Ice-fishing for Cosmic Neutrinos

1. Ice-fishing for Cosmic Neutrinos Subhendu Rakshit TIFR, Mumbai

2. Goals of neutrino astronomy • Astrophysics: To explore astrophysical objects like AGN or GRBs. Find out sources of high energy cosmic rays. Main aim.. • Particle physics: To explore beyond standard model physics options which may affect neutrino nucleon cross-sections at high energy. Other possibilities… Appeared in US particle physics roadmap! First step: To determine the incoming neutrino flux

3. Astrophysical motivations • Historically looking at the same astrophysical object at different wavelengths revealed many details regarding their internal mechanisms • A 3-pronged approach involving conventional photon astronomy, cosmic ray astronomy and neutrino astronomy will yield better results

4. Conventional astronomy with photons • Ranges from 104 cm radio-waves to 10-14 cm high energy gamma rays • Pros: • Photons are neutral particles. So they can point back to their sources • photons are easy to detect as they interact electromagnetically with charged particles • Cons: • Due to the same reason they get absorbed by dust or get obstructed • Very high energy photons on its way interact with cosmic microwave background radiation and cannot reach us

5. Cosmic ray astronomy • Very high energy cosmic rays (protons, heavy nuclei,..) do reach us from the sky • It is difficult to produce such energetic particles in the laboratory • It is puzzling where they are produced and how they get accelerated to such energies!! • Although they can be detected on Earth, it is not possible to identify the sources as their paths get scrambled in magnetic fields  A serious disadvantage! • Only very high energy(>1010 GeV) cosmic rays point back to their sources

6. Neutrino astronomy • The suspected sources of very high energy photons and cosmic rays are believed to be the sources of neutrinos as well • Pros: Neutrinos being weakly interacting reaches Earth rather easily • Cons: Due to the same reason it also interacts rarely with the detector material ⇒ Large detector size!! • Successful neutrino astronomy with the sun and supernova. Now it is time to explore objects like Active Galactic Nuclei or Gamma Ray Bursts • Impressive range for future neutrino telescopes: 102 GeV to 1012 GeV!

7. Neutrino detectors Underground Air shower Underwater / ice GeV TeV PeV EeV 1 PeV = 106 GeV 1 EeV = 109 GeV

8. Why a Km3 detector? • Estimations of the expected amount of UHE neutrinos can be made from the observed flux of cosmic rays at high energies. This limits the size of the detector • However such estimations are quite difficult as many assumptions go in • There can be hidden sources of neutrinos!! • So the neutrino flux can always be higher!

10. A Km3 detector • PMTs detect Cherenkov light emitted by charged particles created by neutrino interactions IceCube • 1KM^3  The Cherenkov cone needs to be reconstructed to determine the energy and direction of the muon

11. Used for calibration, background rejection and air-shower physics - The predecessor of IceCube

12. IceCube is optimised for detection of muon neutrinos above 1 TeV as: • We get better signal to noise ratio • Neutrino cross-section and muon range increases with energy. Larger the muon range, the larger is the effective detection volume • The mean angle between muon and neutrino decreases with energy like 1/√E, with a pointing accuracy of about 1◦at 1 TeV • The energy loss of muons increases with energy. For energies above 1 TeV, this allows us to estimate the muon energy from the larger light emission along the track

13. Detection strategy • Cosmic rays produce muons in our atmosphere, which can fake a neutrino-induced muon signal  background • So we use the Earth to filter them out! • Upto PeV neutrinos can cross the Earth to reach IceCube • For high energy neutrinos Earth becomes opaque as the probability that the neutrinos will interact becomes higher with  energy • So very high energy neutrinos can reach Icecube only from the sky or from horizontal directions!  IceCube

14. Sources of neutrinos • Signal: The neutrinos from astrophysical sources: AGN or GRBs for example • Background: Atmospheric neutrinos. They are produced from cosmic ray interactions with the atmosphere  A guaranteed flux well measured in AMANDA. Agrees with expectations. As the ATM  flux falls rather rapidly(∝ E-3) with energy, at higher energy we can observe the ‘signal’ neutrinos from AGN or GRBs free of these background neutrinos

15. Neutrino spectra Note: At higher energies the flux is smaller. But higher energy neutrinos also have higher cross-section. So detection probability is also higher!

16. Another background • Cosmogenic or GZK neutrinos: UHE cosmic ray protons interact with CMBR photons to produce these neutrinos via charged pion decay However at IceCube the rate would be quite small

17. Eliminating backgrounds • Energy cuts • Directional cuts • Directional signals • Temporal considerations

18. Delving into the details...

19. Production at astrophysical sources: Initial flavour ratio • Propagation through space: Massive neutrinos undergo quantum mechanical oscillations. So neutrinos reach Earth with a flavour ratio • Propagation through the Earth: Neutrinos while propagating may interact with the Earth. CC or NC interactions. τpropagation is more elaborate:τ→τ→τ→τ... • Detection at IceCube: Muon neutrinos produce muons via CC interactions. All neutrinos produce showers through NC interactions. A CC interaction by a τmay produce spectacular signatures!

