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. AMISIC

Amisic can simulate the interaction of three different combinations of incoming particles: proton–proton, photon–proton and photon–photon collision. The parameters for the simulation of photonic multiple interactions can be found in [SS97]. It has several parameters to control the simulation of the multiple-parton interactions, they are listed below. Each of these parameters has to be set in the subsetting AMISIC, like so

  PT_0: 2.5

The usual rules for yaml structure apply, c.f. Input structure.


Value \(p_\text{T,0}^\text{(ref)}\) for the calculation of the IR regulator, see formula below. Defaults to 2.5.


Value \(p_\text{T,min}^\text{(ref)}\) for the calculation of the IR cutoff, see formula below. Defaults to 3.


The pseudorapidity \(\eta\) used to calculate the IR cutoff and regulator, \(p_\text{T,min}\) and \(p_\text{T,0}\). Defaults to 0.16.


Reference energy to normalise the actual cms energy for the calculation of the IR cutoff and regulator. Defaults to 7000.


The IR cut-off for the 2->2 scatters. It is calculated as

\[p_\text{T,min} = p_\text{T,min}^\text{(ref)} \left( \frac{E_\text{cms}}{E_\text{cms}^\text{(ref)}} \right)^\eta\]

but can also be set explicitly.


IR regulator \(p_\text{T,0}\) in the propagator and in the strong coupling. It is calculated as

\[p_\text{T,0} = p_\text{T,0}^\text{(ref)} \left( \frac{E_\text{cms}}{E_\text{cms}^\text{(ref)}} \right)^\eta\]

but can also be set explicitly.


Defaults to PT scheme. More schemes have yet to be added.


Factor to scale the renormalisation scale \(\mu_R\), defaults to 0.5.


Factor to scale the factorisation scale \(\mu_F\), defaults to 1.0.


Specifies the factor to scale the non-diffractive cross section calculated in the MPI initialisation. Defaults to 0.4.


Only to be used for double-gaussian matter form, where it will control the distribution of matter over the two gaussians. It assumes that a fraction \(f^2\) is distributed by the inner gaussian \(r_1\), another fraction \((1-f)^2\) is distributed by the outer gaussian \(r_2\), and the remaining fraction \(2f(1-f)\) is distributed by the combined radius \(r_\text{tot} = \sqrt{\frac{r_1^2+r_2^2}{2}}\). Defaults to 0.5.


Defaults to 0.4. Is used to control the radius of the (inner) gaussian. If used with the double-gaussian matter form, this value must be smaller than MATTER_RADIUS2.


Defaults to 1.0. It is only used for the case of a double-gaussian overlap, see below.


Defaults to Single_Gaussian. Alternatively, Double_Gaussian can be used to model the overlap between the colliding particles, however, it has not been tested yet.


Controls the number of bins for the numerical integration of

\[\int_{p_T^2}^{s/4} dp_T^2 \frac{d \sigma}{dp_T^2}\]

Defaults to 200.


Number of points to estimate the the cross-section during the integration. The error should behave as \(\frac{1}{\sqrt{n_\text{MC}}}\). Defaults to 1000.


Number of points to sample in the center-of-mass energy \(\sqrt{s}\). This is only used if the energy is not fixed, i.e. in the case of EPA photons. Defaults to 100.

5.11.3. 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.