1 <chapter name="Timelike Showers">
3 <h2>Timelike Showers</h2>
5 The PYTHIA algorithm for timelike final-state showers is based on
6 the article <ref>Sjo05</ref>, where a transverse-momentum-ordered
7 evolution scheme is introduced, with the extension to fully interleaved
8 evolution covered in <ref>Cor10a</ref>. This algorithm is influenced by
9 the previous mass-ordered algorithm in PYTHIA <ref>Ben87</ref> and by
10 the dipole-emission formulation in Ariadne <ref>Gus86</ref>. From the
11 mass-ordered algorithm it inherits a merging procedure for first-order
12 gluon-emission matrix elements in essentially all two-body decays
13 in the standard model and its minimal supersymmetric extension
17 The normal user is not expected to call <code>TimeShower</code> directly,
18 but only have it called from <code>Pythia</code>. Some of the parameters
19 below, in particular <code>TimeShower:alphaSvalue</code>, would be of
20 interest for a tuning exercise, however.
22 <h3>Main variables</h3>
24 Often the maximum scale of the FSR shower evolution is understood from the
25 context. For instance, in a resonace decay half the resonance mass sets an
26 absolute upper limit. For a hard process in a hadronic collision the choice
27 is not as unique. Here the <aloc href="CouplingsAndScales">factorization
28 scale</aloc> has been chosen as the maximum evolution scale. This would be
29 the <ei>pT</ei> for a <ei>2 -> 2</ei> process, supplemented by mass terms
30 for massive outgoing particles. For some special applications we do allow
33 <modepick name="TimeShower:pTmaxMatch" default="1" min="0" max="2">
34 Way in which the maximum shower evolution scale is set to match the
35 scale of the hard process itself.
36 <option value="0"><b>(i)</b> if the final state of the hard process
37 (not counting subsequent resonance decays) contains at least one quark
38 (<ei>u, d, s, c ,b</ei>), gluon or photon then <ei>pT_max</ei>
39 is chosen to be the factorization scale for internal processes
40 and the <code>scale</code> value for Les Houches input;
41 <b>(ii)</b> if not, emissions are allowed to go all the way up to
42 the kinematical limit (i.e. to half the dipole mass).
43 This option agrees with the corresponding one for
44 <aloc href="SpacelikeShowers">spacelike showers</aloc>. There the
45 reasoning is that in the former set of processes the ISR
46 emission of yet another quark, gluon or photon could lead to
47 doublecounting, while no such danger exists in the latter case.
48 The argument is less compelling for timelike showers, but could
49 be a reasonable starting point.
51 <option value="1">always use the factorization scale for an internal
52 process and the <code>scale</code> value for Les Houches input,
53 i.e. the lower value. This should avoid doublecounting, but
54 may leave out some emissions that ought to have been simulated.
55 (Also known as wimpy showers.)
57 <option value="2">always allow emissions up to the kinematical limit
58 (i.e. to half the dipole mass). This will simulate all possible event
59 topologies, but may lead to doublecounting.
60 (Also known as power showers.)
62 <note>Note:</note> These options only apply to the hard interaction.
63 Emissions off subsequent multiparton interactions are always constrainted
64 to be below the factorization scale of the process itself. They also
65 assume you use interleaved evolution, so that FSR is in direct
66 competition with ISR for the hardest emission. If you already
67 generated a number of ISR partons at low <ei>pT</ei>, it would not
68 make sense to have a later FSR shower up to the kinematical for all
72 <parm name="TimeShower:pTmaxFudge" default="1.0" min="0.25" max="2.0">
73 In cases where the above <code>pTmaxMatch</code> rules would imply
74 that <ei>pT_max = pT_factorization</ei>, <code>pTmaxFudge</code>
75 introduces a multiplicative factor <ei>f</ei> such that instead
76 <ei>pT_max = f * pT_factorization</ei>. Only applies to the hardest
77 interaction in an event, cf. below. It is strongly suggested that
78 <ei>f = 1</ei>, but variations around this default can be useful to
80 <note>Note:</note>Scales for resonance decays are not affected, but can
81 be set separately by <aloc href="UserHooks">user hooks</aloc>.
