arXiv subjects

Stefan Dittmaier

Publications and source records attributed to Stefan Dittmaier.

At least 37 records · Page 2Linked to original sources

arXiv2021

Electric charge renormalization to all orders

The electric charge renormalization constant, as defined in the Thomson limit, is expressed in terms of self-energies of the photon-Z-boson system in an arbitrary R_ξ-gauge to all perturbative orders. The derivation as carried out in the Standard Model holds in all spontaneously broken gauge theories with the SU(2)_w \times U(1)_Y gauge group in the electroweak sector and is based on the application of charge universality to a fake fermion with infinitesimal weak hypercharge and vanishing weak isospin, which effectively decouples from all other particles. Charge universality, for instance, follows from the known universal form of the charge renormalization constant as derived within the background-field formalism. Finally, we have generalized the described procedure to gauge theories with gauge group U(1)_Y \times G with any Lie group G, only assuming that electromagnetic gauge symmetry is unbroken and mixes with U(1)_Y transformations in a non-trivial way.

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arXiv2020

Mixed NNLO QCD x electroweak corrections of O(N_f α_s α) to single-W/Z production at the LHC

First results on the radiative corrections of order O(N_fα_sα) are presented for the off-shell production of W or Z bosons at the LHC, where N_f is the number of fermion flavours. These corrections comprise all diagrams at O(α_sα) with closed fermion loops, form a gauge-invariant part of the next-to-next-to-leading-order corrections of mixed QCD x electroweak type, and are the ones that concern the issue of mass renormalization of the W and Z resonances. The occurring irreducible two-loop diagrams, which involve only self-energy insertions, are calculated with current standard techniques, and explicit analytical results on the electroweak gauge-boson self-energies at O(α_sα) are given. Moreover, the generalization of the complex-mass scheme for a gauge-invariant treatment of the W/Z resonances is described for the order O(α_sα). While the corrections, which are implemented in the Monte Carlo program RADY, are negligible for observables that are dominated by resonant W/Z bosons, they affect invariant-mass distributions at the level of up to 2% for invariant masses of >500 GeV and are, thus, phenomenologically relevant. The impact on transverse-momentum distributions is similar, taking into account that leading-order predictions to those distributions underestimate the spectrum.

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arXiv2020

Electroweak Radiative Corrections for Collider Physics

Current particle phenomenology is characterized by the spectacular agreement of the predictions of the Standard Model of particle physics (SM) with all results from collider experiments and by the absence of significant signals of non-standard physics, despite the fact that we know that the SM cannot be the ultimate theory of nature. In this situation, confronting theory and experiment with high precision is a promising direction to look for potential traces of physics beyond the SM. On the theory side, the calculation of radiative corrections of the strong and electroweak interactions is at the heart of this task, a field that has seen tremendous conceptual and technical progress in the last decades. This review aims at a coherent introduction to the field of electroweak corrections and tries to fill gaps in the literature between standard textbook knowledge and the current state of the art. The SM and the machinery for its perturbative evaluation are reviewed in detail, putting particular emphasis on renormalization, on one-loop techniques, on modern amplitude methods and tools, on the separation of infrared singularities in real-emission corrections, on electroweak issues connected with hadronic initial or final states in collisions, and on the issue of unstable particles in quantum field theory together with corresponding practical solutions.

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arXiv2019

Next-to-leading-order QCD and electroweak corrections to triple-W production with leptonic decays at the LHC

We present a calculation of the next-to-leading-order QCD and electroweak corrections to WWW production with leptonically decaying $\mathrm{W}$ bosons at the LHC, fully taking into account off-shell contributions, intermediary resonances, and spin correlations. The contributions of the quark-antiquark-induced electroweak correction to typical fiducial cross sections at the LHC are of the order of 5-8% and grow to tens of percent in the high-energy tails of distributions. We observe strong cancellations among the positive quark-photon and negative quark-antiquark-induced electroweak corrections. In addition to results based on full $2\to 6/7$-particle next-to-leading-order matrix elements, we present a calculation based on the triple-pole approximation, which expands the matrix elements around the poles of three simultaneously resonant $\mathrm{W}$ bosons. The triple-pole approximation performs particularly well for integrated cross sections and for differential cross sections that are insensitive to off-shell effects, such as angular and rapidity distributions.

