Constraints on the Z-Z′ mixing angle from data measured for the process e+e? → W+W? at the LEP2 collider

An analysis of effects induced by new neutral gauge Z′ bosons was performed on the basis of data from the OPAL, DELPHI, ALEPH, and L3 experiments devoted to measuring differential cross sections for the process of the annihilation production of pairs of charged gauge W ^± bosons at the LEP2 collider. By using these experimental data, constraints on the Z′-boson mass and on the angle of Z-Z′ mixing were obtained for a number of extended gauge models.     

Signals of supersymmetry at the Z0 resonance

The Z⁰ boson can have a sizeable branching ratio into a pair of neutralinos (the fermionic superpartners of the neutral gauge and Higgs bosons). All the subsequent decays of the heavier neutralinos lead to very distinctive signatures in e⁺e⁻ collisions at the Z⁰ pole. This may even be an efficient way to produce the Higgs boson. We study the various branching ratios as functions of the parameters of the underlying supersymmetric theory, making no ad hoc assumptions on the actual composition of the physical neutralino states.     

Flavour-changing neutral currents in models with extra<Emphasis Type="Italic">Z′</Emphasis> boson

New neutral gauge bosonsZ′ are the features of many models addressing the physics beyond the standard model. Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict new charged fermions also. A mixing of the known fermions with new states, with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree-level flavour-changing neutral interactions mediated byZ exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour-changing neutral currents are mainly due to the exchange of the new neutral gauge bosonZ′. We review flavour-changing neutral currents in models with extraZ′ boson. Then we discuss some flavour-changing processes forbidden in the standard model and new contributions to standard model processes.     

Relations between the masses of the superpartners of leptons and quarks,the goldstino coupling and the neutral currents

The mass² splittings between leptons and quarks and their spin-0 partners under supersymmetry are related to the goldstino couplings. The bosonic partner of the goldstino cannot be the photon itself. But it should be, in part, a linear combination of the various neutral gauge bosons. As a result, mass relations constrain the neutral current structure of the theory. They require the existence of at least two neutral gauge bosons in addition to the photon and suggest the possibility of a universal mass² splitting between leptons and quarks and their spin-0 partners.     

Searches for new neutral gauge Z′ bosons at the e+e? International Linear Collider and their identification

The potential of the electron-positron International Linear Collider for searches for and the separation of signals induced by new neutral gauge bosons predicted by various classes of models featuring an extended gauge sector is investigated. The analysis presented in this article was performed for processes of annihilation fermion-pair production and was based on the use of differential polarization observables, which ensure a higher sensitivity (in relation to integrated observables) of the processes being considered to Z′-boson parameters. Thresholds for discovering and identifying new neutral gauge bosons associated with models belonging to the E ₆ and LR, as well as the ALR and SSM, classes are determined. In particular, it is shown that polarization experiments at a 0.5-TeV electron-positron collider of integrated luminosity 100 fb⁻¹ would make it possible to identify unambiguously the entire set of Z′-boson models (Z′_SSM, Z′ _φ , Z′ _η , Z′ _χ , Z′_LRS, and Z′_ALR) for M _Z′ < 6√s and to improve considerably the respective estimates expected from experiments with unpolarized particles.     

Spin asymmetries in large-pT production of gauge bosons based on quantum chromodynamics and electroweak gauge model

We make predictions for spin asymmetries in the production of gauge bosons, such as Z⁰, Wsu± and γ, at large p_T from the collision of longitudinally polarized hadrons using QCD perturbation theory and the electroweak gauge model in the framework of a hard scattering model. Asymmetries typically at the 20% and 4% level are predicted for the weak bosons and photon, respectively. Experimental tests should be possible in the near future, e.g., at Fermilab, ISABELLE and colliding SPS.     

O(5)×U(1) electroweak gauge theory and the neutrino oscillations in matter

A study is made of the groupO(5)×U(1). The group is economical in the number of gauge bosons, which we associate with each of its generators, and is anomaly-free. The left-handed leptonsL _L ^T (v _e ,e,,v ) _L are assigned to the four-dimensional spinorial representations ofO(5). The right-handed particles are taken to be the singlets of the group. The theory has three sets of gauge bosons: (1) analogues of the GWS model, (2) additional charged gauge bosons, and (3) a set of three additional neutral gauge bosons as compared to the GWS model. We introduce neutrino mixing by mixing the additional charged gauge bosons. We develope a theory of neutrino oscillations in matter in such a way that in the absence of matter the scattering length reduces to the usual scattering length in vacuum. Even if the neutrino masses are equal or the neutrinos are massless, we still have neutrino oscillations in matter, a result already noted by Wolfenstein.     

SU(2) × SU(2) Electroweak Theory II: Chiral and Vector Fields on the Physical Vacuum

In this letter a discussion is offered on how symmetry breaking of a theory with twisted bundle of two chiral SU(2) bundles leads to a set of gauge potentials from each group on the physical vacuum that are vector and chiral. The result is that symmetry breaking of this theory leads to massive A ³ transverse modes of the 3-photon along with electromagnetic photons plus the massive neutral and charged weakly interacting bosons. The electromagnetic sector is demonstrated to be a massless vector field and the remainder is a broken chiral field theory.     

