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Higgs mass - Why is it a Gaussian distribution?

  1. May 5, 2015 #1
    Hi, so my question is along the lines of the following:

    If i'm looking at the decay channel H=>ZZ=>4 Muons, why would the resultant distribution for the mass of the parent Higgs be a Gaussian?

    Is it a case of the peak is the actual value of the Higgs at rest; those of greater mass are Higgs that are moving; those with smaller mass are 'off shell' Higgs particles?

    Thank you kindly for any responses.
  2. jcsd
  3. May 5, 2015 #2
    The spread in mass values you see in the plots comes from limitations in detector resolution. To determine the mass of the parent particle from its daughters, you need to measure the directions and energies of the outgoing leptons, and then determine the invariant mass of that system (in this case, four leptons). Those measurements are only so good, leading to a spread in calculated mass values.

    The intrinsic width of the Standard Model Higgs is only 4 MeV, much smaller than the width you can read off the invariant mass plots.
  4. May 5, 2015 #3


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    The plotted mass values are always the invariant mass, which is the energy of the Higgs in its rest frame.

    The experimental shape is dominated by detector resolution. If you take the Z, on the other hand, its shape is a Breit-Wigner function and comes from its finite lifetime (it can be a bit off-shell). The Higgs would show the same effect, but with a width of (predicted) about 4 MeV it is way too narrow to see it directly at the LHC.
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