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1<html>
2<head>
3<title>Particle Decays</title>
4<link rel="stylesheet" type="text/css" href="pythia.css"/>
5<link rel="shortcut icon" href="pythia32.gif"/>
6</head>
7<body>
8
9<h2>Particle Decays</h2>
10
11The <code>ParticleDecays</code> class performs the sequential decays of
12all unstable hadrons produced in the string fragmentation stage,
13i.e. up to and including <i>b</i> hadrons and their decay products,
14such as the <i>tau</i> lepton. It is not to be used for the decay of
15more massive <a href="ResonanceDecays.html" target="page">resonances</a>, such as top,
16<i>Z^0</i> or SUSY, where decays must be performed already at the
17<code>ProcessLevel</code> of the event generation.
18
19<p/>
20The decay description essentially copies the one present in
21PYTHIA since many years, but with some improvements, e.g. in the decay
22tables and the number of decay models available. Recently a more
23sophisticated handling of <i>tau</i> decays has also been introduced.
24Some issues may need further polishing.
25
26<h3>Variables determining whether a particle decays</h3>
27
28Before a particle is actually decayed, a number of checks are made.
29
30<p/>
31(i) Decay modes must have been defined for the particle kind;   
32tested by the <code>canDecay()</code> method of <code>Event</code> 
33(and <code>ParticleData</code>).       
34
35<p/>
36(ii) The main switch for allowing this particle kind to decay must
37be on; tested by the <code>mayDecay()</code> method of <code>Event</code> 
38(and <code>ParticleData</code>).
39
40<p/>
41(iii) Particles may be requested to have a nominal proper lifetime
42<i>tau0</i> below a threshold.
43
44<p/><code>flag&nbsp; </code><strong> ParticleDecays:limitTau0 &nbsp;</strong> 
45 (<code>default = <strong>off</strong></code>)<br/>
46When on, only particles with <i>tau0 &lt; tau0Max</i> are decayed.
47 
48
49<p/><code>parm&nbsp; </code><strong> ParticleDecays:tau0Max &nbsp;</strong> 
50 (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
51The above <i>tau0Max</i>, expressed in mm/c.
52 
53
54<p/>
55(iv) Particles may be requested to have an actual proper lifetime
56<i>tau</i> below a threshold.
57
58<p/><code>flag&nbsp; </code><strong> ParticleDecays:limitTau &nbsp;</strong> 
59 (<code>default = <strong>off</strong></code>)<br/>
60When on, only particles with <i>tau &lt; tauMax</i> are decayed.
61 
62
63<p/><code>parm&nbsp; </code><strong> ParticleDecays:tauMax &nbsp;</strong> 
64 (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
65The above <i>tauMax</i>, expressed in mm/c.<br/>
66In order for this and the subsequent tests to work, a <i>tau</i> 
67is selected and stored for each particle, whether in the end it
68decays or not. (If each test would use a different temporary
69<i>tau</i> it would lead to inconsistencies.)
70 
71
72<p/>
73(v) Particles may be requested to decay within a given distance
74of the origin.
75
76<p/><code>flag&nbsp; </code><strong> ParticleDecays:limitRadius &nbsp;</strong> 
77 (<code>default = <strong>off</strong></code>)<br/>
78When on, only particles with a decay within a radius <i>r &lt; rMax</i> 
79are decayed. There is assumed to be no magnetic field or other
80detector effects.
81 
82
83<p/><code>parm&nbsp; </code><strong> ParticleDecays:rMax &nbsp;</strong> 
84 (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
85The above <i>rMax</i>, expressed in mm.
86   
87
88<p/>
89(vi) Particles may be requested to decay within a given cylidrical
90volume around the origin.
91
92<p/><code>flag&nbsp; </code><strong> ParticleDecays:limitCylinder &nbsp;</strong> 
93 (<code>default = <strong>off</strong></code>)<br/>
94When on, only particles with a decay within a volume limited by
95<i>rho = sqrt(x^2 + y^2) &lt; xyMax</i> and <i>|z| &lt; zMax</i> 
96are decayed. There is assumed to be no magnetic field or other
97detector effects.
98 
99
100<p/><code>parm&nbsp; </code><strong> ParticleDecays:xyMax &nbsp;</strong> 
101 (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
102The above <i>xyMax</i>, expressed in mm.
103   
104
105<p/><code>parm&nbsp; </code><strong> ParticleDecays:zMax &nbsp;</strong> 
106 (<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
107The above <i>zMax</i>, expressed in mm.
108   
109
110<h3>Mixing</h3>
111
112<p/><code>flag&nbsp; </code><strong> ParticleDecays:mixB &nbsp;</strong> 
113 (<code>default = <strong>on</strong></code>)<br/>
114Allow or not <i>B^0 - B^0bar</i> and <i>B_s^0 - B_s^0bar</i> mixing.
