A feasibility study for the detection of SuperNova explosions with an Undersea Neutrino Telescope

A. Leisos, A. G. Tsirigotis, S. E. TzamariasPhysics Laboratory H.O.U

SN (GARCHING) Model In this study we consider only the flux and the reaction

HOURS(HOU Reconstruction& Simulation)

α is a function of time

e e p e n

e p e n

2 ( ), ( )0

1 1

( ), ( )

1 ( ),4 ( )

11

e

e ee e

e

e e

eee e

T

a t E t

Ea aE

a t E t

d L tE d f dtdE d E t

Eaf E e

E E

Detector Geometrical Layout

154 Towers with a mean horizontal distance of 180mEach Tower consists of 20 bars, 6m in length and 40m apartOne MultiPMT OM at each end of the bar.

Detectors Footprint

31 3” PMTs (~30% max QE) inside a 17” glass sphere with 31 bases (total ~6.5W) Cooling shield and stemSingle vs multi-photon hit

separation Large (1260 cm2) photocade

area per OM First full prototype under test

Neutrino Energy (MeV)

σ IBD x

10-4

1 cm

2

Neutrino Energy (MeV)

Spectrum at the source <E>=12.54 MeV

Spectrum of IBD neutrinos<E>=18.87 MeV

e p e n

Neutrino Energy (MeV)<P

ositr

on E

nerg

y> (M

eV)

Neutrino Energy (MeV)

<Pos

itron

cos

θ>Positron Energy (MeV)

Num

ber o

f eve

nts

generated

At least 1 hit in OM

<E>=17.19 MeV

<E>=19.23MeV

Nobs int Veff

00

0

00

( ) ~:2

) 1( 0Tt

bck

tTt d S T t e

dt TS T s e dt

T 2.95s

d 10kpcT 2.95s

ρint 0.078m-3

Significance Level of discovery:Assuming that μbck is the expected number of background events then for 5σ discovery the minimum number of observed events, m, is defined as:

1

bcki e bck

i!i0

m

1 erf5

2

Time Integrated Differential Flux(d: SN distance in kpc)

number of e / cm2 / MeV

int arg0

( )( )e e

e e

e

t et

d EE dE

dE

Interaction Density

Veff nobs

ngen

Vgen

Effective Volume

5bck

bcbck bc

kkbut Tm m m T

Assuming Gaussian statistics:

Nobs T bck T bck T

S T bck T

maxGeneral Condition:

Garching Model: t0=2.35

2 ( ), ( )0

1 ( ),4 ( )

e

e ee e

T

a t E t

d L tE d f dtdE d E t

Coincidence Level (Multiplicity)

Rate

(Hz)

K40 Coincidence rate per OM Background-I: Only K40 Decays

K40 Decay Distance (m)

Even

ts p

er O

M

(arb

. Uni

ts) Multiplicity>2

Multiplicity>4

Energy(MeV)

electron

Gamma (1.46 MeV)

K40 event sample in full WPD detector (T=2.95s)

Expe

cted

Num

ber o

f K40

bac

kgro

und

Even

tsCoincidence Level (Multiplicity)

Coincidence between the small PMTs of the same OM, within 100 ns

e.g. “Coincidence Level” or “Multiplicity” 3

IBD event sample in full WPD detector for (Integration Time : T=2.95s )

OM Multiplicity (100ns coincidence window)

Signal: SN @ 10 kpc

Expe

cted

Num

ber o

f Eve

nts

Multiplicity>4

Multiplicity>6

Multiplicity>2

Interaction Point Distance (m) to OM

Arb

. uni

ts

K40

SN @ 10kpc

Inverse Beta Decay IBD :e p e n

K40 Decay Distance (m) to OM

Even

ts p

er O

M

(arb

. Uni

ts) Multiplicity>2

Multiplicity>4

Background Rejection-I: K40 Decays

OM Multiplicity > 5

Q-Threshold (Q of a Small PMT > Q–Threshold) = b

Number of Small PMTS with Q > Q–Threshold > c

OM Multiplicity (100ns coincidence window)

Expe

cted

Num

ber o

f Eve

nts

K40

SN @ 10kpc

Active OM: Selection Criteria

K40

K40

K40

SN

SN

SN

Number of PMT hits per Active OM

Q-Threshold: 1 pe

Q-Threshold: 2 pe

Q-Threshold: 3 pe

Q>Q-Threshold

Q>Q-Threshold

Q>Q-Threshold

PMT hit

Background-II: Atmopsheric muons

Εμ (GeV)

Num

ber o

f Eve

nts

Generated (MuPage)

At least 1 active OM

OM Multiplicity

K40

SN

Atmospheric μ

Num

ber o

f Eve

nts

Expected Number of Εvents in a full WPD detector (6160 Oms) in T=2.95s

K40

SN

μ

Atmospheric muons seems to be tough guys !

