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<title>Timelike Showers</title>
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<link rel="shortcut icon" href="pythia32.gif"/>
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<h2>Timelike Showers</h2>
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The PYTHIA algorithm for timelike final-state showers is based on
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the article [<a href="Bibliography.html" target="page">Sjo05</a>], where a transverse-momentum-ordered
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evolution scheme is introduced, with the extension to fully interleaved
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evolution covered in [<a href="Bibliography.html" target="page">Cor10a</a>]. This algorithm is influenced by
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the previous mass-ordered algorithm in PYTHIA [<a href="Bibliography.html" target="page">Ben87</a>] and by
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the dipole-emission formulation in Ariadne [<a href="Bibliography.html" target="page">Gus86</a>]. From the
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mass-ordered algorithm it inherits a merging procedure for first-order
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gluon-emission matrix elements in essentially all two-body decays
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in the standard model and its minimal supersymmetric extension
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[<a href="Bibliography.html" target="page">Nor01</a>].
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The normal user is not expected to call <code>TimeShower</code> directly,
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but only have it called from <code>Pythia</code>. Some of the parameters
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below, in particular <code>TimeShower:alphaSvalue</code>, would be of
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interest for a tuning exercise, however.
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<h3>Main variables</h3>
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Often the maximum scale of the FSR shower evolution is understood from the
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context. For instance, in a resonace decay half the resonance mass sets an
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absolute upper limit. For a hard process in a hadronic collision the choice
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is not as unique. Here the <a href="CouplingsAndScales.html" target="page">factorization
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scale</a> has been chosen as the maximum evolution scale. This would be
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the <i>pT</i> for a <i>2 -> 2</i> process, supplemented by mass terms
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for massive outgoing particles. For some special applications we do allow
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<p/><code>mode </code><strong> TimeShower:pTmaxMatch </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
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Way in which the maximum shower evolution scale is set to match the
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scale of the hard process itself.
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<br/><code>option </code><strong> 0</strong> : <b>(i)</b> if the final state of the hard process
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(not counting subsequent resonance decays) contains at least one quark
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(<i>u, d, s, c ,b</i>), gluon or photon then <i>pT_max</i>
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is chosen to be the factorization scale for internal processes
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and the <code>scale</code> value for Les Houches input;
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<b>(ii)</b> if not, emissions are allowed to go all the way up to
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the kinematical limit (i.e. to half the dipole mass).
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This option agrees with the corresponding one for
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<a href="SpacelikeShowers.html" target="page">spacelike showers</a>. There the
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reasoning is that in the former set of processes the ISR
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emission of yet another quark, gluon or photon could lead to
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doublecounting, while no such danger exists in the latter case.
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The argument is less compelling for timelike showers, but could
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be a reasonable starting point.
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<br/><code>option </code><strong> 1</strong> : always use the factorization scale for an internal
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process and the <code>scale</code> value for Les Houches input,
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i.e. the lower value. This should avoid doublecounting, but
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may leave out some emissions that ought to have been simulated.
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(Also known as wimpy showers.)
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<br/><code>option </code><strong> 2</strong> : always allow emissions up to the kinematical limit
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(i.e. to half the dipole mass). This will simulate all possible event
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topologies, but may lead to doublecounting.
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(Also known as power showers.)
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<br/><b>Note:</b> These options only apply to the hard interaction.
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Emissions off subsequent multiparton interactions are always constrainted
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to be below the factorization scale of the process itself. They also
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assume you use interleaved evolution, so that FSR is in direct
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competition with ISR for the hardest emission. If you already
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generated a number of ISR partons at low <i>pT</i>, it would not
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make sense to have a later FSR shower up to the kinematical for all
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<p/><code>parm </code><strong> TimeShower:pTmaxFudge </strong>
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(<code>default = <strong>1.0</strong></code>; <code>minimum = 0.25</code>; <code>maximum = 2.0</code>)<br/>
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In cases where the above <code>pTmaxMatch</code> rules would imply
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that <i>pT_max = pT_factorization</i>, <code>pTmaxFudge</code>
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introduces a multiplicative factor <i>f</i> such that instead
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<i>pT_max = f * pT_factorization</i>. Only applies to the hardest
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interaction in an event, cf. below. It is strongly suggested that
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<i>f = 1</i>, but variations around this default can be useful to
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<br/><b>Note:</b>Scales for resonance decays are not affected, but can
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be set separately by <a href="UserHooks.html" target="page">user hooks</a>.