115 
116
117<p/><code>parm&nbsp; </code><strong> ParticleDecays:xBdMix &nbsp;</strong> 
118 (<code>default = <strong>0.776</strong></code>; <code>minimum = 0.74</code>; <code>maximum = 0.81</code>)<br/>
119The mixing parameter <i>x_d = Delta(m_B^0)/Gamma_B^0</i> in the
120<i>B^0 - B^0bar</i> system. (Default from RPP2006.)
121   
122
123<p/><code>parm&nbsp; </code><strong> ParticleDecays:xBsMix &nbsp;</strong> 
124 (<code>default = <strong>26.05</strong></code>; <code>minimum = 22.0</code>; <code>maximum = 30.0</code>)<br/>
125The mixing parameter <i>x_s = Delta(m_B_s^0)/Gamma_B_s^0</i> in the
126<i>B_s^0 - B_s^0bar</i> system. (Delta-m from CDF hep-ex-0609040,
127Gamma from RPP2006.)
128   
129
130<h3>Tau decays</h3>
131
132A new machinery has been introduced to handle <i>tau</i> lepton decays,
133with helicity information related to the production process and with
134the form of the hadronic current fitted to data. It is largely based
135on the corresponding Herwig++ implementation [<a href="Bibliography.html" target="page">Gre07</a>], with
136some input from Tauola [<a href="Bibliography.html" target="page">Jad90</a>]. A complete writeup is
137in preparation [<a href="Bibliography.html" target="page">Ilt11</a>].
138
139<p/>
140For <i>tau</i>s in external processes, interfaced with Les Houches
141Acccord information available, e.g. via Les Houches Event Files (LHEF),
142the new machinery interprets the SPINUP number for <i>tau</i> leptons
143as giving their helicity, and decays them accordingly. The only exceptions
144are when a specific polarization is forced by the user (see below),
145which then overrides the SPINUP value, or when SPINUP has the special
146value 9 (unpolarized). In the latter case, PYTHIA defaults back to
147attempting to determine the helicity structure from the production
148process, in the same way as for internal processes.
149
150<p/>
151This new machinery is on by default, but it is possible to revert to
152the simpler old decay handling, e.g. to study differences. Furthermore
153the spin tracing framework does not yet cover all possibilities; notably
154it cannot handle taus coming from SUSY decay chains
155(except via LHEF), so it makes sense
156to switch off the new machinery in such instances, for speed reasons if
157nothing else. In case only one tau mother species is undefined, the
158polarization involved can be set by hand.
159
160<p/><code>mode&nbsp; </code><strong> ParticleDecays:sophisticatedTau &nbsp;</strong> 
161 (<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 3</code>)<br/>
162Choice of <i>tau</i> decay model.
163<br/><code>option </code><strong> 0</strong> : old decay model, with isotropic decays.
164When reading LHEF files, the SPINUP digit will be ignored. 
165<br/><code>option </code><strong> 1</strong> : sophisticated decays where <i>tau</i> polarization is
166calculated from the <i>tau</i> production mechanism.
167When reading LHEF files, the SPINUP digit will be used.
168 
169<br/><code>option </code><strong> 2</strong> : sophisticated decays as above, but additionally <i>tau</i> 
170polarization is set to <code>ParticleDecaus:tauPolarization</code> for
171<i>tau</i>s produced from <code>ParticleDecays:tauMother</code>.
172When reading LHEF files, this overrides the SPINUP digit.
173 
174<br/><code>option </code><strong> 3</strong> : sophisticated decays where <i>tau</i> polarization is set
175to <code>ParticleDecaus:tauPolarization</code> for all <i>tau</i> decays.
176When reading LHEF files, this overrides the SPINUP digit.
177 
178<br/><b>Note</b>: options <code>2</code> and <code>3</code>,
179to force a specific <i>tau</i> polarization, only affect the decay
180of the <i>tau</i>. The angular distribution of the <i>tau</i> itself,
181given by its production, is not modified by these options. If you want, e.g.,
182a righthanded <i>W</i>, or a SUSY decay chain, the kinematics should
183be handled by the corresponding cross section class(es), supplemented by
184the resonance decay one(s). The options here could then still be used
185to ensure the correct polarization at the <i>tau</i> decay stage.
186 
187
188<p/><code>parm&nbsp; </code><strong> ParticleDecays:tauPolarization &nbsp;</strong> 
189 (<code>default = <strong>0</strong></code>; <code>minimum = -1.</code>; <code>maximum = 1.</code>)<br/>
190Polarization of the <i>tau</i> when mode <i>2</i> or <i>3</i> of
191<code>ParticleDecays:sophisticatedTau</code> is selected.
