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<title>Particle Decays</title>
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<h2>Particle Decays</h2>
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The <code>ParticleDecays</code> class performs the sequential decays of
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all unstable hadrons produced in the string fragmentation stage,
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i.e. up to and including <i>b</i> hadrons and their decay products,
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such as the <i>tau</i> lepton. It is not to be used for the decay of
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more massive <a href="ResonanceDecays.html" target="page">resonances</a>, such as top,
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<i>Z^0</i> or SUSY, where decays must be performed already at the
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<code>ProcessLevel</code> of the event generation.
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The decay description essentially copies the one present in
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PYTHIA since many years, but with some improvements, e.g. in the decay
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tables and the number of decay models available. Recently a more
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sophisticated handling of <i>tau</i> decays has also been introduced.
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Some issues may need further polishing.
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<h3>Variables determining whether a particle decays</h3>
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Before a particle is actually decayed, a number of checks are made.
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(i) Decay modes must have been defined for the particle kind;
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tested by the <code>canDecay()</code> method of <code>Event</code>
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(and <code>ParticleData</code>).
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(ii) The main switch for allowing this particle kind to decay must
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be on; tested by the <code>mayDecay()</code> method of <code>Event</code>
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(and <code>ParticleData</code>).
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(iii) Particles may be requested to have a nominal proper lifetime
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<i>tau0</i> below a threshold.
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<p/><code>flag </code><strong> ParticleDecays:limitTau0 </strong>
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(<code>default = <strong>off</strong></code>)<br/>
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When on, only particles with <i>tau0 < tau0Max</i> are decayed.
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<p/><code>parm </code><strong> ParticleDecays:tau0Max </strong>
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(<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
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The above <i>tau0Max</i>, expressed in mm/c.
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(iv) Particles may be requested to have an actual proper lifetime
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<i>tau</i> below a threshold.
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<p/><code>flag </code><strong> ParticleDecays:limitTau </strong>
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(<code>default = <strong>off</strong></code>)<br/>
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When on, only particles with <i>tau < tauMax</i> are decayed.
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<p/><code>parm </code><strong> ParticleDecays:tauMax </strong>
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(<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
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The above <i>tauMax</i>, expressed in mm/c.<br/>
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In order for this and the subsequent tests to work, a <i>tau</i>
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is selected and stored for each particle, whether in the end it
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decays or not. (If each test would use a different temporary
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<i>tau</i> it would lead to inconsistencies.)
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(v) Particles may be requested to decay within a given distance
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<p/><code>flag </code><strong> ParticleDecays:limitRadius </strong>
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(<code>default = <strong>off</strong></code>)<br/>
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When on, only particles with a decay within a radius <i>r < rMax</i>
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are decayed. There is assumed to be no magnetic field or other
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<p/><code>parm </code><strong> ParticleDecays:rMax </strong>
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(<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
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The above <i>rMax</i>, expressed in mm.
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(vi) Particles may be requested to decay within a given cylidrical
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volume around the origin.
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<p/><code>flag </code><strong> ParticleDecays:limitCylinder </strong>
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(<code>default = <strong>off</strong></code>)<br/>
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When on, only particles with a decay within a volume limited by
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<i>rho = sqrt(x^2 + y^2) < xyMax</i> and <i>|z| < zMax</i>
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are decayed. There is assumed to be no magnetic field or other
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<p/><code>parm </code><strong> ParticleDecays:xyMax </strong>
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(<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
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The above <i>xyMax</i>, expressed in mm.
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<p/><code>parm </code><strong> ParticleDecays:zMax </strong>
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(<code>default = <strong>10.</strong></code>; <code>minimum = 0.</code>)<br/>
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The above <i>zMax</i>, expressed in mm.
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<p/><code>flag </code><strong> ParticleDecays:mixB </strong>
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(<code>default = <strong>on</strong></code>)<br/>
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Allow or not <i>B^0 - B^0bar</i> and <i>B_s^0 - B_s^0bar</i> mixing.
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<p/><code>parm </code><strong> ParticleDecays:xBdMix </strong>
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(<code>default = <strong>0.776</strong></code>; <code>minimum = 0.74</code>; <code>maximum = 0.81</code>)<br/>
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The mixing parameter <i>x_d = Delta(m_B^0)/Gamma_B^0</i> in the
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<i>B^0 - B^0bar</i> system. (Default from RPP2006.)
