The Surface Detector Array of the Pierre Auger Observatory

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The Surface Detector Array of the Pierre Auger
Observatory

• Aaron S. Chou for the Pierre Auger
  Collaboration
• APS Meeting, Tampa, FL
• April 18, 2005

• Detector description
• Calibrations, triggers
• Detector response and mass spectroscopy
• Angular accuracy
• Energy determination.

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Deployment Status as of April 2, 2005
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Detectors as far as the eye can see.
1.5 km
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Auger Water Cherenkov Detectors
Plastic tank with 12 tons of very pure water
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Auger Tanks are Self-Calibrating

• Atmospheric muons ? standard candle for an
  end-to-end calibration of the detector response
• Measure signal strength in units of
  Vertical Equivalent Muons (VEM)
• All trigger thresholds are conveniently expressed
  in these physical units instead of arbitrary
  electronics units.

Minimum bias data taken with low threshold
triggering, 500ns traces
Single muon peak
Number of evts
Integrated FADC signal (arbitrary units)
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Local triggers are performed by each tank

• 1) Threshold single 25nsec FADC bin with
  Signalgt3.2VEM in all 3 PMTS.
• 20Hz of mostly atmospheric muon background

2) Time over Threshold 10 bins above
0.2VEM in 3msec window. Detects EM portion of
shower extended in time from scattering. 1Hz
rate dominated by small showers
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Local Trigger Bits are Radiod to the Control Room

• Global trigger requires gt2 triggered tanks
  passing topological criteria. If satisfied,
  then the central computer signals the tanks to
  send their buffered data.
• Alternatively, the Fluorescence detector triggers
  can also initiate the readout of buffered surface
  detector data.

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Physics Event Rate 600/day (so far)
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Auger Tanks are Hyper-sensitive to Muons!

• Tank height 120cm 3X0
• Signal is proportional to energy deposit
  m240MeV, e10MeV, g10MeV
• Auger signal contributions from m, EM are
  comparable at gt 1 km
• Contrast with AGASA scintillators which sample
  mainly the EM component.

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Young Old Shower
Young shower See sparse muon spikes above the
EM shower front. (Detector response to muons is
enhanced.)
Old shower (1st interaction far away from
ground) The EM is completely attenuated. See
only a compact muon pulse in detectors near the
shower core. Get muon content from a shape
analysis of the FADC traces.
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Angular reconstruction with SD

• Obtain cosmic ray arrival direction from simple
  geometry
• Use the differences in arrival times of the
  shower front at each detector
• If 4 tanks are hit fit also for radius of
  curvature

By comparing trigger times of signals from pairs
of tanks, measured timing error 7ns
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Angular resolution with SD

• Estimation from simulations
• Angular resolution 1 at E gt1019eV
• Resolution improves with zenith angle ?
• Resolution improves with increasing E

Preliminary Simulation Showers injected at 45o
(Aires - SDSim)
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As Advertised, Auger views the Southern Sky!
Preliminary raw event map gt4 tanks
hit, Equatorial coordinates 3 smoothing
Preliminary
events
Using water tanks instead of flat scintillators
gives a large increase in exposure at larger
zenith angles.
Zenith angle
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Reconstucting the shower footprint Vertical
(q35o) Inclined (q72o) Events
35 tanks
14 tanks
14 km
7 km
13 km
Shift the assumed core position until the data
points lie on a single curve.
This curve is called the lateral distribution
function (LDF)
Signal(VEM)
Transverse Core Distance(m)
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A Big One q 60 , 34 tanks
The fitted signal S(1000) at 1 km
transverse core distance is chosen as an
optimized measure of the energy. (Optimized
w.r.t. array spacing, statistical fluctuations in
shower development, systematic errors.) Use
simulations to predict S(1000)?Energy conversion
34 tanks
60
8 km
(m)
14 km
Lateral Distribution Function
1020eV (preliminary)
1?1020eV
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Summary

• The Auger Surface Detector is being rapidly
  deployed and has already accumulated a large
  physics data set.
• Auger data provides for good measurements of
• Composition
• Direction
• Energy. We will have the statistics to probe the
  trans-GZK region!
• Expect the first measurement results this summer!

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Muon Sensitivity and Primary Composition

• The muon flux depends on the type of primary
  particle.
• Gamma-induced showers are purely electromagnetic
• Fe56 showers gives more muons than protons
  showers
• With muon flux sensitivity, we gain information
  about the composition and interactions of the
  primary cosmic ray!
• Measurement of gamma/non-gamma is a stringent
  test of top-down models including decays of
  topological defects or of Super-Heavy Dark
  Matter.

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dS/S (from fluctuations in shower development)
has a shallow minimum at large core distances
AGASA 2x1020eV
Closer to ground
Farther from ground
m
EM
d(Signal)/Signal
Shower-shower fluct.
Poisson sampling
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Detect UHE Neutrinos via Horizontal Air
Showers(HAS)

• Large zenith angle hadronic showers have lost
  most of their EM component by the time they reach
  the detector. Only the hard muons are left.
• Neutrinos can shower deep in the atmosphere,
  right over the detector ? Easy to identify
• Tau neutrinos can also interact in the earth, and
  the escaping Tau particle can decay and form an
  upward-going shower.

SM Interaction Prob 10-4 ? Auger event rate
few/year. Good chance to check for enhanced
interaction rate (10-100x) at TeV scale. ?
Large extra dimensions, etc. (Feng and Shapere,
hep-ph/0109106)
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Neutrino Sensitivities
Expected no. per year
?e and ?? Sensitivity
?? Sensitivity
High
DIS
None
?? Limit (E-2) for 5 years
X. Bertou et. al. Astropart. Phys. 17 (2002) 183