Neutrino Astronomy:

Seeing the Cosmos in a Light

Matthew MalekImperial College London

Advances in Astronomy17-April-2010

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Outline

• Introduction– Ways of looking at the sky– What is this neutrino thing, anyway?

• What has been done in astronomy with neutrinos?– Solar neutrinos– Supernova 1987a

• What are we doing now?– Supernovae: Bursts and relics– Point sources: AGN, GRB, etc.– High energy neutrino astronomy

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How Do We Look At The Sky?

• For most of our history, humanity could only observe space via visible light…

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How Do We Look At The Sky?

• Then came other messengers: – Infrared, X-ray, Microwave, Gamma rays, et cetera…

Hale-Bopp in IR (Palomar)

CMB (WMAP 2008)

SNR in Centaurus (Chandra)

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Halzen, Ressell & Turner

e+e-

Radio

CMB

Visible

GeV

-rays

IRB

X-rays

Astrophysics with cosmic rays

Astrophysics with photons

Protons @1021 eV point as far as 40Mpc

How Else Can We Look?

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What Is This “Neutrino” Thing?

…and why do we care?

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• 1910s - 1920s: Studies of nuclear β decays N1 → N2 + e-

Did not appear to conserve energy!

• 1930: Wolfgang Pauli postulated Neutrinos in order to save energy conservation

N1 → N2 + e- + “I have done a terrible thing. I have postulated a

particle that cannot be detected”

has no charge, no mass, very feeble interaction, just a bit of energy

• 1956: finally discovered by Cowan and Reines.

Used nuclear reactor as source of neutrinos. Nobel prize 1995

A Brief History of Neutrinos

nuclei electron

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Neutrino Interactions only interact ‘weakly’ – how weak is this? mean free path

(i.e., average distance travelled before interacting) is:

~ 1 light year of lead!• 1 light year ~ 1016 m = 10,000,000,000,000,000 m

Interaction

n + e→ p + e –

Mediated by W boson

neutronu u

d d

d(-1/3) u(2/3)

W

e –

e

proton

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Cosmic GallNeutrinos they are very small.

They have no charge and have no massAnd do not interact at all.

The earth is just a silly ballTo them, through which they simply pass,

Like dustmaids down a drafty hallOr photons through a sheet of glass.They snub the most exquisite gas,Ignore the most substantial wall,

Cold-shoulder steel and sounding brass,Insult the stallion in his stall,

And, scorning barriers of class,Infiltrate you and me! Like tall

And painless guillotines, they fallDown through our heads into the grass.

At night, they enter at NepalAnd pierce the lover and his lass

From underneath the bed – you callIt wonderful; I call it crass.

– by John Updike (1960)

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Why Study Neutrinos?• Second only to the photon in abundance

• Produced in the Big Bang in numbers comparable to photons

• Neutrinos are crucial to understanding how the Sun shines

• Neutrinos provide a unique window into exploding stars (supernovae)

• Neutrino astronomy: used to study distant objects

• Recent surprise: neutrinos have non-zero mass! We don’t know what the mass is but it is less than:

0.00000000000000000000000000000001 g

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• Atmospheric – from cosmic rays

• Artificially created (reactors, accelerators)

• Natural background radiation (from rocks, etc.)

• Solar – from nuclear reactions within the sun

• Supernovae – core collapse of massive stars

• Cosmic background – relics from Big Bang

• Other sources: AGNs? GRBs?

Sources of Neutrinos

2002002 Nobel Physics prize! Ray Davis & Masatoshi Koshiba share with Riccardo Giacconi

for “pioneering contributions to astrophysics”.

}

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Solar Neutrinos:

Dawn of a Era!

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What Makes The Sun Shine?

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What Makes The Sun Shine?

15In The Mine, But Looking At The Stars…

• First solar neutrino detector:• Homestake mine, S. Dakota• Ray Davis, Brookhaven • 1967 – 1998• 615 tons of C2Cl4

(cleaning fluid!)• “Radiochemical” detector:

e + 37Cl → 37Ar* + e-

Good News: First discovery of solar !

Bad News: Far fewer than anticipated!

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Some Questions RemainQ1: How do you know the neutrinos came from the sun?

A: Need a differenttype of neutrino telescope!