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arXiv2019

Renormalization schemes for mixing angles in extended Higgs sectors

The proper renormalization of mixing angles in quantum field theories is a long-standing problem. It is relevant for the renormalization of the quark mixing matrix in the Standard Model and for various mixing scenarios in theories beyond. In this contribution we specifically consider theories with extended scalar sectors. We describe renormalization schemes for mixing angles based on combinations of observables or symmetry requirements such as rigid or background-field gauge invariance and compare their properties to previous approaches such as $\bar{MS}$ schemes. We formulate specific renormalization conditions for the mixing angles in the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model and calculate electroweak corrections to Higgs-boson decays via W- or Z-boson pairs within these models for a selection of (new and old) renormalization schemes.

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arXiv2019

PROPHECY4F 3.0: A Monte Carlo program for Higgs-boson decays into four-fermion final states in and beyond the Standard Model

The Monte Carlo generator PPROPHECY4F provides a PROPer description of the Higgs dECaY into 4 Fermions within the Standard Model, the Standard Model with a fourth fermion generation, a simple Higgs-singlet extension of the Standard Model, and the Two-Higgs-Doublet Model. The fully differential predictions include the full QCD and electroweak next-to-leading-order corrections, all interference contributions between different WW/ZZ channels, and all off-shell effects of intermediate W/Z bosons. PPROPHECY4F computes the inclusive partial decay widths and allows for the computation of binned differential distributions of the decay products. For leptonic final states also unweighted events are provided.

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arXiv2019

Renormalization of mixing angles

We discuss the renormalization of mixing angles for theories with extended scalar sectors. Motivated by shortcomings of existing schemes for mixing angles, we review existing renormalization schemes and introduce new ones based on on-shell conditions or symmetry requirements such as rigid or background-field gauge invariance. Considering in particular the renormalization of the mixing angles in the Two-Higgs-Doublet Model and the Higgs-Singlet Extension of the Standard Model, we compare electroweak corrections within these models for a selection of renormalization schemes. As specific examples, we present next-to-leading-order results on the four-fermion decays of heavy and light CP-even Higgs bosons, $H_1/H_2\to WW/Z Z\to4f$, and on electroweak Higgs-boson production processes, i.e. Higgs-strahlung and vector-boson fusion. We find that our new proposals for on-shell and symmetry-based renormalization conditions are well-behaved for the considered benchmark scenarios in both models.

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arXiv2019

Low-virtuality photon transitions $γ^*\to f\bar f$ and the photon-to-jet conversion function

The calculation of electroweak corrections to processes with jets in the final state involves contributions of low-virtuality photons leading to jets in the final state via the singular splitting $γ^* \to q\bar q$. These singularities can be absorbed into a photon-to-jet "fragmentation function", better called "conversion function", since the physical final state is any hadronic activity rather than an identified hadron. Using unitarity and a dispersion relation, we relate this $γ^* \to q\bar q$ conversion contribution to an integral over the imaginary part of the hadronic vacuum polarization and thus to the experimentally known quantity $Δα^{(5)}_{\mathrm{had}}(M^2_{\rm Z})$. Therefore no unknown non-perturbative contribution remains that has to be taken from experiment. We also describe practical procedures following subtraction and phase-space-slicing approaches for isolating and cancelling the $γ^* \to q\bar q$ singularities against the photon-to-jet conversion function. The production of Z+jet at the LHC is considered as an example, where the photon-to-jet conversion is part of a correction of the order $α^2/α_{\rm s}$ relative to the leading-order cross section.

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arXiv2019

QCD and electroweak corrections to WZ scattering at the LHC

We present the first computation of the full next-to-leading-order QCD and electroweak corrections to the WZ scattering process at the LHC. All off-shell, gauge-boson-decay, and interference effects are taken into account for the process $\mathrm{p} \mathrm{p} \to μ^+μ^-\mathrm{e}^+ν_\mathrm{e} \mathrm{j} \mathrm{j} + X$ at the orders $\mathcal{O}{\left( α_\mathrm{s} α^6 \right)}$ and $\mathcal{O}{\left( α^7 \right)}$. The electroweak corrections feature the typical Sudakov behaviour towards high energy and amount to $-16\%$ relative to the electroweak contribution to the integrated cross section. Moreover, the corrections induce significant shape distortions in differential distributions. The next-to-leading-order analysis of the quark- and gluon-induced channels is supplemented by a leading-order study of all possible contributions to the full $4\ell+2\mbox{jets}$ production cross section in a realistic fiducial phase-space volume.

