arXiv subjects

Christian T. Preuss

Publications and source records attributed to Christian T. Preuss.

16 recordsLinked to original sources

arXiv2026

Multiplicative matching of neutral current deep-inelastic scattering processes at next-to-leading order in PYTHIA 8

We introduce a method for matching the neutral-current deep inelastic scattering process with parton showers in Pythia 8, at first order in the strong coupling. The implementation, based on the well-established multiplicative matching approach, is achieved by reweighting leading-order Born-level events and requires that the first parton-shower emission is distributed according to the real matrix-element. The method is implemented as an internal matching strategy in the Pythia 8 event generator applicable to both currently available parton-shower algorithms, the default one and Vincia. The validity of higher-order corrections is verified with comparisons against existing next-to-leading order simulations. The strategy is used to describe reduced cross-sections measured at the HERA collider, and we find better overall agreement and reduced uncertainties with the matching.

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arXiv2026

Multiplicative NLO matching in DIS with Pythia 8

We present a new multiplicative next-to-leading order (NLO) matching scheme in Pythia 8 for deep-inelastic scattering (DIS) processes in electron-proton collisions. The implementation is based on numerical event-by-event integration over the virtual corrections and real-emission phase-space and matrix-element corrections (MECs) for the first parton-shower emission. The latter has been included for two parton-shower options currently available in Pythia 8 to handle DIS processes, the default shower with dipole-recoil option and Vincia. The implementation has been validated against other available NLO-capable event generators and compared to experimental data from the HERA collider. In all cases we find very good agreement confirming the validity of the new Pythia implementation. We also study the impact of the MECs in each shower and consider the implications for the upcoming Electron-Ion collider.

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arXiv2026

Top Pair Threshold Revisited

Recently the CMS and ATLAS collaborations have found evidence for an unexpectedly large ttbar cross section in the threshold region, with an excess of the order of 5-10 pb relative to continuum perturbative calculations. A convenient approach to the theoretical study of this region, unifying the above- and below-threshold behaviour, is the non-relativistic Green's function formalism. It was first applied to top production more than 35 years ago, well before the discovery of the top. We therefore revive and dissect the old formalism, and put it back together in a more consistent form, suited for Monte Carlo event generation. Combined with some practical prescriptions, it can be applied to current conditions. As an example, the below-threshold cross section comes out to be of the order of 6.5 pb, i.e. comparable with the CMS and ATLAS numbers. The new code is publicly available in the Pythia event generator so can be used for more detailed comparisons.

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arXiv2026

Theory uncertainties of the irreducible background to VBF Higgs production

Higgs boson production through gluon fusion in association with two jets is an irreducible background to Higgs boson production through vector boson fusion, one of the most important channels for analyzing and understanding the Higgs boson properties at the Large Hadron Collider. Despite a range of available simulation tools, precise predictions for the corresponding final states are notoriously hard to achieve. Using state-of-the-art fixed-order calculations as the baseline for a comparison, we perform a detailed study of similarities and differences in existing event generators. We provide consistent setups for the simulations that can be used to obtain identical parametric precision in various programs used by experiments. We find that NLO calculations for the two-jet final state are essential to achieve reliable predictions.

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arXiv2026

QCD splitting functions beyond kinematical limits

We present a systematic decomposition of QCD splitting functions into scalar dipole radiators and pure splitting remainders up to second order in the strong coupling. The individual components contain terms that are formally sub-leading in soft or collinear scaling parameters, but well understood and universal due to their origin in scalar QCD. The multipole radiator functions which we derive share essential features of the known double-soft and one-loop soft gluon currents, and are not based on kinematical approximations.