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<p/><code>parm </code><strong> ParticleDecays:xBsMix </strong>
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(<code>default = <strong>26.05</strong></code>; <code>minimum = 22.0</code>; <code>maximum = 30.0</code>)<br/>
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The mixing parameter <i>x_s = Delta(m_B_s^0)/Gamma_B_s^0</i> in the
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<i>B_s^0 - B_s^0bar</i> system. (Delta-m from CDF hep-ex-0609040,
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A new machinery has been introduced to handle <i>tau</i> lepton decays,
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with helicity information related to the production process and with
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the form of the hadronic current fitted to data. It is largely based
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on the corresponding Herwig++ implementation [<a href="Bibliography.html" target="page">Gre07</a>], with
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some input from Tauola [<a href="Bibliography.html" target="page">Jad90</a>]. A complete writeup is
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in preparation [<a href="Bibliography.html" target="page">Ilt11</a>].
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For <i>tau</i>s in external processes, interfaced with Les Houches
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Acccord information available, e.g. via Les Houches Event Files (LHEF),
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the new machinery interprets the SPINUP number for <i>tau</i> leptons
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as giving their helicity, and decays them accordingly. The only exceptions
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are when a specific polarization is forced by the user (see below),
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which then overrides the SPINUP value, or when SPINUP has the special
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value 9 (unpolarized). In the latter case, PYTHIA defaults back to
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attempting to determine the helicity structure from the production
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process, in the same way as for internal processes.
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This new machinery is on by default, but it is possible to revert to
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the simpler old decay handling, e.g. to study differences. Furthermore
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the spin tracing framework does not yet cover all possibilities; notably
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it cannot handle taus coming from SUSY decay chains
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(except via LHEF), so it makes sense
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to switch off the new machinery in such instances, for speed reasons if
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nothing else. In case only one tau mother species is undefined, the
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polarization involved can be set by hand.
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<p/><code>mode </code><strong> ParticleDecays:sophisticatedTau </strong>
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(<code>default = <strong>1</strong></code>; <code>minimum = 0</code>; <code>maximum = 3</code>)<br/>
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Choice of <i>tau</i> decay model.
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<br/><code>option </code><strong> 0</strong> : old decay model, with isotropic decays.
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When reading LHEF files, the SPINUP digit will be ignored.
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<br/><code>option </code><strong> 1</strong> : sophisticated decays where <i>tau</i> polarization is
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calculated from the <i>tau</i> production mechanism.
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When reading LHEF files, the SPINUP digit will be used.
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<br/><code>option </code><strong> 2</strong> : sophisticated decays as above, but additionally <i>tau</i>
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polarization is set to <code>ParticleDecaus:tauPolarization</code> for
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<i>tau</i>s produced from <code>ParticleDecays:tauMother</code>.
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When reading LHEF files, this overrides the SPINUP digit.
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<br/><code>option </code><strong> 3</strong> : sophisticated decays where <i>tau</i> polarization is set
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to <code>ParticleDecaus:tauPolarization</code> for all <i>tau</i> decays.
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When reading LHEF files, this overrides the SPINUP digit.
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<br/><b>Note</b>: options <code>2</code> and <code>3</code>,
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to force a specific <i>tau</i> polarization, only affect the decay
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of the <i>tau</i>. The angular distribution of the <i>tau</i> itself,
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given by its production, is not modified by these options. If you want, e.g.,
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a righthanded <i>W</i>, or a SUSY decay chain, the kinematics should
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be handled by the corresponding cross section class(es), supplemented by
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the resonance decay one(s). The options here could then still be used
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to ensure the correct polarization at the <i>tau</i> decay stage.
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<p/><code>parm </code><strong> ParticleDecays:tauPolarization </strong>
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(<code>default = <strong>0</strong></code>; <code>minimum = -1.</code>; <code>maximum = 1.</code>)<br/>
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Polarization of the <i>tau</i> when mode <i>2</i> or <i>3</i> of
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<code>ParticleDecays:sophisticatedTau</code> is selected.
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<p/><code>mode </code><strong> ParticleDecays:tauMother </strong>
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(<code>default = <strong>0</strong></code>; <code>minimum = 0</code>)<br/>
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Mother of the <i>tau</i> for forced polarization when mode <i>2</i> of
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<code>ParticleDecays:sophisticatedTau</code> is selected. You should give the
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positive identity code; to the extent an antiparticle exists it will
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automatically obtain the inverse polarization.
