3 <title>Resonance Decays</title>
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30 <h2>Resonance Decays</h2>
32 The <code>ResonanceDecays</code> class performs the sequential decays of
33 all resonances formed in the hard process. Note the important distinction
34 between "resonances" and other "particles" made in PYTHIA.
37 The list of resonances contains <i>gamma^*/Z^0</i>, <i>W^+-</i>, top,
38 the Higgs, and essentially all new particles of Beyond-the-Standard-Model
39 physics: further Higgses, sfermions, gauginos, techniparticles, and so on.
40 The partial widths to different decay channels are perturbatively
41 calculable, given the parameters of the respective model, and branching
42 ratios may be allowed to vary across a (reasonably broad) resonance peak.
43 Usually resonances are short-lived, and therefore it makes sense to consider
44 their decays immediately after the primary hard process has been set up.
45 Furthermore, in several cases the decay angular distributions are encoded
46 as part of the specific process, e.g. the <i>W</i> decays differently in
47 <i>f fbar -> W^+-</i>, <i>f fbar -> W^+ W^-</i> and
48 <i>h^0 -> W^+ W^- </i>. All of these particles are (in PYTHIA) only
49 produced as part of the hard process itself, i.e. they are not produced
50 in showers or hadronization processes. Therefore the restriction to
51 specific decay channels can be consistently taken into account as a
52 corresponding reduction in the cross section of a process. Finally, note
53 that all of these resonances have an on-shell mass above 20 GeV, with the
54 exception of some hypothetical weakly interacting and stable particles
55 such as the gravitino.
58 The other particles include normal hadrons and the Standard-Model leptons,
59 including the <i>tau^+-</i>. These can be produced in the normal
60 hadronization and decay description, which involve unknown nonperturbative
61 parameters and multistep chains that cannot be predicted beforehand:
62 a hard process like <i>g g -> g g</i> can develop a shower with a
63 <i>g -> b bbar</i> branching, where the <i>b</i> hadronizes to a
64 <i>B^0bar</i> that oscillates to a <i>B^0</i> that decays to a
65 <i>tau^+</i>. Therefore any change of branching ratios - most of which
66 are determined from data rather than from first principles anyway -
67 will not be taken into account in the cross section of a process.
68 Exceptions exist, but most particles in this class are made to decay
69 isotropically. Finally, note that all of these particles have a mass
74 There is one ambiguous case in this classification, namely the photon.
75 The <i>gamma^*/Z^0</i> combination contains a low-mass peak when
76 produced in a hard process. On the other hand, photons can participate
77 in shower evolution, and therefore a photon originally assumed
78 massless can be assigned an arbitrarily high mass when it is allowed
79 to branch into a fermion pair. In some cases this could lead to
80 doublecounting, e.g. between processes such as
81 <i>f fbar -> (gamma^*/Z^0) (gamma^*/Z^0)</i>,
82 <i>f fbar -> (gamma^*/Z^0) gamma</i> and
83 <i>f fbar -> gamma gamma</i>. Here it make sense to limit the
84 lower mass allowed for the <i>gamma^*/Z^0</i> combination,
85 in <code>23:mMin</code>, to be the same as the upper limit allowed
86 for an off-shell photon in the shower evolution, in
87 <code>TimeShower:mMaxGamma</code>. By default this matching is done
91 In spite of the above-mentioned differences, the resonances and the
92 other particles are all stored in one common
93 <?php $filepath = $_GET["filepath"];
94 echo "<a href='ParticleData.php?filepath=".$filepath."' target='page'>";?>particle data table</a>, so as to offer a
95 uniform interface to <?php $filepath = $_GET["filepath"];
96 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>setting and
97 getting</a> properties such as name, mass, charge and decay modes,
98 also for the <?php $filepath = $_GET["filepath"];
99 echo "<a href='ParticleProperties.php?filepath=".$filepath."' target='page'>";?>particle properties</a>
100 in the event record. Some methods are specific to resonances, however,
101 in particular for the calculation of partial widths and thereby of
102 branching ratio. For resonances these can be calculated dynamically,
103 set up at initialization for the nominal mass and then updated to the
104 current mass when these are picked according to a Breit-Wigner resonance
107 <h3>Resonance Decays and Cross Sections</h3>
109 As already hinted above, you have the possibility to set the allowed
110 decay channels of resonances, see
111 <?php $filepath = $_GET["filepath"];
112 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>Particle Data Scheme</a> description.
