Off-shell Effects Research Articles

Theoretical study of the π+p→pa0+ϕ reaction

We study the production of hyperon resonances in the π+p→pa0+ϕ reaction within an effective Lagrangian approach. The model includes the production of Δ(1940) and Δ(1920) in the intermediate state excited by the ρ and p exchanges between the initial proton and π meson. Because of the large contribution of ρ exchange in Δ(1940) production, Δ(1940) is found to be a significant contribution near the threshold in this reaction. We provide total and differential cross section predictions for the reaction and discuss the possible influence of model parameters, off-shell effects, and branching ratios for Δ(1940)→pa0 and Δ(1920)→pa0 decays, which will be useful in future experimental studies. This reaction can provide a platform for studying the features of Δ(1940) resonance, especially the coupling to the pa0 channel. Published by the American Physical Society 2024

QCD effects in electroweak WZjj\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$WZjj$$\\end{document} production at current and future hadron colliders

We present an update of an existing implementation of WZjj production via vector-boson scattering in the framework of the POWHEG BOX program. In particular, previously unavailable semi-leptonic and fully hadronic decay modes of the intermediate vector bosons are provided, and operators of dimension six in an effective-field theory approach to account for physics beyond the Standard Model in the electroweak sector are included. For selected applications phenomenological results are provided to illustrate the capabilities of the new program. The impact of the considered dimension-six operators on experimentally accessible distributions is found to be small for current LHC energies, but enhanced in the kinematic reach of a potential future hadron collider with an energy of 100 TeV. The relevance of fully accounting for spin correlations and off-shell effects in the decay system is explored by a comparison with results obtained with the MadSpin tool that are based on an approximate treatment of the leptonic final state resulting from vector boson scattering processes. For selected semi-leptonic and hadronic decay modes we demonstrate the sensitivity of realistic signal selection procedures on QCD corrections and parton-shower effects.

Non-resonant anomaly detection with background extrapolation

Complete anomaly detection strategies that are both signal sensitive and compatible with background estimation have largely focused on resonant signals. Non-resonant new physics scenarios are relatively under-explored and may arise from off-shell effects or final states with significant missing energy. In this paper, we extend a class of weakly supervised anomaly detection strategies developed for resonant physics to the non-resonant case. Machine learning models are trained to reweight, generate, or morph the background, extrapolated from a control region. A classifier is then trained in a signal region to distinguish the estimated background from the data. The new methods are demonstrated using a semi-visible jet signature as a benchmark signal model, and are shown to automatically identify the anomalous events without specifying the signal ahead of time.

Complete NLO corrections to off-shell tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \ extrm{t}\\overline{\ extrm{t}}\ extrm{Z} $$\\end{document} production at the LHC

Measuring precisely top-pair-associated processes at hadron colliders will become possible with the upcoming LHC running stages. The increased data statistics will especially enable differential measurements leading to an improved characterisation of such processes. Aiming at a consistent data-theory comparison, precise Standard-Model predictions are needed, including higher-order corrections and full off-shell effects. In this work we present NLO-accurate predictions for the production and decay of a top-antitop pair in association with a Z boson at the LHC, in the multi-lepton decay channel. The complete set of LO contributions and NLO corrections of EW and QCD origin is included. The calculation is based on full matrix elements, computed with all resonant and non-resonant contributions, complete spin correlations and interference effects. Integrated and differential cross-sections are presented for a realistic fiducial setup.

LEX-EFT: the Light Exotics Effective Field Theory

We propose the creation of a Light Exotics Effective Field Theory (LEX-EFT) catalog. LEX-EFT is a generic framework to capture all interactions between the Standard Model (SM) and all (or at least a large class of) theoretically allowed exotic states beyond the Standard Model (bSM), indexed by their SM and bSM charges. These states are light enough to be on or nearly on shell in some collider processes. This framework, which subsumes beyond the Standard Model paradigms as generally as possible, is meant to extend recent successful implementations of bSM EFTs and complement e.g. the Standard Model Effective Field Theory (SMEFT), which can capture the off-shell effects of exotic fields. In this work, we review a general method for the construction of a complete list of gauge-invariant operators involving SM interactions with light exotics via iterative tensor product decomposition, up to the desired order in mass dimension. Each operator is characterized by specific Clebsch-Gordan coefficients determined by the charge flow; we show how this charge flow affects the range of EFT validity and cross sections associated with an effective operator. We create an example catalog of exotic scalars coupling to SM gauge boson pairs, and we highlight some operators with exotic weak SU(2)L charges that can produce spectacular LHC phenomenology. We further demonstrate the utility of the LEX-EFT approach with several examples of effects on kinematic distributions and cross sections that would not be captured by EFTs agnostic to the exotic degrees of freedom and may evade the main inclusive collider searches tailored to the existing preferred set of standard bSM theories.

