Z Boson Research Articles

Strong Coupling from Inclusive Semi-leptonic Decay of Charmed Mesons

Abstract Employing the heavy quark expansion model with the kinetic scheme, we evaluate αS(mc 2), the strong coupling constant at the charm quark mass mc with data on inclusive semileptonic decays of charmed mesons. Using the experimental values of semileptonic decay widths of the D0 and the D+, the value of αS(mc 2) is determined to be 0.445±0.009±0.114, where the first uncertainty is experimental and the second systematic. This reported αS(mc 2) is in good agreement with the value of αS(mc 2) calculated by running αS(mZ 2) at the Z0 boson mass mZ with the renormalization group evolution equation. In addition, values of αS(mc 2) obtained individually from each of the D0, D+, and D+ s mesons are found to be consistent being of the same origin.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

Phenomenology of TMD parton distributions in Drell-Yan and Z0 boson production in a hadron structure oriented approach

We present a first practical implementation of a recently proposed hadron structure oriented (HSO) approach to transverse momentum dependent (TMD) phenomenology applied to Drell-Yan-like processes, including lepton pair production at moderate Q2 and Z0 boson production. We compare and contrast general features of our methodology with other common practices and emphasize aspects that we view as improvements. Separately, we discuss the importance of treatments that preserve various theoretical properties of the nonperturbative parts of TMD parametrizations for future applications that have the extraction of hadron structure as a primary goal. These include the HSO’s preservation of a basic TMD parton-model-like framework even while accounting for full TMD factorization and evolution, explicit preservation of the integral relationship between TMD and collinear parton density functions, and the ability to meaningfully compare different theoretical models of nonperturbative TMD parton distributions. In our examples, we show that there is significant sensitivity at moderate Q2 to both the form of the nonperturbative transverse momentum dependence and the parametrization of collinear parton densities. However, we also find that evolving to Q2=MZ2, without fitting, results in a satisfactory postdiction of existing data for Z0 production, nearly independently of the modeling of nonperturbative transverse momentum behavior. We argue that this demonstrates that moderate Q measurements should be given greater weight than high Q measurements in extractions of nonperturbative transverse momentum dependence. We also obtain new extractions of the nonperturbative Collins-Soper kernel within the HSO approach. We discuss its features and compare with some earlier extractions. Published by the American Physical Society 2024

QCD corrections to Higgs boson production and H→bb¯ decay in weak boson fusion

We study QCD corrections to the process where a Higgs boson is produced in weak boson fusion and then decays into a pair of massive b quarks. We find that typical experimental criteria used to identify b jets in this process affect QCD corrections to the decay, making it necessary to account for them in the proper description of this process. Indeed, if corrections to the production and decay are combined, the fiducial cross section of the weak boson fusion process pp→H(→bb¯)jj is reduced by about 40% relative to leading-order predictions, compared to just about 8% if only corrections to the production process are considered. We investigate the origin of these large corrections through next-to-next-to-leading order and conclude that they appear because a number of independent moderately large effects conspire to significantly reduce the fiducial cross section for this process. Published by the American Physical Society 2024

Automatic generation of helicity amplitudes in the Feynman-diagram gauge

We develop a method to calculate helicity amplitudes of an arbitrary tree-level process in the Feynman-diagram (FD) gauge for an arbitrary gauge model with adraph5_a@. We start from the ’t Hooft–Feynman gauge Lagrangian in eynules and generate scattering amplitudes by identifying the Goldstone boson as the 5th component of each weak boson. All the vertices of the 5-component weak bosons are then created automatically by assembling the relevant weak boson and Goldstone boson vertices in the Feynman gauge. The 5-component weak boson vertices are then connected by the 5×5 matrix propagator in the FD gauge. As a demonstration of the method we calculate the cross section for the process μ−μ+→νμν¯μtt¯H with complex top Yukawa coupling, which can be obtained by adding a gauge invariant dimension-6 operator to the Standard Model (SM) Lagrangian. The FD gauge and the unitary (U) gauge amplitudes give exactly the same cross section, and subtle gauge theory cancellation among diagrams in the U gauge at high energies is absent in the FD gauge, as has been observed for various SM processes. In addition, we find that the total cross sections at high energies are dominated by a single, or a set of nonvanishing Feynman amplitudes with the higher dimensional vertices in the FD gauge. Published by the American Physical Society 2024