84 <parm name="TimeShower:pTmaxFudgeMPI" default="1.0" min="0.25" max="2.0">
85 A multiplicative factor <ei>f</ei> such that
86 <ei>pT_max = f * pT_factorization</ei>, as above, but here for the
87 non-hardest interactions (when multiparton interactions are allowed).
90 <modepick name="TimeShower:pTdampMatch" default="0" min="0" max="2">
91 These options only take effect when a process is allowed to radiate up
92 to the kinematical limit by the above <code>pTmaxMatch</code> choice,
93 and no matrix-element corrections are available. Then, in many processes,
94 the fall-off in <ei>pT</ei> will be too slow by one factor of <ei>pT^2</ei>.
95 That is, while showers have an approximate <ei>dpT^2/pT^2</ei> shape, often
96 it should become more like <ei>dpT^2/pT^4</ei> at <ei>pT</ei> values above
97 the scale of the hard process. This argument is more obvious for ISR,
98 but is taken over unchanged for FSR to have a symmetric description.
99 <option value="0">emissions go up to the kinematical limit,
100 with no special dampening.
102 <option value="1">emissions go up to the kinematical limit,
103 but dampened by a factor <ei>k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac)</ei>,
104 where <ei>Q_fac</ei> is the factorization scale and <ei>k</ei> is a
105 multiplicative fudge factor stored in <code>pTdampFudge</code> below.
107 <option value="2">emissions go up to the kinematical limit,
108 but dampened by a factor <ei>k^2 Q^2_ren/(pT^2 + k^2 Q^2_ren)</ei>,
109 where <ei>Q_ren</ei> is the renormalization scale and <ei>k</ei> is a
110 multiplicative fudge factor stored in <code>pTdampFudge</code> below.
112 <note>Note:</note> These options only apply to the hard interaction.
113 Emissions off subsequent multiparton interactions are always constrainted
114 to be below the factorization scale of the process itself.
117 <parm name="TimeShower:pTdampFudge" default="1.0" min="0.25" max="4.0">
118 In cases 1 and 2 above, where a dampening is imposed at around the
119 factorization or renormalization scale, respectively, this allows the
120 <ei>pT</ei> scale of dampening of radiation by a half to be shifted
121 by this factor relative to the default <ei>Q_fac</ei> or <ei>Q_ren</ei>.
122 This number ought to be in the neighbourhood of unity, but variations
123 away from this value could do better in some processes.
127 The amount of QCD radiation in the shower is determined by
128 <parm name="TimeShower:alphaSvalue" default="0.1383"
129 min="0.06" max="0.25">
130 The <ei>alpha_strong</ei> value at scale <ei>M_Z^2</ei>. The default
131 value corresponds to a crude tuning to LEP data, to be improved.
135 The actual value is then regulated by the running to the scale
136 <ei>pT^2</ei>, at which the shower evaluates <ei>alpha_strong</ei>.
138 <modepick name="TimeShower:alphaSorder" default="1" min="0" max="2">
139 Order at which <ei>alpha_strong</ei> runs,
140 <option value="0">zeroth order, i.e. <ei>alpha_strong</ei> is kept
142 <option value="1">first order, which is the normal value.</option>
143 <option value="2">second order. Since other parts of the code do
144 not go to second order there is no strong reason to use this option,
145 but there is also nothing wrong with it.</option>
149 QED radiation is regulated by the <ei>alpha_electromagnetic</ei>
150 value at the <ei>pT^2</ei> scale of a branching.
152 <modepick name="TimeShower:alphaEMorder" default="1" min="-1" max="1">
153 The running of <ei>alpha_em</ei>.
154 <option value="1">first-order running, constrained to agree with
155 <code>StandardModel:alphaEMmZ</code> at the <ei>Z^0</ei> mass.
