# What is a Decay width really, qualitatively?

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1. May 22, 2015

### 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.

2. May 22, 2015

### Simon Bridge

3. May 23, 2015

### Staff: Mentor

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.

4. May 24, 2015

### AlanKirby

That really helps, thank you.