L EPTONIC NEUTRINOS Arunava Bhadra High Energy & Cosmic Ray Research Ctr. North Bengal University My collaborators: Prabir Banik and Biplab Bijay

T HE ERA OF NEUTRINO ASTRONOMYING THE U NIVERSE 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 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 2

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 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 3

T HE REASONS BEHIND SUCH ENTHUSIASM 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 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. 4

E XTRATERRESTRIAL E NERGETIC NEUTRINOS : C OSMIC 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  (He) nuclei (other elements too, in much shorter supply) 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 5

E NERGY SPECTRUM 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 6

N UCLEAR ABUNDANCE 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 7

T WO K EY ( UNANSWERED ) QUESTIONS  Where they come from –The problem of cosmic ray origin  How they are produced – The problem of acceleration mechanism 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 8

P OTENTIAL SOURCES OF COSMIC RAYS Colliding galaxies Colliding galaxies Gamma ray bursts Gamma ray bursts Giant black holes spinning rapidly Giant black holes spinning rapidly Supernova Supernova Magnetized spinning neutron stars Magnetized spinning neutron stars AGN AGN 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 9

DIRECT DETECTION POINTING AT THE ACCELERATION SITES Cosmic rays interact with ambient matter/radiation Giving rise to energetic gamma rays p + p → π o + X p +  →  o  π o + p π o →   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 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 10

T E V GAMMA RAYS – LEPTONIC OR HADRONIC ORIGIN ? SNR RX J /18/2013 WAPP 2013 Mayapuri, Darjeeling 11

G AMMA RAY ASSOCIATED NEUTRINOS 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 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 12

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. 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 13

E XPECTED NEUTRINO FLUX 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 14

L EPTONIC 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 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 15

Another production channel is that e+  RF  e+ ,   RF  µ + µ -, whereas at low energies  e + e -. Instead of muon pair pion pair also can be produced. 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 16

F EASIBILITY : The threshold energy for such a muon pair production reaction E th =  s > 2m µ ~ 0.21 GeV Therefore,  e   > 0.02 f g GeV 2 where f g =(1-cos  e  ) -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. 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 17

For s >> m e 2 inelasticity of triplet production is very small  =1.768(s/ m e 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 /  12/18/2013 WAPP 2013 Mayapuri, Darjeeling 18

The mean free path for muon pair production l(r)=(  n  ) -1. For muon pair production  MPP is around 0.1 µb for s  20m 2 µ [Athar et al, PRD 2001]. For an astrophysical object of temperature T n  (R) = (a/2.8k)([1+z g ]T) 3 ~ 9 x T cm -3. This gives l(r) ~ T keV cm. which is well within the radial extent of different celestial sources like SNR, AGN 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 19

F LUX : The cross-section in delta resonance is about 5 x cm 2. So MPP is more than 10 3 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. 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 20

AGN jet composition – Multi-wavelength observations of the powerful gamma-ray quasar PKS 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) 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 21

C ONCLUSION : 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. 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 22

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 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 23

Thank you 12/18/2013 WAPP 2013 Mayapuri, Darjeeling 24

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