The existence of geons, physical states of self-bound gravitons, has long been proposed. In the context of four-dimensional causal dynamical triangulation simulations we investigate this possibility by measuring curvature-curvature correlators of different gravitational operators. We find a behavior consistent with a massive state, independent of the operators considered, over a certain distance window. While at most a hint, this is tantalizing due to its possible implications for dark matter or (primordial) black holes. We also find indications that the phase of rapid expansion of the obtained de Sitter universe impacts the mass, and relates to quantum fluctuations of space-time.

We compare the three general-purpose Monte Carlo event generators, HERWIG, PYTHIA, and SHERPA for jet photoproduction processes in e^+e^- e + e − and ep e p collisions. Due to the lower energy scales probed, photoproduction is particularly sensitive to non-perturbative corrections. In a systematic analysis we disentangle and quantify the differences between the generators in these processes, i.e. contributions from beam remnants, parton showers, multiparton interactions (MPIs), and hadronisation modelling. We outline the default inputs and implementation differences and compare the computations with experimental data from ≤p and HERA. We find that all generators provide a decent description of the data within the uncertainties, with particularly good descriptions by the LO-accurate PYTHIA and the NLO-accurate SHERPA. Finally, we also present predictions for the upcoming EIC for jet observables and event shapes and conclude that a modern global refit of the photon parton distributions and dedicated experimental measurements ported to the RIVET framework to constrain non-perturbative parameters are the key prerequisites for precision photoproduction phenomenology at the EIC.

We have implemented two recently proposed dipole shower algorithms that have next-to-leading-logarithmic accuracy at leading colour in the Herwig event generator. We study their properties and compare them to Herwig’s existing dipole and angular ordered parton shower algorithms. In addition to their improved properties in the logarithmic regime, we find important roles for their extrapolations into the hard regime, where we perform NLO matching, and into the infrared regime, where we perform cluster hadronization. We emphasise the importance of this infrared regime and the precise definition of the infrared cutoff used by each shower as the initial state for Herwig’s hadronization model. Studying the results at the hadron level, we find important consequences of this infrared cutoff difference and propose it as a starting point for further study of the interplay between parton showers and hadronization models. We conclude by studying the models’ tunability and identifying the best-fit parameters for each.

It has been a long entertained idea that self-bound gravitons, so-called geons, could be a dark matter candidate or form (primordial) black holes. The development of viable candidates for quantum gravity allows now to investigate these ideas. Analytic methods show that the description of geons needs to be based on composite operators made out of the graviton field. We present results from a numerical investigation into this idea using causal dynamical triangulations, an ab-initio non-perturbative definition of quantum gravity based on general relativity, and accessible in lattice-gauge-theory-like simulations. Our results suggest an interesting dependence on cosmological time and other unexpected features. Finally, we extend the analytic part of the setting to a supergravity scenario. This provides hints which, if confirmed, could explain why supersymmetry may in a realistic universe in principle not be observable at low (collider) energy scales.

We perform a systematic and comprehensive analysis of sub-leading colour corrections in perturbative QCD processes involving multiple soft gluon emissions. This necessitates going beyond the standard parton shower paradigm in order to incorporate interference effects and is accomplished using the CVolver program, which simulates parton showers at the amplitude level. We can compute cross-sections with full-colour precision and also broken down explicitly in terms of their Nc dependence. In this paper, we focus on the jet cross-section with a veto of additional jets in some fixed region of phase-space, since this is sensitive to wide-angle, soft gluon radiation. We consider Z→ qq , H → gg, qq→ qq , qg → qg, gg → gg and colour-singlet → qqqq . We find that non-trivial sub-leading colour effects are generally important at the 5–30

