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2 | <head> |
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3 | <title>Beam Shape</title> |
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29 | |
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30 | <h2>Beam Shape</h2> |
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31 | |
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32 | The <?php $filepath = $_GET["filepath"]; |
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33 | echo "<a href='BeamParameters.php?filepath=".$filepath."' target='page'>";?>Beam Parameters</a> |
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34 | page explains how you can set a momentum spread of the two incoming |
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35 | beams, and a spread and offset for the location of the interaction |
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36 | vertex. The spread is based on a simple parametrization in terms of |
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37 | independent Gaussians, however, which is likely to be too primitive |
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38 | for realistic applications. |
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39 | |
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40 | <p/> |
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41 | It is therefore possible to define your own class, derived from the |
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42 | <code>BeamShape</code> base class, and hand it in to Pythia with the |
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43 | <code><?php $filepath = $_GET["filepath"]; |
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44 | echo "<a href='ProgramFlow.php?filepath=".$filepath."' target='page'>";?> |
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45 | pythia.setBeamShapePtr( BeamShape*)</a></code> method. |
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46 | Below we describe what such a class has to do. An explicit toy example |
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47 | is shown in <code>main23.cc</code>. |
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48 | |
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49 | <p/> |
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50 | The <code>BeamShape</code> base class has a very simple structure. |
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51 | It only has two main virtual methods. The first, <code>init()</code>, is |
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52 | used for initialization. The second, <code>pick()</code>, selects |
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53 | beam momentum and production vertex in the current event. |
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54 | |
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55 | <a name="method1"></a> |
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56 | <p/><strong>BeamShape::BeamShape() </strong> <br/> |
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57 | |
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58 | <strong>virtual BeamShape::~BeamShape() </strong> <br/> |
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59 | the constructor and destructor do not need to do anything. |
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60 | |
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61 | |
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62 | <a name="method2"></a> |
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63 | <p/><strong>virtual void BeamShape::init( Settings& settings, Rndm* rndmPtrIn) </strong> <br/> |
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64 | the base-class method simply reads in the relevant values stored |
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65 | in the <code>Settings</code> data base, and saves a pointer to the |
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66 | random-number generator. You are free to write your own |
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67 | derived initialization routine, or use the existing one. In the |
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68 | latter case you can then give your own modified interpretation |
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69 | to the beam spread parameters defined there. |
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70 | <br/>The two flags <code>Beams:allowMomentumSpread</code> and |
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71 | <code>Beams:allowVertexSpread</code> should not be tampered with, |
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72 | however. These are checked elsewhere to determine whether the beam |
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73 | shape should be set or not, whereas the other momentum-spread |
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74 | and vertex-spread parameters are local to this class. |
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75 | |
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76 | |
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77 | <a name="method3"></a> |
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78 | <p/><strong>virtual void BeamShape::pick() </strong> <br/> |
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79 | this method is the key one to supply in the derived class. Here you |
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80 | are free to pick whatever parametrization you desire for beam momenta |
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81 | and vertex position, including correlations between the two. |
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82 | At the end of the day, you should set a few protected |
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83 | <code>double</code> numbers: |
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84 | <br/><code>deltaPxA, deltaPyA, deltaPzA</code> for the three-momentum |
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85 | shift of the first incoming beam, relative to the nominal values; |
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86 | <br/><code>deltaPxB, deltaPyB, deltaPzB</code> for the three-momentum |
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87 | shift of the second incoming beam, relative to the nominal values; |
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88 | <br/><code>vertexX, vertexY, vertexZ, vertexT</code> for the |
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89 | production-vertex position and time. |
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90 | <br/>As usual, momentum is given in GeV, and space and time in mm, |
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91 | with <i>c = 1</i>. |
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92 | |
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93 | |
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94 | <a name="method4"></a> |
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95 | <p/><strong>Vec4 BeamShape::deltaPA() </strong> <br/> |
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96 | |
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97 | <strong>Vec4 BeamShape::deltaPB() </strong> <br/> |
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98 | read out the three-momentum shifts for beams A and B that were set by |
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99 | <code>pick()</code>. The energy components are put to zero at this stage, |
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100 | since they are most conveniently calculated after the original and the |
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101 | shift three-momenta have been added. |
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102 | |
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103 | |
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104 | <a name="method5"></a> |
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105 | <p/><strong>Vec4 BeamShape::vertex() </strong> <br/> |
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106 | read out the production-vertex position and time that were set by |
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107 | <code>pick()</code>. |
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108 | |
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109 | |
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110 | </body> |
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111 | </html> |
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112 | |
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113 | <!-- Copyright (C) 2012 Torbjorn Sjostrand --> |
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