NLO + parton-shower generator for varvec{Wc} production in the POWHEG BOX RES

We present the implementation of a next-to-leading-order plus parton-shower event generator for the hadronic production of a heavy charm quark accompanied by a leptonically-decaying W boson in the POWHEG BOX RES framework. We consider both signatures, i.e. pprightarrow c, ell ^- {bar{nu }}_{ell } and pprightarrow {bar{c}} ,ell ^+ {nu }_{ell }, and we include exactly off-shell and spin-correlation effects, as well as off-diagonal Cabibbo–Kobayashi–Maskawa contributions. We present particle-level results, obtained interfacing our code with the Herwig7.2 and Pythia8.3 shower Monte Carlo event generators, including hadronization and underlying-event effects, and compare them against the data collected by the CMS Collaboration at sqrt{s}=13 TeV.

On cancellation of non-adiabatic and off-shell effects in the antiproton annihilation in deuteron

On cancellation of non-adiabatic and off-shell effects in the antiproton annihilation in deuteron

Off-shell generalized parton distributions and form factors of the pion

Off-shell effects in generalized parton distributions (GPDs) of the pion, appearing, e.g., in the Sullivan process, are considered. Due to the lack of crossing symmetry, the moments of GPDs involve also odd powers of the skewness (longitudinal momentum transfer) parameter, which results in emergence of new off-shell form factors. With current-algebra techniques, we derive exact relations between the four off-shell gravitational form factors of the pion, in analogy to the electromagnetic case. Our results place stringent constraints on the off-shell GPDs of the pion. We provide an explicit realization in terms of a chiral quark model, where we show that the off-shell effects in GPDs are potentially significant in modeling physical processes and should not be neglected.

The π−p→a0−ηp reaction in an effective Lagrangian model

In this work, we perform a study of the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{a}_{0}^{\ensuremath{-}}\ensuremath{\eta}p$ reaction within an effective Lagrangian approach and isobar model. In our model, we consider the excitation of the ${N}^{*}$ and ${\mathrm{\ensuremath{\Delta}}}^{*}$ resonances in the intermediate states and background terms. Based on our model, it is found that this reaction is dominated by the production of the $N(1535)$ in the near threshold region. And the contributions from other well-established ${N}^{*}$ or ${\mathrm{\ensuremath{\Delta}}}^{*}$ resonances only play minor roles. Besides, we discuss the possibility of verifying the existence of a narrow nucleon resonance $N(1685)$/$N(1700)$ proposed by some recent works in the present reaction. With adopting different quantum numbers, we find that it may contribute significantly in this reaction. Thus this reaction may provide a good platform to study the properties of nucleon resonances, e.g., $N(1535)$ and $N(1685)$/$N(1700)$, which couple strongly with the $\ensuremath{\eta}N$ channel. To compare with future experiments, the predictions of the total cross sections, the angular distributions, and invariant mass spectrums of final particles are also presented. Furthermore, the off-shell effects due to high-spin states are also discussed.

Tt¯bb¯ at the LHC: On the size of off-shell effects and prompt b -jet identification