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arXiv2018

PHOTON-2017 conference proceedings

This document collects the proceedings of the PHOTON 2017 conference ("International Conference on the Structure and the Interactions of the Photon", including the 22th "International Workshop on Photon-Photon Collisions", and the "International Workshop on High Energy Photon Colliders") held at CERN (Geneva) in May 2017. The latest experimental and theoretical developments on the topics of the PHOTON conference series are covered: (i) $γ\,γ$ processes in e$^+$e$^-$, proton-proton (pp) and nucleus-nucleus (AA) collisions at current and future colliders, (ii) $γ$-hadron interactions in e$^\pm$p, pp, and AA collisions, (iii) final-state photon production (including Standard Model studies and searches beyond it) in pp and AA collisions, and (iv) high-energy $γ$-ray astrophysics. These proceedings are dedicated to the memory of Maria Krawczyk.

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arXiv2018

Electroweak corrections in the Two-Higgs-Doublet Model and a Singlet Extension of the Standard Model

We present the next-to-leading-order calculation of the partial decay widths of light CP-even Higgs bosons decaying into four fermions in the Two-Higgs-Doublet Model and a Singlet Extension of the Standard Model. Different renormalization schemes are applied in the calculation, which is implemented into the analysis tool PROPHECY4F. Some sample results on the Higgs->4fermions decay widths illustrate how the corrections reduce the dependence on the renormalization scale and the choice of the scheme.

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arXiv2018

Precision calculations for $h \to WW/ZZ \to 4$ fermions in a Singlet Extension of the Standard Model with Prophecy4f

We consider an extension of the Standard Model by a real singlet scalar field with a $Z_2$-symmetric Lagrangian and spontaneous symmetry breaking with vacuum expectation value for the singlet. Considering the lighter of the two scalars of the theory to be the 125GeV Higgs particle, we parametrize the scalar sector by the mass of the heavy Higgs boson, a mixing angle $α$, and a scalar Higgs self-coupling $λ_{12}$. Taking into account theoretical constraints from perturbativity and vacuum stability, we compute next-to-leading-order electroweak and QCD corrections to the decays $h\to WW/ZZ \to 4$ fermions of the light Higgs boson for some scenarios proposed in the literature. We formulate two renormalization schemes and investigate the conversion of the input parameters between the schemes, finding sizeable effects. Solving the renormalization-group equations for the MSbar parameters $α$ and $λ_{12}$, we observe a significantly reduced scale and scheme dependence in the next-to-leading-order results. For some scenarios suggested in the literature, the total decay width for the process $h \to 4f$ is computed as a function of the mixing angle and compared to the width of a corresponding Standard Model Higgs boson, revealing deviations below 10%. Differential distributions do not show significant distortions by effects beyond the Standard Model. The calculations are implemented in the Monte Carlo generator Prophecy4f, which is ready for applications in data analyses in the framework of the singlet extension.

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arXiv2018

Higgs-boson decay to four fermions in the Two-Higgs-Doublet Model and Prophecy4f

We present the next-to-leading-order calculation of the partial decay widths of the light CP-even Higgs boson decaying into four fermions in the Two-Higgs-Doublet Model. The four different renormalization schemes applied in the calculation are described as well as the calculation and its implementation into the analysis tool Prophecy4f. Some sample results show the size of the next-to-leading-order correction as well as the overall size of the deviation from the Standard Model prediction.