157 <option value="0">zeroth order, i.e. <ei>alpha_em</ei> is kept
158 fixed at its value at vanishing momentum transfer.</option>
159 <option value="-1">zeroth order, i.e. <ei>alpha_em</ei> is kept
160 fixed, but at <code>StandardModel:alphaEMmZ</code>, i.e. its value
161 at the <ei>Z^0</ei> mass.
166 The natural scale for couplings, and PDFs for dipoles stretching out
167 to the beam remnants, is <ei>pT^2</ei>. To explore uncertainties it
168 is possibly to vary around this value, however, in analogy with what
169 can be done for <aloc href="CouplingsAndScales">hard processes</aloc>.
171 <parm name="TimeShower:renormMultFac" default="1." min="0.1" max="10.">
172 The default <ei>pT^2</ei> renormalization scale is multiplied by
173 this prefactor. For QCD this is equivalent to a change of
174 <ei>Lambda^2</ei> in the opposite direction, i.e. to a change of
175 <ei>alpha_strong(M_Z^2)</ei> (except that flavour thresholds
176 remain at fixed scales).
179 <parm name="TimeShower:factorMultFac" default="1." min="0.1" max="10.">
180 The default <ei>pT^2</ei> factorization scale is multiplied by
185 The rate of radiation if divergent in the <ei>pT -> 0</ei> limit. Here,
186 however, perturbation theory is expected to break down. Therefore an
187 effective <ei>pT_min</ei> cutoff parameter is introduced, below which
188 no emissions are allowed. The cutoff may be different for QCD and QED
189 radiation off quarks, and is mainly a technical parameter for QED
190 radiation off leptons.
192 <parm name="TimeShower:pTmin" default="0.4" min="0.1" max="2.0">
193 Parton shower cut-off <ei>pT</ei> for QCD emissions.
196 <parm name="TimeShower:pTminChgQ" default="0.4" min="0.1" max="2.0">
197 Parton shower cut-off <ei>pT</ei> for photon coupling to coloured particle.
200 <parm name="TimeShower:pTminChgL" default="0.0005" min="0.0001" max="2.0">
201 Parton shower cut-off <ei>pT</ei> for pure QED branchings.
202 Assumed smaller than (or equal to) <code>pTminChgQ</code>.
206 Shower branchings <ei>gamma -> f fbar</ei>, where <ei>f</ei> is a
207 quark or lepton, in part compete with the hard processes involving
208 <ei>gamma^*/Z^0</ei> production. In order to avoid overlap it makes
209 sense to correlate the maximum <ei>gamma</ei> mass allowed in showers
210 with the minumum <ei>gamma^*/Z^0</ei> mass allowed in hard processes.
211 In addition, the shower contribution only contains the pure
212 <ei>gamma^*</ei> contribution, i.e. not the <ei>Z^0</ei> part, so
213 the mass spectrum above 50 GeV or so would not be well described.
215 <parm name="TimeShower:mMaxGamma" default="10.0" min="0.001"
217 Maximum invariant mass allowed for the created fermion pair in a
218 <ei>gamma -> f fbar</ei> branching in the shower.
221 <h3>Interleaved evolution</h3>
223 Multiparton interactions (MPI) and initial-state showers (ISR) are
224 always interleaved, as follows. Starting from the hard interaction,
225 the complete event is constructed by a set of steps. In each step
226 the <ei>pT</ei> scale of the previous step is used as starting scale
227 for a downwards evolution. The MPI and ISR components each make
228 their respective Monte Carlo choices for the next lower <ei>pT</ei>
229 value. The one with larger <ei>pT</ei> is allowed to carry out its
230 proposed action, thereby modifying the conditions for the next steps.