Electroweak corrections to Higgs boson production via Z Z fusion at the future LHeC

An important mechanism for production of the Higgs boson at the prospective Large Hadron-electron Collider (LHeC) is via neutral current (NC) weak boson fusion (WBF) processes. Aside from its role in measurements of Higgs couplings within the standard model, this production mode is particularly useful in searchings of Higgs decays into invisible particles in various models for the Higg portal dark matter. In this work we compute the electroweak corrections for the NC WBF at the LHeC up to the one-loop level. For a center-of-mass energy of 1.98 TeV, the magnitudes of the relative corrections for the total cross section at next-to-leading (NLO) order are respectively 8% and 17%, in the two renormalization schemes we use. The NLO terms also distort various distributions (notably, those for Higgs and electron observables) computed at the leading order. Along with our previous treatment of the charge current processes, this paper completes the calculation of the NLO electroweak effects for the dominant Higgs production modes at the LHeC. Published by the American Physical Society 2024

Tripartite entanglement in H→ZZ,WW decays

The decays of the Higgs boson produce a state in which the spins of the decay products and the orbital angular momentum (L) are highly entangled. We obtain the tripartite density operator, as well as reduced operators, for the decay into two weak bosons. For H→ZZ in the four-lepton final state we also estimate the statistical sensitivity at the Large Hadron Collider and future upgrades, using a binned method to reconstruct the density operators from distributions. With the expected Run 3 data, establishing genuine tripartite entanglement would be possible beyond the 5σ level. The violation of Bell inequalities involving the spins of the two weak bosons could also be established beyond the 5σ level. Published by the American Physical Society 2024

Polarized ZZ pairs in gluon fusion and vector boson fusion at the LHC

Pair production of helicity-polarized weak bosons (Vλ=Wλ±,Zλ) from gluon fusion (gg→VλVλ′′) and weak boson fusion (V1V2→VλVλ′′) are powerful probes of the Standard Model, new physics, and properties of quantum systems. Measuring cross sections of polarized processes is a chief objective of the Large Hadron Collider's (LHC) Run 3 and high luminosity programs, but progress is limited by the simulation tools that are presently available. We propose a method for computing polarized cross sections that works by directly modifying Feynman rules instead of (squared) amplitudes. The method is applicable to loop-induced processes, and can capture the interference between arbitrary polarization configurations, interference with non-resonant diagrams, as well as off-shell/finite-width effects. By construction, previous results that work at the (squared) amplitude level are recoverable. As a demonstration, we report the prospect of observing and studying polarized ZλZλ′ pairs when produced via gluon fusion and electroweak processes in final-states with four charged leptons at the LHC, using the new method to simulate the gluon fusion process. Our Feynman rules are publicly available as a set of Universal FeynRules Object libraries called SM_Loop_VPolar.

The LHC as a Neutrino-Ion Collider

Proton-proton collisions at the LHC generate a high-intensity collimated beam of neutrinos in the forward (beam) direction, characterised by energies of up to several TeV. The recent observation of LHC neutrinos by FASERν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u $$\\end{document} and SND@LHC signifies that this previously overlooked particle beam is now available for scientific investigation. Here we quantify the impact that neutrino deep-inelastic scattering (DIS) measurements at the LHC would have on the parton distributions (PDFs) of protons and heavy nuclei. We generate projections for DIS structure functions for FASERν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u $$\\end{document} and SND@LHC at Run III, as well as for the FASERν\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ u $$\\end{document}2, AdvSND, and FLArE experiments to be hosted at the proposed Forward Physics Facility (FPF) operating concurrently with the High-Luminosity LHC (HL-LHC). We determine that up to one million electron-neutrino and muon-neutrino DIS interactions within detector acceptance can be expected by the end of the HL-LHC, covering a kinematic region in x and Q2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Q^2$$\\end{document} overlapping with that of the Electron-Ion Collider. Including these DIS projections in global (n)PDF analyses, specifically PDF4LHC21, NNPDF4.0, and EPPS21, reveals a significant reduction in PDF uncertainties, in particular for strangeness and the up and down valence PDFs. We show that LHC neutrino data enable improved theoretical predictions for core processes at the HL-LHC, such as Higgs and weak gauge boson production. Our analysis demonstrates that exploiting the LHC neutrino beam effectively provides CERN with a “Neutrino-Ion Collider” without requiring modifications in its accelerator infrastructure.

