[Paper Review] Towards Diffraction in Herwig

Stefan Gieseke, Frashër Loshaj+1 more|arXiv (Cornell University)|Feb 15, 2016

Particle physics theoretical and experimental studies6 references3 citations

TL;DR

This paper proposes a reconfiguration of Herwig's colour reconnection model to suppress spurious large pseudorapidity gaps in soft multiple parton interactions, which arise from artificial cluster splitting. It introduces explicit soft diffraction via Regge-theory-based matrix elements for single and double diffraction, showing that collinear quark-diquark emission in dissociated protons better reproduces the flat forward pseudorapidity gap cross section than isotropic decay.

ABSTRACT

We propose changes to the colour reconnection model in the Monte Carlo event generator Herwig in order to remove the quasi diffractive events from the soft multiple parton interactions. We then implement explicitly soft diffraction and show some preliminary results.

Motivation & Objective

• To address the unphysical enhancement of large pseudorapidity gaps in Herwig's soft multiple parton interaction (MPI) model, which contradicts data from ATLAS.
• To identify that the colour reconnection model artificially generates large gaps by splitting large colour clusters into forward-moving fragments.
• To propose new colour connection topologies that suppress large gaps in non-diffractive events by restricting reconnections involving forward clusters.
• To implement explicit soft diffraction in Herwig using Regge theory, with matrix elements for single and double diffraction.
• To improve the forward pseudorapidity gap distribution by modeling dissociated proton decay with collinear quark-diquark systems, reducing unphysical hadronization effects.

Proposed method

• Modify the colour reconnection model in Herwig to prevent reconnections that split large clusters into forward-moving fragments, thereby reducing spurious large pseudorapidity gaps.
• Introduce new colour connection topologies (e.g., A, H types) that naturally lead to small pseudorapidity gaps in non-diffractive events.
• Implement single and double diffraction using Regge-theory-based cross sections: $\frac{d^{2}\sigma^{SD}}{dM^{2}dt} \propto \left(\frac{s}{M^{2}}\right)^{2\alpha_{\mathbb{P}}(t)-1}$ and $\frac{d^{3}\sigma^{DD}}{dM_{1}^{2}dM_{2}^{2}dt} \propto \left(\frac{s}{M_{1}^{2}M_{2}^{2}}\right)^{2\alpha_{\mathbb{P}}(t)-1}$.
• Generate diffractive events via two-to-two matrix elements: $pp \to p^*p$, $pp \to pp^*$, $pp \to p^*p^*$, where $p^*$ is a dissociated proton.
• Model the dissociation of $p^*$ as a cluster of quark and diquark, with two cases: isotropic decay and collinear emission of quark-diquark with the proton.
• Use cluster hadronization to simulate final-state particles and assess the impact on the forward pseudorapidity gap distribution.

Experimental results

Research questions

• RQ1 Why does Herwig’s current colour reconnection model produce an unphysical enhancement of large pseudorapidity gaps in minimum bias events?
• RQ2 Can new colour connection topologies suppress large gaps in non-diffractive soft MPI without altering the underlying eikonal model?
• RQ3 How can soft diffraction be consistently implemented in Herwig to reproduce the observed flat behavior of the forward pseudorapidity gap cross section?
• RQ4 Does collinear quark-diquark emission in dissociated protons lead to a more realistic forward gap distribution than isotropic decay?
• RQ5 To what extent do hadronization effects distort the diffraction cross section at small and large pseudorapidity gaps?

Key findings

• The colour reconnection model in Herwig artificially enhances large pseudorapidity gaps due to splitting of large colour clusters into forward-moving fragments.
• New colour connection topologies (e.g., A, H) suppress large gaps in non-diffractive events, though none alone fully reproduce the expected exponential fall-off.
• Isotropic decay of the dissociated proton in diffraction leads to a rapid fall-off of the cross section at small $\Delta\eta^F$, inconsistent with data.
• Collinear emission of quark and diquark with the outgoing proton produces a flatter forward pseudorapidity gap distribution, better matching the observed constant behavior.
• Hadronization effects in the collinear case shift the onset of particle production to higher $\Delta\eta^F$, but the flat region does not extend fully to small gaps.
• Future improvements may come from including gluon emission in the dissociated proton decay, enabling more isotropic final-state distributions and better agreement with data.