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arXiv2025

NNLO+NNLL Predictions for Heavy-Jet Mass and C-parameter in Higgs Decays to Quarks and Gluons

We consider the resummation of large logarithmic corrections arising in the two-particle limit at next-to-next-to-leading logarithmic (NNLL) accuracy for the heavy-jet mass and $C$-parameter distributions in the decay of a Higgs boson to quarks and gluons: $H\to b\bar{b}$, $H\to c\bar{c}$, and $H\to gg$. We demonstrate how the matched NNLO+NNLL results clarify the relative contributions of key hadronic Higgs-decay channels ($H\to b\bar{b}$, $H\to c\bar{c}$, $H\to gg$) yielding reduced uncertainties for both event-shape observables -- especially for heavy-jet mass -- while revealing substantial effects that shift the $C$-parameter peak in gluonic decays.

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arXiv2025

Higgs production via vector-boson fusion at the LHC

In this article, we summarise the recent experimental measurements and theoretical work on Higgs boson production via vector-boson fusion at the LHC. Along with this, we provide state-of-the-art predictions at fixed order as well as with parton-shower corrections within the Standard Model at 13.6 TeV. The results are presented in the form of multi-differential distributions as well as in the Simplified Template Cross Section bins. All materials and outputs of this study are available on public repositories. Finally, following findings in the literature, recommendations are made to estimate theoretical uncertainties related to parton-shower corrections.

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arXiv2025

Matching Z $\to$ Hadrons at NNLO with Sector Showers

We present a detailed technical derivation of matching conditions at next-to-next-to-leading order in the sectorised VINCIA parton shower, by considering leading-colour 2-, 3- and 4-jet rates in hadronic Z-boson decays. In particular, we introduce a full subtraction-based calculation of the matching coefficient required to obtain the NLO 3-jet rate. This is achieved through a judicious choice of the counter-terms, which optimises the numerical evaluation of the subtracted double-real matrix element. We additionally give a consistent prescription for incorporating interference effects due to higher-order mixing between Born states with different flavour contents. Finally, we briefly comment on higher-order uncertainty estimates.

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arXiv2025

The Thrust Distribution at NNLO+NNLL in Higgs Decays to Quarks and Gluons

We present a calculation of the thrust distribution in Higgs decays to quarks and gluons, $H\to b\bar{b}$, $H\to c\bar{c}$, and $H\to gg$, including the resummation of large logarithmic corrections that arise in the two-particle limit at next-to-next-to-leading logarithmic (NNLL) accuracy, and match it to fixed-order results for three-particle decays at next-to-next-to-leading order (NNLO) in the strong coupling. The resummation is performed analytically within the ARES framework and combined with the fixed-order results using the logR matching technique. The fixed-order calculation is carried out numerically with the NNLOJET parton-level event generator, using the antenna subtraction method. We perform detailed cross-validation in the two-particle region, demonstrating that the expansion of the NNLL resummed result correctly reproduces the logarithmic structure of the fixed-order calculation to $\mathcal{O}(α_\mathrm{s}^3)$, up to a predictable N$^{3}$LL term at $\mathcal{O}(α_\mathrm{s}^3L)$. In addition to providing the first NNLO+NNLL accurate predictions for the thrust distribution in Higgs decays to quarks and gluons, we analytically extract the $\mathcal{O}(α_\mathrm{s}^2)$ hard-virtual correction $c_2$ and the $α_\mathrm{s}^3L$ term $G_{31}$ in both the $H\to q\bar{q}$ ($q=b,c$) and $H\to gg$ decay channels.

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arXiv2025

Precise Predictions for Event Shapes in Hadronic Higgs Decays

We present NNLO QCD predictions for a wide range of event-shape observables in hadronic Higgs decays, taking into account the two dominant decay modes $H\to gg$ and $H\to b\bar{b}$. Specifically, we consider the six classical event shapes thrust, heavy jet mass, $C$-parameter, total and wide jet broadening, and the three-jet resolution $y_{23}$ in the Durham algorithm. We also present results for the soft-drop variant of thrust. Decays of the Higgs boson to two gluons are treated in the heavy-top limit, whereas decays to a bottom-quark pair are mediated by a non-vanishing Yukawa coupling, despite considering kinematically massless quarks. Our results highlight the importance of NNLO QCD corrections in the calculation of event-shape observables and provide means to quantify the intrinsic difference between the two Higgs decay modes.

