Modern Physics Letters A

We review the present bounds on Kaluza–Klein (KK)-excitations of Standard Model gauge bosons, predicted by TeV size extra dimensions, which have been set by the ElectroWeak Precision Test (EWPT) and recently by the direct searches performed at the CERN Large Hadron Collider (LHC). Concerning the theoretical framework we present results for the Non-Universal Extra Dimension (NUED) model with matter localized at the boundaries and all gauge bosons propagating into the bulk and acquiring KK-excitations. We also discuss its simplified version, NUED(EW), where only the ElectroWeak SM gauge bosons are allowed to propagate in the extra dimension, and its next-to-minimal version ElectroWeak Universal Extra Dimension (UED)(EW) where SU(3) is localized on the brane while the SU(2)×U(1) particle content fully propagates into the bulk.

Although they do not address the hierarchy problem, models with Universal Extra Dimensions have attracted a lot of attention as simple benchmark models characterized by small mass splittings and a dark matter (DM) WIMP played by the Lightest Kaluza–Klein particle (LKP). We review their status, with emphasis on minimal implementation in five dimensions (MUED) in which the LKP is a massive hypercharge gauge boson. In this case, the mass range accounting for the correct DM abundance (around 1.4 TeV) remains untouched by LHC8 and is out of reach of present DM direct detection experiments. However, LHC14 can probe the relevant region in the 3-lepton channel.

In this paper, motivated by the recent discovery of a Higgs-like boson at the Large Hadron Collider (LHC) with a mass m_H≃125 GeV, we review different models where the hierarchy problem is solved by means of a warped extra dimension. In the Randall–Sundrum (RS) model electroweak observables provide very strong bounds on the mass of KK modes which motivates extensions to overcome this problem. Two extensions are briefly discussed. One particular extension is based on the deformation of the metric such that it strongly departs from the AdS₅ structure in the IR region while it goes asymptotically to AdS₅ in the UV brane. This model has the IR brane close to a naked metric singularity (which is outside the physical interval) characteristic of soft-walls constructions. The proximity of the singularity provides a strong wave function renormalization for the Higgs field which suppresses the T and S parameters. The second class of considered extensions are based on the introduction of an extra gauge group in the bulk such that the custodial SU(2)_R symmetry is gauged and protects the T parameter. By further enlarging the bulk gauge symmetry one can find models where the Higgs is identified with the fifth component of gauge fields and for which the Higgs potential along with the Higgs mass can be dynamically determined by the Coleman–Weinberg mechanism.

We review constraints from quark and lepton flavor violation on extra dimensional models with warped geometry, both in the minimal and the custodial model. For both scenarios, Kaluza–Klein (KK) masses that are large enough to suppress constraints from electroweak precision tests (EWPT) also sufficiently suppress all quark flavor and CP violation, with the exception of CP violation in mixing and (to a lesser extend) in mixing. In the lepton sector the minimal scenario leads to excessively large contributions to μ→eγ transitions, requiring KK masses of at least 20 TeV or larger.

The first run of the LHC has been tremendously successful, having discovered the Higgs boson as well as having set stringent limits on superpartner masses. This has severely constrained supersymmetric models. A consistent scenario based on LHC data suggests that the third generation sfermions, Higgsinos and the gluino must be light while the first two generation sfermions are heavy. Furthermore, an extra contribution to the Higgs quartic coupling is needed to explain the 126 GeV Higgs mass. Interestingly this spectrum can naturally arise from supersymmetry breaking in a warped geometry where the sfermion spectrum is related to the fermion mass hierarchy. Various possible scenarios are reviewed and compared with the data from the first run at the LHC.

We explain what features of string theory can be tested at the LHC. We review the present bounds on the string mass and on extra gauge bosons as well as the prospects for the upcoming experiments.

The quest for hints about the existence of extra dimensions has recently made tremendous progress with the analysis of the data registered during the first run of the LHC, at center-of-mass energies of 7 TeV and 8 TeV. While we are awaiting the LHC run at nominal energy, it is a good time to review the results and constraints already obtained in a large variety of scenarios and signatures.

The success of the first three years of operations of the CERN Large Hadron Collider (LHC) at center-of-mass energies of 7 TeV and 8 TeV radically changed the landscape of searches for new physics beyond the Standard Model (BSM) and our very way of thinking about its possible origin and its hiding place. Among the paradigms of new physics that have been probed quite extensively at the LHC, are various models that predict the existence of extra spatial dimensions. In this review, the current status of searches for extra dimensions with the Compact Muon Solenoid (CMS) detector is presented, along with prospects for future searches at the full energy of the LHC, expected to be reached in the next few years.