231 This is relevant since the two components compete for the energy
232 contained in the beam remnants: both an interaction and an emission
233 take avay some of the energy, leaving less for the future. The end
234 result is a combined chain of decreasing <ei>pT</ei> values, where
235 ones associated with new interactions and ones with new emissions
239 There is no corresponding requirement for final-state radiation (FSR)
240 to be interleaved. Such an FSR emission does not compete directly for
241 beam energy (but see below), and also can be viewed as occuring after
242 the other two components in some kind of time sense. Interleaving is
243 allowed, however, since it can be argued that a high-<ei>pT</ei> FSR
244 occurs on shorter time scales than a low-<ei>pT</ei> MPI, say.
245 Backwards evolution of ISR is also an example that physical time
246 is not the only possible ordering principle, but that one can work
247 with conditional probabilities: given the partonic picture at a
248 specific <ei>pT</ei> resolution scale, what possibilities are open
249 for a modified picture at a slightly lower <ei>pT</ei> scale, either
250 by MPI, ISR or FSR? Complete interleaving of the three components also
251 offers advantages if one aims at matching to higher-order matrix
252 elements above some given scale.
254 <flag name="TimeShower:interleave" default="on">
255 If on, final-state emissions are interleaved in the same
256 decreasing-<ei>pT</ei> chain as multiparton interactions and initial-state
257 emissions. If off, final-state emissions are only addressed after the
258 multiparton interactions and initial-state radiation have been considered.
262 As an aside, it should be noted that such interleaving does not affect
263 showering in resonance decays, such as a <ei>Z^0</ei>. These decays are
264 only introduced after the production process has been considered in full,
265 and the subsequent FSR is carried out inside the resonance, with
266 preserved resonance mass.
269 One aspect of FSR for a hard process in hadron collisions is that often
270 colour diples are formed between a scattered parton and a beam remnant,
271 or rather the hole left behind by an incoming partons. If such holes
272 are allowed as dipole ends and take the recoil when the scattered parton
273 undergoes a branching then this translates into the need to take some
274 amount of remnant energy also in the case of FSR, i.e. the roles of
275 ISR and FSR are not completely decoupled. The energy taken away is
276 bokkept by increasing the <ei>x</ei> value assigned to the incoming
277 scattering parton, and a reweighting factor
278 <ei>x_new f(x_new, pT^2) / x_old f(x_old, pT^2)</ei>
279 in the emission probability ensures that not unphysically large
280 <ei>x_new</ei> values are reached. Usually such <ei>x</ei> changes are
281 small, and they can be viewed as a higher-order effect beyond the
282 accuracy of the leading-log initial-state showers.
285 This choice is not unique, however. As an alternative, if nothing else
286 useful for cross-checks, one could imagine that the FSR is completely
287 decoupled from the ISR and beam remnants.
289 <flag name="TimeShower:allowBeamRecoil" default="on">
290 If on, the final-state shower is allowed to borrow energy from
291 the beam remnants as described above, thereby changing the mass of the
292 scattering subsystem. If off, the partons in the scattering subsystem
293 are constrained to borrow energy from each other, such that the total
294 four-momentum of the system is preserved. This flag has no effect
295 on resonance decays, where the shower always preserves the resonance
296 mass, cf. the comment above about showers for resonances never being
300 <flag name="TimeShower:dampenBeamRecoil" default="on">
301 When beam recoil is allowed there is still some ambiguity how far
302 into the beam end of the dipole that emission should be allowed.
303 It is dampened in the beam region, but probably not enough.
304 When on an additional suppression factor
305 <ei>4 pT2_hard / (4 pT2_hard + m2)</ei> is multiplied on to the
306 emission probability. Here <ei>pT_hard</ei> is the transverse momentum
307 of the radiating parton and <ei>m</ei> the off-shell mass it acquires
308 by the branching, <ei>m2 = pT2/(z(1-z))</ei>. Note that
309 <ei>m2 = 4 pT2_hard</ei> is the kinematical limit for a scattering
310 at 90 degrees without beam recoil.
