Brane Localized Kinetic Terms Research Articles

Flavor-violating charged lepton decays in the little Randall-Sundrum model

The little Randall-Sundrum (little RS) model receives significantly stronger constraints from the flavor observables in comparison to the Randall-Sundrum (RS) model. In this paper, we analyze the effect of the electroweak sector in little RS on flavor-changing decays of charged leptons. We compare the predictions of the model with the current limits on the flavor-violating branching ratios of $\ensuremath{\mu}\ensuremath{\rightarrow}eee$, $\ensuremath{\tau}\ensuremath{\rightarrow}eee$, $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu}\ensuremath{\mu}$, $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}ee$, $\ensuremath{\tau}\ensuremath{\rightarrow}e\ensuremath{\mu}\ensuremath{\mu}$, $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$, and $\ensuremath{\mu}Ti\ensuremath{\rightarrow}eTi$, and we show that the dominant constraint arises from the $\ensuremath{\mu}Ti\ensuremath{\rightarrow}eTi$ process which strongly limits the KK-1 gauge boson mass (${M}_{KK}$) to be $\ensuremath{\gtrsim}30.7\text{ }\text{ }\mathrm{TeV}$. We then derive and show that generalizing the electroweak gauge sector to include the brane localized kinetic term relaxes this constraint to $\ensuremath{\gtrsim}12\text{ }\text{ }\mathrm{TeV}$. Toward the conclusion, we comment on the possibility that the large flavor-violating currents can be mitigated by relaxing the assumption regarding the unnatural thinness and rigidity of the UV brane and discuss the possibility of suppression of these currents in the presence of fat fluctuating branes.

Taming εK in little Randall-Sundrum models

The Randall Sundrum models receive significant constraints from the neutral kaon system. The $CP$ violating observable ${\ensuremath{\epsilon}}_{K}$, in the Randall Sundrum scenario, requires the lightest $KK$ gluon to be heavier than $\ensuremath{\sim}24\text{ }\text{ }\mathrm{TeV}$. The constraint is even stronger in the little Randall Sundrum (LRS) models, $\ensuremath{\gtrsim}32\text{ }\text{ }\mathrm{TeV}$. The LRS models are motivated for their possible visibility at the LHC. We show that the stringent constraints from $K$ physics can be relaxed in the LRS models, in the presence of the brane localized kinetic terms (BLKT). In particular, for a range of values, a UV BLKT could significantly modify the lightest $KK$ gluon wave function such that the limit can reduces to 5 TeV. We also show that such a relaxation of the constraints can also be achieved by imposing flavor symmetries \`a la minimal flavor protection.

Fat brane and seesaw mechanism in extra dimensions

In this paper we present a higher-dimensional seesaw mechanism. We consider a single, flat extra dimension, where a fat brane is localized and contains the standard model (SM) fields, similar to Universal Extra Dimension models. There is only one Dirac fermion in the bulk, and in four dimensions it results in two towers of Kaluza-Klein (KK) Majorana sterile neutrinos, whose mass mixing with the SM neutrinos is suppressed due to a brane-localized kinetic term. The interaction between the sterile neutrinos and the SM is through the usual coupling with the Higgs boson, where the coupling depends upon the compactification radius $R^{-1}=10^{-2}-1$ GeV and the width of the fat brane $L^{-1}=2$ TeV, where the latter value is chosen to avoid LHC constraints. Due to this suppression mechanism the mass of the lightest sterile neutrinos can be of order $\mathcal{O}(1-10)$~\text{TeV} while naturally explaining the small SM neutrino mass, which in turn is easily obtained for a large range of parameter choices. Furthermore, neutrino oscillations are not substantially influenced by the tower of sterile KK particles. Finally, leptogenesis is investigated in this setup, and it is viable for some values within the parameter space.

