arXiv subjects

Andreas Papaefstathiou

Publications and source records attributed to Andreas Papaefstathiou.

At least 19 recordsLinked to original sources

arXiv2026

Higgs Boson Pair Production via Gluon Fusion: Higher-Order Corrections and Theoretical Uncertainties

In this contribution, the higher-order QCD and electroweak corrections to Standard Model Higgs boson pair production via the gluon-fusion mechanism, $gg\to hh$, are summarized and the different sources of theoretical uncertainty are assessed. The discussion includes finite top quark mass effects, matching to parton showers, approximate NNLO and N$^3$LO QCD corrections, NLO electroweak effects, and uncertainties associated with the top quark mass scheme and perturbative scale choices. In addition, we provide an updated state-of-the-art recommendation for the inclusive gluon-fusion Higgs boson pair production cross section and the corresponding Higgs boson pair invariant-mass distribution.

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arXiv2026

Quadruple-Higgs boson production at the high-luminosity LHC

We investigate the production of four Higgs bosons via gluon fusion at the high-luminosity LHC. We construct a phenomenological analysis of the eight-$b$-jet final state and use a multivariate analysis to derive expected simultaneous constraints on the triple and quartic Higgs boson self-coupling modifications. At 95% confidence level, the resulting region projects onto $-10.3<c_3<11.2$ and $-222<d_4<234$, where $c_3=\kappa_3-1$ and $d_4=\kappa_4-1$. We also examine how quadruple-Higgs boson production can enter a triple-Higgs boson signal region requiring at least six $b$-tagged jets. In addition, we consider two extensions of the Standard Model scalar sector: a one-real-singlet extension yielding the direct resonant process $pp\rightarrow S\rightarrow hhhh$, and a two-real-singlet extension yielding the cascade $pp\rightarrow h_3\rightarrow h_2h_2\rightarrow (hh)(hh)$, where $S$, $h_2$ and $h_3$ are new scalar resonances. For these processes, we derive expected upper limits on the quadruple-Higgs boson production cross sections as functions of the new scalar masses.

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arXiv2025

Extracting Higgs Self-Coupling Constraints through Triple Higgs Boson Production at Future Hadron Colliders

We present a systematic study of triple Higgs boson production at future high-energy hadron colliders, using the six-$b$-jet final state as a probe of the Higgs self-interactions. We conduct, under realistic detector smearing assumptions, both a traditional cut-based analysis, and a multivariate one using gradient boosting. The multivariate strategy is found to enhance sensitivity to beyond the Standard Model effects on the Higgs boson's self-couplings, while preserving large signal event yields, thus enabling more robust statistical inference. This allows us to assess the impact of detector effects, systematic uncertainties, background normalisation, as well as different truncation choices in an effective-field-theory description of the new physics effects possibly affecting the Higgs boson's self-interactions. Our results demonstrate that statistically-meaningful and perturbative-unitarity-compatible constraints on the trilinear and quartic Higgs boson self-couplings can be achieved, provided that systematic uncertainties are controlled at the few-percent level. Finally, we extrapolate our results to various collider energies and luminosities, demonstrating in particular that an 85 TeV proton-proton collider performs comparably to a 100 TeV machine. Altogether, our findings therefore establish the six-$b$ channel as a viable probe of the Higgs self-interactions at most future hadron collider options currently being examined by the high-energy physics community.

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arXiv2025

Triple Higgs Boson Production with Two Heavy Scalars at the LHC via a Simplified Approach

We investigate triple Higgs boson production at the CERN LHC, in models containing two new heavy neutral scalar particles. We apply a simplified factorized approach in the narrow-width approximation to double-resonant triple Higgs boson production, and demonstrate that relevant constraints can be derived on models with extended scalar sectors, during the high-luminosity phase of the LHC. We also find, within an explicit two-real-singlet model and for the cut-based 6 $b$-jet analysis considered here, that the double-resonant contribution captures the dominant features of the signal, while non-resonant components have a limited impact on the expected sensitivity. The method therefore provides a fast approximate framework for models in which triple Higgs boson production is dominated by a narrow double-resonant topology.

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arXiv2024

Investigating the Electroweak Phase Transition with a Real Scalar Singlet at a Muon Collider

A strong first-order electroweak phase transition (SFOEWPT) is essential for explaining baryogenesis and for potentially generating observable gravitational waves. This study investigates the potential of a high-energy muon collider to examine the occurrence of SFOEWPT within the context of a Standard Model extended by a real scalar singlet (xSM). We analyzed all possible decay modes of the singlet to constrain the valid parameter space of SFOEWPT, which was extracted numerically at different renormalization scales to account for theoretical uncertainties, thereby determining the sensitivity of a muon collider to the production and decay channels of novel heavy scalar particles that emerge in the xSM. The findings demonstrate that a 3 TeV muon collider can directly examine the nature of electroweak symmetry breaking by efficiently detecting novel scalar particles associated with a first-order electroweak phase transition through jet-rich final states, thus complementing the indirect constraints from gravitational wave experiments.

