Decayer help

You can give the decayer any list of particles. It will look through the list, find the heaviest unstable particle and decay it. This involves going through all the known decays, from the highest branching fraction to the lowest, and replacing this particle with its daughters. Then the whole process starts again with the new list of particles. Subsequent decays are nested using lists, and once a final state has been reached that satisfies all the criteria, it is shown in a white box with the text aligned to the right. Once it has decayed all the particles into all the possible final states, it adds up all the branching fractions for each final state and displays them all, so that you can see how many times D⁺→ π⁺π⁺π⁺ π^-π^- takes place via all intermediate processes.

The decayer is a work in progress, so it may take longer than you think it would to reach stable particles. In fact, it frequently times out. After 29s it gives up, jumps out of all the nested lists and gives the (incomplete) output as it appears at that time.

To decay a list of particles you need to type in their Monte Carlo numbers into the first box.

To make the decayer useful to as many people as possible, the Monte Carlo numbering scheme used is the one set out by the PDG. You can see it [pdf]here.

Branching fraction threshold

(Unless otherwise specified, a branching fraction means the product of all the individual branching fractions between the current list of particles and the initial list of particles that the user enters.) You can choose to exclude final states with branching fractions below a certain value. This is very important if you want to prevent the decayer from timing out, because it is trying to combine many rare processes.

Single step branching fraction threshold

This is the same as the branching fraction threshold, except that it applies to just one step in the process. If you set the single step branching fraction threshold to 1×10^-8 and decay a π⁰ the decayer will not include the decay to positronium, which has a branching fraction of 1.82×10^-9, even if you set the branching fraction threshold to 0.

Require

You can insist that certain particles be in the final state by listing their Monte Carlo numbers here, separated by semicolons. At the moment the decayer inteprets a semicolon as a boolean OR operator, so if one of the particles listed appears in the final state, that final state will be included.

Exclude

Exclude works in the same way as require, and with the same logic, except that it does not wait until the final state. If the decayer finds any of the particles in the excluded list it gives up on that decay and moves to the next one. This makes exclude "quicker" than require. There are also some tick-box options so that you can exclude who classes of particles.

Supress upper limits

You can either include or exclude upper limits in the decays. It's recommended that you exclude upper limits by selecting "Supress upper limits", as it saves a lot of time.

Supress upper limits

You can either include or exclude upper limits in the decays. It's recommended that you exclude upper limits by selecting "Supress upper limits", as it saves a lot of time.

Stack graph sensitivity

At the bottom of the page the decayer shows a stack-graph of all the final states that satisfy the criteria. You can change how large a final state branching fraction has to be before its shown on the stack graph using this option. You cannot affect the performance of the decayer by changing this variable, so it's worth changing it to a small value if you want to see how the final states compare to each other. (I hope to add an SVG pie chart as well at some point in the future.)

Decay depth

You can force the decayer to stop decaying after a given number of steps. This can be useful if you want to look at the branching fractions of a single particle, but not its daughters, or if you want to look at the inclusive decays of a particle.

Invisible particles

The only difference between an invisible particle and a visible one is that an invisible particle shows up in red. It can be quite enlightening to see how many final states contain an invisible particle (ie one that passes through the detector unseen.)

Stable particles

You can force the decayer to not decay certain particles. This really helps with performance, so if you're familiar with how, say, the η decays then you can add it to this list and save lots of time and thus avoiding timeouts.