We investigate full off-shell effects in $t\overline{t}b\overline{b}$ production in the dilepton channel at the LHC with the center-of-mass energy $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$. Specifically, we compute next-to-leading-order (NLO) QCD corrections to the $pp\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}b\overline{b}b\overline{b}+X$ process and provide a prescription for $b$-jet identification to distinguish prompt $b$ jets from $b$ jets originating from the decay of the top quarks. As an important irreducible background to $pp\ensuremath{\rightarrow}t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$, $t\overline{t}$ production in association with two prompt $b$ jets is a primary source of uncertainty in the measurement of $t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$. In quantifying full off-shell effects, we perform comparisons between the state-of-the-art full off-shell computation and the calculation in the narrow width approximation. The former includes all double-, single- and nonresonant Feynman diagrams, interferences as well as finite-width effects of the top quarks and $W$ gauge bosons. The latter restricts the unstable top quarks and $W$ gauge bosons to on-shell states and includes for the first time NLO QCD corrections to both production and decays. We observe that full off-shell effects are subdominant compared to the scale uncertainties for the integrated fiducial cross section and for the majority of differential observables in the phase-space regions that we investigated. However, for a number of observables related to beyond the Standard Model searches, full off-shell effects are significant. Furthermore, with our $b$-jet labeling prescription, the prompt $b$ jets and the $b$ jets from top-quark decays can be successfully disentangled.

W±Z production at NNLO QCD and NLO EW matched to parton showers with MiNNLOPS

We consider W±Z production in hadronic collisions and present high-precision predictions in QCD and electroweak (EW) perturbation theory matched to parton showers. To this end, we match next-to-next-to-leading order QCD corrections to parton showers using the MiNNLOPS method and consistently combine them with next-to-leading order EW corrections matched to parton showers. This is the first time such accuracy in the event generation is achieved for any collider process, and we study in detail the impact of different choices in the combination of QCD and EW corrections as well as QCD and QED showers. Spin correlations, interferences and off-shell effects are retained by considering the full leptonic processes ppto {ell}^{+}{ell}^{-}{ell}^{prime pm }{nu}_{ell}^{prime } with ℓ′ ≠ ℓ and ℓ′ = ℓ without approximations, and the matching to QED radiation is performed preserving the resonance structure of the process. We find that NNLO QCD predictions including QCD and QED shower effects provide a very good approximation in the bulk-region of the phase space, while EW effects become increasingly important in the high-energy tails of kinematic distributions. Our default predictions are in excellent agreement with recent ATLAS data.

NLO QCD and EW corrections to off-shell tZj production at the LHC

The production of a single top quark in association with a Z boson (tZj production) at the LHC is a relevant probe of the electroweak sector of the Standard Model as well as a window to possible new-physics effects. The growing experimental interest in performing differential measurements for this process demands an improved theoretical modelling in realistic fiducial regions. In this article we present an NLO-accurate tZj calculation that includes complete off-shell effects and spin correlations, combining QCD and electroweak radiative corrections to the LO signal. Integrated and differential cross sections are shown for a fiducial setup characterised by three charged leptons, two jets, and missing energy.

Mathcal{CP} structure of the top-quark Yukawa interaction: NLO QCD corrections and off-shell effects

Since its discovery at the Large Hadron Collider in 2012 the Higgs boson has arguably become the most famous of the Standard Model particles and many measurements have been performed in order to assess its properties. Among others, these include measurements of the Higgs boson’s mathcal{CP} state which is predicted to be mathcal{CP} -even. Even though a pure mathcal{CP} -odd state has been ruled out, a possible admixture of a mathcal{CP} -odd Higgs state has yet to be excluded. In this work we present predictions for the associated production of a leptonically decaying top quark pair and a stable Higgs boson pp → e+νeμ− overline{nu} μb overline{b} H with possible mixing between mathcal{CP} -even and mathcal{CP} -odd states at NLO in QCD for the LHC with sqrt{s} = 13 TeV. Finite top-quark and gauge-boson width effects as well as all double-, single- and non-resonant Feynman diagrams including their interference effects are taken into account. We compare the behaviour of the mathcal{CP} -even, -odd and -mixed scenarios for the integrated fiducial cross-sections as well as several key differential distributions. In addition, we show that both NLO corrections and off-shell effects play an important role even at the level of integrated fiducial cross-sections and that these are further enhanced in differential distributions. Even though we focus here on the Standard Model Higgs boson, the calculations could be straightforwardly applied to models that have an extended Higgs-boson sector and predict the existence of mathcal{CP} -odd Higgs-like particles, such as the two-Higgs-doublet model.

