arXiv:0910.4462v2 [hep-ph] 8 Dec 2009
GENXICC2.0: An Upgraded Version of the Generator for
Hadronic Production of Double Heavy Baryons Ξcc, Ξbcand Ξbb
Chao-Hsi Chang1,2,3∗Jian-Xiong Wang4†and Xing-Gang Wu1‡
1Department of Physics, Chongqing University, Chongqing 400044, China
2CCAST (World Laboratory), P.O.Box 8730, Beijing 100080, China
3Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China
4Institute of High Energy Physics, P.O.Box 918(4), Beijing 100049, P.R. China
(Dated: December 8, 2009)
An upgraded (second) version of the package GENXICC (A Generator for Hadronic
Production of the Double Heavy Baryons Ξcc, Ξbcand Ξbbby C.H. Chang, J.X. Wang
and X.G. Wu, [its first version: in Comput. Phys. Commun. 177 (2007) 467-478]) is presented.
Users, with this version being implemented in PYTHIA and a GNU C compiler, may simulate
full events of the production in various experimental environments conveniently. In comparison
with the previous version, in order to implement it in PYTHIA properly, a subprogram for the
fragmentation of the produced double heavy diquark to the relevant baryon is complemented
and the interphase of the generator to PYTHIA is changed accordingly.In the subprogram,
with explanation, certain necessary assumptions (approximations) are made so as to conserve the
momenta and the QCD ‘color’ flow for the fragmentation.
NEW VERSION PROGRAM SUMMARY
Title of program : GENXICC2.0
Program obtained from : CPC Program Library or the Institute of Theoretical Physics,
Chinese Academy of Sciences, Beijing, P.R. China: www.itp.ac.cn/?zhangzx/genxicc2.0.
Reference to original program : GENXICC
Reference in CPC : Comput. Phys. Commun. 177, 467(2007)
Does the new version supersede the old program: No
Computer : Any LINUX based on PC with FORTRAN 77 or FORTRAN 90 and GNU C
compiler as well.
Operating systems : LINUX.
Programming language used : FORTRAN 77/90.
Memory required to execute with typical data : About 2.0 MB.
No. of bytes in distributed program, (including PYTHIA6.4) : About 1.5 MB.
Distribution format : .tar.gz .
Nature of physical problem : Hadronic production of double heavy baryons Ξcc, Ξbcand Ξbb.
Method of solution : The code is based on NRQCD framework. With proper option, it can
generate weighted and un-weighted events of hadronic double heavy baryon production.
When the hadronizations of the produced jets and double heavy diquark are taken into
account in the production, the upgraded version with proper interface to PYTHIA can well
generate the events in full.
Restrictions on the complexity of the problem : The color flow, particularly, in the piece of
programming the fragmentation from the produced colorful double heavy diquark into a
relevant double heavy baryon, is treated carefully so as to implement it in PYTHIA properly.
Reasons for new version : Responding to the feedback from users, we improve the gener-
ator mainly by careful completing the ‘final nonperturbative process’ i.e. the formulation
of the double heavy baryon from relevant intermediate diquark. In the present version, the
information for fragmentation about momentum-flow and the-color flow, that is necessary
for PYTHIA to generate full events, is retained although reasonable approximations are
made. In comparison with the original version, the upgraded one can implement it in
PYTHIA properly to do the full event simulation of the double heavy baryon production.
Typical running time : It depends on which option is chosen to match PYTHIA when
generating the events in full and also on which mechanism is chosen to generate the events.
Typically, for the most complicated case with gluon-gluon fusion mechanism to generate
the mixed events via the intermediate diquark in (cc)[3S1]¯3and (cc)[1S0]6 states, and to
generate 1000 events, it takes about 20 hours on a 1.8 GHz Intel P4-processor machine
if IDWTUP=1, whereas to generate 106events it takes about 40 minutes only if IDWTUP=3.
Keywords : Event generator; Hadronic production; double heavy baryons.
Summary of the changes (improvements) : 1) We try to explain the treatment of the mo-
mentum distribution of the process more clearly than the original version, and show how the
final baryon is generated through the typical intermediate diquark precisely. 2) We present
color flow of the involved processes precisely and the corresponding changes for the program
are made. 3). The corresponding changes of the program are explained in the paper.
