We present the implementation of a fully automated quarkonium parton shower in Herwig 7, based on non-relativistic QCD (NRQCD) factorisation with spin-colour projections. The framework systematically incorporates colour-singlet and colour-octet production mechanisms, gluon fragmentation, and diquark production processes. Perturbative short-distance coefficients are combined with non-perturbative NRQCD matrix elements to simulate heavy-quark bound state formation. Splitting functions for $\textit{S}$-, $\textit{P}$- and $\textit{D}$-wave states are explicitly derived and integrated into the angular-ordered shower evolution. The implementation preserves spin correlations and polarisation effects while accurately accounting for feed-down contributions. Results demonstrate improved agreement with the existing LHC data. This quarkonium parton shower will become publicly available with the release of Herwig-7.4.0.

United Kingdom

M. Masouminia

P. Richardson

Quarkonium parton shower in Herwig 7

The modelling of the formation of colour-singlet hadrons from coloured partons, known as Hadronization, is crucial for generating realistic events in Monte Carlo Event Generators. Due to limited understanding of the non-perturbative regime, physically motivated phenomenological hadronization models with tunable parameters are used and later tuned to the experimental data. Modern Monte Carlo generators primarily employ one of two hadronization models: the Lund string model, which is the default in , and the cluster model, which is the default in and . In this work, we combine the Lund string hadronization model, as implemented in , with using interface. We tune the string model with ’s Angular Ordered Parton Shower (AOPS) to lepton and hadron collision data, resulting in the Les Houches Tune ( ), which shows good performance across a wide range of observables. The will be included in the release. (It can also be used with , provided that the relevant patch is applied to the installation script which can be provided by the authors.) This development enables a direct comparative study of the two hadronization models within , both interfaced with the Angular Ordered Parton Shower, which serves as the main motivation behind this work.

Czechia

Michaela Divisova

Miroslav Myska

Poland

Pratixan Sarmah

Switzerland

Andrzej Siódmok

Herwig 7 with the Lund string model: tuning and comparative hadronization studies

The Light Dark Matter eXperiment (LDMX) is proposed to employ a thin tungsten target and a multi-GeV electron beam to carry out a missing momentum search for the production of dark matter candidate particles. We study the sensitivity for a complementary missing-energy-based search using the LDMX Electromagnetic Calorimeter as an active target with a focus on early running. In this context, we construct an event selection from a limited set of variables that projects sensitivity into previously-unexplored regions of light dark matter phase space — down to an effective dark photon interaction strength $\textit{y}$ of approximately 2 $\textit{×}$ 10 (5 $\textit{×}$ 10 ) for a 1 MeV (10 MeV) dark matter candidate mass.

Sweden

Torsten Åkesson

United States

Elizabeth Berzin

Cameron Bravo

Liam Brennan

Lene Bryngemark

Pierfrancesco Butti

Filippo Delzanno

E. Dukes

Valentina Dutta

Bertrand Echenard

Ralf Ehrlich

Thomas Eichlersmith

Einar Elén

Andrew Furmanski

Victor Gomez

Matt Graham

Chiara Grieco

Craig Group

Hannah Herde

Christian Herwig

David Hitlin

Tyler Horoho

Joseph Incandela

Nathan Jay

Asahi Jige

Wesley Ketchum

Gordan Krnjaic

Amina Li

Zihan Ma

Jeremiah Mans

Cristina Mantilla Suarez

Sanjit Masanam

Phillip Masterson

Steven Metallo

Sophie Middleton

Joseph Muse

Timothy Nelson

Rory O’Dwyer

James Oyang

Jessica Pascadlo

Emrys Peets

Luis Pico

Ruth Pöttgen

Philip Schuster

Chris Sellgren

Lauren Tompkins

Natalia Toro

Nhan Tran

Tamas Vami

Erik Wallin

Yuxuan Wang

Andrew Whitbeck

Duncan Wilmot

Xinyi Xu

Danyi Zhang

Sensitivity of an early dark matter search using the electromagnetic calorimeter as a target for the Light Dark Matter eXperiment

