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SemiInternalResonances.php @ 1

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first import of structure, PYTHIA8 and DELPHES
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3	<title>Semi-Internal Resonances</title>
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30	<h2>Semi-Internal Resonances</h2>
31
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.
45
46	<p/>
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
55	channel.
56
57	<p/>
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>main22.cc</code>, that illustrates how you could combine
63	a new process and a new resonance.
64
65	<p/>
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.
75
76	<h3>The ResonanceWidths Class</h3>
77
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.
85
86	<p/>
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.
92
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>.
105
106	<p/>
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
116	not needed.
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.
128
129	<p/>
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.
135
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.
142
143
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).
162
163
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,
200	expressed in GeV.
201
202
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.
209
210
211	<h3>Access to resonance widths</h3>
212
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
218	<pre>
219	ResonanceWidths* myResonance = new MyResonance();
220	pythia.setResonancePtr( myResonance);
221	</pre>
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
227	Flow</a>
228	description.
229
230	<p/>
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>.
244
245	</body>
246	</html>
247
248	<!-- Copyright (C) 2012 Torbjorn Sjostrand -->

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