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<p/><code>parm </code><strong> TimeShower:pTmaxFudgeMPI </strong>
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(<code>default = <strong>1.0</strong></code>; <code>minimum = 0.25</code>; <code>maximum = 2.0</code>)<br/>
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A multiplicative factor <i>f</i> such that
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<i>pT_max = f * pT_factorization</i>, as above, but here for the
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non-hardest interactions (when multiparton interactions are allowed).
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<p/><code>mode </code><strong> TimeShower:pTdampMatch </strong>
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(<code>default = <strong>0</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
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These options only take effect when a process is allowed to radiate up
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to the kinematical limit by the above <code>pTmaxMatch</code> choice,
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and no matrix-element corrections are available. Then, in many processes,
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the fall-off in <i>pT</i> will be too slow by one factor of <i>pT^2</i>.
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That is, while showers have an approximate <i>dpT^2/pT^2</i> shape, often
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it should become more like <i>dpT^2/pT^4</i> at <i>pT</i> values above
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the scale of the hard process. This argument is more obvious for ISR,
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but is taken over unchanged for FSR to have a symmetric description.
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<br/><code>option </code><strong> 0</strong> : emissions go up to the kinematical limit,
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with no special dampening.
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<br/><code>option </code><strong> 1</strong> : emissions go up to the kinematical limit,
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but dampened by a factor <i>k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac)</i>,
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where <i>Q_fac</i> is the factorization scale and <i>k</i> is a
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multiplicative fudge factor stored in <code>pTdampFudge</code> below.
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<br/><code>option </code><strong> 2</strong> : emissions go up to the kinematical limit,
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but dampened by a factor <i>k^2 Q^2_ren/(pT^2 + k^2 Q^2_ren)</i>,
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where <i>Q_ren</i> is the renormalization scale and <i>k</i> is a
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multiplicative fudge factor stored in <code>pTdampFudge</code> below.
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<br/><b>Note:</b> These options only apply to the hard interaction.
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Emissions off subsequent multiparton interactions are always constrainted
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to be below the factorization scale of the process itself.
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<p/><code>parm </code><strong> TimeShower:pTdampFudge </strong>
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(<code>default = <strong>1.0</strong></code>; <code>minimum = 0.25</code>; <code>maximum = 4.0</code>)<br/>
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In cases 1 and 2 above, where a dampening is imposed at around the
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factorization or renormalization scale, respectively, this allows the
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<i>pT</i> scale of dampening of radiation by a half to be shifted
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by this factor relative to the default <i>Q_fac</i> or <i>Q_ren</i>.
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This number ought to be in the neighbourhood of unity, but variations
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away from this value could do better in some processes.
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The amount of QCD radiation in the shower is determined by
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<p/><code>parm </code><strong> TimeShower:alphaSvalue </strong>
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(<code>default = <strong>0.1383</strong></code>; <code>minimum = 0.06</code>; <code>maximum = 0.25</code>)<br/>
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The <i>alpha_strong</i> value at scale <i>M_Z^2</i>. The default
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value corresponds to a crude tuning to LEP data, to be improved.
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The actual value is then regulated by the running to the scale
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<i>pT^2</i>, at which the shower evaluates <i>alpha_strong</i>
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<p/><code>mode </code><strong> TimeShower:alphaSorder </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 2</code>)<br/>
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Order at which <i>alpha_strong</i> runs,
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<br/><code>option </code><strong> 0</strong> : zeroth order, i.e. <i>alpha_strong</i> is kept
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<br/><code>option </code><strong> 1</strong> : first order, which is the normal value.
