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International Workshop on UHE Neutrino Telescopes Chiba July 28-29, 2003. Energy Reconstruction Algorithms for the ANTARES Neutrino Telescope. J.D. Zornoza 1 , A. Romeyer 2 , R. Bruijn 3 on Behalf of the ANTARES Collaboration. 1 IFIC (CSIC-Universitat de València), Spain - PowerPoint PPT Presentation

Energy Reconstruction Algorithms for the ANTARES Neutrino TelescopeJ.D. Zornoza1, A. Romeyer2, R. Bruijn3on Behalf of the ANTARES Collaboration1IFIC (CSIC-Universitat de Valncia), Spain2CEA/SPP Saclay, France3NIKHEF, The NetherlandsInternational Workshop on UHE Neutrino TelescopesChiba July 28-29, 2003

IntroductionNeutrinos 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.

Juan-de-Dios Zornoza - IFIC

ANTARES Layout 12 lines 25 storeys / line 3 PMT / storey~60-75 m350 m100 m14.5 mJunctionboxReadout cables40 km toshore

Juan-de-Dios Zornoza - IFIC

Energy lossThe 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: ionizationStochastic: Pair production, bremstrahlung, photonuclear interactions Above the critical energy (600GeV in water) stochastic losses dominate.Energy loss vs. muon energy:

Juan-de-Dios Zornoza - IFIC

Time distributionThere 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.Photon arrival time distributions

Juan-de-Dios Zornoza - IFIC

Reconstruction algorithmsThree algorithms have been developed to reconstruct the muon energy:

MIM comparison method

Estimation based on dE/dx

Neural networks

Juan-de-Dios Zornoza - IFIC

MIM Comparison method1. 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.log x = p0 + p1 logE + p2 (logE)24. This parameterization is used to estimate the energy of a new MC sample.3. This dependence is parameterized by the fit to a parabola:

Juan-de-Dios Zornoza - IFIC

Reconstructed energyTwo 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 < Ahit/AMIP < 100High energy estimator: 10 < Ahit/AMIP < 1000There is a good correlation between the reconstructed and the generated energy.The resolution is constrained by the stochastic nature of the energy loss process.Erec vs EgenEstimator distributions

Juan-de-Dios Zornoza - IFIC

MIM Resultsvs. muon generated energy:vs. muon reconstructed 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.

Juan-de-Dios Zornoza - IFIC

Estimation based on dE/dxAn new estimator is defined as follows:L = muon path length in the sensitive volumeA = A=total hit amplitudeR = detector responseR(r, , ) 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, , )light attenuation and diffusion (att ~ 55 m)PMT angular responseThis method also uses the dE/dx dependence on the muon energy.

Juan-de-Dios Zornoza - IFIC

Detector response and sensitive volumeThe detector response is defined as:NPMT=number of PMTs in the detectorj=PMT angular responser=distance to the PMTThe 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

Juan-de-Dios Zornoza - IFIC

Results of the dE/dx methodAbove 10 TeV, the energy resolution is a factor 2-3.

Juan-de-Dios Zornoza - IFIC

Neural networksThere are 11 inputs in this method:Hit amplitude and timeHit time residue distributionReconstructed 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.

Juan-de-Dios Zornoza - IFIC

Results of neural network methodThe energy resolution is a factor ~2 above 1 TeV.From 100 GeV to 1 TeV, the energy resolution is ~3.After fitting each x-slice of the log10 Erec/Egen distribution to a Gaussian, we can plot the mean and the sigma:

Juan-de-Dios Zornoza - IFIC

Spectrum reconstruction (I)Atmospheric neutrinosDiffuse flux in E-2 (Waxman & Bahcall)dE/dx energy reconstruction methodUsing 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).

Juan-de-Dios Zornoza - IFIC

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.preliminaryCause: E log10 EEffect: X log10 xlow (MIM method)1 G. D'Agostini NIM A362(1995) 487-498

Juan-de-Dios Zornoza - IFIC

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 E > 50 TeV, after 1 year- 3.910-8 E-2 GeV-1 cm-2 s-1 sr-1 with E > 125 TeV, after 3 yearsThese values are comparable with AMANDA II

Juan-de-Dios Zornoza - IFIC

ConclusionsThree 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 E > 50 TeV.

This value will be similar to AMANDA II.

Juan-de-Dios Zornoza - IFIC