Close-packing
If strings are closely packed, e.g. as a consequence of MPIs, it is
likely that they receive an increased string tension, which translates
into a broader pT spectrum, see further
[Fis16]. It also means an enhanced rate (or rather reduced
suppression) of strange quark production relative to up/down quark
production. Close-packing effects are scaled by the effective number
of near strings, which is pT suppressed and given by
κeff = (1 + (kpp + kqq)
/(1+p2T had/p2T0))
2rκ0.
This scales the probability StringFlav:probStoUD by
P'(s:u/d) = P(s:u/d)(κ0/κeff)
StringFlav:probSQtoQQ and StringFlav:probQQ1toQQ0
also scale in this way. The width of the pT spectrum, given by
σ^2 is scaled by
σ'2 = σ2
κeff/κ0.
The diquark production probability StringFlav:probQQtoQ
scales according to
P'(qq:q)=α'(P(qq:q)/α)κeff,
and α' is α recalculated given
κeff.
We allow different parameters to set kp depending
on what the effective string tension is scaling. In principle if the
close-packing mechanism were exact, enhanceStrange,
enhanceStrange, and enhanceStrange would be
equal, however we have built in extra degrees of freedom in the model.
Note: for the purpose of close-packing, string overlaps are
determined according to rapidity measured along the z axis. This
should be quite appropriate for multi-string
systems produced by MPI in pp collisions but would not make much sense
to apply to processes such as ee→WW→hadrons, for which the z
axis does not play any special role; such studies would be
interesting but would require a generalisation of the current
implementation.
flag ClosePacking:doClosePacking
(default = off)
Turns on the close-packing model.
parm ClosePacking:fluxRatio
(default = 0.5; minimum = 0.0; maximum = 1.0)
kq/kp ratio.
parm ClosePacking:PT0
(default = 2.0; minimum = 0.0)
pT0 parameter above.
parm ClosePacking:enhanceStrange
(default = 0.08; minimum = 0.0)
kp parameter above used for scaling
StringFlav:probStoUD, StringFlav:probSQtoQQ
and StringFlav:probQQ1toQQ0.
parm ClosePacking:enhancePT
(default = 0.5; minimum = 0.0)
kp parameter above used for scaling
StringPT:sigma.
parm ClosePacking:enhanceDiquark
(default = 0.5; minimum = 0.0)
kp parameter above used for scaling
StringFlav:probQQtoQ.
flag ClosePacking:doEnhanceDiquark
(default = on)
As probQQtoQ is scaled by κ by α' regardless of
ClosePacking:enhanceDiquark value, here we allow
the scaling of probQQtoQ with an effective κ to be switched off
altogether to capture the ambiguity in the scaling of probQQtoQ given
the popcorn mechanism for diquark creation.
Parameters ClosePacking:expNSP and
ClosePacking:expMPI are typically used for the thermal
model of string breaks. Normally only one of the options below would
be used, but technically both are allowed and then combine
multiplicatively.
parm ClosePacking:expNSP
(default = 0.5; minimum = 0.0; maximum = 1.0)
r parameter above to allow non-linear scaling of
κeff. Default value is based on Gaussian string
breaks. For the thermal model, the default value is 0.13.
parm ClosePacking:expMPI
(default = 0.0; minimum = 0.0; maximum = 1.0)
Allows scaling of width/temperature by number of MPI rather than number of
near strings. The width/temparture will get the prefactor
N(MPI)expMPI.
Popcorn destructive interference
The popcorn mechanism creates diquarks via a step-wise procedure of
two successive colour fluctuations on a string. By allowing the first
of these colour fluctuaions to break strings given the correct colour
configuration, the probability of forming a diquark is reduced in
densely packed string systems. To modify
StringFlav:probQQtoQ, the survival probability of a
diquark is considered, with fq
and fp being the probability of the colour fluctuation
to connect with an antiparallel or parallel nearby string
respectively. Connections with antiparallel strings is expected to be
dominant due to the favoured colour orientation. These probabilities
are pT suppressed the same way as
κeff is for close-packing, using parameter
ClosePacking:PT0.
As colour indices used in colour reconnections are currently not
stored in the event, an equal distribution of colours is assumed
instead. Considering the survival probability of the first fluctuation
for a given colour configuration, the probability
StringFlav:probQQtoQ is modified by
P'(qq:q) = ((1-fq)nq/9
(1-fp)np/9) P(qq:q).
parm ClosePacking:baryonSup
(default = 0.5; minimum = 0.; maximum = 1.0)
fq parameter above.
parm ClosePacking:parallelBaryonSup
(default = 0.; minimum = 0.; maximum = 1.0)
fp parameter above.