Electroweak Bosons Research Articles

Measurement of electroweak boson production in pp, p+Pb and Pb+Pb collisions with the ATLAS detector

Measurement of electroweak boson production in different collision systems are of great interest to understanding the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. Their production yields in Pb+Pb with respect to pp collisions provide direct tests of both binary collision scaling and possible modification of parton distribution functions (nPDF) due to nuclear effects. Further, the p+Pb collisions provide a relatively clean environment to study nPDFs in detail. The ATLAS detector has a broad acceptance with excellent performance even in the high occupancy environment of central heavy-ion collisions. In this proceedings the latest ATLAS results on W and Z boson production at the centre-of-mass energy of 5.02 TeV are presented, including updated precise result production in pp collisions. Also photon yields are reported in 8.16 TeV in different collision systems are presented.

Exotic decays of top partners: Mind the search gap

Many standard model extensions, including composite Goldstone Higgs models, predict vector-like fermionic top-partners at the TeV scale. The intensive search programmes by ATLAS and CMS focus on decays into a 3rd generation quark and an electroweak boson (W,Z,h). However, underlying models of partial compositeness contain additional states that give rise to exotic top partner decays. We consider a well-motivated scenario in which a charge-2/3 top-partner decays into a pseudo-scalar, T→ta, with a→gg or bb¯ dominating below the tt¯ threshold. We show that the constraints on the top partner mass from QCD pair production are substantially weakened, still allowing a top partner mass as light as 400 GeV.

Dynamical Generation of Elementary Fermion Mass: First Lattice Evidence.

Using lattice simulations we demonstrate from first principles the existence of a nonperturbative mechanism for elementary particle mass generation in models with gauge fields, fermions, and scalars, if an exact invariance forbids power divergent fermion masses and fermionic chiral symmetries broken at UV scale are maximally restored. We show that in the Nambu-Goldstone phase a fermion mass term, unrelated to the Yukawa operator, is dynamically generated. In models with electroweak interactions weak boson masses are also generated, opening new scenarios for beyond the standard model physics.

Coupling QCD-Scale Axionlike Particles to Gluons.

We present a novel data-driven method for determining the hadronic interaction strengths of axionlike particles (ALPs) with QCD-scale masses. Using our method, it is possible to calculate the hadronic production and decay rates of ALPs, along with many of the largest ALP decay rates to exclusive final states. To illustrate the impact on QCD-scale ALP phenomenology, we consider the scenario where the ALP-gluon coupling is dominant over the ALP coupling to photons, electroweak bosons, and all fermions for m_{π}≲m_{a}≲3 GeV. We emphasize, however, that our method can easily be generalized to any set of ALP couplings to standard model particles. Finally, using the approach developed here, we provide calculations for the branching fractions of η_{c}→VV decays; i.e., η_{c} decays into two vector mesons, which are consistent with the known experimental values.

Measurements of the Higgs boson width and anomalous HVV couplings from on-shell and off-shell production in the four-lepton final state

Studies of on-shell and off-shell Higgs boson production in the four-lepton final state are presented, using data from the CMS experiment at the LHC that correspond to an integrated luminosity of 80.2 fb−1 at a center-of-mass energy of 13 TeV. Joint constraints are set on the Higgs boson total width and parameters that express its anomalous couplings to two electroweak vector bosons. These results are combined with those obtained from the data collected at center-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of 5.1 and 19.7 fb−1, respectively. Kinematic information from the decay particles and the associated jets are combined using matrix element techniques to identify the production mechanism and to increase sensitivity to the Higgs boson couplings in both production and decay. The constraints on anomalous HVV couplings are found to be consistent with the standard model expectation in both the on-shell and off-shell regions. Under the assumption of a coupling structure similar to that in the standard model, the Higgs boson width is constrained to be 3.2−2.2+2.8 MeV while the expected constraint based on simulation is 4.1−4.0+5.0 MeV. The constraints on the width remain similar with the inclusion of the tested anomalous HVV interactions.Received 1 January 2019DOI:https://doi.org/10.1103/PhysRevD.99.112003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2019 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Physical SystemsHiggs bosonsTechniquesHadron collidersParticles & Fields

Measurement of electroweak WZ boson production and search for new physics in WZ + two jets events in pp collisions at [formula omitted

A measurement of WZ electroweak (EW) vector boson scattering is presented. The measurement is performed in the leptonic decay modes WZ→ℓνℓ′ℓ′, where ℓ,ℓ′=e,μ. The analysis is based on a data sample of proton-proton collisions at s=13 TeV at the LHC collected with the CMS detector and corresponding to an integrated luminosity of 35.9 fb−1. The WZ plus two jet production cross section is measured in fiducial regions with enhanced contributions from EW production and found to be consistent with standard model predictions. The EW WZ production in association with two jets is measured with an observed (expected) significance of 2.2 (2.5) standard deviations. Constraints on charged Higgs boson production and on anomalous quartic gauge couplings in terms of dimension-eight effective field theory operators are also presented.

