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The advanced popcorn code for baryon production
In section a new advanced popcorn code for
baryon production model was presented, based on [Edé97]. It partly
overwrites and redefines the meaning of some of the parameters above.
Therefore the full description of these new options are given separately
in this section, together with a listing of the new routines involved.
In order to use the new options, a few possibilities are open.
- Use of the old diquark and popcorn models, MSTJ(12) = 1 and
= 2, is essentially unchanged. Note, however, that PARJ(19) is
available for an ad-hoc suppression of first-rank baryon production.
- Use of the old popcorn model with new SU(6) weighting:
- Set MSTJ(12) = 3.
- Increase PARJ(1) by approximately a factor 1.2 to retain
about the same effective baryon production rate as in MSTJ(12) = 2.
- Note: the new SU(6) weighting e.g. implies that the total
production rate of charm and bottom baryons is reduced.
- Use of the old flavour model with new SU(6) treatment and
modified fragmentation function for diquark vertices (which softens
- Set MSTJ(12) = 4.
- Increase PARJ(1) by about a factor 1.7 and PARJ(5) by
about a factor 1.2 to restore the baryon and popcorn rates of the
MSTJ(12) = 2 default.
- Use of the new flavour model (automatically with modified diquark
- Set MSTJ(12) = 5.
- Increase PARJ(1) by approximately a factor 2.
- Change PARJ(18) from 1 to approx. 0.19.
- Instead of PARJ(3) - PARJ(7), tune PARJ(8), PARJ(9),
PARJ(10) and PARJ(18). (Here PARJ(10) is used
only in collisions having remnants of baryon beam particles.)
- Note: the proposed parameter values are based on a global fit to
all baryon production rates. This e.g. means that the proton rate
is lower than in the MSTJ(12) = 2 option, with current data
somewhere in between. The PARJ(1) value would have to be about
3 times higher in MSTJ(12) = 5 than in = 2 to have the same total
baryon production rate (=proton+neutron), but then other baryon
rates would not match at all.
- The new options MSTJ(12) = 4 and = 5 (and, to some extent,
= 3) soften baryon spectra in such a way that PARJ(45)
(the change of for diquarks in the Lund symmetric fragmentation
function) is available for a retune. It affects i.e. baryon-antibaryon
rapidity correlations and the baryon excess over antibaryons in quark jets.
The changes in and additions to the common blocks are as follows.
- MSTU(121) - MSTU(125) :
- Internal flags and counters; only
MSTU(123) may be touched by you.
- MSTU(121) :
- Popcorn meson counter.
- MSTU(122) :
- Points at the proper diquark production weights, to
distinguish between ordinary popcorn and rank 0 diquark
systems. Only needed if MSTJ(12) = 5.
- MSTU(123) :
- Initialization flag. If MSTU(123) is 0 in a
PYKFDI call, PYKFIN is called and MSTU(123) set to 1.
Would need to be reset by you if flavour parameters are changed in
the middle of a run.
- MSTU(124) :
- First parton flavour in decay call, stored to easily
find random flavour partner in a popcorn system.
- MSTU(125) :
- Maximum number of popcorn mesons allowed in decay flavour
generation. If a larger popcorn system passes the fake string
suppressions, the error KF = 0 is returned and the flavour
generation for the decay is restarted.
- MSTU(131) - MSTU(140) :
- Store of popcorn meson flavour codes in
decay algorithm. Purely internal.
- MSTJ(12) :
- (D = 2) Main switch for choice of baryon production model.
Suppression of rank 1 baryons by a parameter PARJ(19) is no longer
governed by the MSTJ(12) switch, but instead turned on by setting
PARJ(19) < 1. Three new options are available:
- = 3 :
- as = 2, but additionally the production of first
rank baryons may be suppressed by a factor PARJ(19).
- = 4 :
- as = 2, but diquark vertices suffers an extra
suppression of the form
is stored in PARF(192).
- = 5 :
- Advanced version of the popcorn model. Independent of
PARJ(3 - 7). Instead depending on PARJ(8 - 10). When using
this option PARJ(1) needs to enhanced by approx. a factor 2
(i.e. it losses a bit of its normal meaning),
and PARJ(18) is suggested to be set to 0.19.
- PARJ(8), PARJ(9) :
- (D = 0.6, 1.2 GeV) The new popcorn
Used to suppress popcorn mesons of total invariant mass by
. Larger PARJ(9) leads to a stronger
suppression of popcorn systems surrounded by an
also a little stronger suppression of strangeness in diquarks.
- PARJ(10) :
- (D = 0.6 GeV) Corresponding parameter for
suppression of leading rank mesons of transverse mass in the
fragmentation of diquark jets, used if MSTJ(12) = 5. The
treatment of original diquarks is flavour independent, i.e. PARJ(10)
is used even if the diquark contains or heavier quarks.
- PARF(131) - PARF(190) :
- Different diquark and popcorn weights,
calculated in PYKFIN, which is automatically called from
- PARF(131) :
- Popcorn ratio
in the old model.
- PARF(132-134) :
- Leading rank meson ratio in the old
model, for original diquark with 0, 1 and 2 -quarks, respectively.
- PARF(135-137) :
- Colour fluctuation quark ratio, i.e. the
relative probability that the heavier quark in a diquark fits into the
baryon at the opposite side of the popcorn meson. For
and original diquarks, respectively.
- PARF(138) :
- The extra suppression of strange colour fluctuation
quarks, due to the requirement of surrounding a popcorn meson. (In the
old model, it is simply PARJ(6).)
