# Higgs decay to Z and Z* at LHC

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• Dhmht_Kr
In summary, the conversation discusses the Z* at Higgs decay and its different mass from the Z. It is explained that the Higgs is not able to produce two Zs on-shell in a decay, as it is too light. The concept of on-shell is defined as ##E^2 = p^2 + m^2## and the muon decay is used as an example. The experimental measurement of 4 muons is discussed and how a cut is made to only keep events with an invariant mass equal to mZ. The plot of the invariant mass is described and it is noted that if mH > 2mZ, there would be a similar signal. The importance of the ZZ* decay is highlighted and it
Dhmht_Kr
TL;DR Summary
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 ?

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.

apostolosdt, ohwilleke, topsquark and 2 others
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^{-*}##

Last edited:
ohwilleke, vanhees71 and topsquark
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 treshold 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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topsquark, vanhees71 and ohwilleke
Here is a plot with more data:

topsquark and malawi_glenn
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

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.

mfb
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.

The WW will always be above ZZ. Don't blame me, Blame Clebsch and Gordon.

vanhees71

## 1. What is the Higgs decay to Z and Z* at LHC?

The Higgs decay to Z and Z* at LHC refers to the process in which a Higgs boson particle decays into a Z boson and a Z* (virtual Z) boson at the Large Hadron Collider (LHC) particle accelerator. This process is one of the ways in which the Higgs boson interacts with other particles, and it helps scientists to study the properties of the Higgs boson.

## 2. Why is the Higgs decay to Z and Z* important?

The Higgs decay to Z and Z* is important because it provides evidence for the existence of the Higgs boson and helps to confirm the Standard Model of particle physics. It also allows scientists to study the properties of the Higgs boson, such as its mass and its interactions with other particles, which can provide insights into the fundamental nature of the universe.

## 3. How is the Higgs decay to Z and Z* detected at LHC?

The Higgs decay to Z and Z* is detected at LHC by analyzing the collision data from particle collisions. Scientists use complex detectors to measure the energy and momentum of the particles produced in the collisions, and they look for specific patterns and signatures that indicate the presence of a Higgs boson decaying into a Z boson and a Z* boson.

## 4. What can the Higgs decay to Z and Z* tell us about the universe?

The Higgs decay to Z and Z* can tell us about the fundamental nature of the universe, as it provides insights into the interactions between particles and the origin of mass. It also helps to explain why some particles have mass while others do not, and it can potentially lead to new discoveries and theories about the fundamental laws of nature.

## 5. Are there any other ways in which the Higgs boson can decay?

Yes, the Higgs boson can also decay into other particles such as photons, W bosons, and fermions. These different decay modes can provide additional information about the properties of the Higgs boson and its interactions with other particles. Scientists continue to study and search for new decay modes of the Higgs boson in order to further our understanding of the universe.

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