Tools and Methods for Underwater, High Energy Neutrino TelescopyA.G.Tsirigotis, A. Leisos, S.E.Tzamarias

Physics Laboratory, School of Science and Technology, Hellenic Open University, Greece

1. 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 appear in Nucl. Instrum.Meth.A (2010)

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

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.

2. KM3NeT

3. Optimization and Evaluation of proposed KM3NeT configurations using HOURS

20m30m40m50m

100m ,130m ,180m ,210m

Horizontal layout: Vertical layout: 20 OMs300 strings

Neutrino telescope configuration

Multi PMT OM

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

One of the detector configurations studied consists of 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.

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

Point Source sensitivity vs distance between strings

-60 degrees source declination

/2( ) cE Ee

10cE TeV

50cE TeV

20cE TeV

100cE PeV

100cE TeV

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 in principle 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 optimization of the detector’s configuration parameters is based on minimizing the sensitivity in observing a point source whilst collecting one year

The diffuse flux sensitivity of the neutrino telescope 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.

Detector at4500m depth

4. Expected Neutrinos from Gamma Ray Bursts and Galactic Point Sources

GRB zenith angle (degrees)

dNn

eu

trin

o/d

TdΩ

(y-1

sr-1

)

3500m detector depth7.5ev/y

4500m detector depth8.4ev/y

background

1000 GRBs/year

Source Type Number of sources

Nsignal/year

4500m depth

Nbckg/year

4500m depth

1. Supernova Remnants

13 4.9 6.2

2. X-ray binaries

3 0.2 1.2

3. Unidentified 14 8.1 5.0

4. Pulsar Wind Nebulae

13 12.0 4.5

H.E.S.S. Galactic Gamma Ray Sources – Potential neutrino emmitters

Expected Neutrino fluence from GRBs

PS1+3SignalBackgrounddiscovery limit

GRBSignalBackgrounddiscovery limit

Most of the identified astrophysical point sources have an expected neutrino flux roughly at the sensitivity limit of the studied detector configuration. Although this detector configuration has an instrumented volume of 7.6km3 and a cost of ~100MEuro, many years of observation are required for a significant discovery for any specific point source. In the table the total number of expected signal and background neutrino events are shown for each type of point source visible by the KM3NeT. The objects of category 1, and perhaps also category 3, are most likely candidates for neutrino emission through hadronic interactions. Category 2 and 4 objects are generally treated as leptonic (inverse Compton) sources and therefore are less likely neutrino emitters.If we consider the accumulated data for all point sources of type 1 and 3, during the KM3NeT construction period (assumed to be 4 years), then we can have a significant discovery (more than 2.5sigma – 99% confidence level) after the 3rd construction year.

SignalBackgrounddiscovery limit

PS1+3

Acc

umul

ated

dat

a

Construction yearGamma Ray Bursts are also potential neutrino emmitters according to the fireball model. High energy neutrinos from prompt emission consistent with the detected gamma rays are expected to arrive within a short time window (~2h). The narrow time window results in reduced background noise and with the combination of an appropriate energy cut, the detection of downgoing GRB neutrinos is feasible.

Considering 1000 GRBs per year in 4π solid angle we expect to detect 8.4 neutrinos per year for the fully constructed proposed detector. This results in significant discovery after the second year of KM3NeT construction.

An alternative senario for early discovery of astrophysical neutrinos by the KM3NeT, would be the instrumentation of the full 7.6km3 detector geometrical volume with one fourth (75) of the strings during the first construction year. This sparse detector with an inter-string distance of 360m will have sufficient efficiency for the detection of high energy neutrinos from GRBs. One year of data taking with this detector could lead to a significant discovery.

SignalBackgrounddiscovery limit

GRB

Acc

umul

ated

dat

a

worth of data. In this optimization several exponential cutoffs in the energy spectrum were taken into account.

μ track

ν-Telescope

Top of the atmosphere

Scintillator array at

Sea level

5. Synchronous Detection of Extensive Air Showers with a Deep Sea ν-Telescope and a Floating Scintillator Array

A floating array of Extensive Air Shower (EAS) detectors can be used as a sea-top calibration infrastructure, on top of the KM3NeT neutrino telescope. A large percent of the cosmic showers contain energetic muons able to penetrate the sea water and reach the ν-detector. The standard calibration method is based on the comparison between the reconstructed shower axis and the reconstructed muon track parameters.

Alternatively the floating array can be used to select EAS passing close to the center of the floating platform Then a simple estimation of the muon track direction can be made. That is the straight line connecting the position of the center of the platform with the weighted mean (weighted by the observed charge) of the active optical modules positions.

Depth (m) Offset Sensitivity (deg)

θ φ

2500 0.007 0.02

3500 0.01 0.07

Depth (m) Offset Sensitivity (deg)

θ φ

2500 0.040 0.26

3500 0.045 0.34

Improved methodStandard method

6. Intercalibration

It is possible to inter-callibrate the neutrino telescope by dividing the underwater detector in sub-detectors. The same muon track is then reconstructed by each sub-detector and the resolution of the neutrino telescope is evaluated by comparing the reconstruction results.The accuracy in estimating the zenith angle depends on the number of active OMs which are used in the reconstruction. The information provided by the whole set of OM’s resulted to a zenith angle resolution of 0.062o±0.003o, whilst the angular resolution using the half of the data-points is 0.095o ±0.005o.

The difference between the two zenith angles, reconstructed by the two sub-groups of OMs, was evaluated on a track by track basis and was used to estimate the neutrino telescope’s angular resolution. This zenith angle difference was found to follow a Gaussian distribution with a sigma parameter equal to 0.14o±0.01o. Assuming the same resolution for each of the two zenith angle estimations, the resolution of each sub-group can be estimated as:

which is in very good agreement with the sub-group’s resolution.

Construction year

Construction yearConstruction year

Acc

umul

ated

dat

a