arXiv subjects

Nikolas Kauer

Publications and source records attributed to Nikolas Kauer.

16 recordsLinked to original sources

arXiv2023

Higgs interference effects in top-quark pair production in the 1HSM

We present a next-to-leading-order (NLO) study of the process $pp \; ( \to \{ h_1, h_2 \}) \to t\bar{t} + X$ in the 1-Higgs-singlet extension of the Standard Model with an additional heavy Higgs boson $h_2$ that mixes with the light Higgs boson $h_1$. This process is subject to large interference effects between loop-induced Higgs-mediated amplitudes and the QCD continuum background which tend to overcompensate any resonance contributions. A reliable modelling of the resulting top-pair invariant mass shapes requires the inclusion of higher-order QCD corrections, which are presented here. The computation of these NLO corrections is exact in all contributions but in the class of non-factorisable two-loop diagrams which are included in an approximate way such that all infrared singular limits are preserved. We present numerical results for several benchmark points with heavy Higgs masses in the range $700$--$3000$ GeV considering the production of stable top quarks. We find that the interference effects dominate the BSM signal yielding sharp dip structures instead of resonance peaks. The significance and excludability of the BSM effect is explored for the LHC Run 2, Run 3 and HL-LHC.

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arXiv2022

Off-shell Higgs Interpretations Task Force: Models and Effective Field Theories Subgroup Report

This report presents the results of the Models and Effective Field Theories Subgroup of the Off-Shell Interpretations Task Force in the LHC Higgs Working Group. The main goal of the subgroup was to discuss and advance the potential impact of off-shell Higgs measurements on searches for BSM physics carried out in the EFT framework or as benchmark model studies. In the first contribution, the off-shell potential to resolve flat directions in parameter space for on-shell measurements is studied. Furthermore, the sensitivity of off-shell measurements to SMEFT dimension-6 operators for the gg $\to$ ZZ process is discussed, and studies of explicit models that are testable in off-shell production are reviewed. In the second contribution, the SMEFT effects in the off-shell gluon fusion and electroweak processes are discussed. Subsequently, the computation of integrated and differential effects using SMEFT@NLO and MG5_aMC@NLO, or JHUGen and MCFM, is demonstrated. On that basis, a study of the prospects of obtaining additional SMEFT constraints - beyond those from existing global fits - by utilising the off-shell process is presented. For clarification, a revised introduction, definition and discussion of the Higgs basis parametrisation of the SMEFT is given in the third contribution. In short notes on the SMEFT, the Higgs basis with an additional constraint is discussed and relations between the Higgs and Warsaw bases are presented. Lastly, an overview of EFT calculations and tools is given.

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arXiv2019

BSM WW production with a jet veto

We consider the impact on WW production of the unique dimension-six operator coupling gluons to the Higgs field. In order to study this process, we have to appropriately model the effect of a veto on additional jets. This requires the resummation of large logarithms of the ratio of the maximum jet transverse momentum and the invariant mass of the W boson pair. We have performed such resummation at the appropriate accuracy for the Standard Model (SM) background and for a signal beyond the SM (BSM), and devised a simple method to interface jet-veto resummations with fixed-order event generators. This resulted in the fast numerical code MCFM-RE, the Resummation Edition of the fixed-order code MCFM. We compared our resummed predictions with parton-shower event generators and assessed the size of effects, such as limited detector acceptances, hadronisation and the underlying event, that were not included in our resummation. We have then used the code to compare the sensitivity of WW and ZZ production at the HL-LHC to the considered higher-dimension operator. We have found that WW can provide complementary sensitivity with respect to ZZ, provided one is able to control theory uncertainties at the percent-level. Our method is general and can be applied to the production of any colour singlet, both within and beyond the SM.

