LEPTONIC NEUTRINOS

Arunava Bhadra

High Energy & Cosmic Ray Research Ctr.

North Bengal University

My collaborators: Prabir Banik and Biplab Bijay

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THE ERA OF NEUTRINO ASTRONOMYING THE UNIVERSE

In April 2012 IceCube detected two high-energy events above 1 PeV

28 events with energies around and above 30 TeV were observed in an all-sky search, conducted between May 2010 and May 2012, with the IceCube neutrino detector

This is the first indication of very high-energy neutrinos coming from outside our solar system,“ - Halzen

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It is gratifying to finally see what we have been looking for. This is the dawn of a new age of astronomy.“ – Halzen

Science Daily Nov. 21, 2013 heading - “The era of neutrino astronomy has begun”

The IceCube project has been awarded the 2013 Breakthrough of the Year by the British magazine Physics World for making the first observation of cosmic neutrinos

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THE REASONS BEHIND SUCH ENTHUSIASM 1

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Neutrinos come from astrophysical sources as close as the Earth and Sun, to as far away as distant galaxies, and even as remnants from the Big Bang.

Since neutrinos only interact weakly, they are unique messengers from the astrophysical sources allowing us to probe deep into the astrophysical body.

Extraterrestrial energetic neutrinos are expected to give direct pointing at the acceleration sites of cosmic rays.

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EXTRATERRESTRIAL ENERGETIC NEUTRINOS: COSMIC RAY CONNECTION

Cosmic Rays (CRs) are a highly isotropic flux of relativistic particles that originates somewhere in the cosmos.

The energy spectrum of cosmic rays exhibit power law behavior with two or three breaks (possibly and few structures)

Mostly protons or a (He) nuclei (other elements too, in much shorter supply)

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ENERGY SPECTRUM 12

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NUCLEAR ABUNDANCE 12

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TWO KEY (UNANSWERED) QUESTIONS

Where they come from –The problem of cosmic ray origin

How they are produced – The problem of acceleration mechanism

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POTENTIAL SOURCES OF COSMIC RAYS

Colliding galaxies

Gamma ray bursts Giant black holes spinning rapidly

Supernova Magnetized spinning neutron stars AGN

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DIRECT DETECTION POINTING AT THE ACCELERATION SITES

Cosmic rays interact with ambient matter/radiation Giving rise to energetic gamma rays

p + p → πo + X p + → o πo + p

πo →

Observations by the HESS, VERITAS, MAGIC, HAGAR and few other telescopes shown the existence of (extra-)galactic sources of electromagnetic radiation with an energy spectrum extending up to several tens of TeVs

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TEV GAMMA RAYS – LEPTONIC OR HADRONIC ORIGIN? SNR RX J1713-3946 1

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GAMMA RAY ASSOCIATED NEUTRINOS 12

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Neutrinos are produced in association with the cosmic-ray beam.

The interaction of high-energy protons with ambient matter or radiation also produce charged pions (and kaons)

p + p → π + X

p + + π+ + n

π+ → µ+ + µ e+ + e + bar(µ)+ µ

π- → µ- + bar(µ) e- + bar(e) + bar(µ) + µ

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Generic cosmic-ray sources produce a neutrino flux comparable to their flux of cosmic rays and pionic TeV gamma rays

Neutrinos from theorized cosmic-ray accelerators

dominate the steeply falling atmospheric neutrino flux above an energy of 100TeV.

Detection of extraterrestrial PeV neutrinos therefore is expected to trace back the cosmic ray source.

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EXPECTED NEUTRINO FLUX 12

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LEPTONIC NEUTRINOS

At lower energies, inverse Compton scattering, converts high-energy electrons into high-energy photons

at energies above the muon threshold, higher orders processes, such as triplet production (TPP)

e + → e + e+ + e-

and electron muon pair production e + → e + µ+ + µ-

dominate (Kusenko PRL 2001). Subsequently neutrinos are produced (from muon

decay) µ e e µ

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Another production channel is thate+RF e+ ,

RF µ+ µ- ,

whereas at low energies e+ e- .

Instead of muon pair pion pair also can be produced.

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FEASIBILITY: The threshold energy for such a muon pair

production reaction Eth = s > 2mµ ~ 0.21 GeV

Therefore, e > 0.02 fg GeV2

where fg=(1-cose)-1

If e is of the order of PeV, has to be few tens of eV

to satisfy the above threshold conditions which is possible in many cases including pulsars, supernovae, AGN, GRB etc.

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For s >> me2

inelasticity of triplet production is very small =1.768(s/ me

2)-3/4 <10-3

[Anguelov et al J.Phys G1999]

One of the produced electron carries almost all the energy ((1-)e ) of the primary electron and it can interact with the ambient radiation field several times.

Effective energy attenuation length eff=TPP/

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The mean free path for muon pair productionl(r)=( n)-1 .

For muon pair production MPP is around 0.1 µb for s 20m2

µ [Athar et al, PRD 2001].

For an astrophysical object of temperature T n (R) = (a/2.8k)([1+zg]T)3 ~ 9 x 1019T3

0.1 cm-

3 .

This gives l(r) ~ 1011 T30.1 keV cm.

which is well within the radial extent of different celestial sources like SNR, AGN

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FLUX:

The cross-section in delta resonance is about 5 x 1028 cm2. So MPP is more than 103 times less.

But it will also depend on the ratio of electron to proton density at the source.

In accretion flow electron to proton ratio is expected to be one due to charge neutrality.

The strong radio emission from sources like AGN, pulsars implies presence of energetic electrons with high densities.

In the strong magnetic field of these sources electron positron plasma are expected to produce through pair production.

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AGN jet composition – Multi-wavelength observations of the powerful

gamma-ray quasar PKS 1510089 suggest that number of electrons and/or positrons that exceeds the number of protons by a factor of at least 10 (Kataoka et al, APJ 2008).

So neutrino flux from leptonic process could be of the same order of that from hadronic interactions or one to two order less.

The flux due to two PeV neutrinos detected by ICECUBE already exceeds the hadronic neutrino flux estimated by Stecker et al originally (PRL 91)

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CONCLUSION:

Neutrinos also can be produced in leptonic interactions.

There are several potential celestial sources where leptonic neutrinos can be produced which include SNR, AGN.

In favorable environment, leptonic neutrino flux can be of the same order to that of hadronic neutrinos.

Detection of neutrinos does not conclusively mean the presence of energetic hadrons.

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Appropriate fluxes of gamma rays and neutrinos together may give clear signature of hadronic cosmic ray sources as in the case of pulsars (bhadra and Dey, MNRAS, 2009).

Confirmation requires CTA + PINGU GRAPES + ICECUBE

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Thank you

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