- PARF(139) :
- Preliminary suppression of a popcorn meson in the new
model. A system of popcorn mesons is started with weight proportional
to PARF(139). It is then tested against the correct weight,
derived from the mass of the system. For strange colour fluctuation
quarks, the weight is PARF(138)*PARF(139).
- PARF(140) :
- Preliminary suppression of leading rank mesons in
diquark strings, irrespective of flavour. Corresponds to PARF(139).
- PARF(141-145) :
- Maximal SU(6) factors for different types
- PARF(146) :
suppression if MSTJ(12) = 5,
derived from PARJ(18).
- PARF(151-190) :
- Production ratios for different diquarks. Stored
in four groups, handling
in the case of original diquarks. In each group
- 1 : colour fluctuation ratio.
- 2,3 : ratio for the vertex quark if the colour fluctuation
quark is light or strange, respectively.
- 4 :
vertex quark ratio if the colour fluctuation quark is
light and .
- 5-7 : (spin 1)/(spin 0) ratio for , and , where
the first flavour is the colour fluctuation quark.
- 8-10 : Unused.
- PARF(191) :
- (D = 0.2) Non-constituent mass in GeV of a
diquark. Used in combination with diquark constituent mass differences
to derive relative production rates for different diquark flavours in
the MSTJ(12) = 5 option.
- PARF(192) :
- (D = 0.5) Parameter for the low- suppression
of diquark vertices in the MSTJ(12) options. PARF(192)
represents , i.e. the suppression
is of the form 1. - PARF(192), in GeV.
- PARF(193,194) :
- (I) Store of some popcorn weights used by the
present popcorn system.
- PARF(201-1400) :
- (I) Weights for every possible popcorn meson
construction in the MSTJ(12) = 5 option. Calculated from input
parameters and meson masses in PYKFIN. When
, the weights for M and the new
diquark depends not only on and : it is also important if
this is a `true' popcorn system, or a system which started with a diquark
at the string end, and if M is the final meson of the popcorn system,
i.e. if the
diquark will go into a baryon or not. With five
possible flavours for and this gives 80 different situations
when selecting and . However, quarks heavier than only exist
in the string endpoints, and if more popcorn mesons are to be produced,
diquark does not influence the weights and the
dependence reduces to what factor (PARJ(8 - 10)) that is used.
Then 40 distinct situations remains, i.e.:
`true popcorn' final meson
YES YES d,u,s d,u,s
NO s,s d,u,s
NO YES d,u,s,s d,u,s,c,b
NO 1 case d,u,s,c,b
This table also shows the order in which the situations are stored. E.g.
situation no. 1 is `YES,YES,d,d', situation no.11 is `YES,NO,,u'.
In every situation can be , or . if
are in the program three possible flavour mixing states available for the
meson. This gives five possible meson flavours, and for each one of them
there are six possible spin states. Thus 30 PARF positions are
reserved for each situation, and these are used as follows:
For each spin multiplet (in the same order as in PARF(1 - 60)) five
positions are reserved. First are stored the weights for the the
mesons, with in increasing order. If
occupies three spots, and the final two are unused. If
final three spots are used for the diagonal states when
In summary, all common-block variables are completely internal, except
MSTU(123), MSTJ(12), PARJ(8) - PARJ(10) and
PARF(191), PARF(192). Among these, PARF(191) and PARF(192)
should not need to be changed. MSTU(123) should be 0 when starting,
and reset to 0 whenever changing a switch or parameter which influences
flavour weight With MSTJ(12) = 4, PARJ(5) may need to increase.
With MSTJ(12) = 5, a preliminary tune suggests PARJ(8) = 0.6,
PARJ(9) = 1.2, PARJ(10) = 0.6, PARJ(1) = 0.20 and
PARJ(18) = 0.19.
Three new subroutines are added, but are only needed for internal use.
- SUBROUTINE PYKFIN :
- to calculate a large set of diquark and popcorn weights from input
parameters. Is called from PYKFDI if MSTU(123) = 0. Sets
MSTU(123) to 1.
- SUBROUTINE PYNMES(KFDIQ) :
- to calculate number of popcorn mesons to be generated in a popcorn
system, or the number of leading rank mesons when fragmenting a
diquark string. Stores the number in MSTU(121).
Always returns 0 if MSTJ(12) < 2. Returns 0 or 1 if
MSTJ(12) < 5.
- KFDIQ :
- Flavour of the diquark in a diquark string. If starting
a popcorn system inside a string, KFDIQ is 0.
- SUBROUTINE PYDCYK(KFL1,KFL2,KFL3,KF) :
- to generate flavours in the phase space model of hadron decays, and
in cluster decays. Is essentially the same as a PYKFDI call, but
also takes into account the effects of string dynamics in flavour
production in the MSTJ(12) options. This is done in order
to get a reasonable interpretation of the input parameters also for
hadron decays with these options.
- KFL1,KFL2,KFL3,KF :
- See SUBROUTINE PYKFDI.
Internally the diquark codes have been extended to store the necessary
further popcorn information. As before, an initially existing diquark
has a code of the type
, where .
Diquarks created in the fragmentation process now have the longer code
, i.e. one further digit is set.
Here is the curtain quark, i.e. the flavour of the quark-antiquark
pair that is shared between the baryon and the antibaryon, either
or . The non-curtain quark, the other of and , may
have its antiquark partner in a popcorn meson. In case there are no popcorn
mesons this information is not needed, but is still set at random to be
either of and . The extended code is used internally in
PYSTRF and PYDECY and in some routines called by them, but
is not visible in any event listings.
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