Kaluza-Klein Excitations Research Articles

Non-minimal Universal Extra Dimensions with Brane Local Terms: the top quark sector

We study the physics of Kaluza-Klein (KK) top quarks in the framework of a non-minimal Universal Extra Dimension (nmUED) with an orbifolded (S1/Z2) flat extra spatial dimension in the presence of brane-localized terms (BLTs). In general, BLTs affect the masses and the couplings of the KK excitations in a non-trivial way including those for the KK top quarks. On top of that, BLTs also influence the mixing of the top quark chiral states at each KK level and trigger mixings among excitations from different levels with identical KK parity (even or odd). The latter phenomenon of mixing of KK levels is not present in the popular UED scenario known as the minimal UED (mUED) at the tree level. Of particular interest are the mixings among the KK top quarks from level `0' and level `2' (driven by the mass of the Standard Model (SM) top quark). These open up new production modes in the form of single production of a KK top quark and the possibility of its direct decays to Standard Model (SM) particles leading to rather characteristic signals at the colliders. Experimental constraints and the restrictions they impose on the nmUED parameter space are discussed. The scenario is implemented in MadGraph 5 by including the quark, lepton, the gauge-boson and the Higgs sectors up to the second KK level. A few benchmark scenarios are chosen for preliminary studies of the decay patterns of the KK top quarks and their production rates at the LHC in various different modes. Recast of existing experimental analyzes in scenarios having similar states is found to be not so straightforward for the KK top quarks of the nmUED scenario under consideration.

Mass hierarchy, mass gap and corrections to Newton’s law on thick branes with Poincaré symmetry

We consider a scalar thick brane configuration arising in a 5D theory of gravity coupled to a self-interacting scalar field in a Riemannian manifold. We start from known classical solutions of the corresponding field equations and elaborate on the physics of the transverse traceless modes of linear fluctuations of the classical background, which obey a Schroedinger-like equation. We further consider two special cases in which this equation can be solved analytically for any massive mode with m^2>0, in contrast with numerical approaches, allowing us to study in closed form the massive spectrum of Kaluza-Klein (KK) excitations and to compute the corrections to Newton's law in the thin brane limit. In the first case we consider a solution with a mass gap in the spectrum of KK fluctuations with two bound states - the massless 4D graviton free of tachyonic instabilities and a massive KK excitation - as well as a tower of continuous massive KK modes which obey a Legendre equation. The mass gap is defined by the inverse of the brane thickness, allowing us to get rid of the potentially dangerous multiplicity of arbitrarily light KK modes. It is shown that due to this lucky circumstance, the solution of the mass hierarchy problem is much simpler and transparent than in the (thin) Randall-Sundrum (RS) two-brane configuration. In the second case we present a smooth version of the RS model with a single massless bound state, which accounts for the 4D graviton, and a sector of continuous fluctuation modes with no mass gap, which obey a confluent Heun equation in the Ince limit. (The latter seems to have physical applications for the first time within braneworld models). For this solution the mass hierarchy problem is solved as in the Lykken-Randall model and the model is completely free of naked singularities.

Formula omitted] Higgs in next-to-minimal Universal Extra Dimensions

The discovery of a Higgs boson and precise measurements of its properties open a new window to test physics beyond the standard model. Models with Universal Extra Dimensions are not an exception. Kaluza–Klein excitations of the standard model particles contribute to the production and decay of the Higgs boson. In particular, parameters associated with third generation quarks are constrained by Higgs data, which are relatively insensitive to other searches often involving light quarks and leptons. We investigate implications of the 126 GeV Higgs in next-to-minimal Universal Extra Dimensions, and show that boundary terms and bulk masses allow a lower compactification scale as compared to in minimal Universal Extra Dimensions.

Kaluza-Klein gluon searches using the three-b-jet decay channel at the Large Hadron Collider

We study observability of a Kaluza-Klein (KK) excitation of a gluon in a five-dimensional model with a warped geometry at the Large Hadron Collider. In this model, the Standard Model fields reside in the bulk and the third generation quarks couple to the KK gluon strongly. We focus on the processes including three $b$ quarks as a final state where the first KK gluon propagates as an intermediate state. We evaluate a significance of those processes by taking account of kinematical cuts and a detector efficiency at the Large Hadron Collider and find that the significance is lager than $5\ensuremath{\sigma}$ with the integrated luminosity of $10\text{ }(100)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ for a certain range of parameters of the model.

