Multiple Partonic Interaction Developments in Herwig++

TL;DR

The paper advances the modeling of multiple parton interactions in Herwig++ by incorporating non-perturbative color correlations through color reconnection, alongside an eikonal MPI framework that includes soft scatter components. The authors demonstrate that color reconnection improves minimum-bias observables at 0.9 TeV and present a UE7-2 tune that satisfactorily describes ATLAS Underlying Event data at 7 TeV, with explicit tuned parameters and a calculated σ_eff. The work highlights improved agreement with MB and UE data, and discusses remaining questions about energy dependence of MPI parameters and the possibility of a unified UE/MB description. Overall, the developments strengthen Herwig++'s ability to model soft QCD activity at the LHC and provide concrete tunings for comparison with experimental data.

Abstract

We briefly review the status of the multiple partonic interaction model in the Herwig++ event generator. First, we show how a change in the colour structure of an event in Herwig++ results in a significant improvement in the description of soft inclusive observables in $pp$ interactions at $\sqrt{s}=900$ GeV. Then we present a comparison of some model results to ATLAS Underlying Event data at $\sqrt{s}=7$ TeV.

Figures (5)

• Figure 1: Comparison of Herwig++ 2.4.2 and Herwig++ 2.5 to ATLAS minimum-bias distributions at $\sqrt{s}=0.9~\mathrm{TeV}$ with $N_{\mathrm{ch}} \ge 6$, $p_{\perp} > 500~\mathrm{MeV}$ and $|\eta| < 2.5$. The ATLAS data was taken from plots published in Ref. :2010ir.
• Figure 2: Formation of clusters, which are represented by ovals. Colour lines are dashed. (a) shows colour-singlet clusters formed according to the dominating colour structure in the $1/N_{\mathrm{C}}$ expansion. (b) shows a possible colour-reconnected state: the partons of the clusters $A$ and $B$ are arranged in new clusters, $C$ and $D$.
• Figure 3: Charged multiplicity and scalar $\sum p_{\perp}$ density of charged particles with $p_t > 100$ MeV and $|\eta| < 2.5$ in the transverse region. The predictions of the UE7-2 tune are compared to ATLAS UE data at 7 TeV Aad:2010fh.
• Figure 4: Standard deviations of the charged multiplicity density and scalar $\sum p_{\perp}$ density of charged particles with $p_t > 100$ MeV and $|\eta| < 2.5$ in the transverse region. The predictions of the UE7-2 tunes are compared to ATLAS UE data at 7 TeV Aad:2010fh.
• Figure 5: Azimuthal distribution of charged particle multiplicity and $\sum p_{\perp}$ densities, with respect to the direction of the leading charged particle (at $\phi = 0$), for $|\eta| < 2.5$. The densities are shown for $p_{\perp}^{\rm lead}\xspace>1$ GeV, $p_{\perp}^{\rm lead}\xspace>2$ GeV, $p_{\perp}^{\rm lead}\xspace>3$ GeV and $p_{\perp}^{\rm lead}\xspace>5$ GeV. The data is compared to the UE7-2 tune.