High Energy Neutrino Telescopes

E. Chaniotakis

What is a neutrino??• Neutrino is a nearly massless elementary particle. It belongs in the family of leptons

• It has no electric charge

•It interacts veeeeeery weakly with matter

…….Anything else?

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• Neutrinos, like charged leptonscome in three flavours. - Electron neutrino- Muon neutrino-Tau neutrino

The meaning of the above statement Is that when (e.g.) a muon decaysit will always produce a muon neutrino –

It also goes the other way round:When a muon neutrino interacts, there will ALWAYS be a muon in the final state

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How do we know all these?

W.Pauli: He proposed the existence of the neutrino to explain the apparentEnergy non-conservation in beta decay (1930)

Cowan & Reines discovered the electron antineutrino (1956)

Steinberger, Schwartz, Lederman et.aldiscovered the muon neutrino (1957)

DONuT collaboration in FermilabDiscovered the tau neutrino (2001)

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LEP experiment at CERN discovers that there areonly 3 families of light neutrinos (1989)

Super Kamiokande discovers neutrinooscillations (1998) => proves that neutrinos have mass

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So far so good. But what have all these got to do with astrophysics?

All stars produce neutrinos in the nuclear reactions taking place within their core.

Explosive events in the universe (e.g supernovae) also produce neutrinos SN 1987 A’ was observed in neutrino light by IMB, Kamiokande collaborations

Neutrinos are expected to be produced in cataclysmic phenomena in the cosmos (GRBs, AGNs etc..)

Super-K neutrinography of the sun

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After all…

• Neutrinos have zero electric charge -> They are not deflected by galactic magnetic fields• They are stable• They have low interaction cross section ->They are not absorbed by matter (gamma ray case) 6

Neutrinos probe a new energy domain unexplored by conventional optical, X-Ray and Gamma Ray astronomy

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EXTRATERRESTIAL NEUTRINO PRODUCTION

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PHYSICS CASE

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Neutrino Spectra

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DETECTOR REQUIREMENTS Detector has to be huge in order to compensate low astrophysical

neutrino flux and low neutrino-nucleus cross section

The layout must be as simple and cheap as possible: “a poor man’s detector”

- One should use as both target and detector active material, one abundant in nature -> Water and ice!

- The detection principle and technology must be well understood -> Cherenkov radiation detection from the by-products of the neutrino interaction

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A back of the envelope calculation

Assuming a km3 detector, with 1y exposure we shall calculate the 100TeV neutrino interaction rate in our setup.

-Neutrino flux with respect to energy:

-Neutrino-nucleon cross section:

11 2 1 12 2

1002*10 * sec

( )

dNcm TeV

dE eV

2396.7*10 [ ]cmGeV nucleon

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Substituting for Eν=100TeV , we have:

Ν= (Neutrino Flux)*(Nr of Targets)*(Detection Time)*

(Cross Section) =11 2 1 38 7 34 2(2*10 sec )*(6.015*10 )*(3.11*10 sec)*(6.7*10 / ) 250.7cm nucleons cm nucleon

One expects 250 events/yr/km3 for 100TeV neutrinos

Taking into account the detector and reconstruction efficiencyThis number falls to ~10 events!!! (Best Case Scenario)

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Detection principle – Measurement of Cherenkov Radiation

1cos

( )c n

2 2 2

2 1(1 )

( )

dN

dxd n

2

2(1 )

u m

c E

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The detector’s “eye”

- Large photocathode (10-15’’)- Low dark noise rate- High photocathode uniformity- High quantum efficiency- Low TTS (Transit time spread)

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PMTs are sealed in a high pressure resistant vessel, and are equipped with mu-metal cage (magnetic field absorbing material), manometer and optical coupling.This is called an Optical Module (OM)

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Operational Neutrino Telescopes

(PreProduction Phase)

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Particle Identification

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Simulated muon in IceCube : The “Golden Channel”

Simulated tau Simulated electron

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A real up-going event

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Critical parameters

Angular Resolution

Is defined as the median angle between a MC generated and the corresponding reconstructed muon.

Depends on water/ice clarity, detection unit spacing, track length. Since the ultimate goal is to measure neutrino point sources, good angular resolution is essential.

It decreases as energy increases to do 0.1° astronomy you must go to > TeV energies

If for example one has a potential neutrino point source then applies a cone around it comparable to its angular resolution in order to search for some excess above the neutrino background in the cone.

Smaller cone less atm. Nu’s getting in Better Signal/Noise.

In short: Angular Resolution better than 0.1deg->REAL ASTRONOMY!!!

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A standard yet beautiful angular resolution measurement (IceCube)

• Measure the moon shadow in the C.R. muon sky!

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Effective Area:

The key advantage of a neutrino telescope is that it can detect particles OUTSIDE its instrumented volume. If one assumes a detector with cross section L² and a muon, with range Rμ propagating vertically to the center of the detector from a distance < Rμ, then the effective detection volume is Rμ*L²

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One defines the Neutrino Effective Area as the Ratio:

Aeff = (Reconstructed neutrino rate)/ (IncomingFlux) Effective area is energy and zenith dependent and also depends on detector and environmental effects.