113 For instance, if you study the process <i>q qbar -> H^0 Z^0</i>
114 you could specify that the <i>Z^0</i> should decay only to
115 lepton pairs, the <i>H^0</i> only to <i>W^+ W^-</i>, the
116 <i>W^+</i> only to a muon and a neutrino, while the <i>W^-</i>
117 can decay to anything. Unfortunately there are limits to the
118 flexibility: you cannot set a resonance to have different properties
119 in different places of a process, e.g. if instead
120 <i>H^0 -> Z^0 Z^0</i> in the above process then the three
121 <i>Z^0</i>'s would all obey the same rules.
124 The restrictions on the allowed final states of a process is directly
125 reflected in the cross section of it. That is, if some final states
126 are excluded then the cross section is reduced accordingly. Such
127 restrictions are built up recursively in cases of sequential decay
128 chains. The restrictions are also reflected in the compositions of
129 those events that actually do get to be generated. For instance,
130 the relative rates of <i>H^0 -> W^+ W^-</i> and
131 <i>H^0 -> Z^0 Z^0</i> are shifted when the allowed sets of
132 <i>W^+-</i> and <i>Z^0</i> decay channels are changed.
135 We remind that only those particles that Pythia treat as resonances
136 enjoy this property, and only those that are considered as part of the
137 hard process and its assocaited resonance decays.
140 There is one key restriction on resonances:
141 <br/><br/><table><tr><td><strong>ResonanceWidths:minWidth </td><td></td><td> <input type="text" name="1" value="1e-20" size="20"/> (<code>default = <strong>1e-20</strong></code>; <code>minimum = 1e-30</code>)</td></tr></table>
142 Minimal allowed width of a resonance, in GeV. If the width falls below
143 this number the resonance is considered stable and will not be allowed
144 to decay. This is mainly intended as a technical parameter, to avoid
145 disasters in cases where no open decay channels exists at all. It could
146 be used for real-life decisions as well, however, but then typically
147 would have to be much bigger than the default value. Special caution
148 would be needed if coloured resonance particles were made stable, since
149 the program would not necessarily know how to hadronize them, and
150 therefore fail at that stage.
153 <h3>Special properties and methods for resonances</h3>
155 The method <code>ParticleData::isResonance(id)</code> allows you to
156 query whether a given particle species is considered a resonance or not.
157 You can also change the default value of this flag in the normal way,
158 e.g. <code>pythia.readString("id:isResonance = true")</code>.
161 An option with a forced width can be set with the
162 <code>id:doForceWidth</code> flag as above, and queried with
163 <code>ParticleData::doForceWidth(id)</code>. It is by default
164 <code>off</code>, and should normally so remain. If switched
165 <code>on</code> then the width stored in <code>id:mWidth</code> is
166 strictly used to describe the Breit-Wigner of the resonance. This is
167 unlike the normal behaviour of standard resonances such as the
168 <i>Z^0</i>, <i>W^+-</i>, <i>t</i> or <i>h^0</i>, which have
169 explicit decay-widths formulae encoded, in classes derived from the
170 <code><?php $filepath = $_GET["filepath"];
171 echo "<a href='SemiInternalResonances.php?filepath=".$filepath."' target='page'>";?>ResonanceWidths</a></code>
172 base class. These formulae are used, e.g., to derive all the Higgs partial
173 widths as a function of the Higgs mass you choose, and at initialization
174 overwrites the existing total width value. The reason for forcing the
175 width to another value specified by you would normally more have to do
176 with experimental issues than with physics ones, e.g. how sensitive your
177 detector would be to changes in the Higgs width by a factor of two.
178 A warning is that such a rescaling could modify the cross section of
179 a process correspondingly for some processes, while leaving it
180 (essentially) unchanged for others (as would seem most logical),
181 depending on how these were encoded. A further warning is that,
182 if you use this facility for <i>Z^0</i> or <i>Z'^0</i> with
183 <i>gamma^*/Z^0</i> or <i>gamma^*/Z^0/Z'^0</i> interference on,
184 then also the handling of this interference is questionable.
185 So, if you need to use the width-rescaling option, be extremely cautios.
188 If a resonance does not have a class of its own, with hardcoded equations
189 for all relevant partial widths, then a simpler object will be created
190 at initialization. This object will take the total width and branching
191 ratios as is (with the optional variations explained in the next section),
192 and thus the rescaling approach brings no further freedom.