Fat brane, dark matter and localized kinetic terms in six dimensions

Extra dimensions (ED) have been used as attempts to explain several phenomena in particle physics over the years. In this paper we investigate the role of an abelian gauge field as mediator of the interaction between dark matter (DM) and Standard Model (SM) particles, in a model with two flat and transverse ED compactified on the chiral square. DM is confined in a thin brane, localized at the origin of the chiral square, while the SM is localized in a finite width brane, lying in the opposite corner of the square. A brane-localized kinetic term is present in the DM brane, while in the fat brane it is not allowed. In this model the kinetic mixing is not required because we assume that the SM particles couple to the mediator through their B-L charges, while DM couples to it via a dark charge. Assuming a complex scalar field as DM candidate it is possible to obtain the observed DM relic abundance and avoid direct detection constraints for some parameter choices.

Gauge field and brane-localized kinetic terms on the chiral square

Extra dimensions have been used as attempts to explain several phenomena in particle physics. In this paper we investigate the role of brane-localized kinetic terms (BLKT) on thin and thick branes with two flat extra dimensions (ED) compactified on the chiral square, and an abelian gauge field in the bulk. The results for a thin brane have resemblance with the 5-D case, leading to a tower of massive KK particles whose masses depend upon the compactification radius and the BLKT parameter. On the other hand, for the thick brane scenario, there is no solution that satisfy the boundary conditions. Because of this, the mechanism of suppressed couplings due to ED (Landim and Rizzo, in JHEP 06:112, 2019) cannot be extended to 6-D.

Thick branes in extra dimensions and suppressed dark couplings

The nature of dark matter (DM) and how it may interact with the various fields of the Standard Model (SM) remains a mystery. In this paper we show that the interaction between new light dark matter mediators and the SM particles can be naturally suppressed if one employs a single, flat extra dimension (ED). In this setup, the SM fields are localized in a finite width ‘fat’ brane, similar to models of Universal Extra Dimensions (UED), while DM, in turn, is confined to a thin brane at the opposite end of the ED interval. Including brane localized kinetic terms on the fat brane for the mediator fields, the resulting coupling between the SM and these light mediators can be several orders of magnitude smaller than the corresponding ones between the mediators and DM which we assume to be a typical gauge coupling. We investigate the implications of this scenario for both vector (i.e, dark photon, DP) and scalar mediator fields in the 5-D bulk. In this setup kinetic mixing, which is usually employed to suppress light mediator couplings, is not required. Here we assume that the SM particles couple to the DP via their B − L charges while the DP couples to the DM via a dark charge. Both the vector DP couplings and the corresponding Higgs portal couplings with the SM are shown to be natural small in magnitude with a size dependent on ratio of the 5-D compactification radius, R−1 ∼ 0.1-1 GeV, and the SM brane thickness, L−1 ∼ 2-10 TeV, a range chosen to avoid LHC and other experimental constraints. In this framework one can obtain the observed value of the DM relic abundance for a wide range of parameter choices, while the constrains due to direct DM detection and the invisible width of the Higgs do not impose significant challenges to the model. Finally, this mechanism can lead to distinct signatures in both present and upcoming experiments as it combines some common features of UED and DP models in a single ED setup.

Status of a flavor-maximal nonminimal universal extra dimension model

In this paper we consider an $S^{1}/\mathbb{Z}_2$ compactified flat extra dimensional scenario where all the standard model states can access the bulk and have generalised brane localised kinetic terms. The flavour structure of brane kinetic terms for the standard model fermions are dictated by stringent flavour bounds on the first two generations implying an $U(2)_{Q_L} \otimes U(2)_{u_R} \otimes U(2)_{d_R}$ flavour symmetry. We consider the constraints on such a scenario arising from dark matter relic density and direct detection measurements, precision electroweak data, Higgs physics and LHC dilepton searches. We discuss the possibility of such a scenario providing an explanation of the recently measured anomaly in $R_{K^{(\ast)}}$ within the allowed region of the parameter space.

