3 <title>Beam Remnants</title>
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13 The <code>BeamParticle</code> class contains information on all partons
14 extracted from a beam (so far). As each consecutive multiple interaction
15 defines its respective incoming parton to the hard scattering a
16 new slot is added to the list. This information is modified when
17 the backwards evolution of the spacelike shower defines a new
18 initiator parton. It is used, both for the multiple interactions
19 and the spacelike showers, to define rescaled parton densities based
20 on the <i>x</i> and flavours already extracted, and to distinguish
21 between valence, sea and companion quarks. Once the perturbative
22 evolution is finished, further beam remnants are added to obtain a
23 consistent set of flavours. The current physics framework is further
24 described in [<a href="Bibliography.html" target="page">Sjo04</a>].
27 The introduction of <a href="MultipleInteractions.html" target="page">rescattering</a>
28 in the multiple interactions framework further complicates the
29 processing of events. Specifically, when combined with showers,
30 the momentum of an individual parton is no longer uniquely associated
31 with one single subcollision. Nevertheless the parton is classified
32 with one system, owing to the technical and administrative complications
33 of more complete classifications. Therefore the addition of primordial
34 <i>kT</i> to the subsystem initiator partons does not automatically
35 guarantee overall <i>pT</i> conservation. Various tricks are used to
36 minimize the mismatch, with a brute force shift of all parton
37 <i>pT</i>'s as a final step.
40 Much of the above information is stored in a vector of
41 <code>ResolvedParton</code> objects, which each contains flavour and
42 momentum information, as well as valence/companion information and more.
43 The <code>BeamParticle</code> method <code>list()</code> shows the
44 contents of this vector, mainly for debug purposes.
47 The <code>BeamRemnants</code> class takes over for the final step
48 of adding primordial <i>kT</i> to the initiators and remnants,
49 assigning the relative longitudinal momentum sharing among the
50 remnants, and constructing the overall kinematics and colour flow.
51 This step couples the two sides of an event, and could therefore
52 not be covered in the <code>BeamParticle</code> class, which only
53 considers one beam at a time.
56 The methods of these classes are not intended for general use,
57 and so are not described here.
60 In addition to the parameters described on this page, note that the
61 choice of <a href="PDFSelection.html" target="page">parton densities</a> is made
62 in the <code>Pythia</code> class. Then pointers to the pdf's are handed
63 on to <code>BeamParticle</code> at initialization, for all subsequent
66 <h3>Primordial <i>kT</i></h3>
68 The primordial <i>kT</i> of initiators of hard-scattering subsystems
69 are selected according to Gaussian distributions in <i>p_x</i> and
70 <i>p_y</i> separately. The widths of these distributions are chosen
71 to be dependent on the hard scale of the central process and on the mass
72 of the whole subsystem defined by the two initiators:
74 sigma = (sigma_soft * Q_half + sigma_hard * Q) / (Q_half + Q)
77 Here <i>Q</i> is the hard-process renormalization scale for the
78 hardest process and the <i>pT</i> scale for subsequent multiple
79 interactions, <i>m</i> the mass of the system, and
80 <i>sigma_soft</i>, <i>sigma_hard</i>, <i>Q_half</i> and
81 <i>m_half</i> parameters defined below. Furthermore each separately
82 defined beam remnant has a distribution of width <i>sigma_remn</i>,
83 independently of kinematical variables.
85 <p/><code>flag </code><strong> BeamRemnants:primordialKT </strong>
86 (<code>default = <strong>on</strong></code>)<br/>
87 Allow or not selection of primordial <i>kT</i> according to the
88 parameter values below.
91 <p/><code>parm </code><strong> BeamRemnants:primordialKTsoft </strong>
92 (<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>)<br/>
93 The width <i>sigma_soft</i> in the above equation, assigned as a
94 primordial <i>kT</i> to initiators in the soft-interaction limit.