NLO QCD corrections to full off-shell production of toverline{t}Z including leptonic decays

Motivated by ongoing new physics searches in the top-quark sector at the Large Hadron Collider we report on the calculation of NLO QCD corrections to the Standard Model pp → toverline{t}Z + X process in the tetra-lepton decay channel. This calculation is based on the matrix elements for the {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}{tau}^{+}{tau}^{-} final state and includes all resonant and non-resonant Feynman diagrams, interferences and off-shell effects of the top quark as well as the W and Z gauge bosons. Also incorporated are photon-induced contributions. As it is customary for such studies, we show theoretical predictions for both fixed and dynamical factorisation and renormalisation scale choices and different PDF sets. Furthermore, we study the main theoretical uncertainties that are associated with neglected higher-order terms in the perturbative expansion and with the parameterisation of the PDF sets. In order to investigate the size of off-shell effects and higher-order corrections in top-quark decays, we perform a second computation for this process, which is based on the narrow-width-approximation where the top quarks, W and Z gauge bosons are kept on-shell. Results are given for the integrated and differential fiducial cross sections for the LHC Run II center-of-mass energy of 13 TeV.

Hadronic production of top-quark pairs in association with a pair of leptons in the powheg box framework

We present an implementation of $t\bar t \ell^+\ell^-$ ($\ell=e,\mu$) hadronic production at next-to-leading order in QCD matched to parton-shower event generators in the POWHEG BOX framework. The program we developed includes all leading-order contributions of order $\alpha_s^2\alpha^2$ for the specified final state, as well as the corresponding first-order QCD corrections. Decays of the top quarks have been simulated retaining spin-correlations in all tree-level matrix elements. We consider the case of the Large Hadron Collider at $\sqrt{s}=13$ TeV and compare results for $t\bar t \ell^+\ell^-$ production in the fiducial volume where the invariant mass of the lepton pairs is centered around the $Z$-boson mass to corresponding predictions for $t\bar{t}Z$ on-shell production with $Z\rightarrow \ell^+\ell^-$. We find that off-shell effects in $t\bar t \ell^+\ell^-$ are in general small at the level of the total cross section, but can decrease the tail of the leptons' transverse momentum distributions by 10-20% and, in these regions, they are visible beyond the scale uncertainty due to renormalization and factorization scale variation. Moreover, we find that accounting for top-quark decays in the narrow-width approximation with tree-level spin correlations also gives origin to 10-20% effects in specific regions of the kinematic distributions of the $t\bar t \ell^+\ell^-$ decayed final state.

Next-to-next-to-leading order event generation for VH production with H → boverline{b} decay

We consider the Higgsstrahlung process in hadronic collisions and present the computation of next-to-next-to-leading order predictions matched to parton showers for both production and H → boverline{b} decay employing the MiNNLOPS method. We present predictions for ZH and W±H production including spin correlations and off-shell effects by calculating the full processes pp → ℓ+ℓ−H → ℓ+ℓ− boverline{b} , pp → {nu}_{ell }{overline{nu}}_{ell }H → {nu}_{ell }{overline{nu}}_{ell }boverline{b} and pp → ℓ±νℓH → {ell}^{pm }{overline{nu}}_{ell }boverline{b} in the narrow-width approximation for the Higgs boson. For the W±H process, NNLO+PS accuracy in production and decay is achieved for the first time. Our calculations are validated against earlier simulations in the NNLOPS approach that includes NNLO corrections via multi-differential reweighting. The new MiNNLOPS generators for these processes, which evaluate NNLO corrections on-the-fly in the event generation, will supersede those earlier calculations. Our predictions are in good agreement with recent measurements of the Higgsstrahlung cross sections.

NLO electroweak and QCD corrections to the production of a photon with three charged lepton plus missing energy at the LHC

