1

High-energy neutrino detection

with the ANTARES underwater

erenkov telescope

Manuela Vecchi

Supervisor: Prof. Antonio Capone

Manuela Vecchi2

Outline

Neutrinos: a short introductionMultimessenger astronomy: the new frontierNeutrino astronomy: why, how and where?The ANTARES Neutrino TelescopeReconstruction issues in ANTARESConclusions and perspectives

Manuela Vecchi3

Neutrinos in the Universeexpected and measured fluxes

Neutrinos, being slightly massive and weakly interacting particles, can travelunscattered from the source to Earth.

Manuela Vecchi4

Neutrinos in the Universe

Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered fromthe source to Earth.

Manuela Vecchi5

Neutrinos in the Universe

Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered fromthe source to Earth.

Manuela Vecchi6

Neutrinos in the Universe

Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered fromthe source to Earth.

Manuela Vecchi7

Neutrinos in the Universe

Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered fromthe source to Earth.

Manuela Vecchi8

Neutrinos in the Universe

Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered fromthe source to Earth.

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Multimessenger astronomy

High-energy photons: 0.01 - 1 Mpc

Proton s: E > 1019 eV (10 Mpc)

Protons and photons are absorbed or deflected since they interact with matter.Neutrinos can travel unscattered opening a new window on the far Universe.

μ-Quasar

Gravitational waves

Manuela Vecchi10

Production of High- Energy Neutrinos

The main (conventional) production channel is the pion decay…

p + + n/ 0 p

+ μ+ μ e+ e μ μ

0 (E ~TeV)

The outstanding observations made by the H.E.S.S. and the MAGIC -raytelescopes can be used to search for simultaneous emission of -rays and s.

The three brightest galactic TeV sources: CRAB Nebula , SNR RX J1713.7-3946 and Vela Junior seem to be within the discovery possibilities of thepresent neutrino telescopes.

Due to neutrino oscillations we have:

o e : μ : = 1: 2: 0 at the Sourceo e : μ : = 1: 1: 1 on Earth

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Neutrino Astronomy … why?

We can understand production sites and mechanisms for Neutrinos andHigh Energy Cosmic Rays.

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Neutrino astronomy … how ?

μ

43°

interaction

Seabed

erenkov cone

3DPMTarray

μ

They can be detected using the visible erenkov radiation producedas the high-energy charged lepton (final state of CC interactions) passesthrough an opaque medium with superluminal velocity.

450 m

Due to small fluxes and interaction cross section a large detection volume isrequired (~ 1 km3).

e and :electromagnetic

and hadronic

showers

μ ~1,5°

E [TeV ]

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Neutrino Astronomy … where ?

The easiest solution is to use an array of photomultiplier tubeslocated in a natural transparent medium: water/ice.

We need: a target for neutrino-muonconversiona medium for production andpropagation of erenkov radiationa shield against atmosphericmuons (background)a large detection volume

Since it’s a long way to an underwater km3 detector … let’s start with …

Manuela Vecchi14

Neutrino Astronomy … where ?

We need: a target for neutrino-muonconversiona medium for propagation ofCherenkov radiationa shield against atmosphericmuons (background)a large detection volume

The ANTARES Neutrino Telescope

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Observable sky for ANTARESTarget sources for Neutrino Telescopes are the ones “traditionally” detected (radio, IR,

visible, rays…)

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Galactic -ray sources

• SuperNova Remnants (SNR)

Detection of resolved -ray emission from shells

RX J1713-3946: Multiwavelenght analysis points tohadronic emission neutrinos

• Pulsar Wind Nebulae (PWN)

-rays emission is mainly thought to be dued toelectromagnetic processes, but hadronic modelshave also been proposed.

• Binary systems

Strong absorbtion of radiation: neutrinos fluxcould be much more abundant than the -raysone.

• Other sources

No counterpart in other wavelenghts: be open to the unexpected!!

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How does a muon look like?

A typical down-going event(atmospheric muon) and its

erenkov cone as seen in thedetector

Atmospheric neutrino candidate (10 lines-detector) seen as an upgoing event

Time [ns]

Dep

th [

m]

μ

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Research project outline

Why ?A fundamental issue for a Neutrino Telescope is the Angular Resolution,which depends on both the detector “hardware” and “software” issues. Anefficient algorithm leads to a good pointing accuracy which is fundamental inthe search for point-like sources.The event energy will help to identify an excess of neutrinos, over theatmospheric background, that can be attributed to diffuse neutrino sources inthe Universe. Energy reconstruction is a hard job, but important since it is thebest chance we have to be sure that a detected neutrino comes from anastrophysical source, at energies bigger than 1 TeV.

How ?Starting from the time-charge-position informations one can parametrise aprobability function which is linked to the best values of track parameters.Track quality cuts must be developed to enhance the quality of backgroudrejection.Energy evaluation (muon and showers) starts from MonteCarlo studies,looking for reliable estimators.

The aim of my research project is the study of reconstruction algorithms formuon tracks identification and muons and showers energy estimation.

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Muon Track Reconstruction

Arrival time and amplitude are stored for each hiton PMTs, together with their positions ( x ~10cm).

Very good precision: angular resolution of 0.2° forE>10 TeV.μ t

rack

C

C

erenkov light

erenkov light

Optical Module

ˆ q

t0,r r 0,E0

The arrival time of a photon on the OM is given by:

k

Time for the μ to reach the pointof light emission (v μ ~ c)

Time for the to propagate inwater (v = c/n)

We can define time residuals:

that is the difference between the observed hit time and the hit time expected for a “direct”photon, not scattered by the molecules in water.

