P. Bernardini September 10, 2006 ARGO-YBJ experiment and TeV gamma astronomy

ARGO-YBJ detector Cosmic rays VHE  -astronomy Conclusions

YangBaJing (Tibet, China) High Altitude Cosmic Ray Laboratory (4300 m a.s.l.) Longitude 90° 31’ 50” East Latitude 30° 06’ 38” North Astrophysical Radiation Ground-based Observatory  -ray astronomy Gamma Ray Burst physics Cosmic Ray physics Sun and Eliosphere physics

Detector layout 10 Pads = 1 RPC (2.80  1.25 m 2 ) 78 m 111 m 99 m74 m 12 RPC = 1 cluster ( 5.7  7.6 m 2 ) 8 Strips = 1 Pad (56  62 cm 2 ) 42 clusters 104 clusters 130 clusters BIG PAD ADC RPC Read-out of the charge induced on “Big Pads” Layer (  95% active surface) of Resistive Plate Chambers (RPC), covering a large area (  5800 m 2 ) + sampling guard ring cm lead as  -converter December clusters January clusters June clusters guard-ring under construction Fall 2006: 154 clusters in data-taking lead transport by means of the new Tibetan railway Summer 2007: fully operative

Time Resolution (ns) 1 ns level  t (ns) HV (kV) 7.3 kV HV (kV) RPC layout & performance Bakelite RPC (5   m) Operation in streamer mode Ar (15%) Isobuthane (10%) TFE (75%) Efficiency >95 % at 7.5 kV (10 kV at s.l.) Time resolution ~1 ns

Off-line Time Calibration & Angular Resolution Real events are used to calibrate the detector (offline procedure in agreement with sampling hardware procedure) Measured angular resolution (even-odd method) in agreement with expected resolution after before residuals  = 0.09 ns Final configuration

Longitudinal EAS development (Corsika simulation) Primary energy: 1000 TeV Proton Iron Argo-YBJ altitude

Main detector features and performance Resistive Plate Chambers (RPC) as active elements Space information from Strips ( pixel 6.5  62 cm 2 ) Time information from 8-strip Pads ( resolution  1 ns) Large area (  m 2 ) and full coverage (  5800 m 2 ) High altitude (4300 m a.s.l.)  Pointing accuracy ( < 0.5°)  Detailed space-time image of the shower front  Detection of small shower (low threshold energy)  Field of View  and duty-cycle  100% continuous monitoring of the sky in the range -10°<  <70°

Operation modes Shower mode Detection of Extensive Air Showers (direction, size, core …) Trigger requirement: minimum number of fired pads  20 fired pads on the central carpet: rate ~5 kHz Aims :cosmic-ray physics (threshold : few TeV) VHE  -astronomy (threshold ~300 GeV) search for gamma-ray bursts Scaler mode counting rate (n  1,2,3,4) for each cluster, averaged in 0.5 s Aims:detector and environment monitor flaring phenomena (GRB, solar flares) with a threshold of few GeV

Detector Control System RPC current Barometric pressure Temperature Relative humidity Continuous monitor of many detector and site parameters

EAS arrival times Agreement with Poissonian statistics Dead time and spurious effects are under control Distribution of time difference between consecutive events Distribution of event-multiplicity in a time window of 1 s

Unprecedented view of showers 74 m 60 m 90 m 120 ns

Cosmic rays with ARGO-YBJ

Measurements Size as Hit multiplicity (pad & strip) Analog read-out of RPC pulse charges Lateral density profile Shower space-time morphology Angular distributions (azimuth, zenith).....  Energy spectrum  Chemical composition  Proton cross section  Check of the hadron interaction models ..... Flux vs strip size Lateral density profile

By means of the RPC analog measurement, a further extension is possible up to thousand TeV 31 m 3500 particles 35 m Energy up to hundreds of TeV by using the strip size

~10% systematic error on N s Comparison of ARGO-data with JACEE and RUNJOB spectra “Bridge” between direct and indirect measurements Flux versus strip size (data and simulations)

High space/time granularity permits unprecedeted studies on the EAS phenomenology (different topologies and time structures) Very energetic shower

Evidence of strong conical shape in small showers

shower front shower axis shower core Study of the time thickness of the shower front conical fit Average time residuals vs distance from the core residual (ns) core distance (m) 200<Nhit<500 Data Simulation (1-30 TeV) with Corsika + QGSjet