Searching for light Higgs scalar bosons in the next generation of electron-positron collider at LEP

Thee ⁺ e ^–-collider facilities at LEP II, with the CM energy S in the range 100–170 GeV, may be able to detect light Higgs bosons, assuming a high luminosity. The production cross sections of a light Higgs bosonH ⁰ in association with the neutral gauge bosonZ ⁰ are calculated for varying ranges of the CM energy expected to be available to LEP II and VLEEP (Novosibirsk) and for various values of the light Higgs mass. It is found that production cross sections are sizable in comparison with those for the very massive Higgs bosons in proton-anti(proton) supercolliders, Tevatron, Sp¯pS, and SSC, respectively. The implication of this feature is pointed out. Further, prospects for light Higgs production in association with the charged gauge bosonW ^– in ultraenergetic neutrino beams are examined.     

Search for Stueckelberg <Emphasis Type="Italic">Z</Emphasis>′ at LHC

We present the discovery potential for heavy Z′ gauge bosons in the Z′→e ⁺ e ⁻ decay channel at the Large Hadron Collider. The analysis focuses on the direct search for the massive Z′ bosons predicted by the Stueckelberg extension of the Standard Model. Using signal and background simulated events, and taking into account the basic parameters of the Compact Muon Solenoid detector, the discovery reach for the Stueckelberg Z′ is found to be in the range between 800 and 900 GeV/c² for an integrated luminosity of 30 fb⁻¹.     

Anomalously interacting new extra vector bosons and their first LHC constraints

In this review phenomenological consequences of the Standard Model extension by means of new spin-1 chiral fields with the internal quantum numbers of the electroweak Higgs doublets are summarized. The prospects for resonance production and detection of the chiral vector Z* and W*^± bosons at the LHC energies are considered on the basis of quantitative simulations within the CompHEP/CalcHEP package. The Z* boson can be observed as a Breit-Wigner resonance peak in the invariant dilepton mass distributions in the same way as the well-known extra gauge Z?? bosons. However, the Z* bosons have unique signatures in transverse momentum, angular and pseudorapidity distributions of the final leptons, which allow one to distinguish them from other heavy neutral resonances. In 2010, with 40 pb^?1 of the LHC proton-proton data at the energy 7 TeV, the ATLAS detector was used to search for narrow resonances in the invariant mass spectrum of e ⁺ e ^? and ??⁺??^? final states and high-mass charged states decaying to a charged lepton and a neutrino. No statistically significant excess above the Standard Model expectation was observed. The exclusion mass limits of 1.15 and 1.35 TeV/c ² were obtained for the chiral neutral Z* and charged W* bosons, respectively. These are the first direct limits on the W* and Z* boson production. Based on the above, a novel strategy for the chiral boson search in the LHC dijet data is discussed. For almost all currently considered exotic models the relevant signal is expected in the central dijet rapidity region y _1,2 ? 0 and |y ₁ ? y ₂| ? 0. On the contrary, the chiral bosons do not contribute to this region but produce an excess of dijet events far away from it. In particular, for these bosons the appropriate kinematical restrictions lead to a dip in the centrality ratio distribution over the dijet invariant mass instead of a bump expected in the most exotic models.     

Neutral currents in left-right symmetric gauge models

We analyse all the neutral-current phenomena following from the general class of gauge models based on the group SU(2) _L ⊗ SU(2) _R ⊗ U(1). It is found that the neutral-current couplings in these models bear a remarkable similarity to those in the standard Weinberg-Salam gauge model. The parameter which plays the role of sin²ϑ_w is found to lie between 0 and 1/2. Comparison with experimental data shows that even a model with the ratio of the masses of the twoZ bosons as small as 1.9 is not ruled out.     

Hadron collider potential for excited bosons search

The e ⁺ e ^? and μ⁺μ^? dilepton final states are the most clear channels for a new heavy neutral resonance search. Their advantage is that usually in the region of expected heavy-mass resonance peak the main irreducible background, from the Standard Model Drell-Yan process, contributes two orders of magnitude smaller than the signal. In this paper we consider the future prospects for search for the excited neutral Z*-bosons. The bosons can be observed as a Breit-Wigner resonance peak in the dilepton invariant mass distributions in the same way as the well-known extra gauge Z′ bosons. However, the Z* bosons have unique signatures in transverse momentum, angular and pseudorapidity distributions of the final leptons, which allow to distinguish them from the other heavy neutral resonances. At present only the ATLAS Collaboration has looked for such new excitations at the Large Hadron Collider and has published its results for 7 TeV collision energy. After successful comparison of our evaluation with these official results we present our estimations for the discovery potential and the exclusion limits on the Z*-boson search in pp collisions at higher centre-of-mass energies and different luminosities. In particular, LHC Run 2 can discover Z*-boson with its mass up to 5.3 TeV, while the High Luminosity LHC can extend that reach to 6.2 TeV. The High Energy LHC (with collision energy of 33 TeV) will be able to probe two times heavier resonance masses at the same integrated luminosities.     

