Onia Processes
Production of J/psi or Upsilon, directly and via chi states and the
colour-octet mechanism.
In each process the square-bracketed expression specifies the state
in spectroscopic notation, (2S+1) L J, followed by
(1) for colour-singlet states and (8) for
colour-octet ditto.
The original Fortran code for these processes has been contributed
by Stefan Wolf [unpublished]. For the C++ version only the unpolarized
expressions are retained, since the theoretical predictions of the
colour-octet model anyway do not agree with the experimental
observations. Furthermore, the polarization effects are modest,
so isotropic decay is not a bad starting point. Such an event sample
can afterwards be reweighted at will by the user, to test various
assumptions.
The description of
final-state radiation
is in this case based on some further model assumptions.
Most of the processes below are divergent in the limit pT -> 0,
and therefore a pTmin scale should be set. Comparisons with
data indicate that this divergence can be tamed the same way as for
the normal QCD 2 -> 2 cross sections [Bar06,Kra08],
which makes sense, since they are all dominated by the same kind of
t-channel gluon exchange. It is therefore possible to use the
SuppressSmallPT user hook to impose a
reweighting that cancels the low-pT divergence.
An eikonalized description of these processes is included in the
multiparton-interactions framework. Here the low-pT dampening
is automatic, and additionally the framework is more consistent
(e.g. with respect to energy-momentum constraints and the
impact-parameter description) for events where the onium production
is not the hardest subprocess, as would often be the case in the
low-pT limit.
Charmonium
Common switch for the group of charmonium production.
g g -> ccbar[3S1(1)] g.
Code 401.
g g -> ccbar[3P0(1)] g.
Code 402.
g g -> ccbar[3P1(1)] g.
Code 403.
g g -> ccbar[3P2(1)] g.
Code 404.
q g -> ccbar[3P0(1)] q.
Code 405.
q g -> ccbar[3P1(1)] q.
Code 406.
q g -> ccbar[3P2(1)] q.
Code 407.
q qbar -> ccbar[3P0(1)] g.
Code 408.
q qbar -> ccbar[3P1(1)] g.
Code 409.
q qbar -> ccbar[3P2(1)] g.
Code 410.
g g -> ccbar[3S1(8)] g.
Code 411.
g g -> ccbar[3S1(8)] g.
Code 412.
g g -> ccbar[3S1(8)] g.
Code 413.
q g -> ccbar[3S1(8)] q.
Code 414.
q g -> ccbar[3S1(8)] q.
Code 415.
q g -> ccbar[3S1(8)] q.
Code 416.
q qbar -> ccbar[3S1(8)] g.
Code 417.
q qbar -> ccbar[3S1(8)] g.
Code 418.
q qbar -> ccbar[3S1(8)] g.
Code 419.
Bottomonium
Common switch for the group of charmonium production.
g g -> bbbar[3S1(1)] g.
Code 501.
g g -> bbbar[3P0(1)] g.
Code 502.
g g -> bbbar[3P1(1)] g.
Code 503.
g g -> bbbar[3P2(1)] g.
Code 504.
q g -> bbbar[3P0(1)] q.
Code 505.
q g -> bbbar[3P1(1)] q.
Code 506.
q g -> bbbar[3P2(1)] q.
Code 507.
q qbar -> bbbar[3P0(1)] g.
Code 508.
q qbar -> bbbar[3P1(1)] g.
Code 509.
q qbar -> bbbar[3P2(1)] g.
Code 510.
g g -> bbbar[3S1(8)] g.
Code 511.
g g -> bbbar[3S1(8)] g.
Code 512.
g g -> bbbar[3S1(8)] g.
Code 513.
q g -> bbbar[3S1(8)] q.
Code 514.
q g -> bbbar[3S1(8)] q.
Code 515.
q g -> bbbar[3S1(8)] q.
Code 516.
q qbar -> bbbar[3S1(8)] g.
Code 517.
q qbar -> bbbar[3S1(8)] g.
Code 518.
q qbar -> bbbar[3S1(8)] g.
Code 519.
Onium matrix elements
The implementation of charmonium and bottomonium production, including
the colour-octet production mechanism, requires information on NRQCD
matrix elements for the various wavefunctions involved. Default values
for these are encoded in the following ten variables. They
are taken from [Nas00]; see also [Bar06].
<O(J/psi)[3S1(1)]>.
<O(J/psi)[3S1(8)]>.
<O(J/psi)[1S0(8)]>.
<O(J/psi)[3P0(8)]>/m_c^2.
<O(chi_c0)[3P0(8)]>/m_c^2.
<O(Upsilon)[3S1(1)]>.
<O(Upsilon)[3S1(8)]>.
<O(Upsilon)[1S0(8)]>.
<O(Upsilon)[3P0(8)]>/m_b^2.
<O(chi_b0)[3P0(8)]>/m_b^2.