplicity for a given energy changes with shower age)

- Spectrum

- proton-air cross section analysis

protonproton

Shower AgeShower Age

zenith < 15°

Useful for Useful for /hadron separation? /hadron separation?

- Different shower development

- Hadrons have more muons

Larger RMS close to the core for proton

Protons exhibit a flatter front

Conclusions 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

Photon ShowerProton Shower

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

Xmax parameter from the fit of Gaisser-Hillas function fit

Corsika

10 Pads = 1 RPC (2.80 1.25 m2)

78 m

111 m

99 m

74 m

12 RPC = 1 cluster ( 5.7 7.6 m2 ) 8 Strips = 1 Pad

(56 62 cm2)

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