Search fortt¯resonances in the lepton plus jets final state with ATLAS using4.7 fb−1ofppcollisions ats=7 TeV

A search for new particles that decay into top quark pairs (tt¯) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 4.7 fb−1 of proton–proton (pp) collision data collected at a center-of-mass energy s=7 TeV. In the tt¯→WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The tt¯ system is reconstructed using both small-radius and large-radius jets, the latter being supplemented by a jet substructure analysis. A search for local excesses in the number of data events compared to the Standard Model expectation in the tt¯ invariant mass spectrum is performed. No evidence for a tt¯ resonance is found and 95% credibility-level limits on the production rate are determined for massive states predicted in two benchmark models. The upper limits on the cross section times branching ratio of a narrow Z′ resonance range from 5.1 pb for a boson mass of 0.5 TeV to 0.03 pb for a mass of 3 TeV. A narrow leptophobic topcolor Z′ resonance with a mass below 1.74 TeV is excluded. Limits are also derived for a broad color-octet resonance with Γ/m=15.3%. A Kaluza–Klein excitation of the gluon in a Randall–Sundrum model is excluded for masses below 2.07 TeV.4 MoreReceived 13 May 2013DOI:https://doi.org/10.1103/PhysRevD.88.012004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the ATLAS Collaboration

One-loop Chern-Simons terms in five dimensions

Abstract We compute one-loop corrections to five-dimensional gauge and gravitational Chern-Simons terms induced by integrating out charged massive fields. The considered massive fields are spin-1/2 and spin-3/2 fermions, as well as complex two-forms with first order kinetic terms. Consistency with six-dimensional gravitational anomalies of (1, 0) and (2, 0) theories is shown by interpreting the massive fields as excited Kaluza-Klein modes in a circle compactification. The results are in accordance with the geometric predictions of the M-theory to F-theory duality as well as the comparison with an explicit one-loop computation in heterotic string theory compactified on K3 × S 1.

Universal extra-dimensional models with boundary localized kinetic terms: Probing at the LHC

In universal extra-dimensional models a conserved $Z_2$ parity ensures the stability of the lightest Kaluza-Klein particle, a potential dark-matter candidate. We show here that boundary-localized kinetic terms, in general, do not preserve this symmetry. We examine the single production of a Kaluza-Klein excitation of the photon and its decay to zero-mode fermion-antifermion pairs in the presence of such terms. We explore how experiments at the Large Hadron Collider at CERN can help constrain the boundary-localized kinetic terms for different choices of the compactification radius.

General self-tuning solutions and no-go theorem

We consider brane world models with one extra dimension. In the bulkthere is in addition to gravity a three form gauge potential orequivalently a scalar (by generalisation of electric magneticduality). We find classical solutions for which the 4d effectivecosmological constant is adjusted by choice of integrationconstants. No go theorems for such self-tuning mechanism arecircumvented by unorthodox Lagrangians for the three form respectivelythe scalar. It is argued that the corresponding effective 4d theoryalways includes tachyonic Kaluza-Klein excitations or ghosts. Knownno go theorems are extended to a general class of models withunorthodox Lagrangians.

The Standard Model with one universal extra dimension

Effects of universal extra dimensions on Standard Model observables first arise at the one-loop level. The quantization of this class of theories is therefore essential in order to perform predictions. A comprehensive study of the SU C(3) × SU L(2) × U Y(1) Standard Model defined in a space-time manifold with one universal extra dimension, compactified on the oribifold \(S^1/Z_2\), is presented. The fact that the four-dimensional Kaluza–Klein theory is subjected to two types of gauge transformations is stressed and its quantization under the basis of the BRST symmetry discussed. A SU C(3) × SU L(2) × U Y(1)-covariant gauge-fixing procedure for the Kaluza–Klein excitations is introduced. The connection between gauge and mass eigenstate fields is established in an exact way. An exhaustive list of the explicit expressions for all physical couplings induced by the Yang–Mills, Currents, Higgs, and Yukawa sectors is presented. The one-loop renormalizability of the standard Green’s functions, which implies that the Standard Model observables do not depend on a cut-off scale, is stressed.

LHC sensitivity to a Kaluza-Klein gluon in the twob-jets decay channel

We examine a possibility to discover a Kaluza-Klein (KK) excitation of gluon in a warped extra-dimension model at the Large Hadron Collider, focusing on a decay channel of the KK gluon into a $b$-quark pair. It is known that, in a certain extension of the warped extra-dimension model, the third generation quarks could strongly couple to the KK gluon, as a result of appropriate bulk fermion mass parameters. Taking account of kinematical cuts to reduce background events, we show the model parameter space, which leads to a significance larger than $5\ensuremath{\sigma}$ with the integrated luminosity of $10\text{ }(100)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$.