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<h3>Other variables</h3>
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<p/><code>parm </code><strong> ParticleDecays:mSafety </strong>
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(<code>default = <strong>0.0005</strong></code>; <code>minimum = 0.</code>; <code>maximum = 0.01</code>)<br/>
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Minimum mass difference required between the decaying mother mass
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and the sum of the daughter masses, kept as a safety margin to avoid
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numerical problems in the decay generation.
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<p/><code>parm </code><strong> ParticleDecays:sigmaSoft </strong>
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(<code>default = <strong>0.5</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 2.</code>)<br/>
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In semileptonic decays to more than one hadron, such as
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<i>B -> nu l D pi</i>, decay products after the first three are
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dampened in momentum by an explicit weight factor
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<i>exp(-p^2/sigmaSoft^2)</i>, where <i>p</i> is the
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three-momentum in the rest frame of the decaying particle.
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This takes into account that such further particles come from the
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fragmentation of the spectator parton and thus should be soft.
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When a decay mode is defined in terms of a partonic content, a random
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multiplicity (and a random flavour set) of hadrons is to be picked,
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especially for some charm and bottom decays. This is done according to
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a Poissonian distribution, for <i>n_p</i> normal particles and
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<i>n_q</i> quarks the average value is chosen as
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n_p/ 2 + n_q/4 + multIncrease * ln ( mDiff / multRefMass)
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with <i>mDiff</i> the difference between the decaying particle mass
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and the sum of the normal-particle masses and the constituent quark masses.
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For gluonic systems <i>multGoffset</i> offers and optional additional
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term to the multiplicity. The lowest possible multiplicity is
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<i>n_p + n_q/2</i> (but at least 2) and the highest possible 10.
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If the picked hadrons have a summed mass above that of the mother a
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new try is made, including a new multiplicity. These constraints
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imply that the actual average multiplicity does not quite agree with
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<p/><code>parm </code><strong> ParticleDecays:multIncrease </strong>
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(<code>default = <strong>4.</strong></code>; <code>minimum = 2.</code>; <code>maximum = 6.</code>)<br/>
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The above <i>multIncrease</i> parameter, except for
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<code>meMode = 23</code>.
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<p/><code>parm </code><strong> ParticleDecays:multIncreaseWeak </strong>
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(<code>default = <strong>2.5</strong></code>; <code>minimum = 1.</code>; <code>maximum = 4.</code>)<br/>
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The above <i>multIncrease</i> parameter, specifically for
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<code>meMode = 23</code>. Here the weak decay implies that only the
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virtual W mass should contribute to the production of new particles,
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rather than the full meson mass.
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<p/><code>parm </code><strong> ParticleDecays:multRefMass </strong>
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(<code>default = <strong>0.7</strong></code>; <code>minimum = 0.2</code>; <code>maximum = 2.0</code>)<br/>
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The above <i>multRefMass</i> parameter.
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<p/><code>parm </code><strong> ParticleDecays:multGoffset </strong>
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(<code>default = <strong>0.5</strong></code>; <code>minimum = 0.0</code>; <code>maximum = 2.0</code>)<br/>
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The above <i>multGoffset</i> parameter.
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<p/><code>parm </code><strong> ParticleDecays:colRearrange </strong>
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(<code>default = <strong>0.5</strong></code>; <code>minimum = 0.</code>; <code>maximum = 1.0</code>)<br/>
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When a decay is given as a list of four partons to be turned into
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hadrons (primarily for modes 41 - 80) it is assumed that they are
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listed in pairs, as a first and a second colour singlet, which could
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give rise to separate sets of hadrons. Here <i>colRearrange</i> is
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the probability that this original assignment is not respected, and
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default corresponds to no memory of this original colour topology.
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<p/><code>flag </code><strong> ParticleDecays:FSRinDecays </strong>
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(<code>default = <strong>true</strong></code>)<br/>
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When a particle decays to <i>q qbar</i>, <i>g g</i>, <i>g g g</i>
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or <i>gamma g g</i>, with <code>meMode > 90</code>, allow or not a
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shower to develop from it, before the partonic system is hadronized.
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(The typical example is <i>Upsilon</i> decay.)
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In addition, some variables defined for string fragmentation and for
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flavour production are used also here.
