# 5.14. Minimum bias events¶

Minimum bias events are simulated through the Shrimps module in Sherpa.

## 5.14.1. Physics of Shrimps¶

### 5.14.1.1. 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 `Lambda2`

,
`beta_02(mb)`

, `kappa`

and `xi`

.

In this framework the eikonals and the cross sections for the different modes (elastic, inelastic, single- and double-diffractive) are calculated.

## 5.14.2. Parameters and settings¶

Below is a list of all relevant parameters to steer the Shrimps module.

### 5.14.2.1. Generating minimum bias events¶

To generate minimum bias events with Shrimps `EVENT_TYPE`

has to be
set to `MinimumBias`

and `SOFT_COLLISIONS`

to `Shrimps`

.

### 5.14.2.2. Shrimps Mode¶

The setup of minimum bias events is done via top-level settings. The
exact choice is steered through the parameter `Shrimps_Mode`

(default `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’;`Elastic`

generates elastic events at a fixed energy;`Single-diffractive`

generates low-mass single-diffractive events at a fixed energy, modelled by the transition of one of the protons to a N(1440) state;`Double-diffractive`

generates low-mass single-diffractive events at a fixed energy, modelled by the transition of both protons to N(1440) states;`Quasi-elastic`

generates a combination of elastic, single- and double-diffractive events in due proportion;`Inelastic`

generates inelastic minimum bias events through the exchange of t-channel gluons or singlets (pomerons). This mode actually will include large mass diffraction;`All`

generates a combination of quasi-elastic and inelastic events in due proportion.

### 5.14.2.3. 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)