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arXiv2024

Multi-Higgs Boson Production with Anomalous Interactions at Current and Future Proton Colliders

We investigate multi-Higgs boson production at proton colliders, in a framework involving anomalous interactions, focusing on triple Higgs boson production. We consider modifications to the Higgs boson self-couplings, to the Yukawa interactions, as well as new contact interactions of Higgs bosons with either quarks or gluons. To this end, we have developed a MadGraph5_aMC@NLO loop model, publicly available at https://gitlab.com/apapaefs/multihiggs_loop_sm, designed to incorporate the relevant operators in the production of multiple Higgs bosons (and beyond). We have performed cross section fits at various energies over the anomalous interactions, and have derived constraints on the most relevant anomalous coefficients, through detailed phenomenological analyses at proton-proton collision energies of 13.6 TeV and 100 TeV, through the 6 $b$-jet final state.

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arXiv2024

Herwig 7.3 Release Note

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.

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arXiv2024

HHH Whitepaper

We here report on the progress of the HHH Workshop, that took place in Dubrovnik in July 2023. After the discovery of a particle that complies with the properties of the Higgs boson of the Standard Model, all Standard Model (SM) parameters are in principle determined. However, in order to verify or falsify the model, the full form of the potential has to be determined. This includes the measurement of the triple and quartic scalar couplings. We here report on ongoing progress of measurements for multi-scalar final states, with an emphasis on three SM-like scalar bosons at 125 GeV, but also mentioning other options. We discuss both experimental progress and challenges as well as theoretical studies and models that can enhance such rates with respect to the SM predictions

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arXiv2024

$\texttt{Pyresias}$: How To Write a Toy Parton Shower

The aim of this article is to provide an educational introduction to the parton shower approach used in particle physics Monte carlo event generators to simulate radiation via quantum chromodynamics. Following a description of the fundamental theoretical underpinnings, instructions on how to build a toy parton shower are given. The associated $\texttt{Python}$ code, along with a detailed JupyterLab notebook, are publicly available at https://github.com/apapaefs/pyresias.

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arXiv2024

Machine Learning Classification of Sphalerons and Black Holes at the LHC

In models with large extra dimensions, "miniature" black holes (BHs) might be produced in high-energy proton-proton collisions at the Large Hadron Collider (LHC). In the semi-classical regime, those BHs thermally decay, giving rise to large-multiplicity final states with jets and leptons. On the other hand, similar final states are also expected in the production of electroweak sphaleron/instanton-induced processes. We investigate whether one can discriminate these scenarios when BH or sphaleron-like events are observed in the LHC using machine learning (ML) methods. Classification among several BH scenarios with different numbers of extra dimensions and the minimal BH masses is also examined. In this study we consider three ML models: XGBoost algorithms with (1) high- and (2) low-level inputs, and (3) a Residual Convolutional Neural Network. In the latter case, the low-level detector information is converted into an input format of three-layer binned event images, where the value of each bin corresponds to the energy deposited in various detector subsystems. We demonstrate that only a small number of detected events are sufficient to effectively discriminate between the sphaleron and BH processes. Separation between BH scenarios with different minimal masses is possible with an order of 10 events passing the preselection. A sufficient number of events could be observed in combined Run-2 and -3 data, if the production cross section is not much smaller than the present limit ~ 0.1 fb. We find, however, that a large number of events is needed to discriminate between BH hypotheses with the same minimal BH mass, but different numbers of extra dimensions.

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arXiv2024

Triple Higgs boson production and electroweak phase transition in the two-real-singlet model

The production of three Higgs bosons at hadron colliders can be enhanced by a double-resonant effect in the $\mathbb{Z}_2$-symmetric two-real-singlet extension of the Standard Model, making it potentially observable in future LHC runs. The production rate is maximized for large scalar couplings, which prompts us to carefully reconsider the perturbativity constraints on the theory. This leads us to construct a new set of 140 benchmark points that have a triple Higgs boson production cross-section at least 100 times larger than the SM value. Furthermore, we study the dynamics of the electroweak phase transition, both analytically at leading order, and numerically without the high-temperature expansion. Both analyses indicate that a first-order phase transition is incompatible with the requirement that both singlets have a non-zero vev in the present-day vacuum, as required by doubly-enhanced triple Higgs boson production. Allowing instead one of the singlets to remain at zero field value opens up the possibility of a first-order phase transition, while di-Higgs boson production can still be enhanced by a (single) resonance.