195 Mainly for internal usage, the
196 <code><?php $filepath = $_GET["filepath"];
197 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>ParticleData</a></code> contain
198 some special methods that are only meaningful for resonances:
200 <li><code>resInit(...)</code> to initialize a resonance, possibly
201 including a recalculation of the nominal width to match the nominal
203 <li><code>resWidth(...)</code> to calculate the partial and total widths
204 at the currently selected mass;</li>
205 <li><code>resWidthOpen(...)</code> to calculate the partial and total
206 widths of those channels left open by user switches, at the currently
208 <li><code>resWidthStore(...)</code> to calculate the partial and total
209 widths of those channels left open by user switches, at the currently
210 selected mass, and store those as input for a subsequent selection of
212 <li><code>resOpenFrac(...)</code> to return the fraction of the total
213 width that is open by the decay channel selection made by users (based on
214 the choice of <code><?php $filepath = $_GET["filepath"];
215 echo "<a href='ParticleDataScheme.php?filepath=".$filepath."' target='page'>";?>onMode</a></code>
216 for the various decay channels, recursively calculated for sequential
218 <li><code>resWidthRescaleFactor(...)</code> returns the factor by which
219 the internally calculated PYTHIA width has to be rescaled to give the
220 user-enforced width;</li>
221 <li><code>resWidthChan(...)</code> to return the width for one particular
222 channel (currently only used for Higgs decays, to obtain instate coupling
223 from outstate width).</li>
225 These methods actually provide an interface to the classes derived from
226 the <code>ResonanceWidths</code> base class, to describe various
229 <h3>Modes for Matrix Element Processing</h3>
231 The <code>meMode()</code> value for a decay mode is used to specify
232 <?php $filepath = $_GET["filepath"];
233 echo "<a href='ParticleDecays.php?filepath=".$filepath."' target='page'>";?>nonisotropic decays or the conversion of
234 a parton list into a set of hadrons</a> in some channels of normal
235 particles. For resonances it can also take a third function, namely
236 to describe how the branching ratios and widths of a resonance should
237 be rescaled as a function of the current mass of the decaying resonance.
238 The rules are especially useful when new channels are added to an
239 existing particle, or a completely new resonance added.
242 <li>0 : channels for which hardcoded partial-width expressions are
243 expected to exist in the derived class of the respective resonance.
244 Should no such code exist then the partial width defaults to zero.
246 <li>1 - 99 : same as 0, but normally not used for resonances.</li>
247 <li>100 : calculate the partial width of the channel from its stored
248 branching ratio times the stored total width. This value remains unchanged
249 when the resonance fluctuates in mass. Specifically there are no
250 threshold corrections. That is, if the resonance fluctuates down in
251 mass, to below the nominal threshold, it is assumed that one of the
252 daughters could also fluctuate down to keep the channel open. (If not,
253 there may be problems later on.)
255 <li>101 : calculate the partial width of the channel from its stored
256 branching ratio times the stored total width. Multiply by a step threshold,
257 i.e. the channel is switched off when the sum of the daughter on-shell
258 masses is above the current mother mass.</li>
259 <li>102 : calculate the partial width of the channel from its stored
260 branching ratio times the stored total width. Multiply by a smooth
262 <i>beta = sqrt( (1 - m_1^2/m_2 - m_2^2/m^2)^2 - 4 m_1^2 m_2^2/m^4)</i>
263 for two-body decays and <i>sqrt(1 - Sum_i m_i / m)</i> for multibody
264 ones. The former correctly encodes the size of the phase space but
265 misses out on any nontrivial matrix-element behaviour, while the latter
266 obviously is a very crude simplification of the correct phase-space
267 expression. Specifically, it is thereby assumed that the stored branching
268 ratio and total width did not take into account such a factor.</li>
269 <li>103 : use the same kind of behaviour and threshold factor as for
270 102 above, but assume that such a threshold factor has been used when
271 the default branching ratio and total width were calculated, so that one
272 should additionally divide by the on-shell threshold factor. Specifically,
273 this will give back the stored branching ratios for on-shell mass,
274 unlike the 102 option. To avoid division by zero, or in general
275 unreasonably big rescaling factors, a lower limit
276 <code>minThreshold</code> (see below) on the value of the on-shell
277 threshold factor is imposed. (In cases where a big rescaling is
278 intentional, code 102 would be more appropriate.) </li>
281 <br/><br/><table><tr><td><strong>ResonanceWidths:minThreshold </td><td></td><td> <input type="text" name="2" value="0.1" size="20"/> (<code>default = <strong>0.1</strong></code>; <code>minimum = 0.01</code>)</td></tr></table>
282 Used uniquely for <code>meMode = 103</code> to set the minimal value
283 assumed for the threshold factor,
284 <i>sqrt( (1 - m_1^2/m_2 - m_2^2/m^2)^2 - 4 m_1^2 m_2^2/m^4)</i>
285 for two-body decays and <i>sqrt(1 - Sum_i m_i / m)</i> for multibody
286 ones. Thus the inverse of this number sets an upper limit for how
287 much the partial width of a channel can increase from the on-shell
288 value to the value for asymptotically large resonance masses. Is mainly
289 intended as a safety measure, to avoid unintentionally large rescalings.
