Icefishing for Cosmic Neutrinos

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Ice-fishing for Cosmic Neutrinos

• Subhendu Rakshit
• TIFR, Mumbai

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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
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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

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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

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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(gt1010 GeV) cosmic rays
  point back to their sources

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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!

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Neutrino detectors
Underground
Air shower
Underwater / ice
GeV TeV
PeV EeV
1 PeV 106 GeV 1 EeV 109 GeV
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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!

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• A Km3 detector
• PMTs detect Cherenkov light emitted by charged
  particles created by neutrino interactions

IceCube

• 1KM3

?
The Cherenkov cone needs to be reconstructed to
determine the energy and direction of the muon
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Used for calibration, background rejection and
air-shower physics
- The predecessor of IceCube
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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/vE, 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

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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!

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IceCube
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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

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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!
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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

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Eliminating backgrounds

• Energy cuts
• Directional cuts
• Directional signals
• Temporal considerations

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Delving into the details...
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• 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. ?t propagation
  is more elaborate ?t?t? ?t?t...
• Detection at IceCube
• Muon neutrinos produce muons via CC
  interactions. All neutrinos produce showers
  through NC interactions. A CC interaction by a ?t
  may produce spectacular signatures!

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Production at astrophysical sources

• A proton gets accelerated and hits another
  proton or a photon. They produce neutron, p and
  p0.Their decay produces cosmic rays, neutrinos
  and photons respectively
• p ? ? p n
• p ? ? p0 p

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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 t0, we produce a ?e
• After sometime t, the mass eigenstates evolve
  differently
• So the probability of detecting another
  flavour is nonzero

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• Now remember the initial flavour ratio at source
  was
• Recent neutrino experiments have established that
  neutrino flavour states ?µ and ?t mix 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!

At source
On Earth
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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

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?µ detection

• 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?/v(E?/1TeV) and the reconstruction error on
  the muon direction is on the order of 1?
• Better energy determination for contained events.
  More contained events at lower energy

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Km long muon tracks from ?µ
10m long cascades from ?e, ?t
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?e detection

• In a CC interaction, a ?e deposits 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 that the direction can be
  reconstructed but precise to 10?

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?t 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 ?t another ?t of 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

?t
t
?t
t
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• Double bang events CC interaction of ?tfollowed
  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 Etlt 106 GeV, in double bang events showers
  are indistinguishable. For Et 106 GeV, tau range
  is a few hundred meters and the showers can be
  separated.
• For 107 GeV lt Etlt 107.5 GeV, the tau decay
  length is comparable to the instrumented detector
  vol.? lollipop
• Etgt 107.5 GeV tau tracks can be confusing

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Propagation equation of ?µ
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Propagation equations of ?t
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Without energy loss
Including energy loss
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Rakshit, Reya, PRD74,103006(2006)
Characteristic bump
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Expected muon event rate per year at IceCube
?µ induced
?µ ?t induced
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Imprinted Earths matter profile
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Probing New Physics
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• 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. ?t propagation
  is more elaborate ?t?t? ?t?t...
• Detection at IceCube
• Muon neutrinos produce muons via CC
  interactions. All neutrinos produce showers
  through NC interactions. A CC interaction by a ?t
  may produce spectacular signatures!

?N xsection sensitive
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• 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

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• 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.

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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

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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!

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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

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?N cross-sections

• We need PDFs for x lt 10-5 for E?gt108 GeV
• Several options but not much discrepancy!
• GRV and CTEQ cross-sections differ at the most by
  20

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Beacom et al, PRD 68,093005(2003)
?e shower(CCNC)
For downgoing ?µ
Horizontal ?µcreating a detectable µ track
?tlollipop
?tdouble bang
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