arXiv subjects

Stefan Dittmaier

Publications and source records attributed to Stefan Dittmaier.

At least 55 records · Page 3Linked to original sources

arXiv2016

Next-to-leading-order electroweak corrections to the production of four charged leptons at the LHC

We present a state-of-the-art calculation of the next-to-leading-order electroweak corrections to ZZ production, including the leptonic decays of the Z bosons into $μ^+μ^-\mathrm{e}^+\mathrm{e}^-$ or $μ^+μ^-μ^+μ^-$ final states. We use complete leading-order and next-to-leading-order matrix elements for four-lepton production, including contributions of virtual photons and all off-shell effects of Z bosons, where the finite Z-boson width is taken into account using the complex-mass scheme. The matrix elements are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential distributions. We present integrated and differential cross sections for the LHC at 13 TeV both for an inclusive setup where only lepton identification cuts are applied, and for a setup motivated by Higgs-boson analyses in the four-lepton decay channel. The electroweak corrections are divided into photonic and purely weak contributions. The former show the well-known pronounced tails near kinematical thresholds and resonances; the latter are generically at the level of $\sim-5\%$ and reach several $-10\%$ in the high-energy tails of distributions. Comparing the results for $μ^+μ^-\mathrm{e}^+\mathrm{e}^-$ and $μ^+μ^-μ^+μ^-$ final states, we find significant differences mainly in distributions that are sensitive to the $μ^+μ^-$ pairing in the $μ^+μ^-μ^+μ^-$ final state. Differences between $μ^+μ^-\mathrm{e}^+\mathrm{e}^-$ and $μ^+μ^-μ^+μ^-$ channels due to interferences of equal-flavour leptons in the final state can reach up to $10\%$ in off-shell-sensitive regions. Contributions induced by incoming photons, i.e. photon-photon and quark-photon channels, are included, but turn out to be phenomenologically unimportant.

hep-ph↗

arXiv2016

Next-to-leading-order electroweak corrections to $pp \to W^+W^-\to$ 4 leptons at the LHC

We present results of the first calculation of next-to-leading-order electroweak corrections to W-boson pair production at the LHC that fully takes into account leptonic W-boson decays and off-shell effects. Employing realistic event selections, we discuss the corrections in situations that are typical for the study of W-boson pairs as a signal process or of Higgs-boson decays $H\to W W^*$, to which W-boson pair production represents an irreducible background. In particular, we compare the full off-shell results, obtained treating the W-boson resonances in the complex-mass scheme, to previous results in the so-called double-pole approximation, which is based on an expansion of the loop amplitudes about the W resonance poles. At small and intermediate scales, i.e. in particular in angular and rapidity distributions, the two approaches show the expected agreement at the level of fractions of a percent, but larger differences appear in the TeV range. For transverse-momentum distributions, the differences can even exceed the 10% level in the TeV range where "background diagrams" with one instead of two resonant W bosons gain in importance because of recoil effects.

hep-ph↗

arXiv2016

NLO QCD and electroweak corrections to $Z+γ$ production with leptonic Z-boson decays

The next-to-leading-order electroweak corrections to $pp\to l^+l^-/\barνν+γ+X$ production, including all off-shell effects of intermediate Z bosons in the complex-mass scheme, are calculated for LHC energies, revealing the typically expected large corrections of tens of percent in the TeV range. Contributions from quark-photon and photon-photon initial states are taken into account as well, but their impact is found to be moderate or small. Moreover, the known next-to-leading-order QCD corrections are reproduced. In order to separate hard photons from jets, both a quark-to-photon fragmentation function á la Glover/Morgan and Frixione's cone isolation are employed. The calculation is available in the form of Monte Carlo programs allowing for the evaluation of arbitrary differential cross sections. Predictions for integrated cross sections are presented for the LHC at 7 TeV, 8 TeV, and 14 TeV, and differential distributions are discussed at 14 TeV for bare muons and dressed leptons. Finally, we consider the impact of anomalous $ZZγ$ and $Zγγ$ couplings.

hep-ph↗

arXiv2016

Dominant $\mathcal{O}(α_sα)$ corrections to Drell-Yan processes in the resonance region