Background Rejection-II: Atmospheric muons

Number of Neighbor-OMs with multiplicity > 2

Num

ber o

f OM

s with

mul

tiplic

ity >

5

Active OMs due to Atmospheric Muons

Cherenkov Light

• 94% of the background due to atmospheric muons (i.e. active Oms with multiplicity >5) has at least one neighbor OM with multiplicity >2• Only 7% of the signal has at least one neighbor OM with multiplicity >2

OM Multiplicity > 5

Q-Threshold = b

Number of PMTS with Q>Q-Threshold > cNeighbors with multiplicity greater than 2 < 1

Simulate the Cherenkov Light Emision and the photoelectron production on the small PMTs photocathode due to:1. SN neutrinos (following the GARCHING Model) interacting inside the KM3 full detector and

producing positrons in the vicinity of the Oms.2. K40 decays producing electrons and gammas3. Atmospheric muons e.g. 107 fully simulated signal events, 1011 fully simulated K40 background events and 106 fully

simulated atmospheric muon tracks

Apply the signal selection criteria for each optical module and estimate:a) the number of SN induced events (OMs passing Certain Selection Criteria) for T=2.95s as a

function of the SN distanceb) the number of background induced events (due to K40 and atmospheric muons) for T=2.95s

Estimate the SN distance for which the observed number of events (on top of the expected background) corresponds to 5σ discovery, e.g.

a b c Signal @10 kpc

Backr. K40

Backr. Atm. μ

5 1 2 147.5 2.53 18.96

7

0

2 1/ 210max

~

2.53 18.96 21.49

1 5.96 10 47!

147.5 10 ~10 25.5

bck

bck

imbck

i

kpcSN

e mi

d kpckpc

5S

5SN

N

max

N 25.5

dN

24kpc

a b c dmax (kpc)

5 0 0 20.7

5 1 0 21.7

5 1 1 24

5 1 2 24

5 1 3 22

5 2 0 23.5

The background rejection criteria are based on the PMT waveform amplitude/chargeCan the Time over Threshold digitization electronics provide the necessary Information?

Pulse simulation

Pulse characteristics @ 1 p.e.Gaussian Amplitude Distribution

σ/μ 35%

Time jitter 1 ns

Rise time 2.5ns

/( ) b t cf t at e

ns

Threshold value : 0.3 μ

ToT (time over threshold)

pes

ToT

(ns)

The error-bars correspond to the RMS of the ToT distribution

K40

SN

μ

We choose the condition: ToT> 7.5 ns, to select pulses “corresponding” to more

than 1 pe

single photoelectron averaged wave form

μ

OM Multiplicity > 5ToT-Threshold > BNumber of PMTS with ToT>ToT-Threshold > c

Neighbors with multiplicity greater than 2 < 1

Dmax=23 kpc for 5 σ discovery

Background Rejection Criteria

OM Multiplicity > 5

Q-Threshold = b

Number of PMTS with Q>Q-Threshold > c

Neighbors with multiplicity greater than 2 < 1

a b c dmax (kpc)

5 1 2 24

a B c dmax (kpc)

5 7.5ns 2 23Including pulse statistics

a B c Signal @10 kpc

Backr. K40

Backr. Atm. μ

5 7.5 ns 2 118.7 < 0.8 15.4

Livermore model ~30 kpcIcecube: dmax=50 kpc (80 strings) (2000 events) 36.9 (DC) (13 events)

Summary

SN explosions within our Galaxy can be detected by an Underwater Neutrino Telescope .

Preliminary results for the Garching model without oscillations, indicate that a maximum distance of ~ 23 kpc for 5σ significance level can be reached

Further Studies• Other models for SN collapse (eg Livermore)

• Inclusion of matter and vacuum oscillations

• Improve selection efficiency and quality cut criteria

• Apply filters for atmospheric muons

• Directionality ?

• Energy sensitive observables ?