On the symmetries of elementary fermions

This paper deals with the symmetries of elementary fermions and their derivation from fundamental physical principles such as the Lorentz invariance and from the spinor-helicity formalism employed to three-vectors. The generators of the Lorentz group are discussed, and the physics of the associated chiral spins is used to establish the chiral symmetry SU(2). The helicity of spin is defined which, when being applied to the hadronic isospin, yields the symmetry group SU(4). The Kronecker product of these two basic symmetries defines a unified symmetry SU(2)⊗SU(4)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$SU(2) \\otimes SU(4)$$\\end{document} of the basic light fermions, namely the chiral doublets of the single lepton and triple quarks of the first family. Breaking this symmetry, according to mechanisms that are used in the electroweak interactions of the Standard Model (SM) of quantum field theory, yields quantum electrodynamics (QED) with symmetry group U(1), weak interactions with symmetry group SU(2), and what we like to name quantum hadrodynamics (QHD, akin to quantum chromodynamics QCD of the SM) with symmetry group SU(3). The gauge bosons associated with QED and QHD remain massless, but the weak bosons and the V bosons, related to the transformation of the quarks into a lepton and vice versa, become massive by the Higgs mechanism. Their masses are defined by the Higgs vacuum and the two coupling constants involved in the unified model. The V-boson mass is predicted to be 35.4 GeV. Furthermore, a possible explanation of hadron confinement is given in terms of the two hadronic charge operators. Moreover, the concept of hypercharge as used in the SM is not needed. Various versions of the extended Dirac equation that include the above symmetry groups are derived.

On the Breaking of the U(1) Peccei–Quinn Symmetry and Its Implications for Neutrino and Dark Matter Physics

The Standard Model of electroweak interactions is based on the fundamental SU(2)weak × U(1)elect representation. It assumes massless neutrinos and purely left-handed massive W± and Z0 bosons to which one should add the massless photon. The existence, verified experimentally, of neutrino oscillations poses a challenge to this scheme, since the oscillations take place between at least three massive neutrinos belonging to a mass hierarchy still to be determined. One should also take into account the possible existence of sterile neutrino species. In a somehow different context, the fundamental nature of the strong interaction component of the forces in nature is described by the, until now, extremely successful representation based on the SU(3)strong group which, together with the confining rule, give a description of massive hadrons in terms of quarks and gluons. To this is added the minimal U(1) Higgs group to give mass to the otherwise massless generators. This representation may also be challenged by the existence of both dark matter and dark energy, of still unknown composition. In this note, we shall discuss a possible connection between these questions, namely the need to extend the SU(3)strong × SU(2)weak × U(1)elect to account for massive neutrinos and dark matter. The main point of it is related to the role of axions, as postulated by Roberto Peccei and Helen Quinn. The existence of neutral pseudo-scalar bosons, that is, the axions, has been proposed long ago by Peccei and Quinn to explain the suppression of the electric dipole moment of the neutron. The associated U(1)PQ symmetry breaks at very high energy, and it guarantees that the interaction of other particles with axions is very weak. We shall review the axion properties in connection with the apparently different contexts of neutrino and dark matter physics.

