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 (and optionally also diquark) production relative to up/down quark production.
For each string undergoing fragmentation, close-packing effects, for each string break, scale with the effective number of nearby strings, determined according to rapidity measured along the z-axis of the system. This should be quite appropriate for multi-string systems produced by MPIs 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.
 
For each string break, the numbers of 
overlapping strings with flux orientations parallel and 
antiparallel to the string on which the break occurs, are denoted by 
p and q respectively. The effective string tension 
is then assumed to scale like 
 
    κeff = (1 + (kpp + kqq) 
    /(1+p2T had/p2T0)) 
    2rκ0. 
 
where pT had is the transverse momentum of the 
hadron produced in the break. Close-packing effects are thus 
suppressed for particles with pT values greater than the 
reference pT0 value. This reflects the expectation 
that the high-pT parts of hard jets are not 
affected by close-packing. 
 
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 
StringFlav:probQQtoQ 
scales according to 
enhanceStrange, 
enhancePT, and enhanceDiquark would be 
equal, however we have built in extra degrees of freedom in the model. 
 
flag   ClosePacking:doClosePacking   
 (default = off)parm   ClosePacking:fluxRatio   
 (default = 0.5; minimum = 0.0; maximum = 1.0)parm   ClosePacking:PT0   
 (default = 2.0; minimum = 0.0)parm   ClosePacking:enhanceStrange   
 (default = 0.08; minimum = 0.0)StringFlav:probStoUD, StringFlav:probSQtoQQ 
and StringFlav:probQQ1toQQ0. 
   
 
parm   ClosePacking:enhancePT   
 (default = 0.5; minimum = 0.0)StringPT:sigma. 
   
 
parm   ClosePacking:enhanceDiquark   
 (default = 0.5; minimum = 0.0)StringFlav:probQQtoQ. 
   
 
flag   ClosePacking:doEnhanceDiquark   
 (default = on)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)parm   ClosePacking:expMPI   
 (default = 0.0; minimum = 0.0; maximum = 1.0)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 
 
parm   ClosePacking:baryonSup   
 (default = 0.5; minimum = 0.; maximum = 1.0)parm   ClosePacking:parallelBaryonSup   
 (default = 0.; minimum = 0.; maximum = 1.0)