Ghosts- and tachyon-free regions of the Randall-Sundrum model parameter space

Model building within the Randall-Sundrum (RS) framework generally involves placing the Standard Model fields in the bulk. Such fields may possess non-zero values for their associated brane-localized kinetic terms (BLKTs) in addition to possible bulk mass parameters. In this paper we clearly identify the regions of the RS model parameter space where the presence of bulk mass terms and BLKTs yield a setup which is free from both ghost and tachyon instabilities. Such physically acceptable parameter space regions can then be used to construct realistic and phenomenologically viable RS models.

Kinetic mixing, dark photons and an extra dimension. Part I

Extra dimensions (ED) can provide a useful tool for model-building. In this paper we introduce a single, flat ED extension of the kinetic-mixing/dark photon (DP) portal for dark matter (DM) interactions with the Standard Model (SM) assuming a compactification ‘radius’ of order R−1 ∼ 10−1000 MeV and examine the resulting modifications to and augmentation of the usual DP phenomenology. In the present scenario, both the DP and DM experience the full 5-D while the SM fields are constrained to lie on a 4-D brane at the boundary of the ED. Such a setup can naturally yield the observed value of the DM relic density and explain the required rough degeneracy of the DM and DP masses needed to obtain it. Gauge symmetry breaking can occur via boundary conditions without the introduction of an additional singlet Higgs scalar thus avoiding all constraints associated with the coupling of such a field to the usual SM Higgs field in 5-D. The self-consistency in the removal of the kinetic mixing terms is found to lead to a brane localized kinetic term for the 5-D gauge field on the SM brane. Multiple variations of this scenario are found to be possible which are consistent with current experimental constraints but which predict very different phenomenologies. In this paper, we discuss the case of a complex scalar 5-D DM field, consistent with constraints arising from the CMB, which may or may not obtain a vacuum expectation value (vev). This approach can lead to interesting and distinctive signatures while being constrained by a wide array of existing measurements but with the details being dependent upon the model specifics.

HL/HE BÜYÜK HADRON ÇARPIŞTIRICISIN’DA EVRENSEL ÇOK BOYUTAR VE SÜPERSİMETRİ

Universal Extra Dimensional (UED) and Supersymmetric (SUSY) models show very similar signals at hadron colliders. In the minimal UED (mUED) where the mass spectrum is compressed, the production of level-1 KK quarks and/or gluons produce multi-jet and/or multi-lepton + missing transverse momenta final state as in the case of SUSY models with R-parity conservation and compressed mass spectrum. Moreover, this similarity can possibly be carried over to non-degenerate mass spectra in next-to-minimal UED (nmUED) model that is the extension of the mUED model with fermion bulk mass term μ and brane localized kinetic terms r, hence, completing the faking in both models. In this regard, we simulated the production of colored level-1 KK particles and their decays resulting in multi-jet + missing transverse momenta signals in both models at 14 TeV LHC and showed that the similarity of both models is indeed the case even with non-compressed mass spectra

Warped graviton couplings to bulk vectors with brane-localized kinetic terms

In this paper, we realize nonuniversal couplings between Kaluza–Klein (KK) gravitons and bulk Standard Model (SM) vectors in the Randall–Sundrum (RS) model by including both UV and IR brane-localized gauge kinetic terms. We find that such kinetic terms can reduce the couplings of KK gravitons to SM gauge bosons and also ensure the KK vector masses are consistent with electroweak precision constraints.

Bulk Higgs with a heavy diphoton signal

We consider scenarios of warped extra-dimensions with all matter fields in the bulk and in which both the hierarchy and the flavor puzzles of the Standard Model are addressed. The simplest extra dimensional extension of the Standard Model Higgs sector, i.e a 5D bulk Higgs doublet, can be a natural and simple explanation to the 750 GeV excess of diphotons hinted at the LHC, with the resonance responsible for the signal being the lightest CP odd excitation coming from the Higgs sector. No new matter content is invoked, the only new ingredient being the presence of (positive) brane localized kinetic terms associated to the 5D bulk Higgs, which allow to reduce the mass of the lightest CP odd Higgs excitation to 750 GeV. Production and decay of this resonance can naturally fit the observed signal when the mass scale of the rest of extradimensional resonances is of order 1 TeV.

