3 <title>Semi-Internal Resonances</title>
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30 <h2>Semi-Internal Resonances</h2>
32 The introduction of a new <?php $filepath = $_GET["filepath"];
33 echo "<a href='SemiInternalProcesses.php?filepath=".$filepath."' target='page'>";?>
34 semi-internal process</a> may also involve a new particle,
35 not currently implemented in PYTHIA. Often it is then enough to
36 use the <?php $filepath = $_GET["filepath"];
37 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>standard machinery</a>
38 to introduce a new particle (<code>id:all = ...</code>) and new
39 decay channels (<code>id:addChannel = ...</code>). By default this
40 only allows you to define a fixed total width and fixed branching
41 ratios. Using <code><?php $filepath = $_GET["filepath"];
42 echo "<a href='ResonanceDecays.php?filepath=".$filepath."' target='page'>";?>meMode</a></code>
43 values 100 or bigger provides the possibility of a very
44 simple threshold behaviour.
47 If you want to have complete freedom, however, there are two
48 ways to go. One is that you make the resonance decay part of the
49 hard process itself, either using the
50 <?php $filepath = $_GET["filepath"];
51 echo "<a href='LesHouchesAccord.php?filepath=".$filepath."' target='page'>";?>Les Houches interface</a> or
52 a semi-internal process. The other is for you to create a new
53 <code>ResonanceWidths</code> object, where you write the code
54 needed for a calculation of the partial width of a particular
58 Here we will explain what is involved in setting up a resonance.
59 Should you actually go ahead with this, it is strongly recommended
60 to use an existing resonance as a template, to get the correct
61 structure. There also exists a sample main program,
62 <code>main26.cc</code>, that illustrates how you could combine
63 a new process and a new resonance.
66 There are three steps involved in implementing a new resonance:
67 <br/>1) providing the standard particle information, as already
68 outlined above (<code>id:all = ...</code>,
69 <code>id:addChannel = ...</code>), except that now branching
70 ratios need not be specified, since they anyway will be overwritten
71 by the dynamically calculated values.
72 <br/>2) writing the class that calculates the partial widths.
73 <br/>3) handing in a pointer to an instance of this class to PYTHIA.
74 <br/>We consider the latter two aspects in turn.
76 <h3>The ResonanceWidths Class</h3>
78 The resonance-width calculation has to be encoded in a new class.
79 The relevant code could either be put before the main program in the
80 same file, or be stored separately, e.g. in a matched pair
81 of <code>.h</code> and <code>.cc</code> files. The latter may be more
82 convenient, in particular if the calculations are lengthy, or
83 likely to be used in many different runs, but of course requires
84 that these additional files are correctly compiled and linked.
87 The class has to be derived from the <code>ResonanceWidths</code>
88 base class. It can implement a number of methods. The constructor
89 and the <code>calcWidth</code> ones are always needed, while others
90 are for convenience. Much of the administrativ machinery is handled
91 by methods in the base class.
93 <p/>Thus, in particular, you must implement expressions for all
94 possible final states, whether switched on in the current run or not,
95 since all contribute to the total width needed in the denominator of
96 the Breit-Wigner expression. Then the methods in the base class take
97 care of selecting only allowed channels where that is required, and
98 also of including effects of closed channels in secondary decays.
99 These methods can be accessed indirectly via the
100 <code><?php $filepath = $_GET["filepath"];
101 echo "<a href='ResonanceDecays.php?filepath=".$filepath."' target='page'>";?>res...</a></code>
102 methods of the normal
103 <code><?php $filepath = $_GET["filepath"];
104 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>particle database</a></code>.
107 A <b>constructor</b> for the derived class obviously must be available.
108 Here you are quite free to allow a list of arguments, to set
109 the parameters of your model. The constructor must call the
110 base-class <code>initBasic(idResIn)</code> method, where the argument
111 <code>idResIn</code> is the PDG-style identity code you have chosen
112 for the new resonance. When you create several related resonances
113 as instances of the same class you would naturally make
114 <code>idResIn</code> an argument of the constructor; for the
115 PYTHIA classes this convention is used also in cases when it is
117 <br/>The <code>initBasic(...)</code> method will
118 hook up the <code>ResonanceWidths</code> object with the corresponding
119 entry in the generic particle database, i.e. with the normal particle
120 information you set up in point 1) above. It will store, in base-class
121 member variables, a number of quantities that you later may find useful:
122 <br/><code>idRes</code> : the identity code you provide;
123 <br/><code>hasAntiRes</code> : whether there is an antiparticle;
124 <br/><code>mRes</code> : resonance mass;
125 <br/><code>GammaRes</code> resonance width;
126 <br/><code>m2Res</code> : the squared mass;
127 <br/><code>GamMRat</code> : the ratio of width to mass.
130 A <b>destructor</b> is only needed if you plan to delete the resonance
131 before the natural end of the run, and require some special behaviour
132 at that point. If you call such a destructor you will leave a pointer
133 dangling inside the <code>Pythia</code> object you gave it in to,
134 if that still exists.
136 <a name="method1"></a>
137 <p/><strong>void ResonanceWidths::initConstants() </strong> <br/>
138 is called once during initialization, and can then be used to set up
139 further parameters specific to this particle species, such as couplings,
140 and perform calculations that need not be repeated for each new event,
141 thereby saving time. This method needs not be implemented.