• Cherenkov detectors find via emitted light

• Can be water, ice, salt…

• Some directional information is preserved

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Directionality Is Key

e

22,385 Solar events(14.5 events/day)

The Sun (seen in neutrino “light”)

The KamiokaNDE detector (MasatoshiKoshiba) first to prove seen from Sun

18Water Filling At Super-Kamiokande

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What About The Missing ?

71±671+7- 6

71Ga37Cl GALLEX(+GNO)

Homestake

128+9-7 SNU

0.47±0.02

H2O SuperKKamioka

0.55±0.08

1.0 +0.20- 0.16

+1.3- 1.1

2.56 ±0.23

7.6 SNU

D2O

0.35±0.02

SAGE

Theory7Be

8B

Experiments

C.C. N.C.

1.01±0.13

pp

CNO

Q2: The so-called “Solar Neutrino Problem” (1967 - 2001)

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Q3: …but what does this teach us about the Sun??

Neutrino telescopes give us a look inside the sun• Photons (light) take about

1,000,000 years to leave• Neutrinos exit “instantly”

Based on solar , we know:1. Fusion powers the Sun

2. SSM originally verified only by (later aided by helioseismology)

3. “pp” neutrinos strongly correlate with solar light output

4. Other, rarer, types give different information

For instance, from 8B solar measurements, we know the temp.

at the core of the Sun is: 1.5 x 107 K ± 1%

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Supernova Neutrinos:

Things That Go BOOM In The Night

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Supernova Progenitors

Main Sequence

H core

RedGiant

He core + H shell

Supergiant

C & O coreHe & H shells

Carbondeflagrationsupernova

CoreCollapse!

“Onion” Shells(H,He,C,O,Ne,Si,Fe)

Accreting White Dwarf

m > 8 M?

Images taken from:http://astron.berkeley.edu/~bmendez/ay10/2000/cycle/cycle.html

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Supernova Classification

NOTE:

Spectral class ≠Mechanism

Classify by spectral lines:

GotHydrogen?

Type IISupernova

Type ISupernova

YES

NO YES

NO

Type IaSupernova

GotHelium?

NOYESType Ic

SupernovaType Ib

Supernova

(Got Silicon?)

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Supernova Neutrino Emission:Start of Collapse

• Electrons captured on nuclei produce e via:

e– + A(N,Z) → e + A(N+1,Z-1)

• Mean free path of neutrinos > core size• Neutrinos escape promptly

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Supernova Neutrino Emission:Neutrino Trapping

• Core density increases as collapse continues

• Mean free path of shrinks w/ increasing density

• Neutrinos trapped by scattering off nuclei:

+ A(N,Z) → + A(N,Z)

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Supernova Neutrino Emission:Shock Wave Formation

• Inner core reaches nuclear densities• Neutron degeneracy halts gravitation attraction• Inner core rebounds, causing shock wave• Shock wave propagates through infalling outer core

• Larger -sphere; s still emitted from outer core

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Supernova Neutrino Emission:Neutronization Burst

• Shock slows infalling matter and separates nucleons• Shock loses energy (8 MeV) per dissociated nucleon

→ eventually stalls (revives how?)

• Electrons captured on dis. protons produce e via:

e– + p → e + n

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Supernova Neutrino Emission:Neutrino Cooling

• Egrav → Etherm, about 1046 Joules• T 40 MeV 500,000,000,000 K

• (Room temperature = 300 K 1/40 eV)

• Proto-neutron star cools, producing • Unlike previously, all 6 types are generated

• Neutron star (or black hole?) left behind

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What Can Teach Us About Supernovae?

• 99% of the energy from a core-collapse supernova is released as neutrinos

• emitted during SN, giving unique insight into the process of a supernova & neutron star formation

• carry information direct from core; no scattering!

• Only 1% energy appears as (+ tiny fraction as kin. energy)

• Light () emitted hours later, largely from decay of radioactive elements produced in the supernova’s shock wave

• scatters in dense, turbulent gas, losing information about its source

Neutrinos () Photons ()

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Finding Supernovae Neutrinos

• To date, only SN burst came from Sanduleak -69o 202 in Large Mag. Cloud

• Spotted on 23-Feb-1987, it is now more famously known as Supernova 1987a

• 19 (or 20) SN neutrinos seen in two water Cherenkov experiments:

• 11 (or 12) at KamiokaNDE• 8 at the competing IMB

• Hundreds of papers written analysing these few neutrinos!

