The single shower calibration accuracy is about 6.7 degrees but the accuracy on the mean value (full data set calibration accuracy) scales down inversely with the square root of the recorded events.By accumulating events using 3 indepedent floating stations in a 10-day operation, the resolution of estimating a possible angular offset is 0.04 degrees (km3 ν-Telescope at 3500m depth)

Synchronous Detection of Extensive Air Showers with a Deep Sea ν-Telescope and a Floating Scintillator ArrayG. Bourlis, A. Leisos, A.G.Tsirigotis, S.E.Tzamarias

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

μ track

ν-Telescope

3500 m depth

Sea level

Top of the atmosphere

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. Such an array can detect the copiously produced, low energy and small zenith angle, atmospheric showers and the collected data can be used for the reconstruction of the direction and of the impact parameter of the shower axis. A large percent (35%) of the cosmic showers in the energy range of 1014 – 5 1015 eV contain energetic muons able to penetrate the sea water and reach the ν-detector. These muons are detectable by the deep-sea telescope and the muon track parameters can be estimated with high accuracy.

19m

5m

Sea Top station

160 Scintillation Tiles

96 WLS fibers

Single PMT read out

The floating array consists of 16 HELYCON detectors, arranged on a two-dimensional grid (5mx5m cell size) covering an area of about 360m2.

The HELYCON charged particle detector is a scintillation counter of 1m2 active area. It is made of plastic scintillator tiles wrapped in Tyvek reflective paper, while the light is collected by wave shifting fibers embedded inside the grooves of the scintillating tiles and it is detected by a fast photomultiplier tube

Θ telescope

Θ array

φ array

φ telescope

Synchronous detection and Calibration

The calibration method is based on the comparison between the reconstructed shower axis and the reconstructed muon track parameters.For example, the difference between the reconstructed zenith angles should follow a normal distribution with mean value equal to zero. Any statistical significant deviation of this mean value from zero, indicates that the estimations of the neutrino telescope suffer from a systematic angular offset.

ν-Telescope

Scintillator Array

Shower Axis

μ track

The Scintillator array

19m

Shower Reconstruction

GPS

~20 m

Thickness

dt=0

dt1dt2

dt3

Shower axis

di

( ; ( , ))i iL P t d

di: distance from the shower axis

Analogue signal

Comparators’ output

t1t2t3 t4 t5 t6 HPTDC output

Thresholds crossed 1 2 3 4 5 6

Charge estimation resolution (%) 13.6 6.9 5.6 4.3 3.4 2.9

Data Acquisition

Charge & Arrival Time Estimation

Muon Track Reconstruction

Single Shower and Full Data Set Calibration Accuracy

Simple Estimation: An Improved Method

d

(XN,YN)

(Xw,Yw)

Θ w

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 0.01 0.02

3500 0.02 0.06

SeaWiet

νOne

Depth (m) Offset Sensitivity (deg)

θ φ

2500 0.040 0.26

3500 0.045 0.34

Depth (m) Offset Sensitivity (deg)

θ φ

2500 0.040 0.20

3500 0.09 0.46

SeaWiet

νOne

Improved method Standard method

Number of counters>5

Co

llec

ted

C

har

ge

(mip

)

Radial Distance (m)

Nu

mb

er

of

sh

ow

ers

Radial Distance (m)

Number of counters>1

θw-θshower

1 station for ~40 hours of operation

3 stations for 10 days

σc =6.70

In first order approximation the particle front is a plane moving with the speed of light. Actually the particle front is curved with a thickness that depends on the distance from the shower axis.By Monte Carlo integration we have calculated the time distribution of the signals (full detector simulation) with respect to the distance from the showers axis.The typical zenith angle resolution of a HELYCON station (3 scintillator counters at a distance of about 20 m) is 4.5 degrees.

The HELYCON Readout electronics offers up to five analog inputs, each one for a scintillation detector.The input signals are compared to six predefined (remotely adjustable) thresholds and the corresponding times of the PMT waveform-threshold crossings are digitized with an accuracy of 100 ps by the HPTDC.

The trigger is realized in the Field Programmable Gate Array (FPGA) of the Readout card which is also responsible for formatting the data and for communicating with the station (local) PC. The data are saved on the hard disk of the local computer and transmitted on request, via the Internet, to a central server

For the charge estimation, a Multi Time Over Threshold Technique is applied. The measured charge is parameterized as a polynomial function of the sum of the times over threshold. The typical resolution is better than 10% while the arrival time resolution was estimated to be 2ns.

The simple estimation of the muon direction is compared, on an event by event basis, to the fully reconstructed muon track by the neutrino telescope. The distribution of the difference between the simple estimation of the muon parameter and the corresponding parameter of the reconstructed muon track, consists of a central Gaussian structure which is used for the evaluation of the calibration accuracy.

dm

L-dm

(Vx,Vy,Vz) pseudo-vertex

dγ

d

Track Parameters

θ : zenith angle φ: azimuth angle (Vx,Vy,Vz): pseudo-vertex coordinates

θc

(x,y,z)

•Combination of Χ2 fit and Kalman Filter (novel application in this area) using the arrival time information of the hits.•Charge – Direction Likelihood using the charge (number of photons) of the hits.

Energy (log(E/GeV))

An

gu

lar

res

olu

tio

n (

med

ian

de

gre

es

)

θshower-θmuon

Summary & Results

Photonis XP-1912Standard, 10-stage, 19 mm (3/4") round tube

WLS fibers: Bicron (BCF-91A)

Light attenuation length: 330cm

•Online Monitor of a HELYCON Station

kkCharge= a (tot)

2.8

/ 2 2

ns

ns

Each HELYCON autonomous array can be used to select EAS passing close to the center of the floating platform (i.e. by requiring at least 5 active counters and by setting a threshold value (i.e. 20 mips) for the total collected charge).Then a simple estimation of the muon track direction can be made. That is the straight line (simple estimation) connecting the position of the center of the platform with the weighted mean (weighted by the observed charge) of the active1 optical modules positions.In this method, the resolution in estimating the shower axis zenith angle is less than one degree.

μ=0.010±0.045 μ=0.15±0.3

We have studied a new strategy of using floating detector arrays in order to investigate for possible systematic errors in track reconstruction by an underwater neutrino telescope. Assuming that three floating arrays collect, independently of each other, data for a period of ten days, we found that a possible offset in zenith angle estimation can be evaluated with an accuracy of 0.01o, whilst a similar offset in the azimuth angle can be found with an accuracy of 0.07o. Furthermore the coordinates of the center of the neutrino telescope, can be estimated with an accuracy better than 1 m. In this study we did not take into account correlations of the above systematic errors.

Scintillator array