97 <p/><code>parm </code><strong> BeamRemnants:primordialKThard </strong>
98 (<code>default = <strong>2.0</strong></code>; <code>minimum = 0.</code>)<br/>
99 The width <i>sigma_hard</i> in the above equation, assigned as a
100 primordial <i>kT</i> to initiators in the hard-interaction limit.
103 <p/><code>parm </code><strong> BeamRemnants:halfScaleForKT </strong>
104 (<code>default = <strong>1.</strong></code>; <code>minimum = 0.</code>)<br/>
105 The scale <i>Q_half</i> in the equation above, defining the
106 half-way point between hard and soft interactions.
109 <p/><code>parm </code><strong> BeamRemnants:halfMassForKT </strong>
110 (<code>default = <strong>1.</strong></code>; <code>minimum = 0.</code>)<br/>
111 The scale <i>m_half</i> in the equation above, defining the
112 half-way point between low-mass and high-mass subsystems.
113 (Kinematics construction can easily fail if a system is assigned
114 a primordial <i>kT</i> value higher than its mass, so the
115 mass-dampening is intended to reduce some troubles later on.)
118 <p/><code>parm </code><strong> BeamRemnants:primordialKTremnant </strong>
119 (<code>default = <strong>0.4</strong></code>; <code>minimum = 0.</code>)<br/>
120 The width <i>sigma_remn</i>, assigned as a primordial <i>kT</i>
121 to beam-remnant partons.
125 A net <i>kT</i> imbalance is obtained from the vector sum of the
126 primordial <i>kT</i> values of all initiators and all beam remnants.
127 This quantity is compensated by a shift shared equally between
128 all partons, except that the dampening factor <i>m / (m_half + m)</i>
129 is again used to suppress the role of small-mass systems.
132 Note that the current <i>sigma</i> definition implies that
133 <i><pT^2> = <p_x^2>+ <p_y^2> = 2 sigma^2</i>.
134 It thus cannot be compared directly with the <i>sigma</i>
135 of nonperturbative hadronization, where each quark-antiquark
136 breakup corresponds to <i><pT^2> = sigma^2</i> and only
137 for hadrons it holds that <i><pT^2> = 2 sigma^2</i>.
138 The comparison is further complicated by the reduction of
139 primordial <i>kT</i> values by the overall compensation mechanism.
141 <p/><code>flag </code><strong> BeamRemnants:rescatterRestoreY </strong>
142 (<code>default = <strong>off</strong></code>)<br/>
143 Is only relevant when <a href="MultipleInteractions.html" target="page">rescattering</a>
144 is switched on in the multiple interactions scenario. For a normal
145 interaction the rapidity and mass of a system is preserved when
146 primordial <i>kT</i> is introduced, by appropriate modification of the
147 incoming parton momenta. Kinematics construction is more complicated for
148 a rescattering, and two options are offered. Differences between these
149 can be used to explore systematic uncertainties in the rescattering
151 The default behaviour is to keep the incoming rescattered parton as is,
152 but to modify the unrescattered incoming parton so as to preserve the
153 invariant mass of the system. Thereby the rapidity of the rescattering
155 The alternative is to retain the rapidity (and mass) of the rescattered
156 system when primordial <i>kT</i> is introduced. This is made at the
157 expense of a modified longitudinal momentum of the incoming rescattered
158 parton, so that it does not agree with the momentum it ought to have had
159 by the kinematics of the previous interaction.<br/>
160 For a double rescattering, when both incoming partons have already scattered,
161 there is no obvious way to retain the invariant mass of the system in the
162 first approach, so the second is always used.
167 The colour flows in the separate subprocesses defined in the
168 multiple-interactions scenario are tied together via the assignment
169 of colour flow in the beam remnant. This is not an unambiguous
170 procedure, but currently no parameters are directly associated with it.
171 However, a simple "minimal" procedure of colour flow only via the beam
172 remnants does not result in a scenario in
173 agreement with data, notably not a sufficiently steep rise of
174 <i><pT>(n_ch)</i>. The true origin of this behaviour and the
175 correct mechanism to reproduce it remains one of the big unsolved issues
176 at the borderline between perturbative and nonperturbative QCD.
