Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why can't we detect neutrino-antineutrino annihilation?

Tags:
  1. Mar 23, 2014 #1

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Various astrophysical processes produce antineutrinos, which then fly off into outer space. I assume there are pretty accurate estimates of the production rates. I can imagine three possible fates for such an antineutrino: (1) annihilating with a neutrino, (2) interacting with baryonic matter, (3) ending up as the only particle inside its own cosmological horizon. It seems like we ought to have pretty good estimates of the rate of the neutrino-antineutrino annihilation process. Each such annihilation produces two back-to-back photons. If the neutrino masses are on the order of 0.1 eV, then these are infrared photons with wavelengths on the order of 10^4 nm. Why can't we detect these photons and thereby determine the neutrino mass spectrum? Are the peaks too weak? Too spread out by Doppler broadening?
     
    Last edited: Mar 23, 2014
  2. jcsd
  3. Mar 23, 2014 #2
    Surely the Doppler broadening is so great that there would be nothing you could call a "peak" in the spectrum? Essentially all neutrinos are relativistic; the energy of the annihilation photons would be almost entirely determined by the neutrinos' kinetic energies and not their masses.
     
    Last edited: Mar 23, 2014
  4. Mar 23, 2014 #3

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Makes sense!
     
  5. Mar 23, 2014 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    The vv → γγ cross-section is truly microsocpic. (zeptoscopic?)

    It's got to be photons, because everything else is kinematically blocked. It's got to go through the Z-pole, at 90 GeV when the neutrinos are at 1/40,000 of an eV. It also has to go through a loop which gives additional kinematic reduction, higher powers of the couplings, and a 1/16π2, which in light of the other factors, is almost too small to worry about.
     
  6. Mar 23, 2014 #5

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The cross-section is small ... compared to what? Is the probability of this fate small compared to both of the other possibilities listed in #1? Negligibly small?
     
  7. Mar 23, 2014 #6

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2015 Award

    I would say "so small that I don't trust the estimation to be done correctly".

    If you want a ballpark, I'd estimate that the vv cross-section divided by the vp cross-section is alpha x m(v)/m(p) x [m(v)/m(e)]^4 /16 pi^2.
     
  8. Mar 24, 2014 #7

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

  9. Mar 24, 2014 #8

    Bill_K

    User Avatar
    Science Advisor

  10. Mar 24, 2014 #9

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Thanks Bill. I had acquired the notion that Majorana particles do not self annihilate. Upon further review, I find the consensus is that is only true for bosons, not leptons.
     
  11. Mar 24, 2014 #10

    ChrisVer

    User Avatar
    Gold Member

    ? If neutrinos are majorana they can annihilate - we are supposed to find the Majorana nature by neutrinoless double beta decay.
     
  12. Mar 25, 2014 #11
    Don't we know already that neutrinos can self-annihilate through a Z boson, because we observe that the Z can decay to neutrinos? ( simply the reverse process)
     
  13. Mar 25, 2014 #12
    It seems to me that this also isn't true, as two Z bosons ( which are there own antiparticles) can self-annihilate through a higgs boson.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted
Similar Discussions: Why can't we detect neutrino-antineutrino annihilation?
Loading...