Gauge Boson Pair Research Articles

Dilepton and four-lepton signals at the LHC in the Littlest Higgs model with T-parity violation

In the presence of the T-parity violating Wess-Zumino-Witten (WZW) anomaly term, the otherwise stable heavy photon A_H in the Littlest Higgs model with T-parity (LHT) decays to either Standard Model (SM) gauge boson pairs, or to SM fermions via loop diagrams. We make a detailed study of the collider signatures where the A_H can be reconstructed from invariant mass peaks in the opposite sign same flavor dilepton or the four-lepton channels. This enables us to obtain information about the fundamental symmetry breaking scale f in the LHT and thereby the low-lying mass spectrum of the theory. In addition, indication of the presence of the WZW term gives us hints of the possible UV completion of the LHT via strong dynamics. The crucial observation is that the sum of all production processes of heavy T-odd quark pairs has a sizeable cross-section at the LHC and these T-odd particles eventually all cascade decay down to the heavy photon A_H. We show that for certain regions of the parameter space with either a small f of around 500 GeV or relatively light T-odd quarks with a mass of around 400 GeV, one can reconstruct the A_H even at the early LHC run with \sqrt{s}=10 TeV and a modest integrated luminosity of 200 pb^{-1}. At \sqrt{s} = 14 TeV and with an integrated luminosity of 30 fb^{-1}, it is possible to cover a large part of the typical parameter space of the LHT, with the scale f up to 1.5 TeV and with T-odd quark masses almost up to 1 TeV. In this region of the parameter space, the mass of the reconstructed A_H ranges from 66 GeV to 230 GeV.

Octetonium at the LHC

Several models of new physics, such as grand unified theories, Pati-Salam models, chiral color models, etc., predict the existence of an $SU(2)_L$ doublet of color-octet scalars (COS). In the Manohar-Wise model, the Yukawa couplings of the COS are assumed to be consistent with Minimal Flavor Violation ensuring constraints from flavor physics are satisfied even for relatively light scalars. In this simple model we consider the production of color singlet bound states of COS that we call octetonium. Octetonium are mainly produced via gluon-gluon fusion and have significant production cross sections at the LHC. They can decay to pairs of gluons or electroweak gauge bosons. If the masses of the octetonia are 1 TeV or less, these states will be visible as resonances in $\gamma \gamma, W^+W^-, ZZ$, and $ \gamma Z$.

Vectorlike confinement at the LHC

We argue for the plausibility of a broad class of vectorlike confining gauge theories at the TeV scale which interact with the Standard Model predominantly via gauge interactions. These theories have a rich phenomenology at the LHC if confinement occurs at the TeV scale, while ensuring negligible impact on precision electroweak and flavor observables. Spin-1 bound states can be resonantly produced via their mixing with Standard Model gauge bosons. The resonances promptly decay to pseudo-Goldstone bosons, some of which promptly decay to a pair of Standard Model gauge bosons, while others are charged and stable on collider time scales. The diverse set of final states with little background include multiple photons and leptons, missing energy, massive stable charged particles and the possibility of highly displaced vertices in dilepton, leptoquark or diquark decays. Among others, a novel experimental signature of resonance reconstruction out of massive stable charged particles is highlighted. Some of the long-lived states also constitute Dark Matter candidates.

Relic density at one-loop with gauge boson pair production

We have computed the full one-loop corrections (electroweak as well as QCD) to processes contributing to the relic density of dark matter in the MSSM where the LSP is the lightest neutralino. We cover scenarios where the most important channels are those with gauge boson pair production. This includes the case of a bino with some wino admixture, a higgsino and a wino. In this paper we specialise to the case of light dark matter much below the TeV scale. The corrections can have a non-negligible impact on the predictions and should be taken into account in view of the present and forthcoming increasing precision on the relic density measurements. Our calculations are made with the help of \SloopS, an automatic tool for the calculation of one-loop processes in the MSSM. The renormalisation scheme dependence of the results as concerns $\tgb$ is studied.

Signals for new spin-1 resonances in electroweak gauge boson pair production at the LHC

The mechanism of electroweak symmetry breaking (EWSB) will be directly scrutinized soon at the CERN Large Hadron Collider. We analyze the LHC potential to look for new vector bosons associated with the EWSB sector, presenting a possible model independent approach to search for these new spin-1 resonances. We show that the analyses of the processes $pp\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{'}\ensuremath{-}}{\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}$, ${\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}jj{\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}$, ${\ensuremath{\ell}}^{\ensuremath{'}\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}{\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}$, ${\ensuremath{\ell}}^{\ifmmode\pm\else\textpm\fi{}}jj{\mathrm{E\ensuremath{\llap{\not\;}}}}_{T}$, and ${\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}jj$ (with $\ensuremath{\ell}$, ${\ensuremath{\ell}}^{\ensuremath{'}}=e$ or $\ensuremath{\mu}$ and $j=\mathrm{\text{jet}}$) have a large reach at the LHC and can lead to the discovery or exclusion of many EWSB scenarios such as Higgsless models.

