Tools and Methods for simulation and evaluation of Very Large Volume Cherenkov Neutrino detectors: Optimization and Evaluation of proposed KM3NeT designs

A.G.Tsirigotis, A. Leisos, S.E.TzamariasPhysics Laboratory, School of Science and Technology, Hellenic Open University, Greece

KM3NeT (km3 Neutrino Telescope) will be one of the world’s largest particle detectors, built at the bottom of the Mediterranean Sea. It will provide a research infrastructure for a rich and diverse deep-sea scientific program.KM3NeT is in its preparatory phase and is building on experience from 3 current Mediterranean projects: ANTARES, NEMO and NESTOR.

The proposed deep-sea infrastructure will serve as a platform for instrumentation of ocean sciences: Oceanology, Marine Biology, Environmental Science, Geology and Geophysics.

The weak nature of neutrino interactions preserves their energy and directionality potentially allowing them to illuminate parts of the Universe opaque to charged particles and EM radiation. In order to observe the predicted fluxes one must instrument km3s of material sensitive to the resulting secondary radiation. The deep Mediterranean permits observation of Cherenkov photons with attenuation lengths of up to 50m.

KM3NeT will be observing a complimentary to the IceCube neutrino detector part of the Universe.

KM3NeT sky coverage IceCube sky coverage

KM3NeT is composed of a number of vertical structures (the Detection Units), which carry photo-sensors and devices for calibration and environmental measurements, arranged vertically on “Storeys”. The basic photo-sensor unit is an “Optical Module (OM)” housing one or several photomultiplier (PMT)tubes, their high-voltage bases and their interfaces to the data acquisition system with nanosecond timing precision.

The HOU Reconstruction & Simulation (HOURS) software package

CORSIKA(Extensive Air

Shower Simulation)

All Flavor NeutrinoInteraction Events

(Secondary Particles Generation)

Atmospheric MuonGeneration from

CORSIKA

GEANT4(KM3NeT Detector Description

and Simulation)

GEANT4(Muon Propagation

to KM3NeT)

Optical Noise, PMT response and Electronics Simulation

Prefit & Filtering Algorithms

Muon Reconstruction

EAS detector Simulation

Shower direction reconstruction

The HOURS software chain Flow Chart

SeaTop CalibrationNeutrino Telescope

Performance

The HOU Reconstruction & Simulation (HOURS) software package has been developed in order to study in detail the response of very large (km3-scale) underwater neutrino telescopes. HOURS comprises a realistic simulation package of the detector response, including an

accurate description of all the relevant physical processes, the production of signal and background as well as several analysis strategies for triggering and pattern recognition, event reconstruction, tracking and energy estimation. Furthermore, this package provides the tools for simulating calibration techniques as well as other studies to estimate the detector sensitivity to several neutrino sources.

Muon energy resolution

HOURS has been used extensively in evaluating architectures and technologies proposed during the design study of the KM3NeT. HOURS also includes Extensive Air Showers (EAS) detector simulation and relevant shower reconstruction software which has been used in estimating the performance of the SeaTop calibration technique for KM3NeT.

In HOURS Kalman filter is used as a recursive track fitting method and is statistically equivalent to the least squares method. The filtering and the fit techniques, which we have developed based on Kalman filters for muon tracking using the Cherenkov emmitted photons, are described in detail in the following references.•A.G.Tsirigotis et al, Nucl.Instrum.Meth.A 602 (2009) 91•A.G.Tsirigotis et al, to be published in Nucl. Instrum.Meth.A (2010)

Optimization and Evaluation of proposed KM3NeT configurations using HOURS

20m30m40m50m

100m ,130m ,180m ,210m

Horizontal layout: Vertical layout: 20 OMs300 strings

Neutrino telescope configuration A

Multi PMT OM

31- 3inch multi PMT OM housed in a 17inch glass sphere

Tower Geometry Floor Geometry

45o45o

Vertical layout: 20 floors/Tower. 40m between

floors. 8m bar length.

Single PMT OM

8 inch PMT housed in a 17 inch glass sphere

Neutrino telescope configuration B

Horizontal layout: 127 Towers separated by 180m

The detector configurations studied are:A)The configuration A with 300 vertical strings, each string composed of 20 Multi-PMT Optical Modules (OMs). Each Multi-PMT OM contains 31 small, 3" Photomultiplier Tubes (PMTs) (19 of them looking downwards and 12 directed upwards), enclosed in one 17" glass sphere.B)The configuration B comprises 127 towers, with a separation of 180m, on an hexagonal seabed layout. Each tower consists of 20 horizontal structures (floors) 40 meters apart. Each floor contains 3 pairs of 8" PMTs distributed along a bar 8m in length: a pair at each end of the bar - with one downward-looking PMT and the other horizontally-looking - and a third pair at the center with both PMTs looking downward at 45o.

