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

sourcesphotons are easy to detect as they interact

electromagnetically with charged particles

• Cons: Due to the same reason they get absorbed by dust or get

obstructedVery 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!

GeV TeV PeV EeV

1 PeV = 106 GeV

1 EeV = 109 GeV

Underwater / ice

Air showerUnderground

Neutrino detectors

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!

μν

IceCube

o1KM^3 • A Km3 detector

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

The Cherenkov cone needs to be reconstructed to determine the energy and direction of the muon

- The predecessor of IceCube

Used for calibration, background rejection and air-shower physics

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

IceCube

• 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!

Detection strategy

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

• 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. τ propagation 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 τ may produce spectacular signatures!

e μ τν :ν :ν =1:2:0

e μ τν :ν :ν =1:1:1

Production at astrophysical sources:

A proton gets accelerated and hits another proton or a photon. They produce neutron, π+ and π0.Their decay produces cosmic rays, neutrinos and photons respectively

p + → π+ + n

p + → π0 + p

+μμ + ν

+e μe + ν + ν

γ + γ e μ τν :ν :ν =1:2:0

• 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

Propagation through space:

e 1 2ν (0) = a ν + b ν

1 2-iE t -iE te 1 2ν (t) = a e ν + b e ν

• Now remember the initial flavour ratio at source was

• Recent neutrino experiments have established that neutrino flavour states µ and τ 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!

e μ τν :ν :ν =1:2:0

e μ τν :ν :ν =1:1:1

At source

On 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

Propagation through the Earth:

int

1N

A totN

int

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◦/√(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

~ Km long muon tracks from µ

~ 10m long cascades from e, τ

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◦

• 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 τ another τ 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

τ

τ

τ

τ

τ detection

• 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

1

int 0

( , )( , ) 1( , ) ( , )

( ) 1

NNC y

A y

d E yd E X dyE X N E X

dX E y dy

int

1N

A totN

1y

EE

y

Propagation equations of τ

1 1

0 0

( , ) ( , )( , ) ( , ) ( , )

( ) 1 1NC

y y

E X E X dy dyK E X K E y E X

X E y y

1tot

A NN

1y

EE

y

1

0

( , ) ( , ) 1( , ) ( , )

ˆ ( ) 1( )CC

y

E X E X dyK E y E X

X E yE

1 1 1ˆ CC dec

1CCCC

A NN

( , , )dec EE X c

m

,, ( , )1

( , )( )

NC CCN yNC CC

totN

d E yK E y

E dy

( , )1( , )

( )

CCN yCC

totN

d E yK E y

E dy

( , )1( , )

( )X ydec

tot

d E yK E y

E dy

1 1( , ) ( , ) ( , )

( ) ( )CC dec

decK E y K E y K E y

E E

Including energy loss

Without energy loss

Characteristic bump

Rakshit, Reya, PRD74,103006(2006)

Expected muon event rate per year at IceCube

µ induced

µ+ τ induced

Imprinted Earth’s matter profile

• 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. τ propagation 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 τ may produce spectacular signatures!

e μ τν :ν :ν =1:2:0

e μ τν :ν :ν =1:1:1

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

172 14

10CM N

EE M E TeV

eV

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%

2 310x

2 /W

N

M

M E E GeV

Horizontal μcreating a detectable μ track

For downgoing μ

e shower(CC+NC)

τlollipop

τdouble bang

Beacom et al, PRD 68,093005(2003)