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arXiv2024

HEJ 2.2: W boson pairs and Higgs boson plus jet production at high energies

We present version 2.2 of the High Energy Jets (HEJ) Monte Carlo event generator for hadronic scattering processes at high energies. The new version adds support for two further processes of central phenomenological interest, namely the production of a W boson pair with equal charge together with two or more jets and the production of a Higgs boson with at least one jet. Furthermore, a new prediction for charged lepton pair production with high jet multiplicities is provided in the high-energy limit. The accuracy of HEJ 2.2 can be increased further through an enhanced interface to standard predictions based on conventional perturbation theory. We describe all improvements and provide extensive usage examples. HEJ 2.2 can be obtained from https://hej.hepforge.org.

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arXiv2023

High Energy Resummed Predictions for the Production of a Higgs Boson with at least One Jet

We present all-order predictions for Higgs boson production plus at least one jet which are accurate to leading logarithm in $\hat s/|p_\perp|^2$. Our calculation includes full top and bottom quark mass dependence at all orders in the logarithmic part, and to highest available order in the tree-level matching. The calculation is implemented in the framework of High Energy Jets (HEJ). This is the first cross section calculated with $\log(\hat s)$ resummation and matched to fixed order for a process requiring just one jet, and our results also extend the region of resummation for processes with two jets or more. This is possible because the resummation is performed explicitly in phase space. We compare the results of our new calculation to LHC data and to next-to-leading order predictions and find a numerically significant impact of the logarithmic corrections in the shape of key distributions, which remains after normalisation of the cross section.

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arXiv2022

The Effective Potential in Fermi Gauges Beyond the Standard Model

We derive the field-dependent masses in Fermi gauges for arbitrary scalar extensions of the Standard Model. These masses can be used to construct the effective potential for various models of new physics. We release a flexible $\texttt{Mathematica}$ notebook ($\texttt{VefFermi}$) which performs these calculations and renders large-scale phenomenological studies of various models possible. Motivated by the debate on the importance of gauge dependence, we show that, even in relatively simple models, there exist points where the global minimum is discontinuous in the gauge parameter. Such points require some care in discovering, indicating that a gauge-dependent treatment might still give reasonable results when examining the global features of a model.

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arXiv2022

A standard convention for particle-level Monte Carlo event-variation weights

Streams of event weights in particle-level Monte Carlo event generators are a convenient and immensely CPU-efficient approach to express systematic uncertainties in phenomenology calculations, providing systematic variations on the nominal prediction within a single event sample. But the lack of a common standard for labelling these variation streams across different tools has proven to be a major limitation for event-processing tools and analysers alike. Here we propose a well-defined, extensible community standard for the naming, ordering, and interpretation of weight streams that will serve as the basis for semantically correct parsing and combination of such variations in both theoretical and experimental studies.

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arXiv2022

Signal strength and W-boson mass measurements as a probe of the electro-weak phase transition at colliders -- Snowmass White Paper

We consider an extension of the scalar sector of the Standard Model (SM) by an additional gauge singlet, which mixes with a part of the SM-like Higgs doublet. Within this model, parameter-space regions exist that can lead to a strong first-order electro-weak phase transition, a necessary condition for electro-weak baryogenesis. We discuss how such regions of the parameter space can be tested using the SM-like Higgs boson's signal strength measurements, as well as precision observables, such as e.g. the W-boson mass, at current and future colliders.

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arXiv2022

The Electro-Weak Phase Transition at Colliders: Discovery Post-Mortem

We explore the capabilities of a future proton collider to probe the nature of the electro-weak phase transition, following the hypothetical discovery of a new scalar particle. We focus on the real singlet scalar field extension of the Standard Model, representing the most minimal, and challenging to probe, framework that can enable a strong first-order electro-weak phase transition. By constructing detailed phenomenological methods for measuring the mass and accessible couplings of the new scalar particle, we find that a 100 TeV proton collider has the potential to explore the parameter space of the real singlet model and provide meaningful constraints on the electro-weak phase transition. We empirically find some necessary conditions for the realization of a strong first order electro-weak phase transition and conjecture that additional information, including through multi-scalar processes and gravitational wave detectors, are likely needed to gauge the nature of the cosmological electro-weak transition. This study represents the first crucial step towards solving the inverse problem in the context of the electro-weak phase transition.

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arXiv2021

Soft QCD Effects in VBS/VBF Topologies

We consider the impact of multi-parton interactions, colour reconnection and hadronization on the modeling of vector boson fusion and vector boson scattering (VBS) final states at the Large Hadron Collider (LHC). We investigate how the variation of the model parameters, compatible with a reasonable spread of predictions around typical tuning observables, extrapolates into the VBS phase space. We study the implications of this variation on the total uncertainty budget attached to realistic simulation of the final states in current event generator predictions. We find that the variations have a non-trivial phase space dependence and become comparable in size to the perturbative uncertainties once next-to-leading order predictions are combined with parton shower evolution.

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