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<br/><code>option </code><strong> 2</strong> : second order. Since other parts of the code do
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not go to second order there is no strong reason to use this option,
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but there is also nothing wrong with it.
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QED radiation is regulated by the <i>alpha_electromagnetic</i>
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value at the <i>pT^2</i> scale of a branching.
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<p/><code>mode </code><strong> TimeShower:alphaEMorder </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = -1</code>; <code>maximum = 1</code>)<br/>
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The running of <i>alpha_em</i>.
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<br/><code>option </code><strong> 1</strong> : first-order running, constrained to agree with
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<code>StandardModel:alphaEMmZ</code> at the <i>Z^0</i> mass.
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<br/><code>option </code><strong> 0</strong> : zeroth order, i.e. <i>alpha_em</i> is kept
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fixed at its value at vanishing momentum transfer.
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<br/><code>option </code><strong> -1</strong> : zeroth order, i.e. <i>alpha_em</i> is kept
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fixed, but at <code>StandardModel:alphaEMmZ</code>, i.e. its value
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at the <i>Z^0</i> mass.
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The rate of radiation if divergent in the <i>pT -> 0</i> limit. Here,
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however, perturbation theory is expected to break down. Therefore an
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effective <i>pT_min</i> cutoff parameter is introduced, below which
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no emissions are allowed. The cutoff may be different for QCD and QED
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radiation off quarks, and is mainly a technical parameter for QED
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radiation off leptons.
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<p/><code>parm </code><strong> TimeShower:pTmin </strong>
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(<code>default = <strong>0.4</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 2.0</code>)<br/>
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Parton shower cut-off <i>pT</i> for QCD emissions.
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<p/><code>parm </code><strong> TimeShower:pTminChgQ </strong>
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(<code>default = <strong>0.4</strong></code>; <code>minimum = 0.1</code>; <code>maximum = 2.0</code>)<br/>
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Parton shower cut-off <i>pT</i> for photon coupling to coloured particle.
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<p/><code>parm </code><strong> TimeShower:pTminChgL </strong>
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(<code>default = <strong>0.0005</strong></code>; <code>minimum = 0.0001</code>; <code>maximum = 2.0</code>)<br/>
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Parton shower cut-off <i>pT</i> for pure QED branchings.
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Assumed smaller than (or equal to) <code>pTminChgQ</code>.
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Shower branchings <i>gamma -> f fbar</i>, where <i>f</i> is a
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quark or lepton, in part compete with the hard processes involving
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<i>gamma^*/Z^0</i> production. In order to avoid overlap it makes
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sense to correlate the maximum <i>gamma</i> mass allowed in showers
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with the minumum <i>gamma^*/Z^0</i> mass allowed in hard processes.
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In addition, the shower contribution only contains the pure
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<i>gamma^*</i> contribution, i.e. not the <i>Z^0</i> part, so
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the mass spectrum above 50 GeV or so would not be well described.
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<p/><code>parm </code><strong> TimeShower:mMaxGamma </strong>
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(<code>default = <strong>10.0</strong></code>; <code>minimum = 0.001</code>; <code>maximum = 50.0</code>)<br/>
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Maximum invariant mass allowed for the created fermion pair in a
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<i>gamma -> f fbar</i> branching in the shower.
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<h3>Interleaved evolution</h3>
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Multiparton interactions (MPI) and initial-state showers (ISR) are
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always interleaved, as follows. Starting from the hard interaction,
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the complete event is constructed by a set of steps. In each step
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the <i>pT</i> scale of the previous step is used as starting scale
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for a downwards evolution. The MPI and ISR components each make
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their respective Monte Carlo choices for the next lower <i>pT</i>
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value. The one with larger <i>pT</i> is allowed to carry out its
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proposed action, thereby modifying the conditions for the next steps.