192 
193
194<p/><code>mode&nbsp; </code><strong> ParticleDecays:tauMother &nbsp;</strong> 
195 (<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/>
196Mother of the <i>tau</i> for forced polarization when mode <i>2</i> of
197<code>ParticleDecays:sophisticatedTau</code> is selected. You should give the
198positive identity code; to the extent an antiparticle exists it will
199automatically obtain the inverse polarization.
200 
201
202<h3>Other variables</h3>
203
204<p/><code>parm&nbsp; </code><strong> ParticleDecays:mSafety &nbsp;</strong> 
205 (<code>default = <strong>0.0005</strong></code>; <code>minimum = 0.</code>; <code>maximum = 0.01</code>)<br/>
206Minimum mass difference required between the decaying mother mass
207and the sum of the daughter masses, kept as a safety margin to avoid
208numerical problems in the decay generation.
209   
210
211<p/><code>parm&nbsp; </code><strong> ParticleDecays:sigmaSoft &nbsp;</strong> 
212 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 2.</code>)<br/>
213In semileptonic decays to more than one hadron, such as
214<i>B -> nu l D pi</i>, decay products after the first three are
215dampened in momentum by an explicit weight factor
216<i>exp(-p^2/sigmaSoft^2)</i>, where <i>p</i> is the
217three-momentum in the rest frame of the decaying particle.
218This takes into account that such further particles come from the
219fragmentation of the spectator parton and thus should be soft.   
220   
221
222<p/>
223When a decay mode is defined in terms of a partonic content, a random
224multiplicity (and a random flavour set) of hadrons is to be picked,
225especially for some charm and bottom decays. This is done according to
226a Poissonian distribution, for <i>n_p</i> normal particles and
227<i>n_q</i> quarks the average value is chosen as 
228<br/><i>
229  n_p/ 2 + n_q/4 + multIncrease * ln ( mDiff / multRefMass)
230</i><br/>
231with <i>mDiff</i> the difference between the decaying particle mass
232and the sum of the normal-particle masses and the constituent quark masses.
233For gluonic systems <i>multGoffset</i> offers and optional additional
234term to the multiplicity. The lowest possible multiplicity is
235<i>n_p + n_q/2</i> (but at least 2) and the highest possible 10.
236If the picked hadrons have a summed mass above that of the mother a
237new try is made, including a new multiplicity. These constraints
238imply that the actual average multiplicity does not quite agree with
239the formula above.
240
241<p/><code>parm&nbsp; </code><strong> ParticleDecays:multIncrease &nbsp;</strong> 
242 (<code>default = <strong>4.</strong></code>; <code>minimum = 2.</code>; <code>maximum = 6.</code>)<br/>
243The above <i>multIncrease</i> parameter, except for
244<code>meMode = 23</code>.
245   
246
247<p/><code>parm&nbsp; </code><strong> ParticleDecays:multIncreaseWeak &nbsp;</strong> 
248 (<code>default = <strong>2.5</strong></code>; <code>minimum = 1.</code>; <code>maximum = 4.</code>)<br/>
249The above <i>multIncrease</i> parameter, specifically for
250<code>meMode = 23</code>. Here the weak decay implies that only the
251virtual W mass should contribute to the production of new particles,
252rather than the full meson mass.
253   
254
255<p/><code>parm&nbsp; </code><strong> ParticleDecays:multRefMass &nbsp;</strong> 
256 (<code>default = <strong>0.7</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 2.0</code>)<br/>
257The above <i>multRefMass</i> parameter.
258   
259
260<p/><code>parm&nbsp; </code><strong> ParticleDecays:multGoffset &nbsp;</strong> 
261 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
262The above <i>multGoffset</i> parameter.
263   
264
265<p/><code>parm&nbsp; </code><strong> ParticleDecays:colRearrange &nbsp;</strong> 
266 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
267When a decay is given as a list of four partons to be turned into
268hadrons (primarily for modes 41 - 80)  it is assumed that they are
269listed in pairs, as a first and a second colour singlet, which could
270give rise to separate sets of hadrons. Here <i>colRearrange</i> is
271the probability that this original assignment is not respected, and
272default corresponds to no memory of this original colour topology.
273   
274
275<p/><code>flag&nbsp; </code><strong> ParticleDecays:FSRinDecays &nbsp;</strong> 
276 (<code>default = <strong>true</strong></code>)<br/>
277When a particle decays to <i>q qbar</i>, <i>g g</i>, <i>g g g</i> 
278or <i>gamma g g</i>, with <code>meMode > 90</code>, allow or not a
279shower to develop from it, before the partonic system is hadronized.
