# 5.11. Multiple interactions¶

The basic MPI model is described in [SvZ87] while Sherpa’s implementation details are discussed in [A+a].

The following parameters are used to steer the MPI setup:

## 5.11.1. MI_HANDLER¶

Specifies the MPI handler. The two possible values at the moment are None and Amisic.

## 5.11.2. MI ISR parameters¶

The following two parameters can be used to overwrite the ISR parameters in the context of multiple interactions: MPI_PDF_SET, MPI_PDF_SET_VERSIONS.

## 5.11.3. TURNOFF¶

Specifies the transverse momentum turnoff in GeV.

## 5.11.4. SCALE_MIN¶

Specifies the transverse momentum integration cutoff in GeV.

## 5.11.5. PROFILE_FUNCTION¶

Specifies the hadron profile function. The possible values are Exponential, Gaussian and Double_Gaussian. For the double gaussian profile, the relative core size and relative matter fraction can be set using PROFILE_PARAMETERS.

## 5.11.6. PROFILE_PARAMETERS¶

The potential parameters for hadron profile functions, see PROFILE_FUNCTION. For double gaussian profiles there are two parameters, corresponding to the relative core size and relative matter fraction.

## 5.11.7. REFERENCE_SCALE¶

Specifies the centre-of-mass energy at which the transverse momentum integration cutoff is used as is, see SCALE_MIN. This parameter should not be changed by the user. The default is 1800, corresponding to Tevatron Run I energies.

## 5.11.8. RESCALE_EXPONENT¶

Specifies the rescaling exponent for fixing the transverse momentum integration cutoff at centre-of-mass energies different from the reference scale, see SCALE_MIN, REFERENCE_SCALE.

## 5.11.9. TURNOFF_EXPONENT¶

Specifies the rescaling exponent for fixing the transverse momentum turnoff at centre-of-mass energies different from the reference scale, see TURNOFF, REFERENCE_SCALE.

## 5.11.10. SIGMA_ND_FACTOR¶

Specifies the factor to scale the non-diffractive cross section calculated in the MPI initialisation.

## 5.11.11. MI_RESULT_DIRECTORY¶

Specifies the name of the directory where the MPI grid is stored. The default comprises the beam particles, their energies and the PDF used. In its default value, this information safeguards against using unsuitable grids for the current calculation.

## 5.11.12. MI_RESULT_DIRECTORY_SUFFIX¶

Supplements the default directory name for the MPI grid with a suffix.