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This is relevant since the two components compete for the energy
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contained in the beam remnants: both an interaction and an emission
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take avay some of the energy, leaving less for the future. The end
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result is a combined chain of decreasing <i>pT</i> values, where
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ones associated with new interactions and ones with new emissions
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There is no corresponding requirement for final-state radiation (FSR)
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to be interleaved. Such an FSR emission does not compete directly for
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beam energy (but see below), and also can be viewed as occuring after
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the other two components in some kind of time sense. Interleaving is
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allowed, however, since it can be argued that a high-<i>pT</i> FSR
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occurs on shorter time scales than a low-<i>pT</i> MPI, say.
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Backwards evolution of ISR is also an example that physical time
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is not the only possible ordering principle, but that one can work
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with conditional probabilities: given the partonic picture at a
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specific <i>pT</i> resolution scale, what possibilities are open
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for a modified picture at a slightly lower <i>pT</i> scale, either
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by MPI, ISR or FSR? Complete interleaving of the three components also
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offers advantages if one aims at matching to higher-order matrix
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elements above some given scale.
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<p/><code>flag </code><strong> TimeShower:interleave </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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If on, final-state emissions are interleaved in the same
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decreasing-<i>pT</i> chain as multiparton interactions and initial-state
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emissions. If off, final-state emissions are only addressed after the
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multiparton interactions and initial-state radiation have been considered.
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As an aside, it should be noted that such interleaving does not affect
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showering in resonance decays, such as a <i>Z^0</i>. These decays are
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only introduced after the production process has been considered in full,
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and the subsequent FSR is carried out inside the resonance, with
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preserved resonance mass.
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One aspect of FSR for a hard process in hadron collisions is that often
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colour diples are formed between a scattered parton and a beam remnant,
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or rather the hole left behind by an incoming partons. If such holes
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are allowed as dipole ends and take the recoil when the scattered parton
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undergoes a branching then this translates into the need to take some
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amount of remnant energy also in the case of FSR, i.e. the roles of
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ISR and FSR are not completely decoupled. The energy taken away is
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bokkept by increasing the <i>x</i> value assigned to the incoming
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scattering parton, and a reweighting factor
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<i>x_new f(x_new, pT^2) / x_old f(x_old, pT^2)</i>
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in the emission probability ensures that not unphysically large
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<i>x_new</i> values are reached. Usually such <i>x</i> changes are
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small, and they can be viewed as a higher-order effect beyond the
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accuracy of the leading-log initial-state showers.
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This choice is not unique, however. As an alternative, if nothing else
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useful for cross-checks, one could imagine that the FSR is completely
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decoupled from the ISR and beam remnants.
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<p/><code>flag </code><strong> TimeShower:allowBeamRecoil </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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If on, the final-state shower is allowed to borrow energy from
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the beam remnants as described above, thereby changing the mass of the
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scattering subsystem. If off, the partons in the scattering subsystem
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are constrained to borrow energy from each other, such that the total
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four-momentum of the system is preserved. This flag has no effect
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on resonance decays, where the shower always preserves the resonance
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mass, cf. the comment above about showers for resonances never being
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<p/><code>flag </code><strong> TimeShower:dampenBeamRecoil </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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When beam recoil is allowed there is still some ambiguity how far
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into the beam end of the dipole that emission should be allowed.
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It is dampened in the beam region, but probably not enough.
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When on an additional suppression factor
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<i>4 pT2_hard / (4 pT2_hard + m2)</i> is multiplied on to the
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emission probability. Here <i>pT_hard</i> is the transverse momentum
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of the radiating parton and <i>m</i> the off-shell mass it acquires
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by the branching, <i>m2 = pT2/(z(1-z))</i>. Note that
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<i>m2 = 4 pT2_hard</i> is the kinematical limit for a scattering
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at 90 degrees without beam recoil.
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<h3>Global recoil</h3>
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The final-state algorithm is based on dipole-style recoils, where
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one single parton takes the full recoil of a branching. This is unlike
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the initial-state algorithm, where the complete already-existing
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final state shares the recoil of each new emission. As an alternative,
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also the final-state algorithm contains an option where the recoil
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is shared between all partons in the final state. Thus the radiation
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pattern is unrelated to colour correlations. This is especially
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convenient for some matching algorithms, like MC@NLO, where a full
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analytic knowledge of the shower radiation pattern is needed to avoid
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doublecountning. (The <i>pT</i>-ordered shower is described in
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[<a href="Bibliography.html" target="page">Sjo05</a>], and the corrections for massive radiator and recoiler
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in [<a href="Bibliography.html" target="page">Nor01</a>].)