Dedicated strategies for triboson signals from cascade decays of vector resonances

New colorless electroweak (EW) charged spin-1 particles with mass of a few TeV arise in numerous extensions of the Standard Model (SM). Decays of such a vector into a pair of SM particles, either fermions or EW bosons, are well studied. Many of these models have an additional scalar, which can lead to (and even dominate in certain parameter regions) a novel decay channel for the heavy vector particles instead - into a SM EW boson and the scalar, which subsequently decays into a SM EW boson pair. In this work, we focus on the scalar being relatively heavy, roughly factor of two lighter than the vector particles, rendering its decay products well separated. Such a cascade decay results in a final state with three isolated bosons. We argue that for this "triboson" signal the existing diboson searches are not quite optimal due to combinatorial ambiguity for three identical bosons, and in addition, due to a relatively small signal cross-section determined by the heaviness of the decaying vector particle. In order to isolate the signal, we demonstrate that tagging all three bosons, followed by use of the full triboson invariant mass distribution as well as that of appropriate subsets of dibosons, is well motivated. We develop these general strategies in detail within the context of a specific class of models that are based on extensions of the standard warped extra-dimensional scenario. We also point out that a similar analysis would apply to models with an enlarged EW gauge sector in four dimensions, even if they involve a different Lorentz structure for the relevant couplings.

Heavy quarks within the electroweak multiplet

Standard-model fields and their associated electroweak Lagrangian are equivalently expressed in a shared spin basis. The scalar-vector terms are written with scalar-operator components acting on quark-doublet elements, and shown to be parametrization-invariant. Such terms, and the t- and b-quark Yukawa terms are linked by the identification of the common mass-generating Higgs operating upon the other fields, after acquiring a vacuum expectation value $v$. Thus, the customary vector masses are related to the fermions', fixing the t-quark mass $m_t$ with the relation $m^2_t+m^2_b=v^2/2$ either for maximal hierarchy, or given the b-quark mass $m_b$, implying $m_t \simeq 173.9$ GeV, for $v=246$ GeV. A sum rule is derived for all quark masses that generalizes this restriction. An interpretation follows that electroweak bosons and heavy quarks belong in a multiplet.

Looking for nonperturbative effects of the electroweak interaction at the LHC

We are considering a possibility for detecting nonperturbative effect in process of top pair production in association with a high [Formula: see text] photon. Starting from previous results on two solutions for a spontaneous generation of an effective interaction of a top pair with a pair of electroweak bosons, we show that a solution with effective cutoff [Formula: see text] is already contradicting to the existing data, while the other one with [Formula: see text] (just the Planck scale) could be reconciled with data and give predictions for process [Formula: see text], which could be effectively checked at the LHC with [Formula: see text]. The confirmation of the predictions would mean a strong support for the existence of nonperturbative effects in the electroweak interaction.

Axion couplings to electroweak gauge bosons

We determine the model-independent component of the couplings of axions to electroweak gauge bosons, induced by the minimal coupling to QCD inherent to solving the strong CP problem. The case of the invisible QCD axion is developed first, and the impact on W and Z axion couplings is discussed. The analysis is extended next to the generic framework of heavy true axions and low axion scales, corresponding to scenarios with enlarged confining sector. The mass dependence of the coupling of heavy axions to photons, W and Z bosons is determined. Furthermore, we perform a two-coupling-at-a-time phenomenological study where the gluonic coupling together with individual gauge boson couplings are considered. In this way, the regions excluded by experimental data for the axion-WW, axion-ZZ and axion-Zgamma couplings are determined and analyzed together with the usual photonic ones. The phenomenological results apply as well to ALPs which have anomalous couplings to both QCD and the electroweak bosons.

Polarization effects in standard model parton distributions at very high energies

We update the earlier work of refs. [1, 2] on parton distribution functions in the full Standard Model to include gauge boson polarization, non-zero input electroweak boson PDFs and next-to-leading-order resummation of large logarithms.

