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<title>External Decays</title>
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<h2>External Decays</h2>
<code>DecayHandler</code> is a base class for the external handling of
decays. It is intended for normal particle decays, primarily
<i>B</i> mesons and <i>tau</i>, and cannot be used to redirect
decays of heavy resonances like <i>t</i> or <i>Z^0</i>.
The user-written derived class is called if a pointer to it has
been given with the
<code><a href="ProgramFlow.html" target="page">pythia.decayPtr()</a></code>
method, where it also is specified which particles it will be called for.
This particle information is accessible with the
<code><a href="ParticleDataScheme.html" target="page">doExternalDecay()</a></code>
method.
<p/>
There is only one pure virtual method in <code>DecayHandler</code>,
to do the decay:
<a name="method1"></a>
<p/><strong>virtual bool DecayHandler::decay(vector<int>& idProd, vector<double>& mProd, vector<Vec4>& pProd, int iDec, const Event& event) </strong> <br/>
where
<br/><code>argument</code><strong> idProd </strong> : is a list of particle PDG identity codes,
<br/><code>argument</code><strong> mProd </strong> : is a list of their respective masses (in GeV), and
<br/><code>argument</code><strong> pProd </strong> : is a list of their respective four-momenta.
<p/>
At input, these vectors each have size one, so that <code>idProd[0]</code>,
<code>mProd[0]</code> and <code>pProd[0]</code> contain information on the
particle that is to be decayed. At output, the vectors should have
increased by the addition of all the decay products. Even if initially
defined in the rest frame of the mother, the products should have been
boosted so that their four-momenta add up to the <code>pProd[0]</code> of
the decaying particle.
<p/>
Should it be of interest to know the prehistory of the decaying
particle, e.g. to set some helicity information affecting the
decay angular distribution, the full event record is available
read-only, with info in which slot <code>iDec</code> the decaying particle
is stored.
<p/>
The routine should return <code>true</code> if it managed the decay and
<code>false</code> otherwise, in which case <code>Pythia</code> will try
to do the decay itself. This e.g. means you can choose to do some decay
channels yourself, and leave others to <code>Pythia</code>. To avoid
double-counting, the channels you want to handle should be switched off
in the <code>Pythia</code> particle database. In the beginning of the
external <code>decay</code> method you should then return
<code>false</code> with a probability given by the sum of the branching
ratios for those channels you do not want to handle yourself.
<p/>
Note that the decay vertex is always set by <code>Pythia</code>, and that
<i>B-Bbar</i> oscillations have already been taken into account,
if they were switched on. Thus <code>idProd[0]</code> may be the opposite
of <code>event[iDec].id()</code>, where the latter provides the code at
production.
<p/>
A sample test program is available in <code>main17.cc</code>, providing
a simple example of how to use this facility.
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