3 <title>Fragmentation</title>
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11 Fragmentation in PYTHIA is based on the Lund string model
12 [<a href="Bibliography.html" target="page">And83, Sjo84</a>]. Several different aspects are involved in
13 the physics description, which here therefore is split accordingly.
14 This also, at least partly, reflect the set of classes involved in
15 the fragmentation machinery.
18 The variables collected here have a very wide span of usefulness.
19 Some would be central in any hadronization tuning exercise, others
20 should not be touched except by experts.
23 The fragmentation flavour-choice machinery is also used in a few
24 other places of the program, notably particle decays, and is thus
25 described on the separate <a href="FlavourSelection.html" target="page">Flavour
28 <h3>Fragmentation functions</h3>
30 The <code>StringZ</code> class handles the choice of longitudinal
31 lightcone fraction <i>z</i> according to one of two possible
35 The Lund symmetric fragmentation function [<a href="Bibliography.html" target="page">And83</a>] is the
36 only alternative for light quarks. It is of the form
38 f(z) = (1/z) * (1-z)^a * exp(-b m_T^2 / z)
40 with the two main free parameters <i>a</i> and <i>b</i> to be
41 tuned to data. They are stored in
43 <p/><code>parm </code><strong> StringZ:aLund </strong>
44 (<code>default = <strong>0.3</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
45 The <i>a</i> parameter of the Lund symmetric fragmentation function.
48 <p/><code>parm </code><strong> StringZ:bLund </strong>
49 (<code>default = <strong>0.8</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 2.0</code>)<br/>
50 The <i>b</i> parameter of the Lund symmetric fragmentation function.
54 In principle, each flavour can have a different <i>a</i>. Then,
55 for going from an old flavour <i>i</i> to a new <i>j</i> one
58 f(z) = (1/z) * z^{a_i} * ((1-z)/z)^{a_j} * exp(-b * m_T^2 / z)
60 This is only implemented for diquarks relative to normal quarks:
62 <p/><code>parm </code><strong> StringZ:aExtraDiquark </strong>
63 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
64 allows a larger <i>a</i> for diquarks, with total
65 <i>a = aLund + aExtraDiquark</i>.
69 Finally, the Bowler modification [<a href="Bibliography.html" target="page">Bow81</a>] introduces an extra
74 for heavy quarks. To keep some flexibility, a multiplicative factor
75 <i>r_Q</i> is introduced, which ought to be unity (provided that
76 quark masses were uniquely defined) but can be set in
78 <p/><code>parm </code><strong> StringZ:rFactC </strong>
79 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
80 <i>r_c</i>, i.e. the above parameter for <i>c</i> quarks.
83 <p/><code>parm </code><strong> StringZ:rFactB </strong>
84 (<code>default = <strong>0.67</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
85 <i>r_b</i>, i.e. the above parameter for <i>b</i> quarks.
88 <p/><code>parm </code><strong> StringZ:rFactH </strong>
89 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
90 <i>r_h</i>, i.e. the above parameter for heavier hypothetical quarks,
91 or in general any new coloured particle long-lived enough to hadronize.
95 As an alternative, it is possible to switch over to the
96 Peterson/SLAC formula [<a href="Bibliography.html" target="page">Pet83</a>]
98 f(z) = 1 / ( z * (1 - 1/z - epsilon/(1-z))^2 )
100 for charm, bottom and heavier (defined as above) by the three flags
102 <p/><code>flag </code><strong> StringZ:usePetersonC </strong>
103 (<code>default = <strong>off</strong></code>)<br/>
104 use Peterson for <i>c</i> quarks.
107 <p/><code>flag </code><strong> StringZ:usePetersonB </strong>
108 (<code>default = <strong>off</strong></code>)<br/>
109 use Peterson for <i>b</i> quarks.
112 <p/><code>flag </code><strong> StringZ:usePetersonH </strong>
113 (<code>default = <strong>off</strong></code>)<br/>
114 use Peterson for hypothetical heavier quarks.
118 When switched on, the corresponding epsilon values are chosen to be
120 <p/><code>parm </code><strong> StringZ:epsilonC </strong>
121 (<code>default = <strong>0.05</strong></code>; <code>minimum = 0.01</code>; <code>maximum = 0.25</code>)<br/>
122 <i>epsilon_c</i>, i.e. the above parameter for <i>c</i> quarks.
125 <p/><code>parm </code><strong> StringZ:epsilonB </strong>
126 (<code>default = <strong>0.005</strong></code>; <code>minimum = 0.001</code>; <code>maximum = 0.025</code>)<br/>
127 <i>epsilon_b</i>, i.e. the above parameter for <i>b</i> quarks.
130 <p/><code>parm </code><strong> StringZ:epsilonH </strong>
131 (<code>default = <strong>0.005</strong></code>; <code>minimum = 0.0001</code>; <code>maximum = 0.25</code>)<br/>
132 <i>epsilon_h</i>, i.e. the above parameter for hypothetical heavier
133 quarks, normalized to the case where <i>m_h = m_b</i>. The actually
134 used parameter is then <i>epsilon = epsilon_h * (m_b^2 / m_h^2)</i>.