Some consequences of a Higgs triplet

We consider an extension of the scalar sector of the Standard Model with a single complex Higgs triplet X. Such extensions are the most economic, model-independent way of generating neutrino masses through triplet interactions. We show that a term like a0ΦΦX† must be included in the most general potential of such a scenario in order to avoid a massless neutral physical scalar. We also demonstrate that a0 must be real, thus ruling out any additional source of CP violation. We then examine the implications of this term in the mass matrices of the singly and doubly charged scalar, neutral scalar and pseudoscalar fields. We find that, for small values of a0/v2, where v2 is the triplet vev, the spectrum allows the decay of heavier scalars into lighter ones via gauge interactions. For large a0/v2, the doubly charged, singly charged and neutral pseudoscalar bosons become practically degenerate, while the even-parity neutral scalars remain considerably lighter, thus emphasizing the possibility of decay of the singly charged or neutral pseudoscalar states into the neutral scalars. Constraints from the ρ-parameter are used to find nontrivial limits on the charged Higgs mass depending on a0. We also study the couplings of the various physical states in this scenario. For small values of |a0|/v2, we find the lightest neutral scalar field to be triplet dominated, and thus having extremely suppressed interactions with fermion as well as gauge–boson pairs.

Quark-antiquark energy density function applied to Di-Gauge boson production at the LHC

In view of the start up of the 14 TeV pp Large Hadron Collider the quark anti-quark reactions leading to the final states W+W−, W±Z0 and Z0Z0 are studied, in the frame work of the Standard Model (SM), using helicity amplitudes. The differential and total cross sections are first evaluated in the parton anti-parton center of mass system. Subsequently they are transformed to their expected structure in pp collisions through the parton anti-parton Energy Density Functions which are here derived from the known Parton Density Functions of the proton. In particular the single and joint longitudinal polarizations of the di-boson final states are calculated. The effect on these reactions from the presence of s-channel heavy vector bosons, like the W' and Z', are evaluated to explore the possibility to utilize the gauge boson pair production as a probe for these 'Beyond the SM' phenomena.

Trilepton production at the CERN LHC: Standard model sources and beyond

Events with three or more isolated leptons in the final state are known to be signatures of new physics phenomena at high energy collider physics facilities. Standard model sources of isolated trilepton final states include gauge boson pair production such as $WZ$ and $W{\ensuremath{\gamma}}^{*}$, and $t\overline{t}$ production. We demonstrate that leptons from heavy flavor decays, such as $b\ensuremath{\rightarrow}lX$ and $c\ensuremath{\rightarrow}lX$, provide sources of trileptons that can be orders of magnitude larger after cuts than other standard model backgrounds to new physics processes. We explain the physical reason heavy flavor backgrounds survive isolation cuts. We propose new cuts to control the backgrounds in the specific case of chargino plus neutralino pair production in supersymmetric models. After these cuts are imposed, we show that it should be possible to find at least a $4\ensuremath{\sigma}$ excess for supersymmetry parameter space point LM9 with $30\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity.

A mechanism for Kaluza–Klein number violation in UED and implications for LHC

We extend our previous analysis on a model where the Higgs does not propagate into an otherwise universal extra dimension. Previously, we computed the cross section for the pair production of Kaluza–Klein excitations of the top quark based on mixing in the Yukawa interactions between the zero-mode top quark and its lowest-lying Kaluza–Klein excitations. In this work, we show that there is also violation of tree-level Kaluza–Klein number conservation in the gauge sector. This allows Kaluza–Klein excitations of the fermions to be produced singly, and opens up new decay channels in the gauge sector. Here, we compute the cross section for the production of the lowest-lying Kaluza–Klein excitations of the top and bottom quarks in the non-universal Higgs model at the Large Hadron Collider, and consider their subsequent decays, including effects from mixing in the gauge sector. The effect is quite large: for example, the pair production of Kaluza–Klein excitations of the bottom quark leads to a pair of top quarks plus a pair of charged weak gauge bosons. Since the production is via the strong interaction, such final states stand out well against the Standard Model background, and are observable at the LHC for a wide range of compactification scales.