Apart from the well-known NNLO QCD and NLO electroweak corrections to W- and Z-boson production at hadron colliders, the most important fixed-order corrections are given by the mixed QCD-electroweak corrections of $\mathcal{O}(α_sα)$. The knowledge of these corrections is of particular importance to control the theoretical uncertainties in the upcoming high-precision measurements of the W-boson mass and the effective weak mixing angle at the LHC. Since these observables are dominated by the phase-space regions of resonant W/Z bosons, we address the $\mathcal{O}(α_sα)$ corrections in the framework of an expansion about the W/Z poles. Retaining only the leading, resonant contribution in the so-called pole approximation, the corrections can be classified into factorizable and non-factorizable contributions. In this article we review our calculation of the numerically dominant corrections which arise from factorizable corrections of "initial-final" type, i.e. they combine the QCD corrections to the production with the large electroweak corrections to the decay of the W/Z boson. Moreover, we compare our results to simpler approximate combinations of electroweak and QCD corrections based on naive products of NLO QCD and electroweak correction factors and using leading-logarithmic approximations for QED final-state radiation. Finally, we estimate the shift in the W-boson mass that results from the $\mathcal{O}(α_sα)$ corrections to the transverse-mass distribution.

hep-ph↗

arXiv2015

Dominant mixed QCD-electroweak $\mathcal{O}(α_sα)$ corrections to Drell-Yan processes in the resonance region

A precise theoretical description of W- and Z-boson production in the resonance region is essential for the correct interpretation of high-precision measurements of the W-boson mass and the effective weak mixing angle. Currently, the largest unknown fixed-order contribution is given by the mixed QCD-electroweak corrections of $\mathcal{O}(α_sα)$. We argue, using the framework of the pole expansion for the NNLO QCD-electroweak corrections established in a previous paper, that the numerically dominant corrections arise from the combination of large QCD corrections to the production with the large electroweak corrections to the decay of the W/Z boson. We calculate these so-called factorizable corrections of "initial-final" type and estimate the impact on the W-boson mass extraction. We compare our results to simpler approximate combinations of electroweak and QCD corrections in terms of naive products of NLO QCD and electroweak correction factors and using leading-logarithmic approximations for QED final-state radiation as provided by the structure-function approach or QED parton-shower programs. We also compute corrections of "final-final" type, which are given by finite counterterms to the leptonic vectorboson decays and are found to be numerically negligible.

hep-ph↗

arXiv2015

NLO QCD and electroweak corrections to W+γ production with leptonic W-boson decays

We present a calculation of the next-to-leading-order electroweak corrections to W+γ production, including the leptonic decay of the W boson and taking into account all off-shell effects of the W boson, where the finite width of the W boson is implemented using the complex-mass scheme. Corrections induced by incoming photons are fully included and find particular emphasis in the discussion of phenomenological predictions for the LHC. The corresponding next-to-leading-order QCD corrections are reproduced as well. In order to separate hard photons from jets, a quark-to-photon fragmentation function a la Glover and Morgan is employed. Our results are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential cross sections. We present integrated cross sections for the LHC at 7TeV, 8TeV, and 14TeV as well as differential distributions at 14TeV for bare muons and dressed leptons. Finally, we discuss the impact of anomalous WWγ couplings.

hep-ph↗

arXiv2015

Non-factorizable photonic corrections to resonant production and decay of many unstable particles

Electroweak radiative corrections to the production of high-multiplicity final states with several intermediate resonances in most cases can be sufficiently well described by the leading contribution of an expansion about the resonance poles. In this approach, also known as pole approximation, corrections are classified into separately gauge-invariant factorizable and non-factorizable corrections, where the former can be attributed to the production and decay of the unstable particles on their mass shell. The remaining non-factorizable corrections are induced by the exchange of soft photons between different production and decay subprocesses. We give explicit analytical results for the non-factorizable photonic virtual corrections to the production of an arbitrary number of unstable particles at the one-loop level and, thus, deliver an essential building block in the calculation of next-to-leading-order electroweak corrections in pole approximation. The remaining virtual factorizable corrections can be obtained with modern automated one-loop matrix-element generators, while the evaluation of the corresponding real photonic corrections can be evaluated with full matrix elements by multi-purpose Monte Carlo generators. Our results can be easily modified to non-factorizable QCD corrections, which are induced by soft-gluon exchange.