The detector configuration B has been optimized for good sensitivity to astrophysical neutrino point sources (>1TeV) and neutrinos (<1TeV) from the anihhilation of Dark Matter particles in the Sun. The optimization of the detector’s configuration A parameters is based on minimizing the sensitivity in observing a point source whilst collecting one year worth of data. In this optimization several exponential cutoffs in the energy spectrum were taken into account.

We have simulated the detector response to cosmic neutrinos isotropically in 4π solid angle, with energies following a power law with the spectral index -2.0, in the range of 100GeV-100PeV. For the atmospheric neutrinos we use both the conventional flux and the flux from charm decays (prompt component). The atmospheric muon background is included in the simulation and its generation is described using the CORSIKA EAS simulation package. The simulated showers correspond to primary particles (up to the Fe nucleus) in the energy range of 1TeV-5PeV, following the measured energy spectrum.

Although the detector sensitivity depends strongly on the energy cutoff of the neutrino flux spectrum, it is apparent that the sensitivity is better for string separation in the range of 130-180m. Taking into account possible future improvements, e.g. to improve the KM3NeT sensitivity in the low neutrino energy regime (for dark matter searches) by adding more detection

In order to optimize the vertical distance between consecutive OMs in a string, we estimated the detector sensitivity for several vertical distances, in the range 20-50m, whilst we keep fixed the distance between strings. We conclude that the optimum inter-OM distance, for all the energy cutoffs considered, is 50m.

Point Source sensitivity vs distance between strings

-60 degrees source declination

/2( ) cE Ee

10cE TeV

50cE TeV

20cE TeV

100cE PeV

100cE TeV

units among the already deployed strings, an 180m string separation seems to be more suitable.

The detector B angular resolution is better below 1TeV neutrino energy, due to the extended horizontal structure. For higher energies the detector’s A resolution is better, owing to the directional sensitivity of the Multi-PMT OMs.

/2( ) cE Ee -60 degrees source declination

Detector B

Detector A

Point source sensitivity comparison

The optimized detector configuration A exhibits better sensitivity for all the depicted energy cutoffs. However, for lower cutoffs, the difference in sensitivities between the two proposed configura-tions is reduced, due to the better efficiency of the detector B at the low energy regime.

Neutrino angular resolution

Detector Option ADetector Option B

Neutrino effective area

Detector Option ADetector Option B

The two configurations exhibits the same effective area for neutrino energy of 1TeV, while for lower energies the detector B is better. For high energies the considerably larger geometrical size of the detector A results to larger effective area.

100PeV source energy spectrum cutoff 1 year observation

Det. Option B (2.5km3)

Det. Option A (7km3)

With atmospheric muon background

No atmospheric muon background

The point source sensitivity versus the source declination is presented for the two detector configurations and the highest energy cutoff. The galactic center, a region of several identified gamma ray sources and potential neutrino emitters, will be visible (declination of −30o) by the KM3NeT during 65% of the time. A point source around the galactic center is detectable with a sensitivity of 1.5·10−9E−2(cm−2s−1GeV−1) for the detector A, while the detector B has a sensitivity of 2.5·10−9E−2(cm−2s−1GeV−1) at this declination.

3500m detector depth

4500m detector depth

With atmospheric muon background

No atmospheric muon background

For an undersea neutrino telescope the water surrounding the detector, serves as the detection medium, the shielding from the atmospheric muon background and, in the case of almost horizontally (or downgoing) incident neutrinos, as the target. As the deployment depth increases, the shielding and target effect are enhanced and the detector should be inprinciple more sensitive. The improvement in sensitivity due to the target thickness is very small, of the order a few percent, while a much bigger effect is observed due to the shielding from the atmospheric muon background, of the order of 10-30% for declinations above −40o.

The diffuse flux sensitivity of the neutrino telescope A (4500m depth) for one year of observation time has been estimated and is shown together with Waxman-Bahcall limit and sensitivity limits from AMANDA , ANTARES and IceCube.

Most of the identified astrophysical point sources have an expected neutrino flux roughly at the sensitivity limit of the detector configuration A, so for a 5 sigma discovery several year of observation are required.

Expected neutrino flux from 2 known neutrino point sources.

1 km3 undersea Neutrino detector is not enough for the detection of potential astrophysical neutrino emmiters. Even a 7 km3 detector is marginal. We should extend the future Mediterranean Neutrino Telescope to a much larger detector volume (~100km3).

Conclusions

Detector A

Detector A4500m depth