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arXiv2025

Multi-Jet Production in Deep Inelastic Scattering with Pythia

We introduce multi-jet merging for deep inelastic scattering in the Vincia parton shower in the Monte Carlo event generator Pythia 8. Merging combines event samples of different parton multiplicities with logarithmically enhanced parton-shower radiation. We consider up to five outgoing partons using two different merging algorithms. We vary the relevant scale choices and compare to experimental data by H1 collaboration at the HERA collider. Results show that multi-jet merging improves description to data, especially for low virtuality events, and that jet cross-sections converge when the partonic multiplicity exceeds the number of reconstructed jets.

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arXiv2025

Jet rates in Higgs boson decay at third order in QCD

We compute the production rates for two, three, four and five jets in the hadronic decay of a Higgs boson in its two dominant decay modes to bottom quarks and gluons to third order in the QCD coupling constant. The five-, four- and three-jet rates are obtained from a next-to-next-to-leading order (NNLO) calculation of Higgs decay to three jets, while the two-jet rate is inferred at next-to-next-to-next-to-leading order (N$^3$LO) from the inclusive decay rate. Our results show distinct differences in the dependence of the jet rates on the jet resolution parameter between the two decay modes, supporting the aim of discriminating different Higgs boson decay channels via classic QCD observables.

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arXiv2025

EERAD3 version 2: QCD corrections in hadronic colour-singlet decays

We present a major update of the publicly available EERAD3 package to calculate perturbative corrections in the strong coupling in hadronic Higgs and $Z$-boson decays. We describe the theoretical framework underlying the numerical implementation and provide a guide to the usage of the program.

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arXiv2024

Multi-Jet Merging in Deep Inelastic Scattering with Pythia

We aim to improve the modelling of deep inelastic scattering by implementing multi-jet merging capabilities in Vincia parton shower in the general-purpose Monte Carlo event generator, Pythia. Merging allows to combine event samples of different parton multiplicities with logarithmically enhanced radiation from parton shower algorithms, without double-counting. Here we consider events up to five outgoing partons and present results for jet analyses compared to experimental data provided by the ZEUS and H1 collaborations of the HERA collider. The analyses span a wide range in photon virtuality, and the results show that the multi-jet merging improves jet modelling especially for low virtuality events.

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arXiv2023

NNLO Matrix-Element Corrections in VINCIA

We report on a new formalism for parton showers whose fixed-order expansion can be corrected through next-to-next-to-leading order (NNLO) in QCD. It is the first such formalism we are aware of that has no dependence on any auxiliary scales or external resummations and which is fully differential in all of the relevant phase spaces. Since the shower acts as the phase-space generator, the dominant singularity structures are encoded by construction and the method can generate unweighted events with very high efficiency without any significant initialisation time. We argue that the the method should be capable of achieving (at least) NNLO+NNDL accuracy for the shower evolution variable and use hadronic Z decays as a specific example.

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arXiv2022

A comprehensive guide to the physics and usage of PYTHIA 8.3

This manual describes the PYTHIA 8.3 event generator, the most recent version of an evolving physics tool used to answer fundamental questions in particle physics. The program is most often used to generate high-energy-physics collision "events", i.e. sets of particles produced in association with the collision of two incoming high-energy particles, but has several uses beyond that. The guiding philosophy is to produce and reproduce properties of experimentally obtained collisions as accurately as possible. The program includes a wide ranges of reactions within and beyond the Standard Model, and extending to heavy ion physics. Emphasis is put on phenomena where strong interactions play a major role. The manual contains both pedagogical and practical components. All included physics models are described in enough detail to allow the user to obtain a cursory overview of used assumptions and approximations, enabling an informed evaluation of the program output. A number of the most central algorithms are described in enough detail that the main results of the program can be reproduced independently, allowing further development of existing models or the addition of new ones. Finally, a chapter dedicated fully to the user is included towards the end, providing pedagogical examples of standard use cases, and a detailed description of a number of external interfaces. The program code, the online manual, and the latest version of this print manual can be found on the PYTHIA web page: https://www.pythia.org/

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