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Technically, the radiation pattern is most conveniently represented
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in the rest frame of the final state of the hard subprocess. Then, for
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each parton at a time, the rest of the final state can be viewed as
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a single effective parton. This "parton" has a fixed invariant mass
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during the emission process, and takes the recoil without any changed
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direction of motion. The momenta of the individual new recoilers are
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then obtained by a simple common boost of the original ones.
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This alternative approach will miss out on the colour coherence
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phenomena. Specifically, with the whole subcollision mass as "dipole"
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mass, the phase space for subsequent emissions is larger than for
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the normal dipole algorithm. The phase space difference grows as
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more and more gluons are created, and thus leads to a way too steep
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multiplication of soft gluons. Therefore the main application is
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for the first one or few emissions of the shower, where a potential
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overestimate of the emission rate is to be corrected for anyway,
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by matching to the relevant matrix elements. Thereafter, subsequent
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emissions should be handled as before, i.e. with dipoles spanned
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between nearby partons. Furthermore, only the first (hardest)
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subcollision is handled with global recoils, since subsequent MPI's
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would not be subject to matrix element corrections anyway.
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In order for the mid-shower switch from global to local recoils
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to work, colours are traced and bookkept just as for normal showers;
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it is only that this information is not used in those steps where
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a global recoil is requested. (Thus, e.g., a gluon is still bookkept
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as one colour and one anticolour dipole end, with half the charge
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each, but with global recoil those two ends radiate identically.)
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<p/><code>flag </code><strong> TimeShower:globalRecoil </strong>
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(<code>default = <strong>off</strong></code>)<br/>
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Alternative approach as above, where all final-state particles share
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the recoil of an emission.
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<br/>If off, then use the standard dipole-recoil approach.
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<br/>If on, use the alternative global recoil, but only for the first
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interaction, and only while the number of particles in the final state
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is at most <code>TimeShower:nMaxGlobalRecoil</code> before the
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<p/><code>mode </code><strong> TimeShower:nMaxGlobalRecoil </strong>
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(<code>default = <strong>2</strong></code>; <code>minimum = 1</code>)<br/>
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Represents the maximum number of particles in the final state for which
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the next final-state emission can be performed with the global recoil
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strategy. This number counts all particles, whether they are
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allowed to radiate or not, e.g. also <i>Z^0</i>. Also partons
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created by initial-state radiation emissions counts towards this sum,
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as part of the interleaved evolution. Without interleaved evolution
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this option would not make sense, since then a varying and large
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number of partons could already have been created by the initial-state
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radiation before the first final-state one, and then there is not
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likely to be any matrix elements available for matching.
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The global-recoil machinery does not work well with rescattering in the
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MPI machinery, since then the recoiling system is not uniquely defined.
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<code>MultipartonInteractions:allowRescatter = off</code> by default,
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so this is not a main issue. If both options are switched on,
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rescattering will only be allowed to kick in after the global recoil
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has ceased to be active, i.e. once the <code>nMaxGlobalRecoil</code>
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limit has been exceeded. This should not be a major conflict,
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since rescattering is mainly of interest at later stages of the
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downwards <i>pT</i> evolution.
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Further, it is strongly recommended to set
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<code>TimeShower:MEcorrections = off</code> (not default!), i.e. not
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to correct the emission probability to the internal matrix elements.
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The internal ME options do not cover any cases relevant for a multibody
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recoiler anyway, so no guarantees are given what prescription would
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come to be used. Instead, without ME corrections, a process-independent
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emission rate is obtained, and <a href="UserHooks.html" target="page">user hooks</a>
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can provide the desired process-specific rejection factors.