• Today, a SN burst from the galactic centre (10 kpc) could provide up to 10,000 events!

• Additionally, because are emitted first, they can be a useful early warning system for astronomers. SNEWS exists to alert astronomers of a nearby supernova.

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Finding Supernovae Neutrinos

Problem:Cannot predict when next SN burst arrives!→ Waiting > 20 years

Semi-Solution: never stop moving… so the cosmos should be filled with a diffuse background of from all the supernovae that have ever exploded! → Look for it whilst waiting!

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Supernova Relic Neutrinos

Solar 8B

Solar hep

Atmospheric e

SRN predictions

• SRN should be an isotropic background composed of from all SN explosions

• Predictions obtained by taking spectrum from single SN and redshifting according to SN rate • Natural energy window to search • Massive stars – with relatively short lives – die in core-collapse

• Thus, SN rate is a good tracker of star formation rate!

→ Birth of cosmology??

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SRN Search Results

Atmospheric e

Decay electrons

Total background(Atm. + decay e)

• SRN signal would manifest as distortion of BG• No such signal seen yet → some models ruled out• The search continues!

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The Expanding Universe of

Neutrino Astronomy:

Other Topics & Observatories

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Other Sources of Cosmic Thus far, only source of extra-solar is SN1987a.

Other possible types include:

• High E: Collisions of galactic cosmic rays

produce ±, which decay into (& other things…)

• Ultra-High E: From collisions of extra-galactic cosmic rays (see slide 5 and last year’s talk)

• Ultra-Low E: Relics from the Big Bang, with temperature of 1.9 K (equiv. E = 1.7×10−4 eV)

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Looking for Cosmic There are many different neutrino telescopes.

An [incomplete] list includes:

• High E: AMANDA, ANTARES, NESTOR, ICECUBE

• Ultra-High E: ANITA, GLUE, RICE, SALSA, Pierre Auger Observatory

• Ultra-Low E: No current experiments.

(Energy is too low for detection w/ current tech.)

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High Energy Cosmic • Likely to correlate to point sources, such as

Gamma Ray Bursts, Active Galactic Nuclei, etc.• Searches by Super-K, MACRO, etc. find nothing...

GRB 080916C imaged by Fermi LAT

• A typical search involves a catalog (e.g., BATSE)• Check for an excess of events around the time of the GRB

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High Energy Cosmic :New Dedicated Observatories

ANTARES[*] is located in the Mediterranean.It uses 885 eyes, in strings 450 m high to search for upgoing high energy cosmic

[*] Astronomy with a Neutrino Telescope and Abyss Environmental RESearch

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The Sky In High Energy Cosmic

• Each point shows one event in ANTARES• Downgoing events cut to remove cosmic rays• Since 2006, all events consistent with atmospheric

– Thus far, no cosmic sources found…

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High Energy Cosmic :New Dedicated Observatories

• IceCube is an ice Cherenkov observatory at the South Pole, covering 1 km3 of ice

• It replaces and incorporates the former AMANDA-II expt.

• Again, galactic map shows no sign of sources… yet!

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Ultra-High Energy Cosmic • UHE intrinsically interesting,

if discovered– Where do they come from?– What process creates them?

• Unusual detection techniques– GLUE: Uses lunar limb as

target and searches for radio emission

– ANITA: Flies in a balloon over Antarctica and looks for radio pulses in the ice

– Pierre Auger Observatory: Uses the Andes as target. Searches for horizontal events with high EM component

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Ultra-High Energy Cosmic :Current Search Results

• Again, no sources discovered (yet)• GZK seem a “guaranteed” source,

from cosmic rays colliding with CMB• Wait and see…

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Summary• Neutrino astronomy has opened up a fascinating new

window for looking at the cosmos• Solar neutrinos are well established and have taught us

much about stellar astrophysics• Supernova neutrinos have given us a glimpse into the

death of massive stars and the formation of neutron stars; we are ready and waiting for the next burst!

• Supernova relic neutrinos must exist. When found, they will open the door to “Neutrino Cosmology.” Exciting!!

• Many high energy neutrino telescope coming online now• Ultra-high energy neutrinos remain elusive.

Check back in one, five, ten years…

→ Extremely interesting time to be doing astronomy!