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arXiv2019

Higgs interference effects at the one-loop level in the 1-Higgs-Singlet extension of the Standard Model

A detailed study of Higgs interference effects at the one-loop level in the 1-Higgs-Singlet extension of the Standard Model (1HSM) is presented for the WW and tt decay modes with fully leptonic WW decay. We explore interference effects for benchmark points with a heavy Higgs mass that significantly exceeds 2*m_t. In the WW channel, the Higgs signal and the interfering continuum background are loop induced. In the tt channel, which features a tree-level background, we also calculate the interference with the one-loop background, which, due to the appearance of the absorptive part, is found to dominate the normalisation and shape of differential Higgs distributions and should therefore be considered in experimental analyses. The commonly used geometric average K-factor approximation K_interference ~ (K_Higgs*K_background)^(1/2) is not appropriate. We calculate with massive top and bottom quarks. Our 1HSM and SM implementation in Sherpa+OpenLoops is publicly available and can be used as parton-level integrator or event generator.

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arXiv2015

Interference effects for H --> WW --> lvqq and H --> ZZ --> llqq searches in gluon fusion at the LHC

Signal-background interference effects are studied for H --> WW and H --> ZZ searches in gluon fusion at the LHC. More specifically, the interference in the channels with semileptonic weak boson pair decay is analysed for light and heavy Higgs masses with minimal and realistic experimental selection cuts. In the semileptonic decay modes, the interference is affected by tree-level background contributions enhanced by 1/e^2 relative to the gluon-fusion continuum background in the fully leptonic decay modes. We find that for both light and heavy Higgs masses the interference with the loop-induced weak-boson pair background dominates over the interference with the tree-level weak-boson plus jets background for a range of selection cuts. We therefore conclude that higher-order background contributions can induce leading interference effects. With appropriate background suppression cuts the interference can be reduced to the 10% level for heavy Higgs masses, and to the per mille level for the light SM Higgs.

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arXiv2015

Heavy Higgs signal-background interference in gg --> VV in the Standard Model plus real singlet

For the Standard Model extended with a real scalar singlet field, the modification of the heavy Higgs signal due to interference with the continuum background and the off-shell light Higgs contribution is studied for gg --> ZZ, WW --> 4 lepton processes at the Large Hadron Collider. Interference effects can range from O(10%) to O(1) effects for integrated cross sections. Despite a strong cancellation between the heavy Higgs-continuum and the heavy Higgs-light Higgs interference, the full interference is clearly non-negligible and modifies the heavy Higgs line shape. A |M_VV - M_h2| < Gamma_h2 cut mitigates interference effects to O(10%) or less. A public program that allows to simulate the full interference is presented.

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arXiv2015

Off-shell Higgs signal and total width determination at the LHC

A substantial off-shell Higgs boson signal in the gluon fusion and vector boson fusion H --> ZZ and H --> WW channels at the Large Hadron Collider (LHC) facilitates a novel, complementary approach to constraining the total Higgs width Gamma_H. With LHC Run 1 data, experimental analyses by CMS and ATLAS find Gamma_H < 5.4 Gamma_H,SM and Gamma_H < [4.5,7.5] Gamma_H,SM at 95% confidence level, respectively, where Gamma_H,SM is the expected value in the Standard Model at the measured Higgs boson mass. I review the theoretical basis of the new approach and discuss its significance in comparison to other methods to bound and measure the Higgs width at the LHC and future colliders.

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arXiv2013

Interference effects for H --> WW/ZZ --> l anti-nu_l anti-l nu_l searches in gluon fusion at the LHC

WW/ZZ interference for Higgs signal and continuum background as well as signal-background interference is studied for same-flavour l anti-nu_l anti-l nu_l final states produced in gluon-gluon scattering at the LHC for light and heavy Higgs masses with minimal and realistic experimental selection cuts. For the signal cross section, we find WW/ZZ interference effects of O(5%) at M_H = 126 GeV. For M_H >= 200 GeV, we find that WW/ZZ interference is negligible. For the gg continuum background, we also find that WW/ZZ interference is negligible. As general rule, we conclude that non-negligible WW/ZZ interference effects occur only if at least one weak boson of the pair is dominantly off-shell due to kinematic constraints. The subdominant weak boson pair contribution induces a correction to the signal-background interference, which is at the few percentage point level before search selection cuts. Optimised selection cuts for M_H >~ 600 GeV are suggested.