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<h3>Radiation off octet onium states</h3>
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In the current implementation, charmonium and bottomonium production
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can proceed either through colour singlet or colour octet mechanisms,
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both of them implemented in terms of <i>2 -> 2</i> hard processes
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such as <i>g g -> (onium) g</i>.
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In the former case the state does not radiate and the onium therefore
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is produced in isolation, up to normal underlying-event activity. In
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the latter case the situation is not so clear, but it is sensible to
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assume that a shower can evolve. (Assuming, of course, that the
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transverse momentum of the onium state is sufficiently high that
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radiation is of relevance.)
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There could be two parts to such a shower. Firstly a gluon (or even a
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quark, though less likely) produced in a hard <i>2 -> 2</i> process
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can undergo showering into many gluons, whereof one branches into the
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heavy-quark pair. Secondly, once the pair has been produced, each quark
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can radiate further gluons. This latter kind of emission could easily
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break up a semibound quark pair, but might also create a new semibound
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state where before an unbound pair existed, and to some approximation
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these two effects should balance in the onium production rate.
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The showering "off an onium state" as implemented here therefore should
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not be viewed as an accurate description of the emission history
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step by step, but rather as an effective approach to ensure that the
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octet onium produced "in the hard process" is embedded in a realistic
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amount of jet activity.
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Of course both the isolated singlet and embedded octet are likely to
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be extremes, but hopefully the mix of the two will strike a reasonable
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balance. However, it is possible that some part of the octet production
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occurs in channels where it should not be accompanied by (hard) radiation.
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Therefore reducing the fraction of octet onium states allowed to radiate
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is a valid variation to explore uncertainties.
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If an octet onium state is chosen to radiate, the simulation of branchings
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is based on the assumption that the full radiation is provided by an
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incoherent sum of radiation off the quark and off the antiquark of the
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onium state. Thus the splitting kernel is taken to be the normal
445
<i>q -> q g</i> one, multiplied by a factor of two. Obviously this is
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a simplification of a more complex picture, averaging over factors pulling
447
in different directions. Firstly, radiation off a gluon ought
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to be enhanced by a factor 9/4 relative to a quark rather than the 2
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now used, but this is a minor difference. Secondly, our use of the
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<i>q -> q g</i> branching kernel is roughly equivalent to always
451
following the harder gluon in a <i>g -> g g</i> branching. This could
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give us a bias towards producing too hard onia. A soft gluon would have
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little phase space to branch into a heavy-quark pair however, so the
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bias may not be as big as it would seem at first glance. Thirdly,
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once the gluon has branched into a quark pair, each quark carries roughly
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only half of the onium energy. The maximum energy per emitted gluon should
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then be roughly half the onium energy rather than the full, as it is now.
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Thereby the energy of radiated gluons is exaggerated, i.e. onia become too
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soft. So the second and the third points tend to cancel each other.
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Finally, note that the lower cutoff scale of the shower evolution depends
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on the onium mass rather than on the quark mass, as it should be. Gluons
464
below the octet-onium scale should only be part of the octet-to-singlet
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<p/><code>parm </code><strong> TimeShower:octetOniumFraction </strong>
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(<code>default = <strong>1.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.</code>)<br/>
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Allow colour-octet charmonium and bottomonium states to radiate gluons.
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0 means that no octet-onium states radiate, 1 that all do, with possibility
471
to interpolate between these two extremes.
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<p/><code>parm </code><strong> TimeShower:octetOniumColFac </strong>
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(<code>default = <strong>2.</strong></code>; <code>minimum = 0.</code>; <code>maximum = 4.</code>)<br/>
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The colour factor used used in the splitting kernel for those octet onium
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states that are allowed to radiate, normalized to the <i>q -> q g</i>
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splitting kernel. Thus the default corresponds to twice the radiation
479
off a quark. The physically preferred range would be between 1 and 9/4.
482
<h3>Further variables</h3>
484
There are several possibilities you can use to switch on or off selected
485
branching types in the shower, or in other respects simplify the shower.