In the context of Monte Carlo (MC) generators with parton showers that have next-to-leading-logarithmic (NLL) precision, the cutoff Q_0 terminating the shower evolution should be viewed as an infrared factorization scale so that parameters or non-perturbative effects of the MC generator may have a field theoretic interpretation with a controllable scheme dependence. This implies that the generator's parton level should be carefully defined within QCD perturbation theory with subleading order precision. Furthermore, it entails that the shower cut Q_0 is not treated as one of the generator's tuning parameters, but that the tuning can be carried out reliably for a range of Q_0 values and that the hadron level description is Q_0-invariant. This in turn imposes non-trival constraints on the behavior of the generator's hadronization model, so that its parameters can adapt accordingly when the Q_0 value is changed. We investigate these features using the angular ordered parton shower and the cluster hadronization model implemented in the Herwig 7.2 MC generator focusing in particular on the e^+e^- 2-jettiness distribution, where the shower is known to be NLL precise and where QCD factorization imposes stringent constraints on the hadronization corrections. We show that the Herwig default cluster hadronization model does not exhibit these features or consistency with QCD factorization with a satisfying precision. We design a modification of the cluster hadronization model, where some dynamical parton shower aspects are added that are missing in the default model. For this novel dynamical cluster hadronization model these features and consistency with QCD factorization are realized much more accurately.

We introduce building blocks for the cluster hadronization model in light of a new structure, focusing on cluster fission and cluster decay. We propose theoretically motivated matrix elements for cluster fission and decay as building blocks and study some first phenomenological implications at different energies. In particular we develop a set of observables which can be used to dissect the hadronization history and have constraining power on the individual building blocks. Our analysis will be completed by including colour reconnection in a follow-up work.

We present the implementation of a new interface in 3.0 supporting all diboson and triboson processes with fully leptonic final states, enabling next-to-leading order (NLO)+parton shower (PS) matched calculations. To demonstrate its capabilities, we study parton shower effects in the triboson production process pp→e+νeμ−ν¯μγ+X using erwig 7.3 with NLO QCD amplitudes from 3.0. We estimate uncertainties from scale variations and analyze the impact of generation-level cuts on parton shower events. This study showcases the new interface’s potential and provides insights into the interplay between fixed-order calculations and parton shower effects in multiboson production processes, crucial for precision measurements and beyond the Standard Model searches at the LHC and future colliders. Published by the American Physical Society 2025

We study differential intra-jet radiation patterns in jet production at full colour. We present a systematic study of several QCD 2→ 2 processes and also multi-jet production from a colourless initial state. We examine how subleading colour corrections are distributed differentially in phase space and find that mere normalization effects due to subleading colour can be due to subtle cancellations across phase space. In general, we find that subleading colour does affect the shapes of distributions.

The spectrum of nonabelian gauge theories cannot be described in terms of elementary particles, and so must be constructed from gauge-invariant composite operators, even in the presence of a Brout–Englert–Higgs effect. This leads to qualitative discrepancies in the prediction of the spectrum between perturbation theory and a full non-perturbative treatment in many theories. This is especially noticeable for GUTs. We present results corroborating this general statement using lattice simulations for a ”GUT-like” toy theory, SU(3) Yang–Mills theory coupled to a Higgs field in the fundamental representation. Despite the apparent simplicity of the model, we find a rich spectrum with some previously unseen features. We also outline the next steps required to generate a large operator basis to extend this investigation to more realistic GUTs.

We present a comparison of three different general-purpose Monte Carlo event generators, Herwig, Pythia, and Sherpa, with respect to the simulation of photoproduction. We outline the default inputs, implementation differences and compare the results at different stages of the event generation. We find that, despite a similar starting point, the final cross sections do have some differences related to different non-perturbative inputs. We compare the simulations with experimental data for jet production in LEP and HERA and find that all generators provide a decent desription of the data within the considered uncertainties. We also present predictions for the upcoming EIC for jet observables and event shapes and conclude that accurate simulations will require further phenomenological advances.

We provide tools to analyze factorization at the amplitude level for processes involving the entire standard model. We focus in particular on a momentum region, in which the factorization of real and certain virtual corrections appears in a generalized eikonal approximation in which we expand around a quasisoft limit for massive gauge bosons, fermions, and scalars. We use the chirality-flow formalism to express loop exchanges or emissions as operators on chiral structures. This provides key tools for amplitude evolution with parton exchange and branching in the full Standard Model, including the electroweak sector. Published by the American Physical Society 2024

Colour evolution and parton branching at the amplitude level have become important theoretical frameworks to improve parton showers, and are algorithms in their own right: they complement shower development by resummation algorithms capable of including interference effects and subleading colour contributions at an unprecedented level. I summarize recent development in the field, focusing on soft gluon evolution, hadronization, and the CVolver framework.

