3 <title>Hadron-Level Standalone</title>
4 <link rel="stylesheet" type="text/css" href="pythia.css"/>
5 <link rel="shortcut icon" href="pythia32.gif"/>
9 <h2>Hadron-Level Standalone</h2>
11 The Les Houches Accord allows external process-level configurations
12 to be fed in, for subsequent parton-level and hadron-level generation
13 to be handled internally by PYTHIA. There is no correspondingly
14 standardized interface if you have external events that have also
15 been generated through the parton-level stage, so that only the
16 hadron-level remains to be handled. A non-standard way to achieve this
17 exists, however, and can be useful both for real applications and
18 for various tests of the hadronization model on its own.
21 The key trick is to set the flag <code>ProcessLevel:all = off</code>.
22 When <code>pythia.next()</code> is called it then does not try to
23 generate a hard process. Since there are no beams, it is also not
24 possible to perform the normal <code>PartonLevel</code> step.
25 (It is still possible to generate final-state radiation, but this
26 is not automatic. It would have to be done by hand, using the
27 <code>pythia.forceTimeShower(...)</code> method, before
28 <code>pythia.next()</code> is called.)
29 Thus only the <code>HadronLevel</code> methods are
30 called, to take the current content of the event record stored in
31 <code>pythia.event</code> as a starting point for any hadronization
32 and decays that are allowed by the normal parameters of this step.
33 Often the input would consist solely of partons grouped into colour
34 singlets, but also (colour-singlet) particles are allowed.
37 To set up all the parameters, a <code>pythia.init()</code> call has
38 to be used, without any arguments. In brief, the structure of the
39 main program therefore should be something like
41 Pythia pythia; // Declare generator.
42 Event& event = pythia.event // Convenient shorthand.
43 pythia.readString("ProcessLevel:all = off"); // The trick!
44 pythia.init(); // Initialization.
45 for (int iEvent = 0; iEvent < nEvent; ++iEvent) {
46 // Insert filling of event here!
47 pythia.next(); // Do the hadron level.
50 Of course this should be supplemented by analysis of events, error checks,
51 and so on, as for a normal PYTHIA run. The unique aspect is how to fill
52 the <code>event</code> inside the loop, before <code>pythia.next()</code>
55 <h3>Input configuration</h3>
57 To set up a new configuration the first step is to throw away the current
58 one, with <code>event.reset()</code>. This routine will also reserve
59 the zeroth entry in the even record to represent the event as a whole.
62 With the <code>event.append(...)</code> methods a new entry is added at the
63 bottom of the current record, i.e. the first time it is called entry
64 number 1 is filled, and so on. The <code>append</code> method basically
65 exists in four variants, either without or with history information,
66 and with four-momentum provided either as a <code>Vec4</code> four-vector
67 or as four individual components:
69 append( id, status, col, acol, p, m)
70 append( id, status, col, acol, px, py, pz, e, m)
71 append( id, status, mother1, mother2, daughter1, daughter2, col, acol, p, m)
72 append( id, status, mother1, mother2, daughter1, daughter2, col, acol, px, py, pz, e, m)
74 The methods return the index at which the entry has been stored,
75 but normally you would not use this feature.
78 You can find descriptions of the input variables
79 <a href="ParticleProperties.html" target="page">here</a>.
80 The PDG particle code <code>id</code> and the Les Houches Accord colour
81 <code>col</code> and anticolour <code>acol</code> tags must be set
82 correctly. The four-momentum and mass have to be provided in units of GeV;
83 if you omit the mass it defaults to 0.
86 Outgoing particles that should hadronize should be given status code 23.
87 Often this is the only status code you need. You could e.g. also fill in
88 incoming partons with -21 and intermediate ones with -22, if you so wish.
89 Usually the choice of status codes is not crucial, so long as you recall
90 that positive numbers correspond to particles that are still around, while
91 negative numbers denote ones that already hadronized or decayed. However,
92 so as not to run into contradictions with the internal PYTHIA checks
93 (when <code>Check:event = on</code>), or with external formats such as
94 HepMC, we do recommend the above codes. When <code>pythia.next()</code>
95 is called the positive-status particles that hadronize/decay get the sign
96 of the status code flipped to negative but the absolute value is retained.
97 The new particles are added with normal PYTHIA status codes.
100 For normal hadronization/decays in <code>pythia.next()</code> the
101 history encoded in the mother and daughter indices is not used.
102 Therefore the first two <code>append</code> methods, which set all these
103 indices vanishing, should suffice. The subsequent hadronization/decays
104 will still be properly documented.
107 The exception is when you want to include junctions in your string
108 topology, i.e. have three string pieces meet. Then you must insert in
109 your event record the (decayed) particle that is the reason for the
110 presence of a junction, e.g. a baryon beam remnant from which several
111 valence quarks have been kicked out, or a neutralino that underwent a
112 baryon-number-violating decay. This particle must have as daughters
113 the three partons that together carry the baryon number.
116 The sample program in <code>main21.cc</code> illustrates how you can work
117 with this facility, both for simple parton configurations and for more
118 complicated ones with junctions.
121 As an alternative to setting up a topology with the methods above,
122 a <a href="LesHouchesAccord.html" target="page">Les Houches Event File</a> (LHEF)
123 can also provide the configurations. Since no beams or processes are
124 defined, only the <code><event>....</event></code> blocks
125 need to be present, one for each event, so strictly it is not a true
126 LHEF. You need to select <code>Beams:frameType = 4</code>, provide
127 the file name in <code>Beams:LHEF</code> and, as above, set
128 <code>ProcessLevel:all = off</code>. Needless to say, an externally
129 linked <code>LHAup</code> class works as well as an LHEF,
130 with <code>Beams:frameType = 5</code>.