We present a novel simulation of a strongly interacting dark sector also known as the Hidden Valley scenarios using angular ordered showers and the cluster hadronisation model in Herwig 7. We discuss the basics of this implementation and the scale hierarchies underpinning the simulation. With the help of a few benchmarks, we show the effect of variation of dark sector parameters on thrust and angularities within the dark sector, and study correlation functions, which can be helpful for understanding the angular structure of these events. Finally we comment on the uncertainties introduced due to lack of knowledge of hadronisation parameters within the dark sectors.

Austria

Suchita Kulkarni

Simon Plätzer

Germany

Dominic Stafford

Dark sector showers and hadronisation in Herwig 7

A new release of the Monte Carlo event generator Herwig (version 7.3) has been launched. This iteration encompasses several enhancements over its predecessor, version 7.2. Noteworthy upgrades include: the implementation of a process-independent electroweak angular-ordered parton shower integrated with QCD and QED radiation; a new recoil scheme for initial-state radiation improving the behaviour of the angular-ordered parton shower; the incorporation of the heavy quark effective theory to refine the hadronization and decay of excited heavy mesons and heavy baryons; a dynamic strategy to regulate the kinematic threshold of cluster splittings within the cluster hadronization model; several improvements to the structure of the cluster hadronization model allowing for refined models; the possibility to extract event-by-event hadronization corrections in a well-defined way; the possibility of using the string model, with a dedicated tune. Additionally, a new tuning of the parton shower and hadronization parameters has been executed. This article discusses the novel features introduced in version 7.3.0.

Gavin Bewick

CERN

Silvia Ferrario Ravasio

Stefan Gieseke

Stefan Kiebacher

Mohammad Masouminia

Andreas Papaefstathiou

Peter Richardson

Daniel Samitz

Michael Seymour

James Whitehead

Herwig 7.3 release note

This paper presents the inaugural investigation of beyond the Standard Model (BSM) radiation processes, framed as a generalized, process- and model-independent parton shower algorithm within Herwig 7, based on direct translations of Universal FeynRules Output (UFO) constructed via Herwig’s ufo2herwig module. Leveraging the fact that shower kinematics are dictated by the spins of involved particles, we calculate comprehensive helicity-dependent branching kernels for all feasible splittings of scalars, fermions, and vector bosons, tailored to Herwig 7’s angular-ordering (AO) parton shower algorithm. Utilizing these kernels, we derive BSM splitting functions in the quasi-collinear limit, ensuring compatibility with the Standard Model (SM) and supersymmetry (SUSY) splitting functions when analogous parameter conditions are applied. These newly derived functions have been integrated into the Herwig 7 event generator framework. Comparative analyses with fixed-order matrix element calculations show good agreement for single radiation events. Moreover, the results showcase the influence of BSM radiation at the Large Hadron Collider (LHC) and envisage its implications for future collider endeavours. This research augments our comprehension of BSM radiation effects, with significant bearings on present and prospective collider-based inquiries.

South Korea

Joon-Bin Lee

Un-ki Yang

Generalized angular-order parton showers in Herwig 7

We revisit the hadronization and decay of excited heavy mesons and heavy baryons in Herwig 7 general-purpose event-generator, following four distinct steps: (i) Passing through the polarisation of heavy hadrons at the end of parton shower through the application of heavy quark effective theory (HQET), where the emergence of a spin-flavour symmetry allows for the determination of the polarisations of the excited heavy mesons and heavy baryons from the helicity states of the light and heavy quarks. (ii) Improving the strong and radiative decay modes of the excited heavy mesons, where in the absence of conclusive experimental data on many of the decays, one needs to rely on HQET symmetries to determine the favoured decay modes, widths and branching ratios. (iii) Re-examination of the production rates of heavy hadrons using all available experimental data sources and (iv) performing a general tune for Herwig’s free parameters to reflect the implemented changes. We compare our predictions against existing experimental data in the presence/absence of the newly implemented updates. These improvements will be available with Herwig-7.3.0 public release.

