Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
Browse
5 results
Search Results
Now showing 1 - 5 of 5
Article Citation - WoS: 2Citation - Scopus: 2Higgsed Stueckelberg Vector and Higgs Quadratic Divergence(Elsevier Ltd., 2015) Demir, Durmuş Ali; Karahan, Canan Nurhan; Korutlu, BesteHere we show that, a hidden vector field whose gauge invariance is ensured by a Stueckelberg scalar and whose mass is spontaneously generated by the Standard Model Higgs field contributes to quadratic divergences in the Higgs boson mass squared, and even leads to its cancellation at one-loop when Higgs coupling to gauge field is fine-tuned. In contrast to mechanisms based on hidden scalars where a complete cancellation cannot be achieved, stabilization here is complete in that the hidden vector and the accompanying Stueckelberg scalar are both free from quadratic divergences at one-loop. This stability, deriving from hidden exact gauge invariance, can have important implications for modeling dark phenomena like dark matter, dark energy, dark photon and neutrino masses. The hidden fields can be produced at the LHC.Article Citation - WoS: 11Citation - Scopus: 11Conformal Transformations in Metric-Affine Gravity and Ghosts(John Wiley and Sons Inc., 2012) Karahan, Canan Nurhan; Doğangün, Oktay; Demir, Durmuş AliConformal transformations play a widespread role in gravity theories in regard to their cosmological and other implications. In the pure metric theory of gravity, conformal transformations change the frame to a new one wherein one obtains a conformal-invariant scalar-tensor theory such that the scalar field, deriving from the conformal factor, is a ghost. In this work, conformal transformations and ghosts will be analyzed in the framework of the metric-affine theory of gravity. Within this framework, metric and connection are independent variables, and, hence, transform independently under conformal transformations. It will be shown that, if affine connection is invariant under conformal transformations, then the scalar field of concern is a non-ghost, non-dynamical field. It is an auxiliary field at the classical level, and might develop a kinetic term at the quantum level. Alternatively, if connection transforms additively with a structure similar to yet more general than that of the Levi-Civita connection, the resulting action describes the gravitational dynamics correctly, and, more importantly, the scalar field becomes a dynamical non-ghost field. The equations of motion, for generic geometrical and matter-sector variables, do not reduce connection to the Levi-Civita connection, and, hence, independence of connection from metric is maintained. Therefore, metric-affine gravity provides an arena in which ghosts arising from the conformal factor are avoided thanks to the independence of connection from the metric.Article Citation - WoS: 16Citation - Scopus: 16Effects of a Real Singlet Scalar on Veltman Condition(Elsevier Ltd., 2014) Karahan, Canan Nurhan; Korutlu, BesteWe revisit the fine-tuning problem in the Standard Model (SM) and show the modification in the Veltman condition by virtue of a minimally-extended particle spectrum with one real SM gauge singlet scalar field. We demand the new scalar to interact with the SM fields through Higgs portal only, and the new singlet to acquire a vacuum expectation value, resulting in a mixing with the CP-even neutral component of the Higgs doublet in the SM. The experimental bounds on the mixing angle are determined by the observed best-fit signal strength σ/σSM. While, the one-loop radiative corrections to the Higgs mass squared, computed with an ultraviolet cut-off scale Λ, come with a negative coefficient, the quantum corrections to the singlet mass squared acquire both positive and negative values depending on the parameter space chosen, which if positive might be eliminated by introducing singlet or doublet vector-like fermions. However, based upon the fact that there is mixing between the scalars, when transformed into the physical states, the tree-level coupling of the Higgs field to the vector-like fermions worsens the Higgs mass hierarchy problem. Therefore, the common attempt to introduce vector-like fermions to cancel the divergences in the new scalar mass might not be a solution, if there is mixing between the scalars.Article Citation - WoS: 4Citation - Scopus: 4Relativistic Mond From Modified Energetics(Springer Verlag, 2014) Demir, Durmuş Ali; Karahan, Canan NurhanWe begin to investigate the question of what modifications in the energy-momentum tensor can yield the correct MOND regime. As a starting study, we refrain from insisting on an action principle and focus exclusively on the equations of motion. The present work, despite the absence of an explicit action functional, can be regarded to extend Milgrom’s modified inertia approach to relativistic domain. Our results show that a proper MOND limit arises if the energy-momentum tensor is modified to involve the determinant of the metric tensor in reference to the flat metric, where the latter is dynamically generated as in the gravitational Higgs mechanism. This modified energy-momentum tensor is conserved in both Newtonian and MONDian regimes.Article Citation - WoS: 17Citation - Scopus: 19Scalars, Vectors and Tensors From Metric-Affine Gravity(Springer Verlag, 2013) Karahan, Canan Nurhan; Altaş, Aslı; Demir, Durmuş AliThe metric-affine gravity provides a useful framework for analyzing gravitational dynamics since it treats metric tensor and affine connection as fundamentally independent variables. In this work, we show that, a metric-affine gravity theory composed of the invariants formed from non-metricity, torsion and curvature tensors can be decomposed into a theory of scalar, vector and tensor fields. These fields are natural candidates for the ones needed by various cosmological and other phenomena. Indeed, we show that the model accommodates TeVeS gravity (relativistic modified gravity theory), vector inflation, and aether-like models. Detailed analyses of these and other phenomena can lead to a standard metric-affine gravity model encoding scalars, vectors and tensors.