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<h3>Modes for Matrix Element Processing</h3>
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Some decays can be treated better than what pure phase space allows,
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by reweighting with appropriate matrix elements. In others a partonic
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content has to be converted to a set of hadrons. The presence of such
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corrections is signalled by a nonvanishing <code>meMode()</code> value
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for a decay mode in the <a href="ParticleDataScheme.html" target="page">particle
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data table</a>. The list of allowed possibilities almost agrees with the
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PYTHIA 6 ones, but several obsolete choices have been removed,
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a few new introduced, and most have been moved for better consistency.
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Here is the list of currently allowed <code>meMode()</code> codes:
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<li> 0 : pure phase space of produced particles ("default");
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input of partons is allowed and then the partonic content is
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converted into the minimal number of hadrons (i.e. one per
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parton pair, but at least two particles in total)</li>
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<li> 1 : <i>omega</i> and <i>phi -> pi+ pi- pi0</i></li>
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<li> 2 : polarization in <i>V -> PS + PS</i> (<i>V</i> = vector,
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<i>PS</i> = pseudoscalar), when <i>V</i> is produced by
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<i>PS -> PS + V</i> or <i>PS -> gamma + V</i></li>
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<li> 11 : Dalitz decay into one particle, in addition to the
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lepton pair (also allowed to specify a quark-antiquark pair that
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should collapse to a single hadron)</li>
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<li> 12 : Dalitz decay into two or more particles in addition
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to the lepton pair</li>
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<li> 13 : double Dalitz decay into two lepton pairs</li>
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<li> 21 : decay to phase space, but weight up <i>neutrino_tau</i> spectrum
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in <i>tau</i> decay</li>
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<li> 22 : weak decay; if there is a quark spectator system it collapses to
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one hadron; for leptonic/semileptonic decays the <i>V-A</i> matrix element
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is used, for hadronic decays simple phase space</li>
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<li> 23 : as 22, but require at least three particles in decay</li>
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<li> 31 : decays of type B -> gamma X, very primitive simulation where
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X is given in terms of its flavour content, the X multiplicity is picked
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according to a geometrical distribution with average number 2, and
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the photon energy spectrum is weighted up relative to pure phase space</li>
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<li> 42 - 50 : turn partons into a random number of hadrons, picked according
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to a Poissonian with average value as described above, but at least
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<code>code</code> - 40 and at most 10, and then distribute then in pure
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phase space; make a new try with another multiplicity if the sum of daughter
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masses exceed the mother one </li>
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<li> 52 - 60 : as 42 - 50, with multiplicity between <code>code</code> - 50
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and 10, but avoid already explicitly listed non-partonic channels</li>
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<li> 62 - 70 : as 42 - 50, but fixed multiplicity <code>code</code> - 60</li>
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<li> 72 - 80 : as 42 - 50, but fixed multiplicity <code>code</code> - 70,
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and avoid already explicitly listed non-partonic channels</li>
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<li> 91 : decay to <i>q qbar</i> or <i>g g</i>, which should shower
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<li> 92 : decay onium to <i>g g g</i> or <i>g g gamma</i>
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(with matrix element), which should shower and hadronize</li>
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<li> 100 - : reserved for the description of partial widths of
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<a href="ResonanceDecays.html" target="page">resonances</a></li>
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Three special decay product identity codes are defined.
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<li>81: remnant flavour. Used for weak decays of c and b hadrons, where the
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c or b quark decays and the other quarks are considered as a spectator
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remnant in this decay. In practice only used for baryons with multiple
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c and b quarks, which presumably would never be used, but have simple
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(copied) just-in-case decay tables. Assumed to be last decay product.</li>
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<li>82: random flavour, picked by the standard fragmentation flavour
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machinery, used to start a sequence of hadrons, for matrix element
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codes in 41 - 80. Assumed to be first decay product, with -82 as second
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and last. Where multiplicity is free to be picked it is selected as for
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normal quarkonic systems. Currently unused.</li>
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<li>83: as for 82, with matched pair 83, -83 of decay products. The
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difference is that here the pair is supposed to come from a closed gluon
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loop (e.g. <i>eta_c -> g g</i>) and so have a somewhat higher average
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multiplicity than the simple string assumed for 82, see the
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<code>ParticleDecays:multGoffset</code> parameter above.</li>
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