# What is a Decay width really, qualitatively?

• AlanKirby
In summary, the conversation discusses the concept of decay width in relation to the distribution of masses obtained in experiments. The decay width is the inverse mean lifetime and is related to the probability of decay. The physical mass of a particle can be slightly different from the peak value due to off-shell particles, while the reconstructed mass is an estimate with its own uncertainty. For example, a Z boson with a mass of 91 GeV could have an invariant mass of 92 GeV, and the width of the peak in the reconstructed mass values is a combination of decay width and detector resolution.
AlanKirby
Hi there, my question is along the lines of the following.

I understand that in an experiment we obtain a distribution of masses for a given particle, due to the finite resolution of the detectors.

In terms of a fundamental (if that even makes sense) decay width, what is the decay width?
Is it simply the following:
Peak value is the particle rest mass; distribution at higher energy is particles with larger velocity; distribution at lower energy are 'off mass shell' particles?

I'm having quite a bit of trouble really getting around what a decay width really is and how to think about it. I understand (to some extent) the point about resolution in experiments, and the relation of width to lifetime, but I don't truly understand.

Thanks for any replies.

The particle velocity (in which frame?) is irrelevant, the mass is always determined by the energy in the rest frame of the particle.

You have to distinguish between "physical" mass (the physical process) and reconstructed mass (the estimate from the experiments).
The "physical" mass can be a bit below or above the peak value (where peak value is "the mass of the particle") - the particles are a bit off-shell.
The reconstructed mass is an estimate of this "physical" mass, with its own uncertainty.

Take a Z boson, for example: the mass of the Z is 91 GeV and its decay width is 2.5 GeV. An actual Z boson in a collision could have an invariant mass of 92 GeV. It decays, a detector sees the decay products and might get 92.3 GeV as estimate (random example),
If you plot the reconstructed mass values, the width of the peak is the combination of decay width and detector resolution.

AlanKirby
mfb said:
The particle velocity (in which frame?) is irrelevant, the mass is always determined by the energy in the rest frame of the particle.

You have to distinguish between "physical" mass (the physical process) and reconstructed mass (the estimate from the experiments).
The "physical" mass can be a bit below or above the peak value (where peak value is "the mass of the particle") - the particles are a bit off-shell.
The reconstructed mass is an estimate of this "physical" mass, with its own uncertainty.

Take a Z boson, for example: the mass of the Z is 91 GeV and its decay width is 2.5 GeV. An actual Z boson in a collision could have an invariant mass of 92 GeV. It decays, a detector sees the decay products and might get 92.3 GeV as estimate (random example),
If you plot the reconstructed mass values, the width of the peak is the combination of decay width and detector resolution.
That really helps, thank you.

## 1. What is a decay width?

A decay width, also known as a natural width, is a measure of the rate at which a particle decays into other particles. It is a fundamental property of particles that undergo decay and is related to the probability of the decay process occurring.

## 2. How is a decay width measured?

A decay width is typically measured by observing the decay products of a particle and calculating the rate at which they are produced. This can be done through various experimental techniques, such as particle colliders or detectors.

## 3. How does a decay width affect a particle's lifetime?

The decay width is directly related to a particle's lifetime. A larger decay width means a shorter lifetime, as the particle is more likely to decay into other particles. Conversely, a smaller decay width leads to a longer lifetime.

## 4. What factors can affect a decay width?

The decay width of a particle can be influenced by various factors, such as the mass of the particle, the strength of the interaction responsible for the decay, and the available energy for the decay process to occur.

## 5. How does a decay width relate to the uncertainty principle?

The uncertainty principle, which states that there is a limit to the precision with which certain pairs of physical properties can be measured, is related to the decay width. The more precisely the energy of a particle is known, the less precisely its lifetime can be determined, and vice versa.

• High Energy, Nuclear, Particle Physics
Replies
11
Views
1K
• High Energy, Nuclear, Particle Physics
Replies
4
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
13
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
2
Views
1K
• High Energy, Nuclear, Particle Physics
Replies
4
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
1
Views
990
• High Energy, Nuclear, Particle Physics
Replies
1
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
13
Views
2K
• High Energy, Nuclear, Particle Physics
Replies
4
Views
1K
• High Energy, Nuclear, Particle Physics
Replies
1
Views
1K