177 As a simple attempt, an additional step is introduced, wherein the gluons
178 of a lower-<i>pT</i> system are merged with the ones in a higher-pT one.
180 <p/><code>flag </code><strong> BeamRemnants:reconnectColours </strong>
181 (<code>default = <strong>on</strong></code>)<br/>
182 Allow or not a system to be merged with another one.
185 <p/><code>parm </code><strong> BeamRemnants:reconnectRange </strong>
186 (<code>default = <strong>10.0</strong></code>; <code>minimum = 0.</code>; <code>maximum = 10.</code>)<br/>
187 A system with a hard scale <i>pT</i> can be merged with one of a
188 harder scale with a probability that is
189 <i>pT0_Rec^2 / (pT0_Rec^2 + pT^2)</i>, where
190 <i>pT0_Rec</i> is <code>reconnectRange</code> times <i>pT0</i>,
191 the latter being the same energy-dependent dampening parameter as
192 used for multiple interactions.
193 Thus it is easy to merge a low-<i>pT</i> system with any other,
194 but difficult to merge two high-<i>pT</i> ones with each other.
198 The procedure is used iteratively. Thus first the reconnection probability
199 <i>P = pT0_Rec^2 / (pT0_Rec^2 + pT^2)</i> of the lowest-<i>pT</i>
200 system is found, and gives the probability for merger with the
201 second-lowest one. If not merged, it is tested with the third-lowest one,
202 and so on. For the <i>m</i>'th higher system the reconnection
203 probability thus becomes <i>(1 - P)^(m-1) P</i>. That is, there is
204 no explicit dependence on the higher <i>pT</i> scale, but implicitly
205 there is via the survival probability of not already having been merged
206 with a lower-<i>pT</i> system. Also note that the total reconnection
207 probability for the lowest-<i>pT</i> system in an event with <i>n</i>
208 systems becomes <i>1 - (1 - P)^(n-1)</i>. Once the fate of the
209 lowest-<i>pT</i> system has been decided, the second-lowest is considered
210 with respect to the ones above it, then the third-lowest, and so on.
213 Once it has been decided which systems should be joined, the actual merging
214 is carried out in the opposite direction. That is, first the hardest
215 system is studied, and all colour dipoles in it are found (including to
216 the beam remnants, as defined by the holes of the incoming partons).
217 Next each softer system to be merged is studied in turn. Its gluons are,
218 in decreasing <i>pT</i> order, inserted on the colour dipole <i>i,j</i>
219 that gives the smallest <i>(p_g p_i)(p_g p_j)/(p_i p_j)</i>, i.e.
220 minimizes the "disturbance" on the existing dipole, in terms of
221 <i>pT^2</i> or <i>Lambda</i> measure (string length). The insertion
222 of the gluon means that the old dipole is replaced by two new ones.
223 Also the (rather few) quark-antiquark pairs that can be traced back to
224 a gluon splitting are treated in close analogy with the gluon case.
225 Quark lines that attach directly to the beam remnants cannot be merged
229 The joining procedure can be viewed as a more sophisticated variant of
230 the one introduced already in [<a href="Bibliography.html" target="page">Sjo87</a>]. Clearly it is ad hoc.
231 It hopefully captures some elements of truth. The lower <i>pT</i> scale
232 a system has the larger its spatial extent and therefore the larger its
233 overlap with other systems. It could be argued that one should classify
234 individual initial-state partons by <i>pT</i> rather than the system
235 as a whole. However, for final-state radiation, a soft gluon radiated off
236 a hard parton is actually produced at late times and therefore probably
237 less likely to reconnect. In the balance, a classification by system
238 <i>pT</i> scale appears sensible as a first try.
241 Note that the reconnection is carried out before resonance decays are
242 considered. Colour inside a resonance therefore is not reconnected.
243 This is a deliberate choice, but certainly open to discussion and
244 extensions at a later stage, as is the rest of this procedure.