We present the implementation of a new interface in VBFNLO 3.0 supporting all di-boson and tri-boson processes with fully leptonic final states, enabling NLO+PS matched calculations. To demonstrate its capabilities, we study parton shower effects in the tri-boson production process $p p \rightarrow e^{+} \nu_e \mu^{-} \bar{\nu}_{\mu} \gamma + X$ using Herwig 7.3 with NLO QCD amplitudes from VBFNLO 3.0. We estimate uncertainties from scale variations and analyze the impact of generation-level cuts on parton shower events. This study showcases the new interface's potential and provides insights into the interplay between fixed-order calculations and parton shower effects in multi-boson production processes, crucial for precision measurements and BSM searches at the LHC and future colliders.

We present the implementation of a new interface in VBFNLO 3.0 supporting all di-boson and tri-boson processes with fully leptonic final states, enabling NLO+PS matched calculations. To demonstrate its capabilities, we study parton shower effects in the tri-boson production process p p → e^+ν_e μ^-ν̅_μγ + X using Herwig 7.3 with NLO QCD amplitudes from VBFNLO 3.0. We estimate uncertainties from scale variations and analyze the impact of generation-level cuts on parton shower events. This study showcases the new interface's potential and provides insights into the interplay between fixed-order calculations and parton shower effects in multi-boson production processes, crucial for precision measurements and BSM searches at the LHC and future colliders.

AbstractVbfnlo is a flexible parton level Monte Carlo program for the simulation of vector boson fusion (VBF), QCD-induced single and double vector boson production plus two jets, and double and triple vector boson production (plus jet) in hadronic collisions at next-to-leading order (NLO) in the strong coupling constant, as well as Higgs boson plus two and three jet production via gluon fusion at the one-loop level. For the new version – Version 3.0 – several major enhancements have been included. An interface according to the Binoth Les Houches Accord (BLHA) has been added for all VBF and di/tri-boson processes including fully leptonic decays. For all dimension-8 operators affecting vector boson scattering (VBS) processes, a modified T-matrix unitarization procedure has been implemented. Several new production processes have been added, namely the VBS $$Z\gamma jj$$ Z γ j j and $$\gamma \gamma jj$$ γ γ j j processes at NLO, $$\gamma \gamma jj $$ γ γ j j , WWj and ZZj production at NLO including the loop-induced gluon-fusion contributions and the gluon-fusion one-loop induced $$\varPhi jjj$$ Φ j j j ($$\varPhi $$ Φ is a CP-even or CP-odd scalar boson) process at LO, retaining the full top-mass dependence. Finally, the code has been parallelized using Openmpi.

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.

In a few recent papers we introduced the chirality-flow formalism, which builds on the spinor-helicity formalism, but incorporates the Fierz identity into the Feynman rules. Calculations at tree level are thereby trivial, often to the extent that it is possible to immediately write down a tree-level Feynman diagram in terms of spinor inner products. This simplification persists in tree-level computer implementations, giving very sizable speedups. In the present paper, we argue that there is also a significant simplification of the Lorentz structure at the one-loop level when using the four-dimensional formulation of the four-dimensional helicity scheme. As at tree level, the gauge reference vector for external gauge bosons, and the simplified Lorentz structure lead to significant shortening of the calculations. Additionally, we find that the possible terms in a tensor decomposition of loop integrals are highly constrained, and therefore the tensor reduction procedure is simplified. Published by the American Physical Society 2024

We construct a set of Wigner 6j symbols with gluon lines (adjoint representations) in closed form, expressed in terms of similar 6j symbols with quark lines (fundamental representations). Together with Wigner 6j symbols with quark lines, this gives a set of 6j symbols sufficient for treating QCD color structure for any number of external particles, in or beyond perturbation theory. This facilitates a complete treatment of QCD color structure in terms of orthogonal multiplet bases, without the need of ever explicitly constructing the corresponding bases. We thereby open up for a completely representation theory based treatment of SU(N) color structure, with the potential of significantly speeding up the color structure treatment.

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.