135 This allows a sensible scaling to a particle with an unknown higher
136 mass without the need for a user intervention.
139 <h3>Fragmentation <i>pT</i></h3>
141 The <code>StringPT</code> class handles the choice of fragmentation
142 <i>pT</i>. At each string breaking the quark and antiquark of the pair are
143 supposed to receive opposite and compensating <i>pT</i> kicks according
144 to a Gaussian distribution in <i>p_x</i> and <i>p_y</i> separately.
145 Call <i>sigma_q</i> the width of the <i>p_x</i> and <i>p_y</i>
146 distributions separately, i.e.
148 d(Prob) = exp( -(p_x^2 + p_y^2) / 2 sigma_q^2).
150 Then the total squared width is
152 <pT^2> = <p_x^2> + <p_y^2> = 2 sigma_q^2 = sigma^2.
154 It is this latter number that is stored in
156 <p/><code>parm </code><strong> StringPT:sigma </strong>
157 (<code>default = <strong>0.304</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 1.0</code>)<br/>
158 the width <i>sigma</i> in the fragmentation process.
162 Since a normal hadron receives <i>pT</i> contributions for two string
163 breakings, it has a <i><p_x^2>_had = <p_y^2>_had = sigma^2</i>,
164 and thus <i><pT^2>_had = 2 sigma^2</i>.
167 Some studies on isolated particles at LEP has indicated the need for
168 a slightly enhanced rate in the high-<i>pT</i> tail of the above
169 distribution. This would have to be reviewed in the context of a
170 complete retune of parton showers and hadronization, but for the
171 moment we stay with the current recipe, to boost the above <i>pT</i>
172 by a factor <i>enhancedWidth</i> for a small fraction
173 <i>enhancedFraction</i> of the breakups, where
175 <p/><code>parm </code><strong> StringPT:enhancedFraction </strong>
176 (<code>default = <strong>0.01</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 0.1</code>)<br/>
177 <i>enhancedFraction</i>,the fraction of string breaks with enhanced
181 <p/><code>parm </code><strong> StringPT:enhancedWidth </strong>
182 (<code>default = <strong>2.0</strong></code>; <code>minimum = 1.0</code>; <code>maximum = 5.0</code>)<br/>
183 <i>enhancedWidth</i>,the enhancement of the width in this fraction.
186 <h3>Jet joining procedure</h3>
188 String fragmentation is carried out iteratively from both string ends
189 inwards, which means that the two chains of hadrons have to be joined up
190 somewhere in the middle of the event. This joining is described by
191 parameters that in principle follows from the standard fragmentation
192 parameters, but in a way too complicated to parametrize. The dependence
193 is rather mild, however, so for a sensible range of variation the
194 parameters in this section should not be touched.
196 <p/><code>parm </code><strong> StringFragmentation:stopMass </strong>
197 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
198 Is used to define a <i>W_min = m_q1 + m_q2 + stopMass</i>,
199 where <i>m_q1</i> and <i>m_q2</i> are the masses of the two
200 current endpoint quarks or diquarks.
203 <p/><code>parm </code><strong> StringFragmentation:stopNewFlav </strong>
204 (<code>default = <strong>2.0</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
205 Add to <i>W_min</i> an amount <i>stopNewFlav * m_q_last</i>,
206 where <i>q_last</i> is the last <i>q qbar</i> pair produced
207 between the final two hadrons.
210 <p/><code>parm </code><strong> StringFragmentation:stopSmear </strong>
211 (<code>default = <strong>0.2</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 0.5</code>)<br/>
212 The <i>W_min</i> above is then smeared uniformly in the range
213 <i>W_min_smeared = W_min * [ 1 - stopSmear, 1 + stopSmear ]</i>.
217 This <i>W_min_smeared</i> is then compared with the current remaining
218 <i>W_transverse</i> to determine if there is energy left for further
219 particle production. If not, i.e. if
220 <i>W_transverse < W_min_smeared</i>, the final two particles are
221 produced from what is currently left, if possible. (If not, the
222 fragmentation process is started over.)
224 <h3>Simplifying systems</h3>
226 There are a few situations when it is meaningful to simplify the
227 original task, one way or another.
229 <p/><code>parm </code><strong> HadronLevel:mStringMin </strong>
230 (<code>default = <strong>1.</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 1.5</code>)<br/>
231 Decides whether a partonic system should be considered as a normal
232 string or a ministring, the latter only producing one or two primary
233 hadrons. The system mass should be above <i>mStringMin</i> plus the
234 sum of quark/diquark constituent masses for a normal string description,
235 else the ministring scenario is used.
