PYTHIA 6.4 Physics and Manual

Note: when multiple interactions are switched on, the distribution used for the
subsequent interactions vanishes in the old model, and has a RMS width
forced equal to the fragmentation one in the new model, but the shape still
follows the choice of MSTP(91).
MSTP(92) : (D = 3) (C) energy partitioning in hadron or resolved-photon remnant, when
this remnant is split into two jets in the old model. (For a splitting into a hadron
plus a jet, see MSTP(94).) The energy fraction χ taken by one of the two objects,
with conventions as described for PARP(94) and PARP(96), is chosen according to
the different distributions below. Here cmin = 0.6 GeV/Ecm ≈ 2〈mq〉/Ecm.
= 1 : 1 for meson or resolved photon, 2(1 − χ) for baryon, i.e. simple counting
rules.
= 2 : (k + 1)(1 − χ) k , with k given by PARP(94) or PARP(96).
= 3 : proportional to (1−χ) k / 4
χ2 + c2 min , with k given byPARP(94) orPARP(96).
= 4 : proportional to (1−χ) k
/ χ2 + c2 min , with k given by PARP(94) orPARP(96).
= 5 : proportional to (1 − χ) k /(χ 2 + c 2 min )b/2 , with k given by PARP(94) or
PARP(96), and b by PARP(98).
MSTP(93) : (D = 1) (C) primordial k⊥ distribution in photon, either it is one of the
incoming particles or inside an electron.
= 0 : no primordial k⊥.
= 1 : Gaussian, width given in PARP(99), upper cut-off in PARP(100).
= 2 : exponential, width given in PARP(99), upper cut-off in PARP(100).
= 3 : power-like of the type dk 2 ⊥ /(k2 ⊥0 +k2 ⊥ )2 , with k⊥0 in PARP(99) and upper k⊥
cut-off in PARP(100).
= 4 : power-like of the type dk 2 ⊥ /(k2 ⊥0 + k2 ⊥ ), with k⊥0 in PARP(99) and upper k⊥
cut-off in PARP(100).
= 5 : power-like of the type dk 2 ⊥ /(k2 ⊥0 + k2 ⊥ ), with k⊥0 in PARP(99) and upper k⊥
cut-off given by the p⊥ of the hard process or by PARP(100), whichever is
smaller.
Note: for options 1 and 2 the PARP(100) value is of minor importance, once
PARP(100)≫PARP(99). However, options 3 and 4 correspond to distributions
with infinite 〈k 2 ⊥ 〉 if the k⊥ spectrum is not cut off, and therefore the
PARP(100) value is as important for the overall distribution as is PARP(99).
MSTP(94) : (D = 3) (C) energy partitioning in hadron or resolved-photon remnant, when
this remnant is split into a hadron plus a remainder-jet in the old model. The
energy fraction χ is taken by one of the two objects, with conventions as described
below or for PARP(95) and PARP(97).
= 1 : 1 for meson or resolved photon, 2(1 − χ) for baryon, i.e. simple counting
rules.
= 2 : (k + 1)(1 − χ) k , with k given by PARP(95) or PARP(97).
= 3 : the χ of the hadron is selected according to the normal fragmentation function
used for the hadron in jet fragmentation, see MSTJ(11). The possibility
of a changed fragmentation function shape in diquark fragmentation (see
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