Constraining minimal and nonminimal universal extra dimension models with Higgs couplings

Early indications from the LHC for the observed scalar boson imply properties close to the Standard Model Higgs, putting considerable constraints on TeV scale new physics scenarios. In this letter we consider flat extra dimensional scenarios with the fifth spatial dimension compactified on an S^1/Z_2 orbifold. We find in the minimal model the experimentally preferred effective Higgs couplings to gluon and photon at 95% confidence level disfavor the New Physics scale below 1.3 TeV. We demonstrate that a generalization of these models to include brane localized kinetic terms can relieve the tension to accommodate scales as low as 0.4 TeV.

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.

On taming the warped radion with supersymmetry

In warped models that solve the hierarchy problem, there is generally no dynamical relation between the size of the fifth dimension and the scale of electroweak symmetry breaking (EWSB). The establishment of such a relation, without fine-tuning, requires that Casimir contributions to the radion potential not exceed the energy density associated with EWSB. Here, we examine the use of supersymmetry for controlling the Casimir energy density and making quantum contributions calculable. We compute the effects of supersymmetry breaking at the UV and IR boundaries of warped backgrounds, in the presence of brane localized kinetic terms. Various limits of supersymmetry breaking are examined. We find that when supersymmetry is broken on the UV brane, vacuum contributions to the radion potential can be controlled (as likely necessary for EWSB to govern the radion potential) via small soft masses as well as a “double volume suppression.” Our formalism can also provide a setup for radion stabilization by bulk fields, when supersymmetry is broken on both the UV and the IR branes.

Simple flavor protection for the Randall-Sundrum model

We present a simple variation of warped flavor models where the hierarchies of fermion masses and mixings are still explained but dangerous flavor-violating effects in the Kaon sector are greatly reduced. The key new ingredients are two horizontal $U(1)$ symmetries. These symmetries forbid flavor violation in the down-quark sector (with the exception of IR brane localized kinetic mixing terms for the left-handed quarks) while allowing for flavor violation in the up-quark sector. The leading flavor constraints come from ${D}^{0}\ensuremath{-}{\overline{D}}^{0}$ mixing, and are safely satisfied for the Kaluza-Klein mass scale of order 3 TeV. Our analysis of the flavor constraints includes the effects of the (usually ignored) brane localized kinetic terms. We also comment on the effects of the additional $U(1)$ gauge bosons that are present if the horizontal symmetries are gauged.

Flavor-violation tests of the warped/composite standard model in the two-site approach

We study flavor violation in the quark sector in a purely $4D$, two-site effective field theory description of the standard model (SM) and just their first Kaluza-Klein excitations from a warped extra dimension. The warped $5D$ framework can provide solutions to both the Planck-weak and flavor hierarchies of the SM. It is also related (via the AdS/CFT correspondence) to partial compositeness of the SM. We focus on the dominant contributions in the two-site model to two observables which we argue provide the strongest constraints from flavor violation, namely, ${ϵ}_{K}$ and $\mathrm{BR}(b\ensuremath{\rightarrow}s\ensuremath{\gamma})$, where contributions in the two-site model occur at tree and loop-level, respectively. In particular, we demonstrate that a ``tension'' exists between these two observables in the sense that they have opposite dependence on composite site Yukawa couplings, making it difficult to decouple flavor-violating effects using this parameter. We choose the size of the composite site QCD coupling based on the relation of the two-site model to the $5D$ model (addressing the Planck-weak hierarchy), where we match the $5D$ QCD coupling to the $4D$ coupling at the loop-level and assuming negligible tree-level brane-localized kinetic terms. We estimate that a larger size of the $5D$ gauge coupling is constrained by the requirement of $5D$ perturbativity. We find that $\ensuremath{\sim}O(5)\text{ }\text{ }\mathrm{TeV}$ mass scale for the new particles in the two-site model can then be consistent with both observables. We also compare our analysis of ${ϵ}_{K}$ in the two-site model to that in $5D$ models, including both the cases of a brane-localized and bulk Higgs.

