5.15. Minimum bias events¶
Minimum bias events are simulated through the Shrimps module in Sherpa.
5.15.1. Physics of Shrimps¶
126.96.36.199. Inclusive part of the model¶
Shrimps is based on the KMR model [RMK09], which is a multi-channel eikonal model. The incoming hadrons are written as a superposition of Good-Walker states, which are diffractive eigenstates that diagalonise the T-matrix. This allows to include low-mass diffractive excitation. Each combination of colliding Good-Walker states gives rise to a single-channel eikonal. The final eikonal is the superposition of the single-channel eikonals. The number of Good-Walker states is 2 in Shrimps (the original KMR model includes 3 states).
Each single-channel eikonal can be seen as the product of two parton
densities, one from each of the colliding Good-Walker states. The
evolution of the parton densities in rapidity due to extra emissions
and absoption on either of the two hadrons is described by a set of
coupled differential equations. The parameter
Delta, which can be
interpreted as the Pomeron intercept, is the probability for emitting
an extra parton per unit of rapidity. The strength of absorptive
corrections is quantified by the parameter
lambda, which can also
be seen as the triple-Pomeron coupling. A small region of size
deltaY around the beams is excluded from the evolution due to the
finite longitudinal size of the parton densities.
The boundary conditions for the parton densities are form factors,
which have a dipole form characterised by the parameters
In this framework the eikonals and the cross sections for the different modes (elastic, inelastic, single- and double-diffractive) are calculated.
5.15.2. Parameters and settings¶
Below is a list of all relevant parameters to steer the Shrimps module.
188.8.131.52. Generating minimum bias events¶
To generate minimum bias events with Shrimps
EVENT_TYPE has to be
184.108.40.206. Shrimps Mode¶
The setup of minimum bias events is done via top-level settings. The
exact choice is steered through the parameter
Inelastic), which allows the following settings:
Xsecs, which will only calculate total, elastic, inelastic, single- and double-diffractive cross sections at various relevant energies and write them to a file, typically ‘InclusiveQuantities/Xsecs.dat’;
Elasticgenerates elastic events at a fixed energy;
Single-diffractivegenerates low-mass single-diffractive events at a fixed energy, modelled by the transition of one of the protons to a N(1440) state;
Double-diffractivegenerates low-mass single-diffractive events at a fixed energy, modelled by the transition of both protons to N(1440) states;
Quasi-elasticgenerates a combination of elastic, single- and double-diffractive events in due proportion;
Inelasticgenerates inelastic minimum bias events through the exchange of t-channel gluons or singlets (pomerons). This mode actually will include large mass diffraction;
Allgenerates a combination of quasi-elastic and inelastic events in due proportion.
220.127.116.11. Parameters of the eikonals¶
The parameters of the differential equations for the parton densities are
Delta(default 0.3): perturbative Pomeron intercept
lambda(default 0.5): triple Pomeron coupling
deltaY(default 1.5): rapidity interval excluded from evolution
The form factors are of the form:
with the parameters
\(\Lambda^2\) (default 1.7 GeV^2)
\(\beta_0^2(mb)\) (default 25.0 mb)
\(\kappa\) (default 0.6)
\(\xi\) (default 0.2)