Ice fishing for cosmic neutrinos
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Ice-fishing for Cosmic Neutrinos. Subhendu Rakshit TIFR, Mumbai. 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:

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Ice-fishing for Cosmic Neutrinos

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Ice-fishing for Cosmic Neutrinos

Subhendu Rakshit

TIFR, Mumbai


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


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


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


  • 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


    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!


    Neutrino detectors

    Underground

    Air shower

    Underwater / ice

    GeV TeV PeV EeV

    1 PeV = 106 GeV

    1 EeV = 109 GeV


    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!


    • 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


    Used for calibration, background rejection and air-shower physics

    - The predecessor of IceCube


    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


    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


    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


    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!


    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


    Eliminating backgrounds

    • Energy cuts

    • Directional cuts

    • Directional signals

    • Temporal considerations


    Delving into the details...


    • 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!


    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


    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


    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


    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


  • 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


    ~ Km long muon tracks from µ

    ~ 10m long cascades from e, τ


    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◦


    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


    • 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


    Propagation equation of µ


    Propagation equations of τ


    Without energy loss

    Including energy loss


    Rakshit, Reya, PRD74,103006(2006)

    Characteristic bump


    Expected muon event rate per year at IceCube

    µ induced

    µ+ τinduced


    Imprinted Earth’s matter profile


    Probing New Physics


    • 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


    • 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


    • 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.


    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


    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!


    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


    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%


    Beacom et al, PRD 68,093005(2003)

    e shower(CC+NC)

    For downgoing μ

    Horizontal μcreating a detectable μ track

    τlollipop

    τdouble bang


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