Azimuth (  ) distribution Expected behaviour in the angular range where the overburden atmosphere increases as 1/cos  x o vertical depth (606 g/cm 2 at YBJ)  attenuation length of showers Deviations from this behaviour (sec  - 1 > 1) due to misreconstructed events and horizontal air showers Fit: I 0 = (165 ± 9) s -1 sr -1  = 5.6 ± 0.1

k is determined by simulations (interaction model dependent), selecting energy and age ranges by means of the actual experimental observables Flux attenuation and p-Air cross section x0x0  Measurement of the flux attenuation  for fixed energies (and shower ages)  p-Air  p-p

p-Air cross section… ARGO-YBJ approach This analysis is a first test of such technique … to be employed in unexplored energy regions R 70 radius of circle including 70% of hits Event selection based on (a) “shower size” as N hit (pad multiplicity) (b) core reconstructed in a fiducial area (30 x 30 m 2 ) (c) constraints on shower density profile and extension (R 70 < 20 m)  data = (81.3 ± 0.6) g/cm 2  data = (76 ± 2) g/cm 2 (sec  -1) distributions

Full Monte Carlo simulation of proton showers Corsika + QGSjet, detector response, trigger and analysis chain used for real data Energy estimate Nhit<500Nhit>500 = 3.67 ± 0.04 TeV = 14.3 ± 0.2 TeV  MC = (75.4 ± 0.8) g/cm 2  MC = (71 ± 1) g/cm 2 k-factor evaluation k = 0.96 ± 0.05 k = 0.97 ± 0.05

p-N cross section Hit numbers (TeV)k  p-Air (mb)  p-N (mb) < ± ± ± 1540 ± 4 > ± ± ± 2047 ± 5

VHE gamma astronomy

Exciting results in  -astronomy Many important discoveries from Imaging Atmospheric Cerenkov Telescopes (HESS, MAGIC and so on) new sources in the Galactic Plane (SNR, PWN …) source in the Galactic Center new Active Galactic Nuclei many sources have been mapped (spectrum) SNRs as VHE  -radiation sources and sites of cosmic-ray production HESS survey of the Galactic plane LSI Micro-Quasar 1ES1218 (z=0.18) New Source PG 1553 (Z>0.25) New source Some sources detected by MAGIC

Complementary measurements Sky survey looking for emission from unknown gamma-sources Monitor variable sources (i.e. variable luminosity of Mrk 421) Improve the sensitivity to flaring sources and GRB’s Probe structures larger than Cerenkov telescope Field of View Ground-based detectors with: - large effective area - high angular resolution - duty-cycle close to 100% - wide Field of View The energy threshold of sampling EAS arrays is too high (tens of TeV) The high altitude and the full coverage allow to lower the energy threshold large exposure (FoV  time) }

Milagro Water-Cerenkov EAS detector (2630 m a.s.l.) located near Los Alamos  5000 m 2 pond with an external array of 175 water tanks First PMT layer under 1.5 m of water Second PMT layer under 8 m of water (sensitive to hadronic component of the shower, used for background rejection)  HAWC Cygnus Region Mrk 421 Crab Nebula Energy threshold ~2 TeV Angular Resolut. ~0.5°

ARGO-YBj approach  Lower energy threshold as an effect of the altitude ( ~300 GeV)  Continuous monitoring of the entire overhead sky (FoV  duty-cycle  100%)  Angular resolution (  <0.5°)  Search for point-like or extended sources looking for flux excess in proper angular bins  Increase the flux sensitivity with the  /h discrimination (space-time pattern of the showers)

N pad > 500  ~ 20 TeV West East South North Observed significance: n  = 3.9 Expected significance: n  = 4.0 The measured deficit size (0.6°±0.3°) is compatible with the simulated Moon size (0.4°) Observed event deficit compatible with the expected one The Moon shadow

Implementing  /h discrimination (I) The photon signal is statistically identified by looking for an excess, from a given direction, over a background due to charged cosmic rays The study of the shower space-time pattern can allow  /h discrimination and then larger sensitivity Encouraging results from Multiscale analysis + ANN Photon ShowerProton Shower

Implementing  /h discrimination (II) Results obtained with different algorithms based on the analysis of the shower image Multiscale shower image analysisShower topology and Lat. Dens. Funct.  /h discrimination

First results with 42 clusters ( 0.6 billion events in 1000 hours live time ) Mkn 421 Mkn 501 Crab The distribution of the standard deviations do not show any excess for this small sample

Search for GRB’s The data collected in scaler mode (E > 1 GeV) are analyzed searching for possible high energy tails of GRBs The search is performed on satellite-triggered bursts ( Three sigma cut )   ARGO-YBJ  EAS-TOP  Chacaltaya The ARGO sensitivity to GRB’s (42 clusters) Fluence limits for HETE/Swift observed GRBs

Conclusions ARGO-YBJ The detector is almost completed (presently 130/154 clusters in data taking) Data analysis shows good performance in shower reconstruction First results in cosmic ray physics. Promising extension to unexplored E p regions. In the future, check of the hadronic interaction models. Contributions to atmospheric neutrino study ?  -astronomy The  -sky is more and more crowded Atmospheric Cerenkov and full-coverage EAS detectors are complementary to detect steady and transient, point-like and extended  -sources, to discover where and how cosmic rays (and neutrinos) are produced The contribution of ARGO-YBJ is beyond the corner