TABLE I: All considered mechanisms for step A, which are defined by the two parameters mgenxi
and ixiccstate. Here the symbol gg-mechanism stands for the gluon-gluon fusion mechanism and
— mgenxi=1 mgenxi=2mgenxi=3
ixiccstate=1gg-mechanism, (cc)¯ 3(3S1) gg-mechanism, (bc)¯ 3(3S1) gg-mechanism, (bb)¯ 3(3S1)
ixiccstate=2gg-mechanism, (cc)6(1S0) gg-mechanism, (bc)6(1S0) gg-mechanism, (bb)6(1S0)
ixiccstate=3 gc-mechanism, (cc)¯ 3(3S1) gg-mechanism, (bc)6(3S1)—
ixiccstate=4 gc-mechanism, (cc)6(1S0) gg-mechanism, (bc)¯ 3(1S0)—
ixiccstate=5cc-mechanism, (cc)¯ 3(3S1)
ixiccstate=6 cc-mechanism, (cc)6(1S0)——
I. MOMENTUM DISTRIBUTION OF THE PRODUCTION
In fact, in the program we divide the production of a double heavy baryon Ξccor Ξbc
or Ξbbinto two steps: Step-A is up-to the production of a relevant double heavy diquark
and Step-B is followed for the fragmentation of the double heavy diquark into the desired
baryon. Therefore, in Step-A, there are three possible mechanisms: the gluon-gluon fusion
mechanism (g + g), gluon-charm collision mechanism (g + c) and charm-charm collision
mechanism (c+c), so in the program we need to fix one of them to produce the diquarks (cc),
(bc) and (bb) accordingly in term. All the three mechanisms and the calculation techniques
for them are described in Refs.[2, 3, 4]. In Step-B, it is for the fragmentation of the double
heavy diquark into the desired baryon. In the step we assume the intermediate diquark is to
‘decay’ into the baryon plus soft parton(s) exclusively, e.g., either (QQ′)[3S1]¯3→ ΞQQ′+¯ q or
(QQ′)[1S0]¯6→ ΞQQ′ + ¯ q + g (here Q,Q′denote c,b-quark, ¯ q an light anti-quark, g a gluon).
For convenience, in the program we name the mechanisms and intermediate diquarks in
terms of the parameters as those in TAB.I, where mgenxi stands for the double-heavy
diquarks, (cc) or (bc) or (bb), and ixiccstate stands for the mechanisms. Note that in
the previous version of GENXICC (we call it GENXICC1.0), we did not program how the
diquark forms the relevant baryon, but alternatively we simply assumed that the relevant
baryon is formed with 100% efficiency.
According to QCD confinement, the produced diquarks (QQ′), i.e. (cc), (bc), (bb), must
be fragmented into relevant baryons by grabbing a light quark q (even suitable number
of gluons g) with definite probability. Since the fragmentation for the heavy diquarks is
absent from the available version of PYTHIA, thus in the upgraded version of GENXICC we
program the fragmentation precisely and make its interphase still to suit PYTHIA properly.
For consistency, in the upgraded version for the fragmentation we adopt the assumptions
and method similar to those taken by PYTHIA  in the case for the fragmentation of a
color-octet component (c¯ c)8into a colorless charmonium. Namely here the double heavy
quark to grab a light quark (with gluons if necessary) from the ‘environment’ to form a
colorless double heavy baryon with a relative possibility for various flavors of the light quark
as u : d : s : c ≃ 1 : 1 : 0.3 : 10−11. Hence in the program we introduce three new parameters
ratiou (default=1), ratiod(default=1), ratios(default=0.3) so as to dictate the probability
for a double heavy diquark to grab a light quark (antiquark) in forming the relevant baryon
finally. These parameters may be changed by setting the values of the parameters in the
parameter.F, when the relative possibilities for various flavors are assumed precisely. One
more parameter nbound is naturally introduced to dictate which type of baryon: Ξ+,++
bb(nbound=3) is to be generated
from the relevant produced diquark. (nbound=4) is to derive the diquark results that can
be generated by previous version (GENXICC1.0). The relative possibilities for the baryons
ccare decided by the value of ratiou, ratiod and ratios. More precisely,
if the diquark (cc)[3S1]¯3is produced, then it will fragment into Ξ++
with 43% probability,
ccwith 43% probability and Ω+
ccwith 14% probability accordingly, when default values of
ratiou, ratiod and ratios are taken.
Below we shall only take the hadronic production of Ξccas an example to show how the
generator GENXICC works, because the production for the baryon Ξbcor Ξbbis similar.
At the end of the Step-A, the final particles’ momenta are set by using the phase gen
routine, that is based on RAMBO (Random Momentum Booster) program , and the
irrelevant phase space is integrated by VEGAS . For the Step-B, we adopt the ‘decay’
method (as that to deal with the intermediate color octet (c¯ c) states to produce J/ψ in
PYTHIA ). According to the method, we start with assuming the diquark mass to be
slightly bigger than that of the baryon (> 2mc), so the diquark may ‘decay’ into a relevant
baryon by emitting very soft partons (anti-quark and/or gluons), and the soft partons take