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- What is the Z* and why it has different mass from Z ?
Goodmornig.I would like to explain me What is the Z* at higgs decay and why it has different mass from Z ?
Higgs decay to Z and Z* at LHC
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It is an off-shell Z, ie, it is not a Z, but a set of particles produced by a Z-interaction.
The Higgs is too light to produce two Zs on-shell in a decay.
The Higgs is too light to produce two Zs on-shell in a decay.
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on-shell means that ##E^2 = p^2 + m^2## holds (in units of c=1).
you can read more here, section 2.2.2 https://arxiv.org/abs/hep-ph/0503172
With the same notation, we could write the muon decay as ##\mu^- \to \nu_\mu + W^{-*}##
you can read more here, section 2.2.2 https://arxiv.org/abs/hep-ph/0503172
With the same notation, we could write the muon decay as ##\mu^- \to \nu_\mu + W^{-*}##
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What is measured experimentally is 4 muons (2 positive & 2 negative).
Then you make a cut on your data to only keep events which has a muon pair (positive and negative) with an invariant mass = mZ (+/- some threshold due to Z-width and experimental uncertainty).
After that, plot the invariant mass of the 4 muons, you should see a peak centered around mH (which we now know is ≈ 125 GeV).
Then you make a cut on your data to only keep events which has a muon pair (positive and negative) with an invariant mass = mZ (+/- some threshold due to Z-width and experimental uncertainty).
After that, plot the invariant mass of the 4 muons, you should see a peak centered around mH (which we now know is ≈ 125 GeV).
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Here is a plot with more data:
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Just one more flavor. If mH > 2mZ we would have the same signal 4 muons. But then one could make one more cut, namely that you would have two pair of muons with invariant mass = mZ
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I'd turn it around - the fact that ZZ* is visible at all is remarkable. The Z* is off shell by 20-30Γ or so. Why is that decay large enough to see? The answer is that the longitudinal component of the Z is the Higgs.
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Still the WW* Branching ratio is much higher for mH = 125 GeV
which is a much harder signal to measure, since the W decay is more "messy" than Z decays.
which is a much harder signal to measure, since the W decay is more "messy" than Z decays.
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The WW will always be above ZZ. Don't blame me, Blame Clebsch and Gordon.