Associated production of a top quark pair with a heavy electroweak gauge boson at NLO+NNLL accuracy

We perform threshold resummation of soft gluon corrections to the total cross sections and the invariant mass distributions for production of a top-antitop quark pair associated with a heavy electroweak boson V = W^+, W^- or Z in pp collisions at the Large Hadron Collider. The resummation is carried out at next-to-next-to-leading-logarithmic (NNLL) accuracy using the direct QCD Mellin space technique in the three-particle invariant mass kinematics. It is found that for the t{bar{t}} Z process the soft gluon resummation introduces significant corrections to the next-to-leading order (NLO) results. For the central scale equal to the t{bar{t}} Z invariant mass the corrections reach nearly 30%. For this process, the dominant theoretical uncertainty of the cross section due to the scale choice is significantly reduced at the NLO+NNLL level with respect to the NLO results. The effects of resummation are found to be less pronounced in the t{bar{t}} W^{pm } case. The obtained results are compared to recent measurements performed by CMS and ATLAS collaborations at the LHC.

Probing low energy scalar leptoquarks by the leptonic W and Z couplings

We compute the generic one-loop contribution involving scalar leptoquarks (LQ) to the W and Z leptonic decay widths. In our computation we include for the first time the finite terms and the corrections due to the external momenta of the electroweak bosons, which is a step beyond the leading-logarithmic approximation considered in the literature so far. We show that the terms we include can be numerically quite significant. They amount to about 20% for scalar LQ masses below 1.5 TeV, as currently allowed by the direct searches at the LHC. To further illustrate the relevance of our results we revisit a model with two light scalar LQs, proposed to accommodate the B-physics anomalies. We show that the finite terms we computed can reduce the tension with the Z-pole data.

Electroweak boson measurements in p-Pb and Pb-Pb collisions at [formula omitted] with ALICE at the LHC

The ALICE collaboration has measured the electroweak boson production at forward rapidity in Pb-Pb and p-Pb collisions at sNN=5.02TeV. Final results on the Z0 boson production in Pb-Pb collisions are presented as a function of centrality and rapidity. Calculations based on free parton distribution functions overestimate the measurement up to 3σ. In addition, final results on W± and Z0 bosons production in p-Pb collisions at the same energy will be presented and discussed.

Electroweak probes of small and large systems with the ATLAS detector

Measurements of isolated prompt photon and massive electroweak (W and Z) boson production in different collision systems are of great interest to understand the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. These channels are sensitive to a variety of effects such as the modification of the parton densities in nuclei in certain kinematic regions, and the energy loss of partons as they undergo multiple interactions in the nucleus before the hard parton-parton scattering. High-statistics samples of lead–lead and proton–lead collision data at sNN=5.02TeV and8.16TeV, respectively, taken by the ATLAS experiment at the LHC, as well as proton–proton comparison data at analogous collision energies, allow for a detailed study of these phenomena in data and comprehensive comparisons to the predictions of a variety of theoretical approaches. This paper presents the latest ATLAS results in these topics, including updated results on inclusive prompt photon production in proton–lead collisions over a broad kinematic range and high-precision W boson results in lead–lead collisions.

Triggering On Hadronic Signatures In The Atlas Experiment-Developments For 2017 And 2018

Hadronic signatures are critical to the ATLAS physics program, and are used extensively for both the StandardModel measurements and searches for new physics. These signatures include generic quark and gluon jets, as well as jets originating from b-quarks or the decay of massive particles (such as electroweak bosons or top quarks). Additionally, missing transverse momentum from non-interacting particles provides an interesting probe in the search for new physics beyond the Standard Model. Developing trigger selections that target these events is a hugechallenge at the LHC due to the enormous rates associated with hadronic signatures. This challenge is exacerbated by the amount of pile-up activity, which continues to grow. In order to address these challenges, several new techniques were developed to significantly improve the potential of the 2017 dataset. An overview of how we triggeron hadronic signatures at the ATLAS experiment is presented, outlining the challenges of hadronic object triggering and describing the improvements performed over the course of the Run 2 LHC data-taking program. The performance in Run 2 data is shown, including demonstrations of the new techniques being used in 2017. We also discuss further critical developments implemented for the rest of Run 2 and their performance in early 2018 data.

