Energy Reconstruction Algorithms for the ANTARES Neutrino Telescope

1
Energy Reconstruction Algorithms for the ANTARES
Neutrino Telescope
International Workshop on UHE Neutrino
Telescopes Chiba July 28-29, 2003

• J.D. Zornoza1, A. Romeyer2, R. Bruijn3
• on Behalf of the ANTARES Collaboration

1IFIC (CSIC-Universitat de València),
Spain 2CEA/SPP Saclay, France 3NIKHEF, The
Netherlands
2
Introduction

• Neutrinos could be a powerful tool to study very
  far or dense regions of the Universe, since they
  are stable and neutral.
• The aim of the ANTARES experiment is to detect
  high energy neutrinos coming from astrophysical
  sources (supernova remnants, active galactic
  nuclei, gamma ray bursts or micro-quasars).
• At lower energies, searches for dark matter
  (WIMPs) and studies on the oscillation parameters
  can be also carried out.
• The background due to atmospheric neutrinos is
  irreducible. However, at high energies, this
  background is low, so energy reconstruction can
  be used to discriminate it.

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ANTARES Layout

• 12 lines
• 25 storeys / line
• 3 PMT / storey

14.5 m
350 m
Junction box
100 m
40 km to shore
60-75 m
Readout cables
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Energy loss

• The muon energy reconstruction is based on the
  fact that the higher its energy, the higher the
  energy loss along its track.
• There are two kinds of processes
• Continuous ionization
• Stochastic Pair production, bremstrahlung,
  photonuclear interactions
• Above the critical energy (600 GeV in water)
  stochastic losses dominate.

Energy loss vs. muon energy
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Time distribution
Photon arrival time distributions

• There is also an effect of the energy on the
  arrival time distribution of the photons.
• The higher the energy, the more important the
  contribution to the time distribution tail.
• The ratio of the tail hits over the peak hits
  gives information about the muon energy.

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Reconstruction algorithms

• Three algorithms have been developed to
  reconstruct the muon energy
• MIM comparison method
• Estimation based on dE/dx
• Neural networks

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MIM Comparison method

• 1. An estimator is defined, based on a comparison
  between the light produced by the muon and the
  light it would have produced if it was a Minimum
  Ionizing Muon

• 2. A large MC sample is generated to calculate
  the dependence between the muon energy and the
  estimator.

• 3. This dependence is parameterized by the fit to
  a parabola

log x p0 p1 logE? p2 (logE?)2

• 4. This parameterization is used to estimate the
  energy of a new MC sample.

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Reconstructed energy
Estimator distributions

• Two energy regimes have been defined, in order to
  optimize the dynamic range of the method. In the
  calculation of the estimator, we only take the
  hits which fulfill
• Low energy estimator 0.1 lt Ahit/AMIP lt 100
• High energy estimator 10 lt Ahit/AMIP lt 1000

Erec vs Egen

• There is a good correlation between the
  reconstructed and the generated energy.
• The resolution is constrained by the stochastic
  nature of the energy loss process.

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MIM Results
vs. muon generated energy

• Each x-slice of the log10(Erec/Egen) distribution
  is fitted to a Gaussian.
• The mean of the distribution is close to zero.
• The resolution at high energies is a factor 2-3.

vs. muon reconstructed energy
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Estimation based on dE/dx

• This method also uses the dE/dx dependence on the
  muon energy.

• An new estimator is defined as follows

Lµ muon path length in the sensitive volume ?A
?Atotal hit amplitude R detector response

• R(r, ?, f) is the ratio of light seen by the
  overall detector, i.e. a kind of detector
  efficiency to a given track. It is independent of
  the reconstruction, but a function of
• track parameters (x, y, z, ?, f)
• light attenuation and diffusion (?att 55 m)
• PMT angular response

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Detector response and sensitive volume

• The detector response is defined as

NPMTnumber of PMTs in the detector ??jPMT
angular response rdistance to the PMT

• The sensitive volume is the volume where the muon
  Cherenkov light can be detected.
• It is defined as the detection volume 2.5 ?att
  in each direction

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Results of the dE/dx method

• Above 10 TeV, the energy resolution is a factor
  2-3.

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Neural networks

• There are 11 inputs in this method
• Hit amplitude and time
• Hit time residue distribution
• Reconstructed track parameters

• Only events with energy above 1 TeV have been
  used to train the NN.
• After studying several topologies, the best
  performances were obtained by a two layer network
  with 20 units in each layer.

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Results of neural network method

• After fitting each x-slice of the log10 Erec/Egen
  distribution to a Gaussian, we can plot the mean
  and the sigma

• The energy resolution is a factor 2 above 1 TeV.
• From 100 GeV to 1 TeV, the energy resolution is
  3.

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Spectrum reconstruction (I)

• Using the methods previously presented, muon
  spectra can be reconstructed.
• The aim is to compare the atmospheric and the
  signal spectra.
• Atmospheric muon background has been rejected in
  the selection process (quality cuts).

Atmospheric neutrinos
Diffuse flux in E-2 (Waxman Bahcall)
dE/dx energy reconstruction method
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Spectrum reconstruction (II)

• Another approach to reconstruct the spectra is to
  use a deconvolution algorithm.
• An iterative method1 based on the Bayes theorem
  has been used.

preliminary
Cause E ?log10 Eµ Effect X ?log10 xlow
(MIM method)
1 G. D'Agostini NIM A362(1995) 487-498
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ANTARES Sensitivity

• The reconstructed energy can be used as a
  threshold to calculate the sensitivity of the
  experiment.
• The optimum value is the one for which we need
  the lowest number of signal events to exclude the
  background hypothesis at a given confidence level
  (i.e. 90)

• The expected sensitivity is
• - 7.710-8 E-2 GeV-1 cm-2 s-1 sr-1
• with Eµ gt 50 TeV, after 1 year
• - 3.910-8 E-2 GeV-1 cm-2 s-1 sr-1
• with Eµ gt 125 TeV, after 3 years
• These values are comparable with AMANDA II

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Conclusions

• Three methods have been developed to reconstruct
  the muon energy, based on the stochastic muon
  energy loss.
• The energy resolution is a factor 2-3 above 1
  TeV.
• The expected sensitivity after 1 year is 8x10-8
  E-2 GeV-1 cm-2 s-1 sr-1 with Eµ gt 50 TeV.
• This value will be similar to AMANDA II.