This page describes a few simple hadron (re)scattering models.
They are intended to take into account
that the overlap of multiple strings at low transverse dimensions
is likely to lead to some collective effects, not unlike those
observed in heavy-ion collisions, even if not quite as pronounced.
Specifically, it is assumed that the hadrons produced can scatter
against each other on the way out, before the fragmenting system
has had time to expand enough that the hadrons get free. Thereby
heavier particles are shifted to higher transverse momenta, at the
expense of the lighter ones.
The main switch on/off switch for rescattering is
HadronLevel:HadronScatter, which by the default is off,
since all models are rather simplistic and have to be used
with some caution. Currently there are three different options available:
The first two options are variations of the same model, whereas
option 2 respresents a different model.
The New Model for Hadron Scattering
Within the new modle, there are two options available for how hadron
pairs are found:
Rapidity based
This corresponds to HadronScatter:mode = 0.
Probe all hadron pairs with an invariant mass minv <
(m21+p2Max)1/2 +
(m22+p2Max)1/2
with the parameter pMax
If a hadron pair passes this cut, the scattering probability for hadrons of
different strings is PDS(Δy) =
PmaxDS(1 - Δy/Δymax)
with rapidity difference Δy of the hadron pair and the
parameters Δymax
and PmaxDS, see
below. If the hadrons are produced
within the same string the probability is PDS(Δy)
PmaxSS if the hadrons are further apart from each
other as HadronScatter:neighbourFar, 0 if they are
closer together as HadronScatter:neighbourNear, and linear
between the maximum HadronScatter:maxProbSS and minimum
probability HadronScatter:minProbSS inbetween.
Rapidity and Azimuth based
This corresponds to HadronScatter:mode = 1.
All hadron pairs are considered. The scattering probability for hadrons
of different strings is PDS(Δy,Δφ) =
PmaxDS(1 - ((Δy)2
+(Δφ)2)1/2/Rmax)
with rapidity difference Δy and difference in azimuth
Δφ of the hadron pair and the
parameters Rmax
and PmaxDS, see
below.
The probability for hadron pairs from the same string is similar
to the one before.
Common Parameters
The following paramters are used for both the above cases:
If switched on, strings within the same string are allowed to
scatter off each other. Otherwise only hadron pairs that are
not produced on the same string are taken into account.
If switched off, each hadron is only allowed to scatter at most once.
By the way that possible scattering pairs are considered in order of
increasing rapidity separation, this introduces a bias towards pairs
with small y separation.
The maximum probability PmaxDS for the
scattering of two hadrons that are not part of the same string.
If scattering of hadrons within the same string is allowed this
parameter gives the closest neighbour that is allowed. The value 1
corresponds to the direct neighbour. The probability associated
with this potential scattering partner is minProbSS.
If scattering of hadrons within the same string is allowed this
parameter gives the neighbour starting from which the maximum
probability maxProbSS is applied.
The minimum probability PminSS for the
scattering of two hadrons within the same string. (Relative to that for
different strings, i.e. for the total probability the baseline
maxProbDS factor also enters.)
The maximum probability PmaxSS for the
scattering of two hadrons within the same string. (Relative to that for
different strings, i.e. for the total probability the baseline
maxProbDS factor also enters.)
The Old Model for Hadron Scattering
Warning: This is still at an experimental level,
and should not be used unless you know what you are doing.
Perform hadron scattering before or after first round of decays,
involving very short-lived particles like the rho.
The default is to perform scattering directly after the
string fragmentation, before any decays.
Allow two hadrons with same parent hadron to scatter.
Allow hadrons which have already scattered to scatter again.
Even if switched on, the same pair can not scatter off each
other twice.
Hadron selection
Probability that a hadron is soft enough to scatter.
(A high-pT hadron presumably being part of a jet,
and thus produced away from the high-particle-density region
at small transverse dimensions.)
N parameter as above.
k parameter as above.
p parameter as above.
Scattering probability
Probability for a pair of hadrons to scatter.
j parameter as above.
rMax parameter as above.
Use tiling in (eta, phi) to reduce number of pairwise tests.