Muon tracks reconstruction algorithms arebased on maximum likelihood procedures.

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Muon track reconstruction (2)The time residuals distribution contains all the instrumental and environmental

informations on the photon path in water.

Photomultiplier timeresolution t affectsthe tres distributionwidth.

Scattering of light on watermolecules + showers producedelay in the arrival time ofphotons

For each possible set of track parameters the probability to obtain theobserved events can be calculated using the likelihood function.

In case of uncorrelated hits the likelihood of the event can be written as theproduct of the likelihood of the individual hits:

P(event | track) = P(hits | ˆ q ,r r 0) = P

i

(ti | tith ,...)

• What about the background ?

• How to take it into account?

Randomnoisemodifiestails

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“Physical” Background rejection

The performance of the reconstruction algorithm depends on the quality of the“Physical” background selection criteria.

Atmospheric muons

Nearly horizontal muons

Atmospheric neutrinos

Muon bundles

Cascades

Downgoing events

Upgoing eventsZenith angle

DataMC: atmospheric s MC: atmospheric s

Track fit quality cut applied

5-lines data: 01/02 - 25/05/2007 ~ 36.8 days of active time.

trigger rate ~ 1 Hz

Integrated rates:MC Atmospheric ~ 0.1 Hz

Data ~ 0.07 Hz

These are the main sources of background events in theneutrino sample

5-lines detector

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Energy reconstructionMain energy loss processes for TeV muons:

Ionisation (logarithmically increasing with Eμ)Radiative processes (in the HE limit Eμ)

We can express muon energy loss as: dE

dX+ Eμ

: ionisationcontribution

: radiation lossescontribution

Radiative energy loss processes generate secondary charged particles alongthe μ trajectory, which also produce erenkov light.

The light produced along the muon track can give an estimate of the energy ofthe muon.

Manuela Vecchi23

Showers reconstruction

Electromagnetic or hadronic showers can be produced during the propagation of theleptons. Their optical signature is that of an expanding spherical shell of erenkovphotons with a larger intensity in the forward direction.

Their detection can give fundamental informations on the electron and tau channels,provided they are energetic enough.

Shower reconstruction can provide good energy resolution (~ 30% ), since the energydeposition is “localised” in a small region, but can give only poor information on thedirection of the original lepton (angular resolution is ~ some degrees).

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Conclusions

& Perspectives

oANTARES (area ~ 0.1 km2) is currently the largest underwater HE neutrino telescope inoperation. HE Neutrino detection in water is now known to be feasible but a largerdetection volume is needed to have a reasonable detection potential.

o Reconstruction is a critical issue: muons identification, together with their energy, is ofcrucial importance to disentangle signal from “physical” background (atmospheric muonsand atmospheric neutrinos). Data and MC agreement start to be good but not yetcompletely understood.

o Showers detection can give important informations on tau and electrons even if theirdetection is not the main target for such a detector.

oThe capability of the km3 telescopes to open the new window of high-energyneutrino astronomy is good, further improvements are needed to explore Ultra HighEnergy domain (acoustic and radio techniques).

Manuela Vecchi25

Conclusions

& Perspectives

oANTARES (area ~ 0.1 km2) is currently the largest underwater HE neutrino telescope inoperation. HE Neutrino detection in water is now known to be feasible but a largerdetection volume is needed to have a reasonable detection potential.

o Reconstruction is a critical issue: muons identification, together with their energy, is ofcrucial importance to disentangle signal from “physical” background (atmospheric muonsand atmospheric neutrinos). Data and MC agreement start to be good but not yetcompletely understood.

o Showers detection can give important informations on tau and electrons even if theirdetection is not the main target for such a detector.

oThe capability of the km3 telescopes to open the new window of high-energyneutrino astronomy is good, further improvements are needed to explore Ultra HighEnergy domain (acoustic and radio techniques).

Manuela Vecchi26

Conclusions

& Perspectives

oANTARES (area ~ 0.1 km2) is currently the largest underwater HE neutrino telescope inoperation. HE Neutrino detection in water is now known to be feasible but a largerdetection volume is needed to have a reasonable detection potential.

o Reconstruction is a critical issue: muons identification, together with their energy, is ofcrucial importance to disentangle signal from “physical” background (atmospheric muonsand atmospheric neutrinos). Data and MC agreement start to be good but not yetcompletely understood.

o Showers detection can give important informations on tau and electrons even if theirdetection is not the main target for such a detector.

oThe capability of the km3 telescopes to open the new window of high-energyneutrino astronomy is good, further improvements are needed to explore Ultra HighEnergy domain (acoustic and radio techniques).

Manuela Vecchi27

Conclusions

& Perspectives

o ANTARES (area ~ 0.1 km2) is currently the largest underwater HE neutrino telescopein operation. HE Neutrino detection in water is now known to be feasible but a largerdetection volume is needed to have a reasonable detection potential.

o Reconstruction is a critical issue: muons identification, together with their energy, is ofcrucial importance to disentangle signal from “physical” background (atmospheric muonsand atmospheric neutrinos). Data and MC agreement start to be good but not yetcompletely understood.

o Showers detection can give important informations on tau and electrons even if theirdetection is not the main target for such a detector.

o The capability of the km3 telescopes to open the new window of high-energyneutrino astronomy is good, further improvements are needed to explore Ultra HighEnergy domain (acoustic and radio techniques).