246 <h3>Further variables</h3>
248 <p/><code>mode </code><strong> BeamRemnants:maxValQuark </strong>
249 (<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
250 The maximum valence quark kind allowed in acceptable incoming beams,
251 for which multiple interactions are simulated. Default is that hadrons
252 may contain <i>u</i>, <i>d</i> and <i>s</i> quarks,
253 but not <i>c</i> and <i>b</i> ones, since sensible
254 kinematics has not really been worked out for the latter.
257 <p/><code>mode </code><strong> BeamRemnants:companionPower </strong>
258 (<code>default = <strong>4</strong></code>; <code>minimum = 0</code>; <code>maximum = 4</code>)<br/>
259 When a sea quark has been found, a companion antisea quark ought to be
260 nearby in <i>x</i>. The shape of this distribution can be derived
261 from the gluon mother distribution convoluted with the
262 <i>g -> q qbar</i> splitting kernel. In practice, simple solutions
263 are only feasible if the gluon shape is assumed to be of the form
264 <i>g(x) ~ (1 - x)^p / x</i>, where <i>p</i> is an integer power,
265 the parameter above. Allowed values correspond to the cases programmed.
267 Since the whole framework is approximate anyway, this should be good
268 enough. Note that companions typically are found at small <i>Q^2</i>,
269 if at all, so the form is supposed to represent <i>g(x)</i> at small
270 <i>Q^2</i> scales, close to the lower cutoff for multiple interactions.
274 When assigning relative momentum fractions to beam-remnant partons,
275 valence quarks are chosen according to a distribution like
276 <i>(1 - x)^power / sqrt(x)</i>. This <i>power</i> is given below
277 for quarks in mesons, and separately for <i>u</i> and <i>d</i>
278 quarks in the proton, based on the approximate shape of low-<i>Q^2</i>
279 parton densities. The power for other baryons is derived from the
280 proton ones, by an appropriate mixing. The <i>x</i> of a diquark
281 is chosen as the sum of its two constituent <i>x</i> values, and can
282 thus be above unity. (A common rescaling of all remnant partons and
283 particles will fix that.) An additional enhancement of the diquark
284 momentum is obtained by its <i>x</i> value being rescaled by the
285 <code>valenceDiqEnhance</code> factor.
287 <p/><code>parm </code><strong> BeamRemnants:valencePowerMeson </strong>
288 (<code>default = <strong>0.8</strong></code>; <code>minimum = 0.</code>)<br/>
289 The abovementioned power for valence quarks in mesons.
292 <p/><code>parm </code><strong> BeamRemnants:valencePowerUinP </strong>
293 (<code>default = <strong>3.5</strong></code>; <code>minimum = 0.</code>)<br/>
294 The abovementioned power for valence <i>u</i> quarks in protons.
297 <p/><code>parm </code><strong> BeamRemnants:valencePowerDinP </strong>
298 (<code>default = <strong>2.0</strong></code>; <code>minimum = 0.</code>)<br/>
299 The abovementioned power for valence <i>d</i> quarks in protons.
302 <p/><code>parm </code><strong> BeamRemnants:valenceDiqEnhance </strong>
303 (<code>default = <strong>2.0</strong></code>; <code>minimum = 0.5</code>; <code>maximum = 10.</code>)<br/>
304 Enhancement factor for valence diqaurks in baryons, relative to the
305 simple sum of the two constituent quarks.
308 <p/><code>flag </code><strong> BeamRemnants:allowJunction </strong>
309 (<code>default = <strong>on</strong></code>)<br/>
310 The <code>off</code> option is intended for debug purposes only, as
311 follows. When more than one valence quark is kicked out of a baryon
312 beam, as part of the multiple interactions scenario, the subsequent
313 hadronization is described in terms of a junction string topology.
314 This description involves a number of technical complications that
315 may make the program more unstable. As an alternative, by switching
316 this option off, junction configurations are rejected (which gives
317 an error message that the remnant flavour setup failed), and the
318 multiple interactions and showers are redone until a
319 junction-free topology is found.
325 <!-- Copyright (C) 2010 Torbjorn Sjostrand -->