280(The typical example is <i>Upsilon</i> decay.)
281 
282
283In addition, some variables defined for string fragmentation and for
284flavour production are used also here.
285 
286<h3>Modes for Matrix Element Processing</h3>
287
288Some decays can be treated better than what pure phase space allows,
289by reweighting with appropriate matrix elements. In others a partonic
290content has to be converted to a set of hadrons. The presence of such
291corrections is signalled by a nonvanishing <code>meMode()</code> value
292for a decay mode in the <a href="ParticleDataScheme.html" target="page">particle
293data table</a>. The list of allowed possibilities almost agrees with the
294PYTHIA 6 ones, but several obsolete choices have been removed,
295a few new introduced, and most have been moved for better consistency.
296Here is the list of currently allowed <code>meMode()</code> codes:
297<ul>
298<li>  0 : pure phase space of produced particles ("default");
299input of partons is allowed and then the partonic content is
300converted into the minimal number of hadrons (i.e. one per
301parton pair, but at least two particles in total)</li>
302<li>  1 : <i>omega</i> and <i>phi -> pi+ pi- pi0</i></li>
303<li>  2 : polarization in <i>V -> PS + PS</i> (<i>V</i> = vector,
304<i>PS</i> = pseudoscalar), when <i>V</i> is produced by
305<i>PS -> PS + V</i> or <i>PS -> gamma + V</i></li>
306<li> 11 : Dalitz decay into one particle, in addition to the
307lepton pair (also allowed to specify a quark-antiquark pair that
308should collapse to a single hadron)</li>
309<li> 12 : Dalitz decay into two or more particles in addition
310to the lepton pair</li>
311<li> 13 : double Dalitz decay into two lepton pairs</li>
312<li> 21 : decay to phase space, but weight up <i>neutrino_tau</i> spectrum
313in <i>tau</i> decay</li>
314<li> 22 : weak decay; if there is a quark spectator system it collapses to
315one hadron; for leptonic/semileptonic decays the <i>V-A</i> matrix element
316is used, for hadronic decays simple phase space</li>
317<li> 23 : as 22, but require at least three particles in decay</li>
318<li> 31 : decays of type B -> gamma X, very primitive simulation where
319X is given in terms of its flavour content, the X multiplicity is picked
320according to a geometrical distribution with average number 2, and
321the photon energy spectrum is weighted up relative to pure phase space</li>
322<li> 42 - 50 : turn partons into a random number of hadrons, picked according
323to a Poissonian with average value as described above, but at least
324<code>code</code> - 40 and at most 10, and then distribute then in pure
325phase space; make a new try with another multiplicity if the sum of daughter
326masses exceed the mother one </li>
327<li> 52 - 60 : as 42 - 50, with multiplicity between <code>code</code> - 50
328and 10, but avoid already explicitly listed non-partonic channels</li>
329<li> 62 - 70 : as 42 - 50, but fixed multiplicity <code>code</code> - 60</li>
330<li> 72 - 80 : as 42 - 50, but fixed multiplicity <code>code</code> - 70,
331and avoid already explicitly listed non-partonic channels</li>
332<li> 91 : decay to <i>q qbar</i> or <i>g g</i>, which should shower
333and hadronize</li>
334<li> 92 : decay onium to <i>g g g</i> or <i>g g gamma</i> 
335(with matrix element), which should shower and hadronize</li>
336<li> 100 - : reserved for the description of partial widths of
337<a href="ResonanceDecays.html" target="page">resonances</a></li>
338</ul>
339
340Three special decay product identity codes are defined.
341<ul>
342<li>81: remnant flavour. Used for weak decays of c and b hadrons, where the
343c or b quark decays and the other quarks are considered as a spectator
344remnant in this decay. In practice only used for baryons with multiple
345c and b quarks, which presumably would never be used, but have simple
346(copied) just-in-case decay tables. Assumed to be last decay product.</li> 
347<li>82: random flavour, picked by the standard fragmentation flavour
348machinery, used to start a sequence of hadrons, for matrix element
349codes in 41 - 80. Assumed to be first decay product, with -82 as second
350and last. Where multiplicity is free to be picked it is selected as for
351normal quarkonic systems. Currently unused.</li> 
352<li>83: as for 82, with matched pair 83, -83 of decay products. The
353difference is that here the pair is supposed to come from a closed gluon
354loop (e.g. <i>eta_c -> g g</i>) and so have a somewhat higher average
355multiplicity than the simple string assumed for 82, see the
356<code>ParticleDecays:multGoffset</code> parameter above.</li>
357</ul>
358
359</body>
360</html>
361
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