Electroweak (EW) triboson production processes with at least one heavy gauge boson are of increasing interest at the Large Hadron Collider (LHC) as direct precision probes of one of the least-tested sectors of the Standard Model (SM), the quartic couplings of the EW gauge bosons. These processes therefore offer promising opportunities for searches for indirect signals of Beyond-the-SM (BSM) physics. In this paper, we present results for fiducial cross sections at next-to-leading-order (NLO) EW and NLO QCD to $p\;p\to e^{+}\;\nu_{e}\;\mu^{+}\;\mu^{-}\;\gamma$ at the 13 TeV LHC. This signature includes the triboson production process $p\;p \to W^{+}\;Z\;\gamma$ with leptonic decays, $W^{+} \to e^{+}\;\nu_{e}$ and $Z \to \mu^{+}\;\mu^{-}$. The computation is based on the complete set of leading-order (LO) contributions of $\mathcal{O}(\alpha^5)$ and on NLO EW and NLO QCD cross sections of $\mathcal{O}(\alpha^6)$ and $\mathcal{O}(\alpha^5 \alpha_\text{s})$, respectively, and thus off-shell effects, spin correlations and non-resonance contributions are fully taken into account. We construct a Monte Carlo framework which provides total and differential cross sections for a chosen set of basic analysis cuts. We find that while NLO EW corrections enhance the fiducial LO total cross section by only $1\%$, they can significantly change some distributions in certain kinematic regions. For example, the relative NLO EW corrections to the muon transverse momentum distribution at 500 GeV amounts to $-20\%$. To illustrate how missing NLO EW corrections could masquerade as BSM physics, we show examples for the impact of dimension-8 operators in the SM Effective Field Theory framework on selected kinematic distributions.

In-medium kinetic theory of D mesons and heavy-flavor transport coefficients

We extend the kinetic theory of $D$ mesons to accommodate thermal and off-shell effects due to the medium modification of the heavy-meson spectral functions. From the Kadanoff-Baym approach we derive the off-shell Fokker-Planck equation which encodes the heavy-flavor transport coefficients. We analyze the thermal width (damping rate) of $D$ mesons due to their scattering off light mesons, focusing on new in-medium effects: off-shell corrections, inelastic channels, and the contribution of the Landau cut. We obtain that the latter effect (absent for vacuum scattering amplitudes) brings sizable corrections at moderate temperatures. We discuss how the heavy-flavor transport coefficients, like the drag and diffusion coefficients, are modified in matter. We find that the $D$-meson spatial diffusion coefficient matches smoothly to the latest results of lattice-QCD calculations and Bayesian analyses at higher temperatures.

Production and decay of the Higgs boson in association with top quarks

We report on the calculation of the next-lo-leading order QCD corrections to Higgs boson production and decay in association with top quarks. We consider leptonic decays of top quarks leading to the hadronic process pp → e+νeμ− overline{nu} μb overline{b} H(H → X) at the LHC with sqrt{s} = 13 TeV. All resonant as well as non-resonant Feynman diagrams, interferences and off-shell effects are included for the top quark and W gauge boson. Decays of the Higgs boson, on the other hand, are included in the narrow-width approximation. Specifically, we consider Higgs boson decays into b overline{b} , τ+τ−, γγ and e+e−e+e−. Numerical results are given at the integrated and differential fiducial level for various factorisation and renormalisation scale choices and different PDF sets. We study the main theoretical uncertainties that are associated with neglected higher order terms in the perturbative expansion and with different parametrisations of the PDFs. Furthermore, we examine the size of the off-shell effects by an explicit comparison to the calculation in the full narrow-width approximation. Finally, the impact of the contributions induced by the bottom-quark parton density is investigated.

Modeling uncertainties of tt¯W± multilepton signatures

In light of recent discrepancies between the modeling of $t\bar{t} W^\pm$ signatures and measurements reported by the Large Hadron Collider (LHC) experimental collaborations, we investigate in detail theoretical uncertainties for multi-lepton signatures. We compare results from the state-of-the-art full off-shell calculation and its Narrow Width Approximation to results obtained from the on-shell $t\bar{t} W^\pm$ calculation, with approximate spin-correlations in top-quark and $W$ decays, matched to parton showers. In the former case double-, single-, and non-resonant contributions together with interference effects are taken into account, while the latter two cases are only based on the double resonant top-quark contributions. The comparison is performed for the LHC at $\sqrt{s} = 13$ TeV for which we study separately the multi-lepton signatures as predicted from the dominant NLO contributions at the perturbative orders $\mathcal{O}(\alpha_s^3\alpha^6)$ and $\mathcal{O}(\alpha_s\alpha^8)$. Furthermore, we combine both contributions and propose a simple way to approximately incorporate the full off-shell effects in the NLO computation of on-shell $pp\to t\bar{t} W^\pm$ matched to parton showers.