313 <h3>Global recoil</h3>
315 The final-state algorithm is based on dipole-style recoils, where
316 one single parton takes the full recoil of a branching. This is unlike
317 the initial-state algorithm, where the complete already-existing
318 final state shares the recoil of each new emission. As an alternative,
319 also the final-state algorithm contains an option where the recoil
320 is shared between all partons in the final state. Thus the radiation
321 pattern is unrelated to colour correlations. This is especially
322 convenient for some matching algorithms, like MC@NLO, where a full
323 analytic knowledge of the shower radiation pattern is needed to avoid
324 doublecountning. (The <ei>pT</ei>-ordered shower is described in
325 <ref>Sjo05</ref>, and the corrections for massive radiator and recoiler
326 in <ref>Nor01</ref>.)
329 Technically, the radiation pattern is most conveniently represented
330 in the rest frame of the final state of the hard subprocess. Then, for
331 each parton at a time, the rest of the final state can be viewed as
332 a single effective parton. This "parton" has a fixed invariant mass
333 during the emission process, and takes the recoil without any changed
334 direction of motion. The momenta of the individual new recoilers are
335 then obtained by a simple common boost of the original ones.
338 This alternative approach will miss out on the colour coherence
339 phenomena. Specifically, with the whole subcollision mass as "dipole"
340 mass, the phase space for subsequent emissions is larger than for
341 the normal dipole algorithm. The phase space difference grows as
342 more and more gluons are created, and thus leads to a way too steep
343 multiplication of soft gluons. Therefore the main application is
344 for the first one or few emissions of the shower, where a potential
345 overestimate of the emission rate is to be corrected for anyway,
346 by matching to the relevant matrix elements. Thereafter, subsequent
347 emissions should be handled as before, i.e. with dipoles spanned
348 between nearby partons. Furthermore, only the first (hardest)
349 subcollision is handled with global recoils, since subsequent MPI's
350 would not be subject to matrix element corrections anyway.
353 In order for the mid-shower switch from global to local recoils
354 to work, colours are traced and bookkept just as for normal showers;
355 it is only that this information is not used in those steps where
356 a global recoil is requested. (Thus, e.g., a gluon is still bookkept
357 as one colour and one anticolour dipole end, with half the charge
358 each, but with global recoil those two ends radiate identically.)
360 <flag name="TimeShower:globalRecoil" default="off">
361 Alternative approach as above, where all final-state particles share
362 the recoil of an emission.
363 <br/>If off, then use the standard dipole-recoil approach.
364 <br/>If on, use the alternative global recoil, but only for the first
365 interaction, and only while the number of particles in the final state
366 is at most <code>TimeShower:nMaxGlobalRecoil</code> before the
370 <modeopen name="TimeShower:nMaxGlobalRecoil" default="2" min="1">
371 Represents the maximum number of particles in the final state for which
372 the next final-state emission can be performed with the global recoil
373 strategy. This number counts all particles, whether they are
374 allowed to radiate or not, e.g. also <ei>Z^0</ei>. Also partons
375 created by initial-state radiation emissions counts towards this sum,
376 as part of the interleaved evolution. Without interleaved evolution
377 this option would not make sense, since then a varying and large
378 number of partons could already have been created by the initial-state
379 radiation before the first final-state one, and then there is not
380 likely to be any matrix elements available for matching.
384 The global-recoil machinery does not work well with rescattering in the
385 MPI machinery, since then the recoiling system is not uniquely defined.
386 <code>MultipartonInteractions:allowRescatter = off</code> by default,
387 so this is not a main issue. If both options are switched on,
388 rescattering will only be allowed to kick in after the global recoil
389 has ceased to be active, i.e. once the <code>nMaxGlobalRecoil</code>
390 limit has been exceeded. This should not be a major conflict,
391 since rescattering is mainly of interest at later stages of the
392 downwards <ei>pT</ei> evolution.