144 <a name="method2"></a>
145 <p/><strong>void ResonanceWidths::calcPreFac(bool calledFromInit = false) </strong> <br/>
146 is called once a mass has been chosen for the resonance, but before
147 a specific final state is considered. This routine can therefore
148 be used to perform calculations that otherwise might have to be repeated
149 over and over again in <code>calcWidth</code> below. It is optional
150 whether you want to use this method, however, or put
151 everything in <code>calcWidth()</code>.
152 <br/>The optional argument will have the value <code>true</code> when
153 the resonance is initialized, and then be <code>false</code> throughout
154 the event generation, should you wish to make a distinction.
155 In PYTHIA such a distinction is made for <i>gamma^*/Z^0</i> and
156 <i>gamma^*/Z^0/Z'^0</i>, owing to the necessity of a special
157 description of interference effects, but not for other resonances.
158 <br/>In addition to the base-class member variables already described
159 above, <code>mHat</code> contains the current mass of the resonance.
160 At initialization this agrees with the nominal mass <code>mRes</code>,
161 but during the run it will not (in general).
164 <a name="method3"></a>
165 <p/><strong>void ResonanceWidths::calcWidth(bool calledFromInit = false) </strong> <br/>
166 is the key method for width calculations and returns a partial width
167 value, as further described below. It is called for a specific
168 final state, typically in a loop over all allowed final states,
169 subsequent to the <code>calcPreFac(...)</code> call above.
170 Information on the final state is stored in a number of base-class
171 variables, for you to use in your calculations:
172 <br/><code>iChannel</code> : the channel number in the list of
173 possible decay channels;
174 <br/><code>mult</code> : the number of decay products;
175 <br/><code>id1, id2, id3</code> : the identity code of up to the first
176 three decay products, arranged in descending order of the absolute value
177 of the identity code;
178 <br/><code>id1Abs, id2Abs, id3Abs</code> : the absolute value of the
179 above three identity codes;
180 <br/><code>mHat</code> : the current resonance mass, which is the same
181 as in the latest <code>calcPreFac(...)</code> call;
182 <br/><code>mf1, mf2, mf3</code> : masses of the above decay products;
183 <br/><code>mr1, mr2, mr3</code> : squared ratio of the product masses
184 to the resonance mass;
185 <br/><code>ps</code> : is only meaningful for two-body decays, where it
186 gives the phase-space factor
187 <i>ps = sqrt( (1. - mr1 - mr2)^2 - 4. * mr1 * mr2 )</i>;
188 <br/>In two-body decays the third slot is zero for the above properties.
189 Should there be more than three particles in the decay, you would have
190 to take care of the subsequent products yourself, e.g. using
191 <br/><code>particlePtr->decay[iChannel].product(j);</code>
192 <br/>to extract the <code>j</code>'th decay products (with
193 <code>j = 0</code> for the first, etc.). Currently we are not aware
194 of any such examples.
195 <br/>The base class also contains methods for <i>alpha_em</i> and
196 <i>alpha_strong</i> evaluation, and can access many standard-model
197 couplings; see the existing code for examples.
198 <br/>The result of your calculation should be stored in
199 <br/><code>widNow</code> : the partial width of the current channel,
203 <a name="method4"></a>
204 <p/><strong>double ResonanceWidths::widthChan( double mHat, int idAbs1, int idAbs2) </strong> <br/>
205 is not normally used. In PYTHIA the only exception is Higgs decays,
206 where it is used to define the width (except for colour factors)
207 associated with a specific incoming/outgoing state. It allows the
208 results of some loop expressions to be pretabulated.
211 <h3>Access to resonance widths</h3>
213 Once you have implemented a class, it is straightforward to
214 make use of it in a run. Assume you have written a new class
215 <code>MyResonance</code>, which inherits from
216 <code>ResonanceWidths</code>. You then create an instance of
217 this class and hand it in to a <code>pythia</code> object with
219 ResonanceWidths* myResonance = new MyResonance();
220 pythia.setResonancePtr( myResonance);
222 If you have several resonances you can repeat the procedure any number
223 of times. When <code>pythia.init(...)</code> is called these resonances
224 are initialized along with all the internal resonances, and treated in
225 exactly the same manner. See also the <?php $filepath = $_GET["filepath"];
226 echo "<a href='ProgramFlow.php?filepath=".$filepath."' target='page'>";?>Program
231 If the code should be of good quality and general usefulness,
232 it would be simple to include it as a permanently available process
233 in the standard program distribution. The final step of that integration
234 ought to be left for the PYTHIA authors, but basically all that is
235 needed is to add one line in
236 <code>ParticleData::initResonances</code>, where one creates an
237 instance of the resonance in the same way as for the resonances already
238 there. In addition, the particle data and decay table for the new
239 resonance has to be added to the permanent
240 <?php $filepath = $_GET["filepath"];
241 echo "<a href='ParticleData.php?filepath=".$filepath."' target='page'>";?>particle database</a>, and the code itself
242 to <code>include/ResonanceWidths.h</code> and
243 <code>src/ResonanceWidths.cc</code>.
248 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->