CP-violating top-Higgs coupling in SMEFT

The total cross section of the process μ−μ+→νμν¯μtt¯H has strong dependence on the CP phase ξ of the top Yukawa coupling, where the ratio of ξ=π and ξ=0 (SM) grows to 670 at s=30 TeV, 3400 at 100 TeV. We study the cause of the strong energy dependence and identify its origin as the (E/mW)2 growth of the weak boson fusion sub-amplitudes, WL−WL+→tt¯H, with the two W's are longitudinally polarized. We repeat the study in the SMEFT framework where EW gauge invariance is manifest and find that the highest energy cross section is reduced to a quarter of the complex top Yukawa model result, with the same energy power. By applying the Goldstone boson (GB) equivalence theorem, we identify the origin of this strong energy growth of the SMEFT amplitudes as associated with the dimension-6 π−π+ttH vertex, where π± denotes the GB of W±. We obtain the unitarity bound on the coefficient of the SMEFT operator by studying all 2→2 and 2→3 cross sections in the J=0 channel.

Top Yukawa coupling determination at high energy muon collider

The Top Yukawa coupling profoundly influences several core mysteries linked to the electroweak scale and the Higgs boson. We study the feasibility of measuring the Top Yukawa coupling at high energy muon colliders by examining the high energy dynamics of the weak boson fusion to top quark pair processes. A deviation of the Top Yukawa coupling from the Standard Model would lead to a modified VV→tt¯ process, violating unitarity at high energy. Our analysis reveals that utilizing a muon collider with a center-of-mass energy of 10 TeV and an integrated luminosity of 10 ab−1 allows us to investigate the Top Yukawa coupling with a precision surpassing 1.5%, more than one order of magnitude better than the precision from tt¯h channel at muon colliders. This precision represents a notable enhancement compared to the anticipated sensitivities of the High-Luminosity LHC (3.4%) and those at muon colliders derived from the tt¯H process. Published by the American Physical Society 2024

Search for pair production of squarks or gluinos decaying via sleptons or weak bosons in final states with two same-sign or three leptons with the ATLAS detector

A search for pair production of squarks or gluinos decaying via sleptons or weak bosons is reported. The search targets a final state with exactly two leptons with same-sign electric charge or at least three leptons without any charge requirement. The analysed data set corresponds to an integrated luminosity of 139 fb−1 of proton-proton collisions collected at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Multiple signal regions are defined, targeting several SUSY simplified models yielding the desired final states. A single control region is used to constrain the normalisation of the WZ + jets background. No significant excess of events over the Standard Model expectation is observed. The results are interpreted in the context of several supersymmetric models featuring R-parity conservation or R-parity violation, yielding exclusion limits surpassing those from previous searches. In models considering gluino (squark) pair production, gluino (squark) masses up to 2.2 (1.7) TeV are excluded at 95% confidence level.

Minimal Inert Doublet benchmark for dark matter and the baryon asymmetry

In this article we discuss a minimal extension of the Inert Doublet Model (IDM) with an effective CP-violating D=6 operator, involving the inert Higgs and weak gauge bosons, that can lift it to a fully realistic setup for creating the baryon asymmetry of the Universe (BAU). Avoiding the need to stick to an explicit completion, we investigate the potential of such an operator to give rise to the measured BAU during a multi-step electroweak phase transition (EWPhT) while sustaining a viable DM candidate in agreement with the measured relic abundance. We find that the explored extension of the IDM can account quantitatively for both DM and for baryogenesis and has quite unique virtues, as we will argue. It can thus serve as a benchmark for a minimal realistic extension of the SM that solves some of its shortcomings and could represent the low energy limit of a larger set of viable completions.After discussing the impact of a further class of operators that open the possibility for a larger mass splitting (enhancing the EWPhT) while generating the full relic abundance also for heavy inert-Higgs DM, we ultimately provide a quantitative evaluation of the induced lepton electric dipole moments in the minimal benchmark for the BAU. These arise here at the two-loop level and are therefore less problematic compared to the ones that emerge when inducing CP violation via an operator involving the SM-like Higgs.