20. Production at astrophysical sources: A proton gets accelerated and hits another proton or a photon. They produce neutron, π+andπ0.Their decay produces cosmic rays, neutrinos and photons respectively p +  → π+ + n p +  → π0 + p

21. Propagation through space: • For massive neutrinos flavour and mass eigenstates are different. This implies that a neutrino of a given flavour can change its flavour after propagating for sometime! For example: µ ↔ e Neutrino oscillation At time t=0, we produce a e After sometime t, the mass eigenstates evolve differently So the probability of detecting another flavour is nonzero

22. At source • Now remember the initial flavour ratio at source was • Recent neutrino experiments have established that neutrino flavour states µ and τmix maximally • Hence it is of no wonder that after traversing a long distance these two states will arrive at equal proportions • Note that although there were no tau neutrinos at the source, we receive them on Earth! On Earth

23. Propagation through the Earth: • While traversing through the Earth, neutrinos can undergo • a charged current(CC) interaction with matter. The neutrino disappears producing e or mu or tau. The dominant effect • or a neutral current interaction(NC) with matter. The neutrino produces another neutrino of same flavour with lower energy • As a consequence, the number of neutrinos decrease as they propagate through the Earth. • This depends on the energy of the neutrino. Higher energy neutrinos get absorbed more, their mean free path is smaller

25. µdetection • Muons range: few Kms at TeV and tens of Km at EeV • The geometry of the lightpool surrounding the muon track is a Km-long cone with gradually decreasing radius • Initial size of the cone for a 100TeV muon is 130m. At the end of its range it reduces to 10m. • The kinematic angle of µ wrt the neutrino is µ is 1◦/√(E/1TeV)and thereconstruction error on the muon direction is on the order of 1◦ • Better energy determination for contained events. More contained events at lower energy

26. ~ Km long muon tracks from µ ~ 10m long cascades from e, τ

27. edetection • In a CC interaction, a edeposits 0.5-0.8% of their energy in an EM shower initiated by the electron. Then a shower initiated by the fragments of the target • The Cherenkov light generated by shower particles spreads over a vol of radius 130m at 10TeV and 460m at 10EeV. Radius grows by ~50m per decade in energy • Energy measurement is good. The shower energy underestimates the neutrino energy by a factor ~3 at 1 TeV to ~4 at 1 EeV • Angle determination poor! Elongated in the direction of e so thatthe direction can be reconstructed but precise to ~10◦

28. τ detection • The propagation mechanism of a tau neutrino is different, as tau may decay during propagation • As a result the tau neutrino never disappears. For each incoming τanotherτof lower energy reaches the detector • The Earth effectively remains transparent even for high energy tau neutrinos • Tau decays produce secondary flux of e and µ τ τ τ τ

29. Double bang events: CC interaction of τfollowed by tau decay • Lollipop events: second of the two double bang showers with reconstructed tau track • Inverted lollipop events: first of the two double bang showers with reconstructed tau track. Often confused with a hadronic event in which a ~100GeV muon is produced! • For Eτ< 106 GeV, in double bang events showers are indistinguishable. For Eτ~ 106 GeV, tau range is a few hundred meters and the showers can be separated. For 107 GeV < Eτ< 107.5 GeV, the tau decay length is comparable to the instrumented detector vol. lollipop Eτ> 107.5 GeV tau tracks can be confusing

30. Propagation equation of µ

31. Propagation equations of τ

34. Without energy loss Including energy loss

35. Rakshit, Reya, PRD74,103006(2006) Characteristic bump

37. Expected muon event rate per year at IceCube µ induced µ+ τinduced

38. Imprinted Earth’s matter profile

39. Probing New Physics

40. Production at astrophysical sources: Initial flavour ratio ? • Propagation through space: Massive neutrinos undergo quantum mechanical oscillations. So neutrinos reach Earth with a flavour ratio ?? • Propagation through the Earth: Neutrinos while propagating may interact with the Earth. CC or NC interactions. τpropagation is more elaborate:τ→τ→τ→τ... • Detection at IceCube: Muon neutrinos produce muons via CC interactions. All neutrinos produce showers through NC interactions. A CC interaction by a τmay produce spectacular signatures! N xsection sensitive

41. Detection of atm µs will enable us to probe CPTV, LIV,VEP which change the standard 1/E energy dependence of osc length. Due to high threshold of IceCube, osc of these high energy atm neutrinos is less • N xsection can get enhanced in XtraDim models • N xsection can get reduced at high energies in color glass condensate models • Visible changes in muon rates, shower rates • For xtradim upgoing neutrinos get absorbed at some energy and also downgoing for higher energies • For lower N xsection models angular dependence and energy dependence for upgoing events are more important

42. Crude neutrino flux determination from up/down events • OK for fixed power flux, but otherwise contained muon events are better. But poorer statistics • Auger is better for UHE neutrinos. New physics effects will be more dramatic • IceCube can probe neutrino spectrum better as Xsection uncertainties are only at high energies where the flux is smaller • Flavour ratio determination possible at IceCube as different flavours have distinctive signatures.

43. Other possibilities • DM detection: Neutrinos from solar core • SUSY search: look for charged sleptons • RPV, Leptoquarks • Part of supernova early detection system! • New physics interactions at the detector • New physics during propagation

44. Summary • UHE neutrinos: particle physics opportunities for the future • IceCube is a discovery expt. • Determining neutrino spectrum independent of new physics poses a challenge • Even crude measurements at IceCube may provide some clue about drastically different new physics scenarios at high energies • Some success with IceCube will lead to bigger detectors • At present we just need to detect an UHE neutrino event at IceCube!

45. Particle physics motivations LHC CM energy ECM = 14 TeV ⇒ LHC: E=108 GeV Tevatron: E=106 GeV Here we talk about neutrino flux of 1012 GeV! ⇒ ECM = 14 ×100 TeV

46. N cross-sections • We need PDF’s for x < 10-5 for E>108 GeV • Several options but not much discrepancy! • GRV and CTEQ cross-sections differ at the most by 20%

47. Beacom et al, PRD 68,093005(2003) e shower(CC+NC) For downgoing μ Horizontal μcreating a detectable μ track τlollipop τdouble bang