Neutral Electroweak Gauge Bosons Research Articles

Electroweak Kaluza-Klein dark matter

In models with universal extra dimensions (UED), the lightest Kaluza-Klein excitation of neutral electroweak gauge bosons is a stable, weakly interacting massive particle and thus is a candidate for dark matter thanks to Kaluza-Klein parity. We examine concrete model realizations of such dark matter in the context of non-minimal UED extensions. The boundary localized kinetic terms for the electroweak gauge bosons lead to a non-trivial mixing among the first Kaluza-Klein excitations of the SU(2)W and U(1)Y gauge bosons and the resultant low energy phenomenology is rich. We investigate implications of various experiments including low energy electroweak precision measurements, direct and indirect detection of dark matter particles and direct collider searches at the LHC. Notably, we show that the electroweak Kaluza-Klein dark matter can be as heavy as 2.4 TeV, which is significantly higher than 1.3 TeV as is indicated as an upper bound in the minimal UED model.

Electroweak gauge-boson production in association with b jets at Hadron Colliders

The production of both charged and neutral electroweak gauge bosons in association with b jets has attracted a lot of experimental and theoretical attention in recent years because of its central role in the physics programs of both the Fermilab Tevatron and the CERN Large Hadron Collider. The improved level of accuracy achieved both in the theoretical predictions and experimental measurements of these processes can promote crucial developments in modeling b-quark jets and b-quark parton distribution functions, and can provide a more accurate description of some of the most important backgrounds to the measurement of Higgs-boson couplings and several new physics searches. In this paper, we review the status of theoretical predictions for cross sections and kinematic distributions of processes in which an electroweak gauge boson is produced in association with up to two b jets in hadronic collisions, namely [Formula: see text], pp → V + 1b jet and [Formula: see text], pp → V + 2b jets with V = W±, Z/γ*, γ. Available experimental measurements at both the Fermilab Tevatron and the CERN Large Hadron Collider are also reviewed and their comparison with theoretical predictions is discussed.

KK-parity non-conservation in UED confronts LHC data

Kaluza–Klein (KK) parity can be violated in a five-dimensional universal extra-dimensional model with boundary-localised (kinetic or mass) terms (BLTs) at the fixed points of $$S^1/Z_2$$ orbifold. In this framework we study the resonant production of Kaluza–Klein excitations of the neutral electroweak gauge bosons at the LHC and their decay into an electron–positron pair or a muon–antimuon pair. We use the results (for the first time, to our knowledge) given by the LHC experiment to constrain the mass range of the first KK-excitation of the electroweak gauge bosons ( $$B^1\ \mathrm{and } \ W_3^1$$ ). It is interesting to note that the LHC result puts an upper limit on the masses of the $$n=1$$ KK-leptons for positive values of the BLT parameters depending upon the mass of the $$\ell ^{+}\ell ^{-}$$ resonance.

8D likelihood effective Higgs couplings extraction framework in h → 4ℓ

In this paper we build a comprehensive analysis framework to perform direct extraction of all possible effective Higgs couplings to neutral electroweak gauge bosons in the decay to electrons and muons, the so called `golden channel'. Our framework is based on a maximum likelihood method constructed from analytic expressions of the fully differential cross sections for $h \rightarrow 4\ell$ and for the dominant irreducible $q\bar{q} \rightarrow 4\ell$ background, where $4\ell = 2e2\mu, 4e, 4\mu$. Detector effects are included by an explicit convolution of these analytic expressions with the appropriate transfer function over all center of mass variables. Using the full set of decay observables, we construct an unbinned 8-dimensional detector-level likelihood function which is continuous in the effective couplings and includes systematic uncertainties. We consider all possible $ZZ$, $Z\gamma$ and $\gamma\gamma$ couplings, allowing for general CP odd/even admixtures and any possible phases. We describe how the convolution is performed and demonstrate the validity and power of the framework with a number of supporting checks and example fits. The framework can be used to perform a variety of multi-parameter extractions, including their correlations, to determine the Higgs couplings to neutral electroweak gauge bosons using data obtained at the LHC and other future colliders.

Universal Extra-Dimensional models with boundary terms: Probing at the LHC

In universal extra-dimensional models a conserved Z2 parity stabilizes the lightest Kaluza-Klein particle, a dark-matter candidate. Boundary-localized kinetic terms, in general, do not preserve this symmetry. We examine, in the presence of such terms, the single production of Kaluza-Klein excitations of the neutral electroweak gauge bosons and their decay to zero-mode fermion-antifermion pairs. We explore how experiments at the Large Hadron Collider constrain the boundary-localized kinetic terms for different compactification radii.