Nonfactorizable corrections to Higgs boson production in weak boson fusion

We discuss the nonfactorizable corrections to Higgs boson production in weak boson fusion at the Large Hadron Collider. Such corrections depend on the finite part of the two-loop virtual amplitude qQ→q′Q′+H which, up to now, has only been computed in the eikonal approximation. We combine this contribution with real-virtual and double-real nonfactorizable QCD corrections and study their impact on the various observables in weak boson fusion. We find that the nonfactorizable corrections are strongly dominated by the two-loop virtual contributions, while all other contributions play a very minor role. This striking imbalance between real and virtual contributions is caused by a process-specific kinematic suppression of the former and a particular enhancement of the virtual corrections related to a Glauber phase. Published by the American Physical Society 2024

Polarised-boson pairs at the LHC with NLOPS accuracy

We present a calculation of inclusive diboson ({textrm{WZ, ZZ, WW}}) processes at the LHC in the presence of intermediate polarised weak bosons decaying leptonically, matching next-to-leading-order accuracy in QCD with parton-shower effects. Starting from recent developments in polarised-boson simulation based on the helicity selection at the amplitude level, we have carried out the implementation in the Powheg-Box-Res framework, and validated it against independent fixed-order calculations. A phenomenological analysis in realistic LHC setups, as well as a comparison with recent ATLAS measurements, are presented.

Forty years of the discovery of the Z0 boson: the mediator of neutral current interactions

A descoberta dos bósons vetoriais massivos Z0 e W foi um marco na construção do Modelo Padrão das Partículas Elementares e permitiu a compreensão de uma gama de mecanismos de produção medidos em laboratório. Este artigo apresenta os aspectos gerais sobre teoria e instrumentação que levaram à descoberta do bóson vetorial massivo Z0 em 1983, uma das partículas mediadoras da Força Fraca e responsável pelas interações de correntes neutras. Uma breve contextualização histórica da construção da Teoria das Interações Fracas é feita, desde a descoberta da radioatividade até a proposta de sua unificação com a Eletrodinâmica Quântica, bem como a descrição dos principais experimentos envolvidos e discussão dos resultado obtidos. Por fim, os resultados obtidos nos experimentos UA1 e UA2 do S p p ¯ S são apresentados, os quais levaram ao Prêmio Nobel de 1984.

BSM patterns in scalar-sector coupling modifiers

We consider what multiple Higgs interactions may yet reveal about the scalar sector. We estimate the sensitivity of a Feynman topology-templated analysis of weak boson fusion Higgs pair production at present and future colliders — where the signal is a function of the Higgs coupling modifiers κV, κ2V, and κλ. While measurements are statistically limited at the LHC, they are under general perturbative control at present and future colliders, departures from the SM expectation give rise to a significant future potential for BSM discrimination in κ2V. We explore the landscape of BSM models in the space of deviations in κV, κ2V, and κλ, highlighting models that have measurable order-of-magnitude enhancements in either κ2V or κλ, relative to their deviation in the single Higgs coupling κV.

Scale dependence of non-factorizable virtual corrections to Higgs boson production in weak boson fusion

The renormalization-scale dependence of the non-factorizable virtual corrections to Higgs boson production in weak boson fusion at next-to-next-to-leading order in perturbative QCD is unusually strong, due to the peculiar nature of these corrections. To address this problem, we compute the three-loop non-factorizable contribution to this process which accounts for the running of the strong coupling constant, and show that it stabilizes the theoretical prediction.

The neutral current deeply inelastic scattering process and a revisit on the intrinsic asymmetry

The intermediate particle in the neutral current deeply inelastic scattering process can be either a virtual photon or a Z0 boson. In this paper we calculate contributions from the weak interaction in the semi-inclusive deeply inelastic scattering process. Semi-inclusive implies that a jet in the current region is also measured in addition to the scattered electron. Jet can be a good candidate to study the three-dimensional structure of the nucleon in the semi-inclusive deeply inelastic scattering process. In the vector boson-nucleon collinear frame, the transverse momentum of the jet is the same to that of the quark in the nucleon under the leading order approximation. As a result, one can define the intrinsic asymmetry which reveals the imbalance of the transverse momentum of the incident quark to explore the transverse momentum dependent parton distribution functions (TMD PDFs). With the Gaussian ansatz for the TMD PDFs, we restudy the transverse momentum dependent behavior of the intrinsic asymmetry. We find that the asymmetry tends to a linear function with respect to k⊥ when Δu,d2 approach Δ′u,d2.