293 All of these <code>meMode</code>'s may coexist for the same resonance.
294 This would be the case e.g. if you want to add a few new channels to an
295 already existing resonance, where the old partial widths come hardcoded
296 while the new ones are read in from an external file. The typical example
297 would be an MSSM Higgs sector, where partial widths to SM particles are
298 already encoded, <code>meMode = 0</code>, while decay rates to sparticles
299 are read in from some external calculation and maybe would be best
300 approximated by using <code>meMode = 103</code>. Indeed the default
301 particle table in PYTHIA uses 103 for all channels that are expected
302 to be provided by external input.
305 Some further clarification may be useful. At initialization the existing
306 total width and on-shell branching ratios will be updated. For channels
307 with <code>meMode < 100</code> the originally stored branching ratios
308 are irrelevant, since the existing code will anyway be used to calculate
309 the partial widths from scratch. For channels with <code>meMode = 100</code>
310 or bigger, instead the stored branching ratio is used together with the
311 originally stored total width to define the correct on-shell partial width.
312 The sum of partial widths then gives the new total width, and from there
313 new branching ratios are defined.
316 In these operations the original sum of branching ratios need not be
317 normalized to unity. For instance, you may at input have a stored total
318 width of 1 GeV and a sum of branching ratios of 2. After initialization
319 the width will then have been changed to 2 GeV and the sum of branching
320 ratios rescaled to unity. This might happen e.g. if you add a few channels
321 to an existing resonance, without changing the branching ratios of the
322 existing channels or the total width of the resonance.
325 In order to simulate the Breit-Wigner shape correctly, it is important
326 that all channels that contribute to the total width are included in the
327 above operations. This must be kept separate from the issue of which
328 channels you want to have switched on for a particular study, to be
333 In the event-generation process, when an off-shell resonance mass has been
334 selected, the width and branching ratios are re-evaluated for this new mass.
335 At this stage also the effects of restrictions on allowed decay modes are
336 taken into account, as set by the <code>onMode</code> switch for each
337 separate decay channel. Thus a channel may be on or off, with different
338 choices of open channels between the particle and its antiparticle.
339 In addition, even when a channel is on, the decay may be into another
340 resonance with its selection of allowed channels. It is these kinds of
341 restrictions that lead to the <i>Gamma_out</i> possibly being
342 smaller than <i>Gamma_tot</i>. As a reminder, the Breit-Wigner for
343 decays behaves like <i>Gamma_out / ((s - m^2)^2 + s * Gamma_tot^2)</i>,
344 where the width in the numerator is only to those channels being studied,
345 but the one in the denominator to all channels of the particle. These
346 ever-changing numbers are not directly visible to the user, but are only
347 stored in a work area.
349 <input type="hidden" name="saved" value="1"/>
352 echo "<input type='hidden' name='filepath' value='".$_GET["filepath"]."'/>"?>
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359 if($_POST["saved"] == 1)
361 $filepath = $_POST["filepath"];
362 $handle = fopen($filepath, 'a');
364 if($_POST["1"] != "1e-20")
366 $data = "ResonanceWidths:minWidth = ".$_POST["1"]."\n";
367 fwrite($handle,$data);
369 if($_POST["2"] != "0.1")
371 $data = "ResonanceWidths:minThreshold = ".$_POST["2"]."\n";
372 fwrite($handle,$data);
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