High-energy suppression of the Higgsstrahlung cross section in the Minimal Composite Higgs Model

If the Higgs boson is composite, signs of this compositeness should appear via a formfactor-like suppression of Higgs scattering cross sections at momentum transfers above the compositeness scale. We explore this by computing the cross section for e+e- ---> ZH (Higgsstrahlung) in a warped five-dimensional gauge-Higgs unification model known as the Minimal Composite Higgs Model (MCHM). We observe that the Higgsstrahlung cross section in the MCHM is strongly suppressed compared to that in the Standard Model at center-of-mass energies above the scale of the first Kaluza-Klein excitations, due to cancellations among the contributions of successive Z boson Kaluza-Klein modes. We also show that the magnitude and sign of the coupling of the first Kaluza-Klein mode can be measured at a future electron-positron collider such as the proposed International Linear Collider or Compact Linear Collider.

Sensitivity of the LHC to Kaluza-Klein gluon in two b-jets decay channel

We study a possibility of observation of the first Kaluza-Klein (KK) excitation of gluon in a warped extra dimension model at the LHC. In our analysis, we adopt the KK gluon mass and the b-quark coupling to the KK gluon as model parameters and study the sensitivity of the ATLAS experiment to observe the KK gluon through the two $b$-jets channel.

Search for heavy resonances, and resonant diboson production with the ATLAS detector

Heavy resonances decaying into a pair of fundamental particles such as jj , l+ l− , γγ , and lV , are among the most common features to search for phenomena beyond the standard model (SM). Electroweak boson pair production, such as WW or ZZ with subsequent decays to lV l′ V′ and lljj respectively, is a powerful test of the spontaneously broken gauge symmetry of the SM and can be also used to search for phenomena beyond the SM. There is a wide spectrum of theoretical models predicting these kinds of resonant signatures. This note covers several searches for these new phenomena conducted within ATLAS in 2011 and 2012 for the LHC 7 and 8 TeV center of mass energies respectively. No significant deviations from the SM have been observed and therefore, limits are set on the characteristic parameters of several new physics models. These benchmark models include new heavy Z ′ /W ′ gauge bosons, chiral excitation of the SM weak gauge bosons, Z * /W * Randal-Sundrum and ADD gravitons, Composite models for quarks, e.g. q * with substructure scale Λ , Quantum black holes, TeV−1 Kaluza-Klein excitation of γ/Z and more.

A search for $t\bar t$ resonance in the lepton plus jets channel with ATLAS using 14 fb−1of proton-proton collisions at √s= 8 TeV

The search for tresonances allows the investigation of a wide range of physics beyond the Standard Model. For a high mass resonance, the top quark is produced with a transverse momentum that is large, compared to its mass, and the decay of such a highly boosted top often leads to a topology that di↵ ers in several respects from that encountered when the top quarks are produced approximately at rest. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 14 fb 1 of proton-proton collisions data collected at center-of-mass energy p s = 8 TeV. No evidence for a t¯ t resonance is found and 95% CL limits on the production rate are determined for massive states in two benchmark models. A narrow leptophobic topcolor Z' boson with a mass below 1.8 TeV is excluded and a Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.0 TeV.

Search for resonant $ \mathrm{t}\overline{\mathrm{t}} $ production in lepton+jets events in pp collisions at $ \sqrt{s}=7 $ TeV

AbstractA model-independent search for the production of heavy resonances decaying into top-antitop quark pairs is presented. The search is based on events containing one lepton (muon or electron) and at least two jets selected from data samples corresponding to an integrated luminosity of 4.4-5.0 fb−1collected in pp collisions at$ \sqrt{s}=7 $TeV. Results are presented from the combination of two dedicated searches optimized for boosted production and production at threshold. No excess of events is observed over the expected yield from the standard model processes. Topcolor Z′ bosons with narrow (wide) width are excluded at 95% confidence level for masses below 1.49 (2.04) TeV and an upper limit of 0.3 (1.3) pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV. Kaluza-Klein excitations of a gluon with masses below 1.82 TeV (at 95% confidence level) in the Randall-Sundrum model are also excluded, and an upper limit of 0.7 pb or lower is set on the production cross section times branching fraction for resonance masses above 1 TeV.