Extracting effective Higgs couplings in the golden channel

Kinematic distributions in Higgs decays to four charged leptons, the so called `golden channel, are a powerful probe of the tensor structure of its couplings to neutral electroweak gauge bosons. In this study we construct the first part of a comprehensive analysis framework designed to maximize the information contained in this channel in order to perform direct extraction of the various possible Higgs couplings. To that end we first complete an earlier analytic calculation of the leading order fully differential cross sections for the golden channel signal and background to include the 4e and $4\mu$ final states with interference between identical final states. We also examine the relative fractions of the different possible combinations of scalar-tensor couplings by integrating the fully differential cross section over all kinematic variables as well as show various doubly differential spectra for both the signal and background. From these analytic expressions we then construct a `generator level' analysis framework based on the maximum likelihood method. We demonstrate the ability of our framework to perform multi-parameter extractions of all the possible effective couplings of a spin-0 scalar to pairs of neutral electroweak gauge bosons including any correlations. This framework provides a powerful method for study of these couplings and can be readily adapted to include the relevant detector and systematic effects which we demonstrate in an accompanying study to follow.

Universal extra dimensions : life with BLKTs

In universal extra dimension (UED) models with one compactified extra dimension, a Z2 symmetry, termed KK-parity, ensures the stability of the lightest Kaluza-Klein particle (LKP) which could be a viable dark matter candidate. This symmetry leads to two fixed points in the extra space like direction. In non-minimal versions of UED boundary-localized kinetic terms (BLKT) of same strength at both fixed points induce a new Z2 symmetry which ensures the stability of LKP. The precision of the dark matter measurements severely correlates and restricts the BLKT parameters of gauge bosons and fermions. Furthermore, BLKT parameters of different strengths at the fixed would induce a non-consevation of KK-parity. We examine, in the presence of such terms, single production and decay of Kaluza-Klein excitations of the neutral electroweak gauge bosons in the context of LHC.

Massive neutral gauge boson production as a probe of nuclear modifications of parton distributions at the LHC

We analyze the role of nuclear modifications of parton distributions, notably, the nuclear shadowing and antishadowing corrections, in production of lepton pairs from decays of neutral electroweak gauge bosons in proton-lead and lead-collisions at the LHC. Using the Collins-Soper-Sterman resummation formalism that we extended to the case of nuclear parton distributions, we observed a direct correlation between the predicted behavior of the transverse momentum and rapidity distributions of the produced vector bosons and the pattern of quark and gluon nuclear modifications. This makes Drell-Yan pair production in $pA$ and $AA$ collisions at the LHC a useful tool for constraining nuclear PDFs in the small-$x$ shadowing and moderate-$x$ antishadowing regions.

Dirac neutralinos and electroweak scalar bosons of N = 1/N = 2 hybrid supersymmetry at colliders

In the N=1 supersymmetric extension of the Standard Model, neutralinos associated in supermultiplets with the neutral electroweak gauge and Higgs bosons are, as well as gluinos, Majorana fermions. They can be paired with the Majorana fermions of novel gaugino/scalar supermultiplets, as suggested by extended N=2 supersymmetry, to Dirac particles. Matter fields are not extended beyond the standard N=1 supermultiplets in N=1/N=2 hybrid supersymmetry to preserve the chiral character of the theory. Complementing earlier analyses in the color sector, central elements of such an electroweak scenario are analyzed in the present study. The decay properties of the Dirac fermions and of the scalar bosons are worked out, and the single and pair production channels of the new particles are described for proton collisions at the LHC, and electron/positron and photon-photon collisions at linear colliders. Special attention is paid to modifications of the Higgs sector, identified with an N=2 hypermultiplet, by the mixing with the novel electroweak scalar sector.