hep-ph↗

arXiv2015

Standard Model Theory

Recent progress in the field of precision calculations for Standard Model processes at the LHC is reviewed, highlighting examples of weak gauge-boson and Higgs-boson production, as discussed at the 27th Rencontres de Blois, 2015.

hep-ph↗

arXiv2015

Techniques for the treatment of IR divergences in decay processes at NLO and application to the top-quark decay

We present the extension of two general algorithms for the treatment of infrared singularities arising in electroweak corrections to decay processes at next-to-leading order: the dipole subtraction formalism and the one-cutoff slicing method. The former is extended to the case of decay kinematics which has not been considered in the literature so far. The latter is generalized to production and decay processes with more than two charged particles, where new "surface" terms arise. Arbitrary patterns of massive and massless external particles are considered, including the treatment of infrared singularities in dimensional or mass regularization. As an application of the two techniques we present the calculation of the next-to-leading-order QCD and electroweak corrections to the top-quark decay width including all off-shell and decay effects of intermediate W bosons. The result, e.g., represents a building block of a future calculation of NLO electroweak effects to off-shell top-quark pair (WWbb) production. Moreover, this calculation can serve as the first step towards an event generator for top-quark decays at next-to-leading order accuracy, which can be used to attach top-quark decays to complicated many-particle top-quark processes, such as for tt+H or tt+jets.

hep-ph↗

arXiv2014

HAWK 2.0: A Monte Carlo program for Higgs production in vector-boson fusion and Higgs strahlung at hadron colliders

The Monte Carlo integrator HAWK provides precision predictions for Higgs production at hadron colliders in vector-boson fusion and Higgs strahlung, i.e. in production processes where the Higgs boson is Attached to WeaK bosons. The fully differential predictions include the full QCD and electroweak next-to-leading-order corrections. Results are computed as integrated cross sections and as binned distributions for important hadron-collider observables.

hep-ph↗

arXiv2014

O(α_sα) corrections to Drell-Yan processes in the resonance region

Drell-Yan-like W-boson and Z-boson production in the resonance region allows for some high-precision measurements that are crucial to carry experimental tests of the Standard Model to the extremes, such as the determinations of the W-boson mass and the effective weak mixing angle. We describe how the Standard Model prediction can be successfully performed in terms of a consistent expansion about the resonance pole, which classifies the corrections in terms of factorizable and non-factorizable contributions. The former can be attributed to the W/Z production and decay subprocesses individually, while the latter link production and decay by soft-photon exchange. At next-to-leading order we compare the full electroweak corrections with the pole-expanded approximations, confirming the validity of the approximation. At O(α_sα), we describe the concept of the expansion and report on results on the non-factorizable contributions, which turn out to be phenomenologically negligible. Moreover, we present first (preliminary) results on the dominant factorizable O(α_sα) corrections, which originate from the interplay of initial-state QCD and final-state electroweak corrections. Numerically those corrections significantly differ from a mere product of the two next-to-leading-order correction factors.

hep-ph↗

arXiv2014

COLLIER -- A fortran-library for one-loop integrals

We introduce the fortran-library COLLIER for the numerical evaluation of one-loop scalar and tensor integrals in perturbative relativistic quantum field theories. Important features are the implementation of dedicated methods to achieve numerical stability for 3- and 4-point tensor integrals, the support of complex masses for internal particles, and the possibility to choose between dimensional and mass regularization for infrared singularities. COLLIER supports one-loop N-point functions up to currently N=6 and has been tested in various NLO QCD and EW calculations.

hep-ph↗

arXiv2014

Neutral MSSM Higgs-boson production with heavy quarks: NLO supersymmetric QCD corrections

Within the minimal supersymmetric extension of the Standard Model (MSSM) the associated production of neutral Higgs bosons with top and bottom quarks belongs to the most important Higgs-boson production processes at the LHC. At large values of tan(beta), in particular, bottom--Higgs associated production constitutes the dominant production channel within the MSSM. We have calculated the next-to-leading-order supersymmetric QCD corrections to neutral Higgs production through the parton processes q qbar, gg -> t tbar / b bbar + h/H/A and present results for the total cross sections. The genuine SUSY-QCD corrections are of moderate size for small tan(beta), but can be sizable for large tan(beta). In the latter case the bulk of these corrections can be absorbed into effective bottom Yukawa couplings.