The effective field theory of codimension-two branes

Distributional sources of matter on codimension-two and higher branes are only well-defined as regularized objects. Nevertheless, intuition from effective field theory suggests that the low-energy physics on such branes should be independent of any high-energy regularization scheme. In this paper, we address this issue in the context of a scalar field model where matter fields (the standard model) living on such a brane interact with bulk fields (gravity). The low-energy effective theory is shown to be consistent and independent of the regularization scheme, provided the brane couplings are renormalized appropriately at the classical level. We perform explicit computations of the classical renormalization group flows at tree and one-loop level, demonstrate that the theory is renormalizable against codimension-two divergences, and extend the analysis to several physical applications such as electrodynamics and brane localized kinetic terms.

Radion phenomenology in realistic warped space models

We investigate the phenomenology of the Randall-Sundrum (RS) radion in realistic models of electroweak symmetry breaking with bulk gauge and fermion fields, since the radion may turn out to be the lightest particle in such models. We calculate the coupling of the radion in such scenarios to bulk fermion and gauge modes. Special attention needs to be devoted to the coupling to massless gauge fields (photon, gluon), since it is well known that loop effects may be important for these fields. We also present a detailed explanation of these couplings from the conformal field theory interpretation. We then use these couplings to determine the radion branching fractions and discuss some of the discovery potential of the LHC for the radion. We find that the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal is enhanced over most of the range of the radion mass over the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal of a standard model Higgs boson, as long as the RS scale is sufficiently low. However, the signal significance depends strongly on free parameters that characterize the magnitude of bare brane-localized kinetic terms for the massless gauge fields. In the absence of such terms, the signal can be enhanced over the traditional RS1 models (where all standard model fields are localized on the IR brane), but the signal can also be reduced compared to RS1 if the brane-localized terms are sizeable. We also show that for larger radion masses, where the $\ensuremath{\gamma}\ensuremath{\gamma}$ signal is no longer significant, one can use the usual 4 lepton signal to discover the radion.

Higher derivatives and brane-localised kinetic terms in gauge theories on orbifolds

We perform a detailed analysis of one-loop corrections to the self-energy of the (off-shell) gauge bosons in six-dimensional N=1 supersymmetric gauge theories on orbifolds. After discussing the Abelian case in the standard Feynman diagram approach, we extend the analysis to the non-Abelian case by employing the method of an orbifold-compatible one-loop effective action for a classical background gauge field. We find that bulk higher derivative and brane-localised gauge kinetic terms are required to cancel one-loop divergences of the gauge boson self energy. After their renormalisation we study the momentum dependence of both the higher derivative coupling h(k^2) and the {\it effective} gauge coupling g_eff(k^2). For momenta smaller than the compactification scales, we obtain the 4D logarithmic running of g_eff(k^2), with suppressed power-like corrections, while the higher derivative coupling is constant. We present in detail the threshold corrections to the low energy gauge coupling, due to the massive bulk modes. At momentum scales above the compactification scales, the higher derivative operator becomes important and leads to a power-like running of g_eff(k^2) with respect to the momentum scale. The coefficient of this running is at all scales equal to the renormalised coupling of the higher derivative operator which ensures the quantum consistency of the model. We discuss the relation to the similar one-loop correction in the heterotic string, to show that the higher derivative operators are relevant in that case too, since the field theory limit of the one-loop string correction does not commute with the infrared regularisation of the (on-shell) string result.