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

Z-Boson reasonance and the number of neutrino varieties

Tags:
  1. Jan 18, 2013 #1
    I've read that the experimental results of Z-Boson resonance confirm the theoretical expectations that there are 3-types of Neutrinos, not 2 or 4.

    How are these theoretical expectations calculated? I.e. how does the number of neutrino varieties affect Z-boson resonance?

    Thank you
     
  2. jcsd
  3. Jan 18, 2013 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

  4. Jan 21, 2013 #3

    jkp

    User Avatar

    At the LEP experiment Z bosons were produced through colliding electrons and positrons. This was a very clean experiment, unlike the LHC, allowing the properties of the Z boson to be studied to high precision. The presence of the Z boson shows up as a peak in the total cross-section as a function of center of mass energy with the peak located at ~91 GeV corresponding to the mass of the Z boson (there is a nice plot of this on p711 fig.20.5 of Peskin & Schroeder). By measuring the Z peak very carefully they can determine the total width (##\Gamma_{tot}##) and the cross-section at the peak (##\sigma_{peak}##).

    The total Z width ##\Gamma_{tot}## is actually made up of two parts. The first is the visible part ##\Gamma_{vis}## made up of decays to charged leptons and hadrons and this is related to the observed peak cross-section ##\sigma_{peak}##. The second is the invisible part ##\Gamma_{inv}## made up of decays to neutrinos which are not observed in the experiment. This second part can be determined by taking ##\Gamma_{tot}-\Gamma_{vis}=\Gamma_{inv}=N_\nu\Gamma(Z\to \nu\overline{\nu})## where ##N_\nu## is the number of active neutrinos. Notice it is "active" neutrinos that matter not the number of neutrinos as there might be "sterile" neutrinos which the Z doesn't decay to. Putting it all together we can get ##N_\nu## from
    $$N_\nu = (\Gamma_{tot}-\Gamma_{vis})/\Gamma(Z\to \nu\overline{\nu})$$
    and the result is very close to ##N_\nu=3##.

    You can try to look at chapter 20 of Peskin & schroeder and in particular do the problems 20.2 and 20.3 on p 728 which will give you a good understanding of the Z resonance and how it relates to the number of active neutrinos.
     
  5. Jan 22, 2013 #4
    This does help a lot. Thanks Vanadium and jkp.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Z-Boson reasonance and the number of neutrino varieties
  1. Z boson (Replies: 7)

Loading...