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Searching for New Physics Inside Jets with the Herwig 7 Generalised Parton Shower

Taehee Kim, Joon-Bin Lee, M. R. Masouminia, Michael H. Seymour, Un-ki Yang

Abstract

This study investigates parton shower evolution incorporating both Standard Model (SM) and beyond-the-Standard-Model (BSM) radiation, focusing on the phenomenology of a massive $Z'$ boson. While traditional approaches typically assume direct $Z'$ production in the hard process, the possibility of $Z'$ production within jets, enabled by subsequent emissions in the parton shower, offers a complementary opportunity to probe new physics through jet substructure and event topology. The newly developed Herwig 7 framework supporting BSM parton showers enables efficient simulation of $Z'$ production in the logarithmically enhanced regime. Using a simple BSM benchmark, the minimal $U(1)_{B{\rm -}L}$ extension of the SM, the interplay between the SM and BSM showers is evaluated to identify kinematic features that distinguish $Z'$-induced jets from conventional signatures. BSM-radiation signatures are contrasted with SM backgrounds such as QCD, top-quark, and Drell-Yan production, identifying potential discriminants for experimental searches. Experimental sensitivity at the LHC and prospective future colliders is estimated via statistical-significance projections. We find that $Z'$ bosons produced through parton shower radiation populate non-isolated regions inside jets, providing an avenue for new-physics searches overlooked in traditional analyses.

Searching for New Physics Inside Jets with the Herwig 7 Generalised Parton Shower

Abstract

This study investigates parton shower evolution incorporating both Standard Model (SM) and beyond-the-Standard-Model (BSM) radiation, focusing on the phenomenology of a massive boson. While traditional approaches typically assume direct production in the hard process, the possibility of production within jets, enabled by subsequent emissions in the parton shower, offers a complementary opportunity to probe new physics through jet substructure and event topology. The newly developed Herwig 7 framework supporting BSM parton showers enables efficient simulation of production in the logarithmically enhanced regime. Using a simple BSM benchmark, the minimal extension of the SM, the interplay between the SM and BSM showers is evaluated to identify kinematic features that distinguish -induced jets from conventional signatures. BSM-radiation signatures are contrasted with SM backgrounds such as QCD, top-quark, and Drell-Yan production, identifying potential discriminants for experimental searches. Experimental sensitivity at the LHC and prospective future colliders is estimated via statistical-significance projections. We find that bosons produced through parton shower radiation populate non-isolated regions inside jets, providing an avenue for new-physics searches overlooked in traditional analyses.

Paper Structure

This paper contains 9 sections, 15 equations, 7 figures, 1 table.

Table of Contents

  1. Introduction
  2. Revisiting Angular-Ordered Parton Shower and $Z'$ Radiation
  3. Sample Generation
  4. Signal Sample
  5. Validation Sample
  6. Background Sample
  7. Kinematics and Event Features of $Z'$ Radiation
  8. Results and Discussions
  9. Summary and Conclusions

Figures (7)

  • Figure 1: Coupling-dependence study of $Z'$ radiation in the parton shower for $M_{Z'}=5$, 10, 20, and 50 GeV. Each panel shows the ratio of events containing more than one $Z'$ boson to those containing exactly one $Z'$ boson, $N(>1~Z')/N(1~Z')$, as a function of the fraction of di-jet events with exactly one $Z'$ boson, $N(1~Z')/N(\text{total})$ normalised by $1/g_{qZ'}^{2}$.
  • Figure 2: Three typical Feynman diagrams that can produce a $Z'$ boson in proton-proton collisions. The left, centre, and right panels show FSR-like, ISR-like, and quark-quark fusion (qqF) topologies, respectively.
  • Figure 3: Parton-level validation of the $Z'$ radiation mechanism for representative mass points, $M_{Z'} =$ 5, 20 and 50 GeV. The distributions show the angular separation $\Delta R(q, Z')$ (left) and the transverse-momentum fraction carried by the quark within the $qZ'$ system, namely $z$ (right). Results are shown for the signal and validation samples discussed in Sec. \ref{['sec:simulation']}. For the $z$ distributions, only events satisfying $\Delta R(q, Z')<1$ are included to suppress ISR-like and qqF topologies and to isolate the logarithmically enhanced phase space relevant for parton shower radiation.
  • Figure 4: Analysis-level observables corresponding to the parton-level variables. The top, middle, and bottom panels show results for $M_{Z'}=5$, 20, and 50 GeV, respectively. Left: $\Delta R$ between the reconstructed jet and the di-muon system forming the $Z'$ candidate. Right: muon energy fraction inside the jet.
  • Figure 5: $\Delta R(\mu,\text{jet})$ distributions for $M_{Z'}=20$ GeV. The top (bottom) panels show the separation between the jet and the leading (subleading) muon. The left column covers the full range, $\Delta R<3.2$, while the right column provides a zoomed view of the near-jet region, $\Delta R<0.5$.
  • ...and 2 more figures