395 Further, it is strongly recommended to set
396 <code>TimeShower:MEcorrections = off</code> (not default!), i.e. not
397 to correct the emission probability to the internal matrix elements.
398 The internal ME options do not cover any cases relevant for a multibody
399 recoiler anyway, so no guarantees are given what prescription would
400 come to be used. Instead, without ME corrections, a process-independent
401 emission rate is obtained, and <aloc href="UserHooks">user hooks</aloc>
402 can provide the desired process-specific rejection factors.
404 <h3>Radiation off octet onium states</h3>
406 In the current implementation, charmonium and bottomonium production
407 can proceed either through colour singlet or colour octet mechanisms,
408 both of them implemented in terms of <ei>2 -> 2</ei> hard processes
409 such as <ei>g g -> (onium) g</ei>.
410 In the former case the state does not radiate and the onium therefore
411 is produced in isolation, up to normal underlying-event activity. In
412 the latter case the situation is not so clear, but it is sensible to
413 assume that a shower can evolve. (Assuming, of course, that the
414 transverse momentum of the onium state is sufficiently high that
415 radiation is of relevance.)
418 There could be two parts to such a shower. Firstly a gluon (or even a
419 quark, though less likely) produced in a hard <ei>2 -> 2</ei> process
420 can undergo showering into many gluons, whereof one branches into the
421 heavy-quark pair. Secondly, once the pair has been produced, each quark
422 can radiate further gluons. This latter kind of emission could easily
423 break up a semibound quark pair, but might also create a new semibound
424 state where before an unbound pair existed, and to some approximation
425 these two effects should balance in the onium production rate.
426 The showering "off an onium state" as implemented here therefore should
427 not be viewed as an accurate description of the emission history
428 step by step, but rather as an effective approach to ensure that the
429 octet onium produced "in the hard process" is embedded in a realistic
430 amount of jet activity.
431 Of course both the isolated singlet and embedded octet are likely to
432 be extremes, but hopefully the mix of the two will strike a reasonable
433 balance. However, it is possible that some part of the octet production
434 occurs in channels where it should not be accompanied by (hard) radiation.
435 Therefore reducing the fraction of octet onium states allowed to radiate
436 is a valid variation to explore uncertainties.
439 If an octet onium state is chosen to radiate, the simulation of branchings
440 is based on the assumption that the full radiation is provided by an
441 incoherent sum of radiation off the quark and off the antiquark of the
442 onium state. Thus the splitting kernel is taken to be the normal
443 <ei>q -> q g</ei> one, multiplied by a factor of two. Obviously this is
444 a simplification of a more complex picture, averaging over factors pulling
445 in different directions. Firstly, radiation off a gluon ought
446 to be enhanced by a factor 9/4 relative to a quark rather than the 2
447 now used, but this is a minor difference. Secondly, our use of the
448 <ei>q -> q g</ei> branching kernel is roughly equivalent to always
449 following the harder gluon in a <ei>g -> g g</ei> branching. This could
450 give us a bias towards producing too hard onia. A soft gluon would have
451 little phase space to branch into a heavy-quark pair however, so the
452 bias may not be as big as it would seem at first glance. Thirdly,
453 once the gluon has branched into a quark pair, each quark carries roughly
454 only half of the onium energy. The maximum energy per emitted gluon should
455 then be roughly half the onium energy rather than the full, as it is now.
456 Thereby the energy of radiated gluons is exaggerated, i.e. onia become too
457 soft. So the second and the third points tend to cancel each other.
460 Finally, note that the lower cutoff scale of the shower evolution depends
461 on the onium mass rather than on the quark mass, as it should be. Gluons
462 below the octet-onium scale should only be part of the octet-to-singlet
465 <parm name="TimeShower:octetOniumFraction" default="1." min="0." max="1." >
466 Allow colour-octet charmonium and bottomonium states to radiate gluons.
467 0 means that no octet-onium states radiate, 1 that all do, with possibility
468 to interpolate between these two extremes.