133 The event information to store in the LHEF, or provide by the
134 <code>LHAup</code>, is essentially the same as above. The only
135 difference is in status codes: outgoing particles should have 1
136 instead of 23, and intermediate resonances 2 instead of -22.
137 Incoming partons, if any, are -1 instead of -21.
139 <h3>Extensions to resonance decays</h3>
141 With the above scheme, <code>pythia.next()</code> will generate
142 hadronization, i.e. string fragmentation and subsequent decays of
143 normal unstable particles. It will not decay
144 <a href="ResonanceDecays.html" target="page">resonances</a>, i.e.
145 <i>W, Z</i>, top, Higgs, SUSY and other massive particles.
148 If the decay products are already provided, of course those
149 products will be hadronized, but without any of the showers
150 that one would expect in <i>Z^0 -> q qbar</i>, say. That is, the
151 presence of a decayed resonance in the event record can be nice
152 for documentation purposes, but otherwise it plays no role.
153 It is possible to change this behaviour with the following flag.
155 <p/><code>flag </code><strong> Standalone:allowResDec </strong>
156 (<code>default = <strong>off</strong></code>)<br/>
157 If off, then resonances are stable if final-state particles,
158 and irrelevant if intermediate particles. If on, then
159 resonances are decayed, sequentially if necessary. After each
160 decay step a parton shower is applied, and subsequent decay
161 kinematics is shifted by the recoil of such branchings.
162 If the decay chain and kinematics of the resonance is provided
163 at input, this information is used, otherwise it is generated
164 internally according to built-in branching ratios etc.
168 The input configuration has to follow the rules described above,
169 with <code>ProcessLevel:all = off</code>. (Terminology not quite
170 consistent, since resonance decays normally is part of the
171 process-level step.) It is possible to combine several resonances,
172 and other coloured or uncoloured particles into the same event.
175 Final-state radiation is applied to each step of the decay
176 sequence by default, but can be switched off with
177 <code>PartonLevel:FSR = off</code> or
178 <code>PartonLevel:FSRinResonances</code>.
180 <h3>Repeated hadronization or decay</h3>
182 An alternative approach is possible with the
183 <code>pythia.forceHadronLevel()</code> routine. This method does
184 a call to the <code>HadronLevel</code> methods, irrespective of the
185 value of the <code>HadronLevel:all</code> flag. If you hadronize
186 externally generated events it is equivalent to a
187 <code>pythia.next()</code> call with
188 <code>ProcessLevel:all = off</code>.
191 This method truly sticks to the hadron level, and thus cannot handle
192 resonance decays. You therefore <b>must not</b> mix it with the
193 <code>Standalone:allowResDec = on</code> framework.
196 The similarity of names indicates that
197 <code>pythia.forceTimeShower( int iBeg, int iEnd, double pTmax,
198 int nBranchMax = 0)</code> is intended to belong to the same set of
199 work-by-hand methods. Here <code>iBeg</code> and <code>iEnd</code>
200 give the range of partons that should be allowed to shower,
201 <code>pTmax</code> the maximum <i>pT</i> scale of emissions,
202 and a nonzero <code>nBranchMax</code> a maximum number of allowed
203 branchings. Additionally, a <code>scale</code> has to be set for each
204 parton that should shower, which requires an additional final argument
205 to the <code>append</code> methods above. This scale limits the maximum
206 <i>pT</i> allowed for each parton, in addition to the global
207 <code>pTmax</code>. When not set the scale defaults to 0, meaning no
208 radiation for that parton.
211 The real application instead is for repeated hadronization of the same
212 PYTHIA process- and parton-level event. This may for some studies
213 help to save time, given that these two first step are more
214 time-consuming than the hadronization one.
217 For repeated hadronization you should first generate an event as usual,
218 but with <code>HadronLevel:all = off</code>. This event you can save
219 in a temporary copy, e.g. <code>Event savedEvent = pythia.event</code>.
220 Inside a loop you copy back with <code>pythia.event = savedEvent</code>,
221 and call <code>pythia.forceHadronLevel()</code> to obtain a new
222 hadronization history.
225 A more limited form of repetition is if you want to decay a given kind
226 of particle repeatedly, without having to generate the rest of the event
227 anew. This could be the case e.g. in <i>B</i> physics applications.
228 Then you can use the <code>pythia.moreDecays()</code> method, which
229 decays all particles in the event record that have not been decayed
230 but should have been done so. The
231 <code>pythia.particleData.mayDecay( id, false/true)</code> method
232 may be used to switch off/on the decays of a particle species
233 <code>id</code>, so that it is not decayed in the
234 <code>pythia.next()</code> call but only inside a loop over a number of
238 Between each loop the newly produced decay products must be
239 removed and the decayed particle status restored to undecayed.
240 The former is simple, since the new products are appended to the
241 end of the event record: <code>event.saveSize()</code> saves the
242 initial size of the event record, and <code>event.restoreSize()</code>
243 can later be used repeatedly to restore this original size, which means
244 that the new particles at the end are thrown away. The latter is more
245 complicated, and requires the user to identify the positions of all
246 particles of the species and restore a positive status code with
247 <code>event[i].statusPos()</code>.
250 The <code>main15.cc</code> program illustrates both these methods,
251 i.e. either repeated hadronization or repeated decay of PYTHIA
257 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->