Topological interactions in the Higgsless model at the LHC

Topological quantum interactions, namely Chern-Simons terms and global Wess-Zumino terms, arise naturally in theories with extra dimensions of space compactified on orbifolds. If the extra dimensions become manifest at the TeV scale, experiments at the Large Hadron Collider (LHC) could observe signatures of topological interactions. Decays of Kaluza-Klein excitations of neutral electroweak gauge bosons into pairs of neutral Standard Model gauge bosons, ZZ and Z+photon, would provide a clean signature, since such decays do not occur at tree level. In this paper, we investigate the prospects for discovering such decays at the LHC, in the context of the Higgsless model of electroweak symmetry breaking. We identify the form of the relevant topological interactions, and estimate their strength. We find that in the minimal version of the model, the signal may be observed with about 100 inverse fb of data at the LHC using the Drell-Yan production process of the Kaluza-Klein gauge bosons. In addition, it is likely that the ultraviolet completion of the model would contain additional massive fermions, which can significantly enhance the signal. With two additional fermion multiplets, observation of the topological decay modes at the 3-sigma level would be possible with about 100 inverse fb of data using the highly model-independent vector boson fusion production channel.

Testing the Majorana nature of gluinos and neutralinos

Gluinos and neutralinos, supersymmetric partners of gluons and neutral electroweak gauge and Higgs bosons, are Majorana particles in the minimal supersymmetric standard model (MSSM). Decays of such self-conjugate particles generate charge symmetric ensembles of final states. Moreover, production channels of supersymmetric particles at colliders are characteristically affected by the Majorana nature of particles exchanged in the production processes. The sensitivity to the Majorana character of the particles can be quantified by comparing the predictions with Dirac exchange mechanisms. A consistent framework for introducing gluino and neutralino Dirac fields can be designed by extending the N=1 supersymmetry of the MSSM to N=2 in the gauge sector. We examine to which extent like-sign dilepton production in the processes qq{yields}q-tildeq-tilde and e{sup -}e{sup -}{yields}e-tilde{sup -}e-tilde{sup -} is affected by the exchange of either Majorana or Dirac gluinos and neutralinos, respectively, at the Large Hadron Collider (LHC) and in the prospective e{sup -}e{sup -} mode of a lepton linear collider.

Unique identification of Lee-Wick gauge bosons at linear colliders

Grinstein, O'Connell and Wise have recently presented an extension of the Standard Model (SM), based on the ideas of Lee and Wick (LW), which demonstrates an interesting way to remove the quadratically divergent contributions to the Higgs mass induced by radiative corrections. This model predicts the existence of negative-norm copies of the usual SM fields at the TeV scale with ghost-like propagators and negative decay widths, but with otherwise SM-like couplings. In earlier work, it was demonstrated that the LW states in the gauge boson sector of these models, though easy to observe, cannot be uniquely identified as such at the LHC. In this paper, we address the issue of whether or not this problem can be resolved at an $e^+e^-$ collider with a suitable center of mass energy range. We find that measurements of the cross section and the left-right polarization asymmetry associated with Bhabha scattering can lead to a unique identification of the neutral electroweak gauge bosons of the Lee-Wick type.

Analysis of the neutralino system in three–body leptonic decays of neutralinos

Neutralinos $\tilde{\chi}^0$ in supersymmetric theories, the spin--1/2 Majorana--type superpartners of the U(1) and SU(2) neutral electroweak gauge bosons and SU(2) neutral Higgs bosons, are expected to be among light supersymmetric particles so that they can be produced copiouslyvia direct pair production and/or from cascade decays of other sparticles such as sleptons at the planned Large Hadron Collider and the prospective International Linear Collider. Considering the prospects of having both highly polarized neutralinos and possibility of reconstructing their decay rest frames, we provide a systematic investigation of the three--body leptonic decays of the neutralinos in the minimal supersymmetric standard model and demonstrate alternative ways for probing the Majorana nature of the neutralinos and CP violation in the neutralino system.

Mass gap effects and higher order electroweak Sudakov logarithms

The infrared structure of spontaneously broken gauge theories is phenomenologically very important and theoretically a challenging problem. Various attempts have been made to calculate the higher-order behavior of large double-logarithmic (DL) corrections originating from the exchange of electroweak gauge bosons resulting in contradictory claims. We present results from two-loop electroweak corrections for the process g→f R f ̄ L to DL accuracy. This process is ideally suited as a theoretical model reaction to study the effect of the mass gap of the neutral electroweak gauge bosons at the two-loop level. Contrary to recent claims in the literature, we find that the calculation performed with the physical Standard Model fields is in perfect agreement with the results from the infrared evolution equation method. In particular, we can confirm the exponentiation of the electroweak Sudakov logarithms through two loops.