Tests of composite models with Z0 decay modes

If Z⁰ and W^± gauge bosons are composite objects, several multiphoton and multigluon Z⁰ decay modes can become well observable and can be used to test the nature of the subconstituents.     

e + e −-Annihilation with polarized beams in the standard model and extended electroweak gauge models

We derive the general expressions for the differential cross section and final state polarization ine ⁺ e ^?→γ,Z ₁ ⁰ ,Z ₂ ⁰ , ...→ff (f: leptons and quarks) with several neutral bosons and with polarized beams. From this also the jet axis distributions ine ⁺ e ^? → 2 jets are obtained. The general formulas are applied to the standard model and extended gauge models of electroweak interactions with one neutral boson lighter than the standard modelZ ⁰. The models differ from each other in the coefficients of angular distributions at energies around the firstZ ⁰ resonance and beyond. Observable effects are expected in certain models already at energies up to 60 GeV if the firstZ ⁰ is not too heavy.     

Confinement and mass gap in abelian gauge

First, we present a simple confining abelian pure gauge theory. Classically, its kinetic term is not positive definite, and it contains a simple UV regularized F⁴ interaction. This provokes the formation of a condensate such that, at the saddle point of the effective potential, the wave function normalization constant of the abelian gauge fields vanishes exactly. Then we study SU(2) pure Yang-Mills theory in an abelian gauge and introduce an auxiliary field for a BRST invariant condensate of dimension 2, which renders the charged sector massive. Under simple assumptions its effective low energy theory reduces to the confining abelian model discussed before, and the VEV of is seen to scale correctly with the renormalization point. Under these assumptions, the confinement condition Z _eff = 0 also holds for the massive charged sector, which suppresses the couplings of the charged fields to the abelian gauge bosons in the infrared regime. Received: 27 November 2002 / Published online: 14 April 2003 RID="a" ID="a" e-mail: Ulrich.Ellwanger@th.u-psud.fr RID="b" ID="b" e-mail: Nicolas.Wschebor@th.u-psud.fr ^* Unité Mixte de Recherche - CNRS - UMR 8627     

Identification of a horizontal gauge boson in hadronic collisions

We compare distributions of leptons produced in hadronic collisions, by a horizontal neutral gauge boson, suggested by the model, to those produced by other theoretically motivated neutral gauge bosons occurring in left–right symmetric models and in superstring-inspired models. Forward–backward asymmetries in the gauge boson leptonic decay are found to be sensitive to specific forms of the couplings. The asymmetries are expected to be maximal for distinguishing it from the other Z's. Received: 17 September 2001 / Published online: 7 December 2001     

Discriminating between Z′-boson effects and effects of anomalous gauge couplings in the double production of W ± bosons at a linear collider

The potential of the International Linear electron-positron Collider (ILC) for seeking, in the annihilation production of W ^±-boson pairs, signals induced by new neutral gauge bosons predicted by models belonging to various classes and featuring an extended gauge sector is studied. Limits that will be obtained at ILC for the parameters and masses of Z′ bosons are compared with present-day and future data from the Large Hadron Collider (LHC). The possibility of discriminating between the effects of Z-Z′ mixing and signals induced by anomalous gauge couplings (AGC) is demonstrated within theoretically motivated trilinear gauge models involving several free anomalous parameters. It is found that the sensitivity of ILC to the effects of Z-Z′ mixing in the process e ⁺ e ^? → W ⁺ W ^? and its ability to discriminate between these two new-physics scenarios, Z′ and AGC, become substantially higher upon employing polarized initial (e ⁺ e ^?) and final (W ^±) states.     

Quantum correlations of bosons and fermions produced in heavy ion collisions

A quantum statistical approach to simulate Bose-Einstein correlations of many boson systems is presented. The extension to fermions and Coulomb-interacting bosons is discussed. This approach appears to be very efficient and is applicable also to cases with very high multiplicities. A technique to analyze pion correlations via their counting distributions is developed. The exact counting distributions for bosons as well as for fermions are derived. The problem of incomplete data occuring in detectors with an acceptance angle <> is studied. The application to Monte Carlo generated pion distributions show that this technique offers a valuable supplement to the usual Hanbury-Brown, Twiss method.     

Minimal left-right symmetric models and new <Emphasis Type="Italic">Z</Emphasis>’ properties at future electron-positron colliders

It was recently shown that left-right symmetric models for elementary particles can be built with only two Higgs doublets. The general consequence of these models is that the left and right fermionic sectors can be connected by a new neutral gauge boson Z having its mass as the only additional new parameter. In this paper we study the influence of the fundamental fermionic representation for this new neutral gauge boson. Signals of possible new heavy neutral gauge bosons are investigated for the future electron-positron colliders at GeV, 1 TeV and 3 TeV. The total cross sections, forward-backward and left-right asymmetries and model differences are calculated for the process . Bounds on Z masses are estimated.Received: 4 May 2004, Revised: 22 September 2004, Published online: 9 November 2004