1 | <chapter name="External Decays"> |
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2 | |
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3 | <h2>External Decays</h2> |
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4 | |
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5 | <code>DecayHandler</code> is a base class for the external handling of |
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6 | decays. It is intended for normal particle decays, primarily |
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7 | <ei>B</ei> mesons and <ei>tau</ei>, and cannot be used to redirect |
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8 | decays of heavy resonances like <ei>t</ei> or <ei>Z^0</ei>. |
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9 | The user-written derived class is called if a pointer to it has |
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10 | been given with the |
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11 | <code><aloc href="ProgramFlow">pythia.decayPtr()</aloc></code> |
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12 | method, where it also is specified which particles it will be called for. |
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13 | This particle information is accessible with the |
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14 | <code><aloc href="ParticleDataScheme">doExternalDecay()</aloc></code> |
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15 | method. |
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16 | |
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17 | <p/> |
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18 | There is only one pure virtual method in <code>DecayHandler</code>, |
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19 | to do the decay: |
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20 | <method name="virtual bool DecayHandler::decay(vector<int>& idProd, v |
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21 | ector<double>& mProd, vector<Vec4>& pProd, int iDec, |
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22 | const Event& event)"> |
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23 | where |
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24 | <argument name="idProd"> is a list of particle PDG identity codes, |
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25 | </argument> |
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26 | <argument name="mProd"> is a list of their respective masses (in GeV), and |
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27 | </argument> |
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28 | <argument name="pProd"> is a list of their respective four-momenta. |
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29 | </argument> |
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30 | </method> |
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31 | |
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32 | <p/> |
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33 | At input, these vectors each have size one, so that <code>idProd[0]</code>, |
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34 | <code>mProd[0]</code> and <code>pProd[0]</code> contain information on the |
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35 | particle that is to be decayed. At output, the vectors should have |
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36 | increased by the addition of all the decay products. Even if initially |
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37 | defined in the rest frame of the mother, the products should have been |
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38 | boosted so that their four-momenta add up to the <code>pProd[0]</code> of |
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39 | the decaying particle. |
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40 | |
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41 | <p/> |
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42 | Should it be of interest to know the prehistory of the decaying |
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43 | particle, e.g. to set some helicity information affecting the |
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44 | decay angular distribution, the full event record is available |
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45 | read-only, with info in which slot <code>iDec</code> the decaying particle |
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46 | is stored. |
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47 | |
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48 | <p/> |
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49 | The routine should return <code>true</code> if it managed the decay and |
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50 | <code>false</code> otherwise, in which case <code>Pythia</code> will try |
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51 | to do the decay itself. This e.g. means you can choose to do some decay |
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52 | channels yourself, and leave others to <code>Pythia</code>. To avoid |
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53 | doublecounting, the channels you want to handle should be switched off |
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54 | in the <code>Pythia</code> particle database. In the beginning of the |
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55 | external <code>decay</code> method you should then return |
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56 | <code>false</code> with a probability given by the sum of the branching |
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57 | ratios for those channels you do not want to handle yourself. |
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58 | |
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59 | <p/> |
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60 | Note that the decay vertex is always set by <code>Pythia</code>, and that |
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61 | <ei>B-Bbar</ei> oscillations have already been taken into account, |
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62 | if they were switched on. Thus <code>idProd[0]</code> may be the opposite |
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63 | of <code>event[iDec].id()</code>, where the latter provides the code at |
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64 | production. |
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65 | |
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66 | <p/> |
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67 | A sample test program is available in <code>main17.cc</code>, providing |
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68 | a simple example of how to use this facility. |
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69 | |
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70 | </chapter> |
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71 | |
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72 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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