486
These should normally not be touched. Their main function is for
489
<p/><code>flag </code><strong> TimeShower:QCDshower </strong>
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(<code>default = <strong>on</strong></code>)<br/>
491
Allow a QCD shower, i.e. branchings <i>q -> q g</i>, <i>g -> g g</i>
492
and <i>g -> q qbar</i>; on/off = true/false.
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<p/><code>mode </code><strong> TimeShower:nGluonToQuark </strong>
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(<code>default = <strong>5</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
497
Number of allowed quark flavours in <i>g -> q qbar</i> branchings
498
(phase space permitting). A change to 4 would exclude
499
<i>g -> b bbar</i>, etc.
502
<p/><code>flag </code><strong> TimeShower:QEDshowerByQ </strong>
503
(<code>default = <strong>on</strong></code>)<br/>
504
Allow quarks to radiate photons, i.e. branchings <i>q -> q gamma</i>;
508
<p/><code>flag </code><strong> TimeShower:QEDshowerByL </strong>
509
(<code>default = <strong>on</strong></code>)<br/>
510
Allow leptons to radiate photons, i.e. branchings <i>l -> l gamma</i>;
514
<p/><code>flag </code><strong> TimeShower:QEDshowerByGamma </strong>
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(<code>default = <strong>on</strong></code>)<br/>
516
Allow photons to branch into lepton or quark pairs, i.e. branchings
517
<i>gamma -> l+ l-</i> and <i>gamma -> q qbar</i>;
521
<p/><code>mode </code><strong> TimeShower:nGammaToQuark </strong>
522
(<code>default = <strong>5</strong></code>; <code>minimum = 0</code>; <code>maximum = 5</code>)<br/>
523
Number of allowed quark flavours in <i>gamma -> q qbar</i> branchings
524
(phase space permitting). A change to 4 would exclude
525
<i>g -> b bbar</i>, etc.
528
<p/><code>mode </code><strong> TimeShower:nGammaToLepton </strong>
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(<code>default = <strong>3</strong></code>; <code>minimum = 0</code>; <code>maximum = 3</code>)<br/>
530
Number of allowed lepton flavours in <i>gamma -> l+ l-</i> branchings
531
(phase space permitting). A change to 2 would exclude
532
<i>gamma -> tau+ tau-</i>, and a change to 1 also
533
<i>gamma -> mu+ mu-</i>.
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<p/><code>flag </code><strong> TimeShower:MEcorrections </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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Use of matrix element corrections where available; on/off = true/false.
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<p/><code>flag </code><strong> TimeShower:MEafterFirst </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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Use of matrix element corrections also after the first emission,
544
for dipole ends of the same system that did not yet radiate.
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Only has a meaning if <code>MEcorrections</code> above is
549
<p/><code>flag </code><strong> TimeShower:phiPolAsym </strong>
550
(<code>default = <strong>on</strong></code>)<br/>
551
Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
554
<p/><code>flag </code><strong> TimeShower:recoilToColoured </strong>
555
(<code>default = <strong>on</strong></code>)<br/>
556
In the decays of coloured resonances, say <i>t -> b W</i>, it is not
557
possible to set up dipoles with matched colours. Originally the
558
<i>b</i> radiator therefore has <i>W</i> as recoiler, and that
559
choice is unique. Once a gluon has been radiated, however, it is
560
possible either to have the unmatched colour (inherited by the gluon)
561
still recoiling against the <i>W</i> (<code>off</code>), or else
562
let it recoil against the <i>b</i> also for this dipole
563
(<code>on</code>). Before version 8.160 the former was the only
564
possibility, which could give unphysical radiation patterns. It is
565
kept as an option to check backwards compatibility. The same issue
566
exists for QED radiation, but obviously is less significant. Consider
567
the example <i>W -> e nu</i>, where originally the <i>nu</i>
568
takes the recoil. In the old (<code>off</code>) scheme the <i>nu</i>
569
would remain recoiler, while in the new (<code>on</code>) instead
570
each newly emitted photon becomes the new recoiler.
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