Two-component Dark Matter in the vectorlike hypercolor extension of the Standard Model

We consider a minimal vectorlike extension of the Standard Model that naturally contains two types of stable neutral particles. They can be interpreted as the Dark Matter candidates. Here, the SM is supplemented by a new fermion (H-quark) sector that is in confilnement providing by the SU(2)HC gauge group. H-quarks interact with the SM particles via standard electroweak bosons. In analogy with the conventional QCD, H-quarks can form bound states, H-hadrons, that emerge in the σ− model framework. Along with the stable neutral H-pion, there is one more pseudo-goldstone (diquark) state, B0, which is stable and also can be a component of the DM. Mass splittings both for components of the H-pion triplet and for neutral H-pion and B0 have been calculated, it is shown that this splittings can be small. The relic abundance for this composition of the DM is analyzed as well as the DM particles scattering off nucleons. A feature of this scenario is that Dark Matter turns out to consist of particles which are close in mass but have different origin and interact differently with ordinary matter. So, the model predicts two-component structure of the DM together with some specifilc manifestations of it.

Computing N -subjettiness for boosted jets

Jet substructure tools have proven useful in a number of high-energy particle-physics studies. A particular case is the discrimination, or tagging, between a boosted jet originated from an electroweak boson (signal), and a standard QCD parton (background). A common way to achieve this is to cut on a measure of the radiation inside the jet, i.e. a jet shape. Over the last few years, analytic calculations of jet substructure have allowed for a deeper understanding of these tools and for the development of more efficient ones. However, analytic calculations are often limited to the region where the jet shape is small. In this paper we introduce a new approach in perturbative QCD to compute jet shapes for a generic boosted jets, waiving the above limitation. We focus on an example common in the substructure literature: the jet mass distribution after a cut on the N -subjettiness τ21 ratio, extending previous works to the region relevant for phenomenology. We compare our analytic predictions to Monte Carlo simulations for both plain and SoftDrop-groomed jets. We use our results to construct analytically a decorrelated tagger.

The Lund jet plane

Lund diagrams, a theoretical representation of the phase space within jets, have long been used in discussing parton showers and resummations. We point out that they can be created for individual jets through repeated Cambridge/Aachen declustering, providing a powerful visual representation of the radiation within any given jet. Concentrating here on the primary Lund plane, we outline some of its analytical properties, highlight its scope for constraining Monte Carlo simulations and comment on its relation with existing observables such as the zg variable and the iterated soft-drop multiplicity. We then examine its use for boosted electroweak boson tagging at high momenta. It provides good performance when used as an input to machine learning. Much of this performance can be reproduced also within a transparent log-likelihood method, whose underlying assumption is that different regions of the primary Lund plane are largely decorrelated. This suggests a potential for unique insight and experimental validation of the features being used by machine-learning approaches.

Gauge properties of hadronic structure of nucleon in neutron radiative beta decay to order O(α/π) in standard V − A effective theory with QED and linear sigma model of strong low-energy interactions

Within the standard [Formula: see text] theory of weak interactions, Quantum electrodynamics (QED) and the linear [Formula: see text]-model (L[Formula: see text]M) of strong low-energy hadronic interactions, we analyze gauge properties of hadronic structure of the neutron and proton in the neutron radiative [Formula: see text]-decay. We show that the Feynman diagrams, describing contributions of hadronic structure to the amplitude of the neutron radiative [Formula: see text]-decay in the tree-approximation for strong low-energy interactions in the L[Formula: see text]M, are gauge invariant. In turn, the complete set of Feynman diagrams, describing the contributions of hadron–photon interactions in the one-hadron-loop approximation, is not gauge invariant. In the infinite limit of the scalar [Formula: see text]-meson, reproducing the current algebra results (S. Weinberg, Phys. Rev. Lett. 18, 188 (1967)), and to leading order in the large nucleon mass expansion the Feynman diagrams, violating gauge invariance, do not contribute to the amplitude of the neutron radiative [Formula: see text]-decay in agreement with Sirlin’s analysis of strong low-energy interactions in neutron [Formula: see text] decays. We assert that the problem of appearance of gauge noninvariant Feynman diagrams of hadronic structure of the neutron and proton is related to the following. The vertex of the effective [Formula: see text] weak interactions does not belong to the combined quantum field theory including the L[Formula: see text]M and QED. We argue that gauge invariant set of Feynman diagrams of hadrons, coupled to real and virtual photons in neutron [Formula: see text] decays, can be obtained within the combined quantum field theory including the Standard Electroweak Model (SEM) and the L[Formula: see text]M, where the effective [Formula: see text] vertex of weak interactions is a result of the [Formula: see text]-electroweak boson exchange.