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Sky Coverage

In order to achieve 4π sky coverage, one must instrument at least two complementary telescopes, one near the equator (e.g: Mediterranean) and one in the South Hemisphere

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Energy Resolution

In neutrino telescopes muon energy is measured by the number of photons coming from Cherenkov radiation per unit length. This way one obtains the mean dE/dx of the muon and matches it to an energy value using the relationship :

dE/dx = a +b*E (E>1 TeV)

For muons one compares this number with the Nr. Of photons for a minimum ionizing muon and multiplies with 1 GeV/m

The process used to measure energy is stochastic (bremsstrahlung, pair production etc) poor energy resolution : Two similar muons deposit different amounts of energy in the detector

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•From the above plot we see that the main contributions to the muon energy loss are from bremsstrahlung and photonuclear interactions. These processes are stochastic and therefore the Nr of photons per GeV bin is not constant. This results in poor energy resolution.

•Furthermore, since a high energy neutrino telescope mainly detects through-going muons, one cannot unfold their energy to their parent neutrino’s with high accuracy. (Poor neutrino vertex resolution). This leads to logarithmic sensitivity in energy measurement-> order of magnitude estimate.

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Below we can see the predicted dE/dx for muons at IceCube resulting from simulation. Muons originate from neutrinos with spectrum at the range 10GeV<Eν<10EeV.

1Ev

About 5.4% of muons have their energy overestimated by a factor ~5!!!!!!30

Detector Optimization

Main particle backgrounds in neutrino telescopes: Atmospheric muons & neutrinos.

Atmospheric neutrino flux remains the same regardless of depth

Downcoming muon flux attenuates as the slant depth they propagate increases.

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Option.1: Turn your OMs downwards!

problem I : you lose events near or above the horizon (i.e : Ultra High Energy Neutrinos) problem II: There is a fraction of downcoming muons erroneously reconstructed as upcoming.

Option.2: Go as deep as possible!

Solution I: Since muon flux scales with depth, you can turn your OMs upwards too. Therefore you can have access to the high energy horizon and above ( Typically: ANTARES can reconstruct up to 5º, NEMO up to 8º andNESTOR up to 10º above the horizon. )

Solution II: Since the misreco. Muons are a fraction of the downcoming muons, they scale with depth too. Therefore they become less as depth increases.

Option.3 : Apply quality cuts

Any physics analysis that respects itself has to use track fit quality cuts to ensure the purity of their signal and maximum background rejection. These cuts include timing, charge and maximum nr of OMs used for reconstruction criteria

Atmospheric Muon Background How to reduce it

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Muon Flux VS Slant Depth

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Backgrounds altogether

Downgoingmuons

Downgoing muons misreconstructed as upgoing

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( Taken by J.P Ernewein: WP2 Session, March 2009, Paris )

Comparison of effective areas of downgoing mu’s between two depths: (4500m) & (3500m)

As depth increases we have more downcoming muonsi.e less of them are misreconstructed as upgoing.

As we use more OMs for thereconstruction: differencedecreases.

Between the two depths,(3500) has ~10%more misreco. than (4500)

37(Declination and water quality not taken into account in this plot)

PRELIMINARY

In a point source search one has to reduce atm. neutrino background: Needs excellent angular resolution

- Kinematic considerations

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Atmospheric Neutrino background: IRREDUCIBLE (?)

However, earth is opaque to neutrinos above ~10PeV due to the rising neutrino-nucleon cross section

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Zenith-energy neutrino distribution for a neutrino telescope

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Environmental parameters Depth: The deployment depth plays major role in selecting a

candidate site for a neutrino telescope in order to filter out atmospheric muon background

- IceCube: 2500m

- Baikal: 1100m

- ANTARES: 2475m

- KM3NeT: ->NESTOR site: 4500m -> NEMO site: 3500m -> ANTARES site: 2475 m

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Ice: BubblesCherenkov photons are heavily backscattered and their tracks randomized after 3-5 m (compared to 25m scattering length in bubble free ice). This way, all timing information is lost as one can see below:

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Water:

Sedimentation Sedimentation is a major problem for marine neutrino telescopes and has

to be carefully monitored before starting a long term measurement. For example, at ANTARES’s site the photocathode for an up-looking OM gets ‘blind’ in a few months’ time

Optical background

- K-40 decays:In marine environment, due to the salinity of water there is finite abundance of beta-radioactive Potassium 40. The decay positrons are above the Cherenkov threshold, thus producing inevitable noise in the detector. This noise is of the order 60kHz in the Mediterranean at single photoelectron level. Therefore, in order to measure neutrino tracks one has to employ offline coincidence techniques.

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- Bioluminescence: comes from microbacteria which happen to be near the detector during data taking. This kind of background has a ‘burst’ signature of 1-10 sec. duration with high (MHz) rate . This kind of background has seasonal behaviour, and varies with depth. In the Mediterranean it has a site dependence too. In the NESTOR (Greece- Pylos) site, the bioluminescence is the lowest compared to the ANTARES (France-Toulon) and NEMO (Italy- Cappo Passero) sites, inducing 1.1% dead time to the detector

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SELECTED RESULTS

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The first PeV cascades ever recorded!!!

Energies estimated to be in the 1-10 PeV range!46

More IceCube cascades

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Neutrino Oscillations

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Typical Skyplot (IceCube)

Arrow indicates possible neutrino point source candidate with small excess above the atmospheric background

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Nu’s from GRBs (IceCube)

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Cosmic Ray spectrum up to 30PeV (IceCube & Ice Top)

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Conclusions High Energy Neutrino Astronomy is after all a reality, ~50

years after its conception and ~40 years after the first neutrino telescope proposal!

IceCube detected the first Ultra High Energy Neutrinos ever observed

Existing neutrino telescopes have started producing data, while a new major effort has started almost 5 years ago to construct a multi km3 neutrino telescope in the Mediterranean (KM3NeT), incorporating NESTOR, NEMO, and ANTARES projects.

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THANK YOU!!

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