Constraints on universal extra dimensions fromW′searches

We obtain constraints on three universal extra-dimensional models utilizing limits from the CMS Collaboration on ${W}^{\ensuremath{'}}$ production and decay into a single-top-quark final state. We find a weak constraint on the minimal universal extra-dimensional model due to small Kaluza-Klein number violating terms. In contrast, the ${W}^{\ensuremath{'}}$ search puts a strong limit on the size of the Dirac mass term of the quarks in split universal extra-dimension models. In nonminimal universal extra-dimension models, the ${W}^{\ensuremath{'}}$ search constrains the splitting between the boundary-localized kinetic terms of the gauge bosons and the quarks. Each of these bounds can be translated into constraints on the mass splitting between the Kaluza-Klein excitations of the $SU(2)$ charged quarks and the Klauza-Klein excitations of the $W$ boson.

Non-minimal universal extra dimensions: the strongly interacting sector at the Large Hadron Collider

We work out the strongly interacting sector of a non-minimal Universal Extra Dimension (nmUED) scenario with one flat extra spatial dimension orbifolded on S^1/Z_2 in the presence of brane-localized kinetic and Yukawa terms. On compactification, these terms are known to have significant, nontrivial impact on the masses and the couplings of the Kaluza-Klein (KK) excitations. We study the masses of the level `1' KK gluon and the quarks and find the modified strong interaction vertices involving these particles. The scenario conserves KK parity. Possibility of significant level-mixing among the quarks from different KK-levels is pointed out with particular reference to the top quark sector. Cross sections for various generic final states involving level `1' KK-gluon and KK-quarks from first two generations are estimated at the Large Hadron Collider (LHC) via an implementation of the scenario in MadGraph-5 with the help of FeynRules. The decay branching fractions of both strong and weakly interacting KK excitations are studied to estimate yields in various different final states involving jets, leptons and missing energy. These are used to put some conservative constraints on the nmUED parameter space using the latest LHC data. Nuances of the scenario are elucidated with reference to the minimal Universal Extra Dimension (mUED) and Supersymmetry (SUSY) and their implications for the LHC are discussed.

Higgs production in a warped extra dimension

Abstract Measurements of the Higgs-boson production cross section at the LHC are an important tool for studying electroweak symmetry breaking at the quantum level, since the main production mechanism gg → h is loop-suppressed in the Standard Model (SM). Higgs production in extra-dimensional extensions of the SM is sensitive to the Kaluza-Klein (KK) excitations of the quarks, which can be exchanged as virtual particles in the loop. In the context of the minimal Randall-Sundrum (RS) model with bulk fields and a brane-localized Higgs sector, we derive closed analytical expressions for the gluon-gluon fusion process, finding that the effect of the infinite tower of virtual KK states can be described in terms of a simple function of the fundamental (5D) Yukawa matrices. Given a specific RS model, this will allow one to easily constrain the parameter space, once a Higgs signal has been established. We explain that discrepancies between existing calculations of Higgs production in RS models are related to the non-commutativity of two limits: taking the number of KK states to infinity and removing the regulator on the Higgs-boson profile, which is required in an intermediate step to make the relevant overlap integrals well defined. Even though the one-loop gg → h amplitude is finite in RS scenarios with a brane-localized Higgs sector, it is important to introduce a consistent ultraviolet regulator in order to obtain the correct result.

LHC signals of noncustodial warped 5D models

We study the implications at the LHC for a recent class of noncustodial warped extra-dimensional models where the ${\mathrm{AdS}}_{5}$ metric is modified near the infrared brane. Such models allow for TeV Kaluza-Klein excitations without conflict with electroweak precision tests. We discuss both the production of electroweak and strong Kaluza-Klein gauge bosons. As we will show, only signals involving the third generation of quarks seem to be feasible in order to probe this scenario.

Higgs dark matter in UEDs: A good WIMP with bad detection prospects

We study the first Kaluza–Klein excitation of the Higgs boson in universal extra dimensions as a dark matter candidate. The first-level Higgs boson could be the lightest Kaluza–Klein particle, which is stable due to the conservation of Kaluza–Klein parity, in non-minimal models where boundary localized terms modify the mass spectrum. We calculate the relic abundance and find that it agrees with the observed dark matter density if the mass of the first-level Higgs boson is slightly above 2 TeV, not considering coannihilations and assuming no relative mass splitting among the first-level Kaluza–Klein modes. In the case of coannihilations and a non-zero mass splitting, the mass of the first-level Higgs boson can range from 1 TeV to 4 TeV. We study also the prospects for detection of this dark matter candidate in direct as well as indirect detection experiments. Although the first-level Higgs boson is a typical weakly interacting massive particle, an observation in any of the conventional experiments is very challenging.