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arXiv2013

Inadequacy of zero-width approximation for a light Higgs boson signal

The zero-width approximation (ZWA) restricts the intermediate unstable particle state to the mass shell and, when combined with the decorrelation approximation, fully factorizes the production and decay of unstable particles. The ZWA uncertainty is expected to be of O(Gamma/M), where M and Gamma are the mass and width of the unstable particle. We review the ZWA and demonstrate that errors can be much larger than expected if a significant modification of the Breit-Wigner lineshape occurs. A thorough examination of the recently discovered candidate Standard Model Higgs boson is in progress. For M_H ~ 125 GeV, one has Gamma_H/M_H < 10^(-4), which suggests an excellent accuracy of the ZWA. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalization at the 1% precision level. For gg -> H -> VV, V = W,Z, O(5-10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to suppress this region in search channels where the Higgs mass cannot be reconstructed. We note that H -> VV decay modes in non-gluon-fusion channels are similarly affected.

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arXiv2012

Inadequacy of zero-width approximation for a light Higgs boson signal

In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130 GeV) is not excluded by experimental data. In this mass range, the width of the Standard Model Higgs boson is more than four orders of magnitude smaller than its mass. The zero-width approximation is hence expected to be an excellent approximation. We show that this is not always the case. The inclusion of off-shell contributions is essential to obtain an accurate Higgs signal normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%) corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable Higgs-continuum interference occurs. We discuss how experimental selection cuts can be used to exclude this region in search channels where the Higgs invariant mass cannot be reconstructed. We note that the H -> VV decay modes in weak boson fusion are similarly affected.

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arXiv2012

NLO automated tools for QCD and beyond

Theoretical predictions for scattering processes with multi-particle final states at next-to-leading order (NLO) in perturbative QCD are essential to fully exploit the physics potential of present and future high-energy colliders. The status of NLO QCD calculations and tools is reviewed.

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arXiv2012

Signal-background interference in gg --> H --> VV

The resonance-continuum interference between the SM Higgs search signal process gg --> H --> VV (V=W,Z) and the irreducible background process gg --> VV is studied at leading order for integrated cross sections and differential distributions in pp collisions at \sqrt{s}=7 TeV and 14 TeV for M_H=400 GeV. Leptonic weak boson decays are included, and realistic experimental selection cuts are applied.

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arXiv2010

Modern Feynman Diagrammatic One-Loop Calculations

In this talk we present techniques for calculating one-loop amplitudes for multi-leg processes using Feynman diagrammatic methods in a semi-algebraic context. Our approach combines the advantages of the different methods allowing for a fast evaluation of the amplitude while monitoring the numerical stability of the calculation. In phase space regions close to singular kinematics we use a method avoiding spurious Gram determinants in the calculation. As an application of our approach we report on the status of the calculation of the amplitude for the process $pp\to b\bar{b}b\bar{b}+X$.

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arXiv2010

Recent Progress in the Golem Project

We report on the current status of the Golem project which aims at the construction of a general one-loop evaluator for matrix elements. We construct the one-loop matrix elements from Feynman diagrams in a highly automated way and provide a library for the reduction and numerically stable evaluation of the tensor integrals involved in this approach. Furthermore, we present applications to physics processes relevant for the LHC.

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arXiv2010

NLO Cross Sections for the LHC using GOLEM: Status and Prospects

In this talk we review the GOLEM approach to one-loop calculations and present an automated implementation of this technique. This method is based on Feynman diagrams and an advanced reduction of one-loop tensor integrals which avoids numerical instabilities. We have extended our one-loop integral library golem95 with an automated one-loop matrix element generator to compute the virtual corrections of the process $q\bar{q}\to b\bar{b}b\bar{b}$. The implementation of the virtual matrix element has been interfaced with tree-level Monte Carlo programs to provide the full result for the above process.

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arXiv2010

ZZ+jet and Graviton+jet at NLO QCD: recent applications using GOLEM methods

In this talk we discuss recent progress concerning precise predictions for hadron colliders. We show results of two applications of tensor reduction using GOLEM methods: the next-to-leading order (NLO) corrections to $pp \to ZZ+$jet as an important background for Higgs particle and new physics searches at hadron colliders, and the NLO corrections to graviton plus jet hadro-production, which is an important channel for graviton searches at the Tevatron and the LHC.

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