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arXiv2017

Precision calculations for h->WW/ZZ->4 fermions in the Two-Higgs-Doublet Model with PROPHECY4F

We have calculated the next-to-leading-order electroweak and QCD corrections to the decay processes h -> WW/ZZ -> 4 fermions of the light CP-even Higgs boson h of various types of Two-Higgs-Doublet Models (Types I and II, "lepton-specific" and "flipped" models). The input parameters are defined in four different renormalization schemes, where parameters that are not directly accessible by experiments are defined in the MSbar scheme. Numerical results are presented for the corrections to partial decay widths for various benchmark scenarios previously motivated in the literature, where we investigate the dependence on the MSbar renormalization scale and on the choice of the renormalization scheme in detail. We find that it is crucial to be precise with these issues in parameter analyses, since parameter conversions between different schemes can involve sizeable or large corrections, especially in scenarios that are close to experimental exclusion limits or theoretical bounds. It even turns out that some renormalization schemes are not applicable in specific regions of parameter space. Our investigation of differential distributions shows that corrections beyond the Standard Model are mostly constant offsets induced by the mixing between the light and heavy CP-even Higgs bosons, so that differential analyses of h -> 4f decay observables do not help to identify Two-Higgs-Doublet Models. Moreover, the decay widths do not significantly depend on the specific type of those models. The calculations are implemented in the public Monte Carlo generator PROPHECY4F and ready for application.

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arXiv2017

Standard Model Theory

The field of precision calculations for Standard Model processes at the LHC has experienced enormous progress in recent years. This talk highlights some examples from the physics of parton distribution functions, jets, electroweak gauge bosons and Higgs bosons.

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arXiv2017

Next-to-leading-order QCD and electroweak corrections to WWW production at proton-proton colliders

Triple-W-boson production in proton-proton collisions allows for a direct access to the triple and quartic gauge couplings and provides a window to the mechanism of electroweak symmetry breaking. It is an important process to test the Standard Model (SM) and might be background to physics beyond the SM. We present a calculation of the next-to-leading order (NLO) electroweak corrections to the production of WWW final states at proton-proton colliders with on-shell W bosons and combine the electroweak with the NLO QCD corrections. We study the impact of the corrections to the integrated cross sections and to kinematic distributions of the W bosons. The electroweak corrections are generically of the size of 5-10% for integrated cross sections and become more pronounced in specific phase-space regions. The real corrections induced by quark-photon scattering turn out to be as important as electroweak loops and photon bremsstrahlung corrections, but can be reduced by phase-space cuts. Considering that prior determinations of the photon parton distribution function (PDF) involve rather large uncertainties, we compare the results obtained with different photon PDFs and discuss the corresponding uncertainties in the NLO predictions. Moreover, we determine the scale and total PDF uncertainties at the LHC and a possible future 100 TeV pp collider.

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arXiv2017

Renormalization schemes for the Two-Higgs-Doublet Model and applications to h -> WW/ZZ -> 4fermions

We perform the renormalization of different types of Two-Higgs-Doublet Models for the calculation of observables at next-to-leading order. In detail, we suggest four different renormalization schemes based on on-shell renormalization conditions as far as possible and on MSbar prescriptions for the remaining field-mixing parameters where no distinguished on-shell condition exists and make contact to existing schemes in the literature. In particular, we treat the tadpole diagrams in different ways and discuss issues of gauge independence and perturbative stability in the considered schemes. The renormalization group equations for the MSbar parameters are solved in each scheme, so that a consistent renormalization scale variation can be performed. We have implemented all Feynman rules including counterterms and the renormalization conditions into a FeynArts model file, so that amplitudes and squared matrix elements can be generated automatically. As an application we compute the decay of the light, CP-even Higgs boson of the Two-Higgs-Doublet Model into four fermions at next-to-leading order. The comparison of different schemes and the investigation of the renormalization scale dependence allows us to test the perturbative consistency in each of the renormalization schemes, and to get a better estimate of the theoretical uncertainty that arises due to the truncation of the perturbation series.

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arXiv2016

Collier: a fortran-based Complex One-Loop LIbrary in Extended Regularizations

We present the library Collier for the numerical evaluation of one-loop scalar and tensor integrals in perturbative relativistic quantum field theories. The code provides numerical results for arbitrary tensor and scalar integrals for scattering processes in general quantum field theories. For tensor integrals either the coefficients in a covariant decomposition or the tensor components themselves are provided. Collier supports complex masses, which are needed in calculations involving unstable particles. Ultraviolet and infrared singularities are treated in dimensional regularization. For soft and collinear singularities mass regularization is available as an alternative.

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