Hadronization and decay of excited heavy hadrons in Herwig 7

The origin of the excess of low-energy events observed by the MiniBooNE experiment remains a mystery, despite exhaustive investigations of backgrounds and a series of null measurements from complementary experiments. One intriguing explanation is the production of beyond-the-Standard-Model particles that could mimic the experimental signature of additional <math><mrow><msub><mrow><mi>ν</mi></mrow><mrow><mi>e</mi></mrow></msub></mrow></math> appearance seen in MiniBooNE. In one proposed mechanism, muon neutrinos up-scatter to produce a new “dark neutrino” state that decays by emitting highly collimated electron-positron pairs. We propose high-energy neutrinos produced from <math><mi>W</mi></math> boson decays at the Large Hadron Collider as an ideal laboratory to study such models. Simple searches for a low-mass, boosted dilepton resonance produced in association with a high-<math><msub><mi>p</mi><mi>T</mi></msub></math> muon from the <math><mi>W</mi></math> decay with run 2 data would already provide unique sensitivity to a range of dark neutrino scenarios, with prompt and displaced searches providing complementarity. Looking farther ahead, we show how the unprecedented sample of <math><mi>W</mi></math> boson decays anticipated at the HL-LHC, together with improved lepton acceptance would explore much of the parameter space most compatible with the MiniBooNE excess.

Joshua Isaacson

Bo Jayatilaka

Pedro A. N. Machado

Allie Reinsvold Hall

Murtaza Safdari

Shedding light on the MiniBooNE excess with searches at the LHC

The Light Dark Matter eXperiment (LDMX) is an electron-beam fixed-target experiment designed to achieve comprehensive model independent sensitivity to dark matter particles in the sub-GeV mass region. An upgrade to the LCLS-II accelerator will increase the beam energy available to LDMX from 4 to 8 GeV. Using detailed GEANT4-based simulations, we investigate the effect of the increased beam energy on the capabilities to separate signal and background, and demonstrate that the veto methodology developed for 4 GeV successfully rejects photon-induced backgrounds for at least 2 × 1014 electrons on target at 8 GeV.

Jonathan Eisch

Cooper Froemming

Niramay Gogate

Omar Moreno

Geoffrey Mullier

Leo Östman

Luis Sarmiento

Matthew Solt

Cristina Suarez

Photon-rejection power of the Light Dark Matter eXperiment in an 8 GeV beam

We introduce a new search strategy for visibly decaying muonphilic particles using a proton beam spectrometer modeled after the SpinQuest experiment at Fermilab. In this setup, a <math><mrow><mo>∼</mo><mn>100</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> primary proton beam impinges on a thick fixed target and yields a secondary muon beam. As these muons traverse the target material, they scatter off nuclei and can radiatively produce hypothetical muonphilic particles as initial- and final-state radiation. If such new states decay to dimuons, their combined invariant mass can be measured with a downstream spectrometer immersed in a Tesla-scale magnetic field. For a representative setup with <math><mn>3</mn><mo>×</mo><msup><mn>10</mn><mn>14</mn></msup></math> muons on target with typical energies of <math><mo>∼</mo><mn>20</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>, a 15% invariant mass resolution, and an effective 100 cm target length, this strategy can probe the entire parameter space for which <math><mrow><mo>∼</mo><mn>200</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi><mo>−</mo><mi>GeV</mi></mrow></math> scalar particles resolve the muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math> anomaly. We present sensitivity to these scalar particles at the SpinQuest experiment where no additional hardware is needed and the search could be parasitically executed within the primary nuclear physics program. Future proton beam dump experiments with optimized beam and detector configurations could have even greater sensitivity.

Diana Forbes

Yonatan Kahn

New searches for muonphilic particles at proton beam dump spectrometers