238 <p/><code>parm </code><strong> FragmentationSystems:mJoin </strong>
239 (<code>default = <strong>0.2</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 1.</code>)<br/>
240 When two colour-connected partons are very nearby, with at least
241 one being a gluon, they can be joined into one, to avoid technical
242 problems of very small string regions. The requirement for joining is
243 that the invariant mass of the pair is below <i>mJoin</i>, where a
244 gluon only counts with half its momentum, i.e. with its contribution
245 to the string region under consideration. (Note that, for technical
246 reasons, the 0.2 GeV lower limit is de facto hardcoded.)
249 <p/><code>parm </code><strong> FragmentationSystems:mJoinJunction </strong>
250 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 2.</code>)<br/>
251 When the invariant mass of two of the quarks in a three-quark junction
252 string system becomes too small, the system is simplified to a
253 quark-diquark simple string. The requirement for this simplification
254 is that the diquark mass, minus the two quark masses, falls below
255 <i>mJoinJunction</i>. Gluons on the string between the junction and
256 the respective quark, if any, are counted as part of the quark
257 four-momentum. Those on the two combined legs are clustered with the
258 diquark when it is formed.
263 The <code>MiniStringFragmentation</code> machinery is only used when a
264 string system has so small invariant mass that normal string fragmentation
265 is difficult/impossible. Instead one or two particles are produced,
266 in the former case shuffling energy-momentum relative to another
267 colour singlet system in the event, while preserving the invariant
268 mass of that system. With one exception parameters are the same as
269 defined for normal string fragmentation, to the extent that they are
270 at all applicable in this case.
272 A discussion of the relevant physics is found in [<a href="Bibliography.html" target="page">Nor00</a>].
273 The current implementation does not completely abide to the scheme
274 presented there, however, but has in part been simplified. (In part
275 for greater clarity, in part since the class is not quite finished yet.)
277 <p/><code>mode </code><strong> MiniStringFragmentation:nTry </strong>
278 (<code>default = <strong>2</strong></code>; <code>minimum = 1</code>; <code>maximum = 10</code>)<br/>
279 Whenever the machinery is called, first this many attempts are made
280 to pick two hadrons that the system fragments to. If the hadrons are
281 too massive the attempt will fail, but a new subsequent try could
282 involve other flavour and hadrons and thus still succeed.
283 After <i>nTry</i> attempts, instead an attempt is made to produce a
284 single hadron from the system. Should also this fail, some further
285 attempts at obtaining two hadrons will be made before eventually
289 <h3>Junction treatment</h3>
291 A junction topology corresponds to an Y arrangement of strings
292 i.e. where three string pieces have to be joined up in a junction.
293 Such topologies can arise if several valence quarks are kicked out
294 from a proton beam, or in baryon-number-violating SUSY decays.
295 Special attention is necessary to handle the region just around
296 the junction, where the baryon number topologically is located.
297 The junction fragmentation scheme is described in [<a href="Bibliography.html" target="page">Sjo03</a>].
298 The parameters in this section should not be touched except by experts.
300 <p/><code>parm </code><strong> StringFragmentation:eNormJunction </strong>
301 (<code>default = <strong>2.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 10</code>)<br/>
302 Used to find the effective rest frame of the junction, which is
303 complicated when the three string legs may contain additional
304 gluons between the junction and the endpoint. To this end,
305 a pull is defined as a weighed sum of the momenta on each leg,
306 where the weight is <i>exp(- eSum / eNormJunction)</i>, with
307 <i>eSum</i> the summed energy of all partons closer to the junction
308 than the currently considered one (in the junction rest frame).
309 Should in principle be (close to) <i>sqrt((1 + a) / b)</i>, with
310 <i>a</i> and <i>b</i> the parameters of the Lund symmetric
311 fragmentation function.
314 <p/><code>parm </code><strong> StringFragmentation:eBothLeftJunction </strong>
315 (<code>default = <strong>1.0</strong></code>; <code>minimum = 0.5</code>)<br/>
316 Retry (up to 10 times) when the first two considered strings in to a
317 junction both have a remaining energy (in the junction rest frame)
321 <p/><code>parm </code><strong> StringFragmentation:eMaxLeftJunction </strong>
322 (<code>default = <strong>10.0</strong></code>; <code>minimum = 0.</code>)<br/>
323 Retry (up to 10 times) when the first two considered strings in to a
324 junction has a highest remaining energy (in the junction rest frame)
325 above a random energy evenly distributed between
326 <i>eBothLeftJunction</i> and
327 <i>eBothLeftJunction + eMaxLeftJunction</i>
328 (drawn anew for each test).
331 <p/><code>parm </code><strong> StringFragmentation:eMinLeftJunction </strong>
332 (<code>default = <strong>0.2</strong></code>; <code>minimum = 0.</code>)<br/>
333 Retry (up to 10 times) when the invariant mass-squared of the final leg
334 and the leftover momentum of the first two treated legs falls below
335 <i>eMinLeftJunction</i> times the energy of the final leg (in the
336 junction rest frame).
342 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->