Neutrino masses and the CERN LHC: Testing the type II seesaw mechanism

We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (type II seesaw) at the LHC, in which a Higgs triplet is introduced. In the optimistic scenarios with a small Higgs triplet vacuum expectation value ${v}_{\ensuremath{\Delta}}<{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, one can look for clean signals of lepton-number violation in the decays of doubly charged (${H}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$) and singly charged (${H}^{\ifmmode\pm\else\textpm\fi{}}$) Higgs bosons to distinguish the normal hierarchy (NH), the inverted hierarchy (IH), and the quasidegenerate (QD) spectrum for the light neutrino masses. The observation of either ${H}^{+}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}\overline{\ensuremath{\nu}}$ or ${H}^{+}\ensuremath{\rightarrow}{e}^{+}\overline{\ensuremath{\nu}}$ will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases. The ${H}^{++}$ decays moderately depend on a Majorana phase ${\ensuremath{\Phi}}_{2}$ in the NH, but sensitively depend on ${\ensuremath{\Phi}}_{1}$ in the IH. In a less favorable scenario ${v}_{\ensuremath{\Delta}}>2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, when the leptonic channels are suppressed, one needs to observe the decays ${H}^{+}\ensuremath{\rightarrow}{W}^{+}{H}_{1}$ and ${H}^{+}\ensuremath{\rightarrow}t\overline{b}$ to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation. In the most optimistic situation, ${v}_{\ensuremath{\Delta}}\ensuremath{\sim}{10}^{\ensuremath{-}4}\text{ }\text{ }\mathrm{GeV}$, both channels of the lepton pairs and gauge boson pairs may be available simultaneously. The determination of their relative branching fractions would give a measurement for the value of ${v}_{\ensuremath{\Delta}}$.

Anomalous VVH interactions at a linear collider

We examine, in a model independent way, the sensitivity of a linear collider to the couplings of a light Higgs boson to a pair of gauge bosons, including the possibility of CP violation. We construct several observables that probe the various possible anomalous couplings. For an intermediate mass Higgs, a collider operating at a center of mass energy of 500 GeV and with an integrated luminosity of 500 fb−1 is shown to be able to constrain the ZZH vertex at the few per cent level, with even higher sensitivity for some of the couplings. However, lack of sufficient number of observables as well as contamination from the ZZH vertex limits the precision to which anomalous part of the WWH coupling can be probed.

Logarithmic electroweak corrections to gauge-boson pair production at the LHC

We have studied the effects of the complete logarithmic electroweak O ( α ) corrections on the production of vector-boson pairs WZ, ZZ, and WW at the LHC. These corrections are implemented into a Monte Carlo program for p p → 4 f ( + γ ) with final states involving four or two leptons using the double-pole approximation. We numerically investigate purely leptonic final states and find that electroweak corrections lower the predictions by 5–30% in the physically interesting region of large di-boson invariant mass and large angle of the produced vector bosons.

First order QCD corrections to the weak gauge boson pair production in hadronic collisions

A complete next-to-leading logarithm (NLL) to the weak gauge boson pair production in hadron colliders pp → ZZ + X, which involves computing the contributions from the 2 → 3 real gluon emission as well as the one loop correction to the 2 → 2 process are presented. The calculation utilizes a combination of analytic and Monte Carlo integration methods.

Two-bodyZ′decays in the minimal 3-3-1 model

The two-body decays of the extra neutral boson Z_2 predicted by the minimal 331 model are analyzed. At the three-level it can decay into standard model particles as well as exotic quarks and the new gauge bosons predicted by the model. The decays into a lepton pair are strongly suppressed, with $Br(Z_2 --> l^+l^-) ~ 10^{-2}$ and $Br(Z_2 --> \bar{\nu}_l \nu) ~ 10^{-3}$. In the bosonic sector, Z_2 would decay mainly into a pair of bilepton gauge bosons, with a branching ratio below the 0.1 level. The Z_2 boson has thus a leptophobic and bileptophobic nature and it would decay dominantly into quark pairs. The anomaly-induced decays $Z_2 --> Z_1\gamma$ and $Z_2 --> Z_1Z_1$, which occurs at the one-loop level are studied. It is found that $Br(Z_2 --> Z_1\gamma) ~ 10^{-9}$ and $Br(Z_2 --> Z_1Z_1) ~ 10^{-6}$ at most. As for the $Z_2 --> W^+W^-$ and $Z_2 --> Z_1H$ decays, with H a relatively light Higgs boson, they are induced via Z'-Z mixing. It is obtained that $Br(Z_2 --> W^+W^-) ~ 10^{-2}$ and $Br (Z_2 --> Z_1H) ~ 10^{-5}$. We also examine the flavor changing neutral current decays $Z_2 --> tc$ and $Z_2 --> tu$, which may have branching fractions as large as $10^{-3}$ and $10^{-5}$, respectively, and thus may be of phenomenological interest.

