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Neutrino Astronomy: Seeing the Cosmos in a Matthew Malek Imperial College London Advances in Astronomy 17-April-2010 Light 2 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 3 How Do We Look At The Sky? • For most of our history, humanity could only observe space via visible light… 4 How Do We Look At The Sky? Hale-Bopp in IR (Palomar) SNR in Centaurus (Chandra) • Then came other messengers: – Infrared, X-ray, Microwave, Gamma rays, et cetera… CMB (WMAP 2008) 5 Astrophysics with cosmic rays GeV -rays Astrophysics with photons Protons @1021 eV point as far as 40Mpc X-rays Visible IRB CMB Radio How Else Can We Look? e+e- Halzen, Ressell & Turner 6 What Is This “Neutrino” Thing? …and why do we care? 7 A Brief History of Neutrinos N1 → N2 + e- • 1910s - 1920s: Studies of nuclear β decays Did not appear to conserve energy! nuclei electron • 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 8 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 neutron Interaction n + e→ p + e – u u d d d(-1/3) proton u(2/3) W e– Mediated by W boson e 9 Cosmic Gall Neutrinos they are very small. They have no charge and have no mass And do not interact at all. The earth is just a silly ball To them, through which they simply pass, Like dustmaids down a drafty hall Or 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 fall Down through our heads into the grass. At night, they enter at Nepal And pierce the lover and his lass From underneath the bed – you call It wonderful; I call it crass. – by John Updike (1960) 10 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 11 Sources of Neutrinos • 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? 2002 Nobel Physics prize! 200 Ray Davis & Masatoshi Koshiba share with Riccardo Giacconi for “pioneering contributions to astrophysics”. 12 Solar Neutrinos: Dawn of a Era! 13 What Makes The Sun Shine? 14 What Makes The Sun Shine? In 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* + eGood News: First discovery of solar ! Bad News: Far fewer than anticipated! 15 16 Some Questions Remain Q1: How do you know the neutrinos came from the sun? A: Need a different type of neutrino telescope! • Cherenkov detectors find via emitted light • Can be water, ice, salt… • Some directional information is preserved 17 Directionality Is Key The KamiokaNDE detector (Masatoshi Koshiba) first to prove seen from Sun e 22,385 Solar events (14.5 events/day) The Sun (seen in neutrino “light”) Water Filling At Super-Kamiokande 18 What About The Missing +0.20 1.0 - 0.16 7.6 +1.3 SNU 19 ? 1.01±0.13 +9 128 -7 SNU - 1.1 Experiments +7 71 - 6 0.55±0.08 Theory 7Be pp 71±6 8B CNO 0.47±0.02 2.56 ±0.23 37Cl Homestake Kamioka 0.35±0.02 H2O SuperK C.C. D2O N.C. SAGE 71Ga GALLEX (+GNO) Q2: The so-called “Solar Neutrino Problem” (1967 - 2001) 20 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% 21 Supernova Neutrinos: Things That Go BOOM In The Night 22 Supernova Progenitors Main Sequence Accreting White Dwarf Carbon deflagration supernova Supergiant H core m > 8 M? Red Giant C & O core He & H shells He core + H shell Images taken from: http://astron.berkeley.edu/~bmendez/ay10/2000/cycle/cycle.html “ Onion” Shells (H,He,C,O,Ne,Si,Fe) Core Collapse! 23 Supernova Classification Classify by spectral lines: Type II Supernova Got Hydrogen? Type I Supernova (Got Silicon?) NOTE: Spectral class ≠Mechanism Type Ib Supernova Type Ia Supernova Got Helium? Type Ic Supernova 24 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 25 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) 26 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 27 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 28 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 What Can Teach Us About Supernovae? 29 Neutrinos () • 99% of the energy from a core-collapse supernova is released as neutrinos Photons () • Only 1% energy appears as (+ tiny fraction as kin. energy) • emitted during SN, giving unique insight into the process of a supernova & neutron star formation • Light () emitted hours later, largely from decay of radioactive elements produced in the supernova’s shock wave • carry information direct from core; no scattering! • scatters in dense, turbulent gas, losing information about its source 30 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. 31 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! 32 Supernova Relic Neutrinos • 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 Solar 8B Solar hep Atmospheric e SRN predictions • 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?? 33 SRN Search Results Total background (Atm. + decay e) Decay electrons Atmospheric e • SRN signal would manifest as distortion of BG • No such signal seen yet → some models ruled out • The search continues! 34 The Expanding Universe of Neutrino Astronomy: Other Topics & Observatories 35 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) 36 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.) 37 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... • A typical search involves a catalog (e.g., BATSE) • Check for an excess of events around the time of the GRB GRB 080916C imaged by Fermi LAT High Energy Cosmic : New Dedicated Observatories 38 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 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… 39 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! 40 41 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 : Current Search Results Ultra-High Energy Cosmic • Again, no sources discovered (yet) • GZK seem a “guaranteed” source, from cosmic rays colliding with CMB • Wait and see… 42 43 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!