hep-ph↗

arXiv2014

Mixed QCD-electroweak O(α_sα) corrections to Drell-Yan processes in the resonance region: pole approximation and non-factorizable corrections

Drell-Yan-like W-boson and Z-boson production in the resonance region allows for high-precision measurements that are crucial to carry experimental tests of the Standard Model to the extremes, such as the determination of the W-boson mass and the effective weak mixing angle. In this article, we establish a framework for the calculation of the mixed QCD-electroweak O(α_sα) corrections to Drell-Yan processes in the resonance region, which are one of the main remaining theoretical uncertainties. We describe how the Standard Model prediction can be successfully performed in terms of a consistent expansion about the resonance poles, which classifies the corrections in terms of factorizable and non-factorizable contributions. The former can be attributed to the W/Z production and decay subprocesses individually, while the latter link production and decay by soft-photon exchange. At next-to-leading order we compare the full electroweak corrections with the pole-expanded approximations, confirming the validity of the approximation. At O(α_sα), we describe the concept of the expansion and explicitly give results on the non-factorizable contributions, which turn out to be phenomenologically negligible. Our results, thus, demonstrate that for phenomenological purposes the O(α_sα) corrections can be factorized into terms associated with initial-state and/or final-state corrections. Moreover, we argue that the factorization properties of the non-factorizable corrections at O(α_sα) from lower-order O(α_s) graphs generalize to any order in O(α_s^nα).

hep-ph↗

arXiv2013

Electroweak Corrections at High Energies

We present a survey of the most abundant processes at the LHC for sensitivity to electroweak corrections at \sqrt{s} = 8, 14, 33, and 100 TeV proton-proton collision energies. The processes studied are pp -> dijet, inclusive W and Z, W/Z+jets, and WW. In each case we compare the experimental uncertainty in the highest kinematic regions of interest with the relative magnitude of electroweak corrections and fixed-order α_S calculations.

hep-ph↗

arXiv2013

Weak radiative corrections to dijet production at the LHC

We summarize the calculation of the weak corrections to dijet production at hadron colliders, comprising tree-level effects of O(α_sα,α^2) and loop corrections of O(α_s^2α). Although suppressed by the small value of the coupling constant α, the weak radiative corrections can become large in the high-energy domain due to the appearance of Sudakov-type and other high-energy logarithms. Generally the corrections to the transverse-momentum distributions are larger by approximately a factor of two compared to the corresponding reach in the invariant-mass distributions, because the invariant-mass distributions are not, unlike the k_T distributions, dominated by the Sudakov regime at high scales. The electroweak tree-level contributions are found to be of the same generic size as the loop corrections.

hep-ph↗

arXiv2013

Electroweak corrections to monojet production at the Tevatron and the LHC

Single-jet production with missing transverse momentum is one of the most promising discovery channels for new physics at the LHC. In the Standard Model, Z + jet production with a Z-boson decay into neutrinos leads to this monojet signature. To improve the corresponding Standard Model predictions, we present the calculation of the full next-to-leading-order (NLO) electroweak corrections and a recalculation of the NLO QCD corrections to monojet production at the Tevatron and the LHC. We discuss the phenomenological impact on the total cross sections as well as on relevant differential distributions.

hep-ph↗

arXiv2012

Gluon-induced Higgs-strahlung at next-to-leading order QCD

Gluon-induced contributions to the associated production of a Higgs and a Z-boson are calculated with NLO accuracy in QCD. They constitute a significant contribution to the cross section for this process. The perturbative correction factor (K-factor) is calculated in the limit of infinite top-quark and vanishing bottom-quark masses. The qualitative similarity of the results to the well-known ones for the gluon-fusion process $gg\to H$ allows to conclude that rescaling the LO prediction by this K-factor leads to a reliable NLO result and realistic error estimate due to missing higher-order perturbative effects. We consider the total inclusive cross section as well as a scenario with a boosted Higgs boson, where the Higgs boson's transverse momentum is restricted to values ptH>200GeV. In both cases, we find large correction factors $K\approx 2$ in most of the parameter space.

hep-ph↗