471 <parm name="TimeShower:octetOniumColFac" default="2." min="0." max="4." >
472 The colour factor used used in the splitting kernel for those octet onium
473 states that are allowed to radiate, normalized to the <ei>q -> q g</ei>
474 splitting kernel. Thus the default corresponds to twice the radiation
475 off a quark. The physically preferred range would be between 1 and 9/4.
478 <h3>Further variables</h3>
480 There are several possibilities you can use to switch on or off selected
481 branching types in the shower, or in other respects simplify the shower.
482 These should normally not be touched. Their main function is for
485 <flag name="TimeShower:QCDshower" default="on">
486 Allow a QCD shower, i.e. branchings <ei>q -> q g</ei>, <ei>g -> g g</ei>
487 and <ei>g -> q qbar</ei>; on/off = true/false.
490 <modeopen name="TimeShower:nGluonToQuark" default="5" min="0" max="5">
491 Number of allowed quark flavours in <ei>g -> q qbar</ei> branchings
492 (phase space permitting). A change to 4 would exclude
493 <ei>g -> b bbar</ei>, etc.
496 <flag name="TimeShower:QEDshowerByQ" default="on">
497 Allow quarks to radiate photons, i.e. branchings <ei>q -> q gamma</ei>;
501 <flag name="TimeShower:QEDshowerByL" default="on">
502 Allow leptons to radiate photons, i.e. branchings <ei>l -> l gamma</ei>;
506 <flag name="TimeShower:QEDshowerByGamma" default="on">
507 Allow photons to branch into lepton or quark pairs, i.e. branchings
508 <ei>gamma -> l+ l-</ei> and <ei>gamma -> q qbar</ei>;
512 <modeopen name="TimeShower:nGammaToQuark" default="5" min="0" max="5">
513 Number of allowed quark flavours in <ei>gamma -> q qbar</ei> branchings
514 (phase space permitting). A change to 4 would exclude
515 <ei>g -> b bbar</ei>, etc.
518 <modeopen name="TimeShower:nGammaToLepton" default="3" min="0" max="3">
519 Number of allowed lepton flavours in <ei>gamma -> l+ l-</ei> branchings
520 (phase space permitting). A change to 2 would exclude
521 <ei>gamma -> tau+ tau-</ei>, and a change to 1 also
522 <ei>gamma -> mu+ mu-</ei>.
525 <flag name="TimeShower:MEcorrections" default="on">
526 Use of matrix element corrections where available; on/off = true/false.
529 <flag name="TimeShower:MEafterFirst" default="on">
530 Use of matrix element corrections also after the first emission,
531 for dipole ends of the same system that did not yet radiate.
532 Only has a meaning if <code>MEcorrections</code> above is
536 <flag name="TimeShower:phiPolAsym" default="on">
537 Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
540 <flag name="TimeShower:recoilToColoured" default="on">
541 In the decays of coloured resonances, say <ei>t -> b W</ei>, it is not
542 possible to set up dipoles with matched colours. Originally the
543 <ei>b</ei> radiator therefore has <ei>W</ei> as recoiler, and that
544 choice is unique. Once a gluon has been radiated, however, it is
545 possible either to have the unmatched colour (inherited by the gluon)
546 still recoiling against the <ei>W</ei> (<code>off</code>), or else
547 let it recoil against the <ei>b</ei> also for this dipole
548 (<code>on</code>). Before version 8.160 the former was the only
549 possibility, which could give unphysical radiation patterns. It is
550 kept as an option to check backwards compatibility. The same issue
551 exists for QED radiation, but obviously is less significant. Consider
552 the example <ei>W -> e nu</ei>, where originally the <ei>nu</ei>
553 takes the recoil. In the old (<code>off</code>) scheme the <ei>nu</ei>
554 would remain recoiler, while in the new (<code>on</code>) instead
555 each newly emitted photon becomes the new recoiler.
560 <!-- Copyright (C) 2012 Torbjorn Sjostrand -->