Matching NLO QCD and parton showers in heavy flavour production

We apply the MC@NLO approach to the process of heavy flavour hadroproduction. MC@NLO is a method for matching next-to-leading order (NLO) QCD calculations and parton shower Monte Carlo (MC) simulations, with the following features: fully exclusive events are generated, with hadronisation according to the MC model; total rates are accurate to NLO; NLO results for distributions are recovered upon expansion in $\as$; hard emissions are treated as in NLO computations while soft/collinear emissions are handled by the MC simulation, with the same logarithmic accuracy as the MC; matching between the hard and soft regions is smooth, and no intermediate integration steps are necessary. The method was applied previously to the hadroproduction of gauge boson pairs, which at NLO involves only initial-state QCD radiation and a unique colour structure. In heavy flavour production, it is necessary to include contributions from final-state QCD radiation and different colour flows. We present illustrative results on top and bottom production at the Tevatron and LHC.

Electroweak-correction effects in gauge-boson pair production at the CERN LHC

We have studied the eect of one-loop logarithmic electroweak radiative corrections on WZ and W production processes at the LHC. We present analytical results for the leading-logarithmic electroweak corrections to the corresponding partonic processes du ! WZ; W. Using the leading-pole approximation we implement these corrections into Monte Carlo programs for pp ! lll 0 l 0 ;l l. We nd that electroweak corrections lower the predictions by 5{20% in the physically interesting region of large transverse momentum and small rapidity separation of the gauge bosons.

Tests of low scale gravity via gauge boson pair production in γγ collisions

Arkani-Hamed, Dimopoulos and Dvali have recently proposed that gravity may become strong at energies near 1 TeV thus removing the hierarchy problem. This scenario can be tested in several ways at present and future colliders. In this paper we examine the exchange of towers of Kaluza-Klein gravitons and their influence on the production of pairs of massive gauge bosons in $\gamma \gamma$ collisions. These tower exchanges are shown to lead to a new dimension-8 operator that can significant alter the Standard Model expectations for these processes. The role of polarization for both the initial state photons and the final state gauge bosons in improving sensitivity to graviton exchange is emphasized. We find that the discovery reach for graviton tower exchange in the $\gamma \gamma \to W^+W^-$ channel to be significantly greater than for any other process so far examined.

Deconstructing angular correlations inZH,ZZ,andWWproduction at CERN LEP 2

We apply a generalized spin-basis analysis to associated Higgs production and gauge boson pair production at LEP. This framework allows us to identify a choice of spin axes for the processes e+ e- --> ZH,ZZ which leads to strikingly different correlations among the decay products, even well above threshold. This spin basis optimizes the difference in the angular correlations for these two processes. In contrast, the same distributions display little contrast when the helicity basis is used. We also apply this technique to the case of W boson pair production.

Limits on anomalousWWγandWWZcouplings

Limits on the anomalous WWγ and WWZ couplings are presented from a simultaneous fit to the data samples of three gauge boson pair final states in pp collisions at √s=1.8TeV: Wγ production with the W boson decaying to eν or μν, W boson pair production with both of the W bosons decaying to eν or μν, and WW or WZ production with one W boson decaying to eν and the other W boson or the Z boson decaying to two jets. Assuming identical WWγ and WWZ couplings, 95% C.L. limits on the anomalous couplings of -0.30<Δκ<0.43 (λ=0) and -0.20<λ<0.20 (Δκ=0) are obtained using a form factor scale Λ=2.0TeV. Limits found under other assumptions on the relationship between the WWγ and WWZ couplings are also presented.

Two-loop electroweak heavy-fermion corrections to Higgs-boson production and decay

The dominant electroweak corrections to the production cross sections and partial decay widths of a light Standard Model Higgs boson, with mass M H ⪡ 2 m t , are due to top-quark loops. By means of a low-energy theorem, we study at the two-loop level the leading shifts in the Higgsboson couplings to pairs of light fermions and gauge bosons induced by a sequential isodoublet of high-mass fermions. For tree-level and loop-induced Higgs-boson couplings, these corrections are of relative order O( G μ 2 m F 4)) and O( G μ m F 2), respectively, where m F represents a generic heavy-fermion mass, with m F ⪢ M W , M H . Except for the Hbb coupling, all results carry over to the case of the top-quark-induced corrections. We discuss possible phenomenological consequences of our results.