Ashmita Das

Bulk Higgs and Gauge fields in a multiply warped braneworld model

We readdress the problems associated with bulk Higgs and the gauge fields in a five-dimensional Randall-Sundrum model by extending the model to six dimensions with double warping along the two extra spatial dimensions. In this six-dimensional model, we have a freedom of two moduli scales as against one modulus in the five-dimensional model. With a little hierarchy between these moduli, we can obtain the right magnitude for W and Z boson masses from the Kaluza-Klein modes of massive bulk gauge fields where the spontaneous symmetry breaking is triggered by bulk Higgs. We also have determined the gauge couplings of the standard model fermions with Kaluza-Klein modes of the gauge fields. Unlike the case of the five-dimensional model with a massless bulk gauge field, here, we have shown that the gauge couplings and the masses of the Kaluza-Klein gauge fields satisfy the precision electroweak constraints and also obey the Tevatron bounds.

Graviton modes in multiply warped geometry

Abstract The negative results in the search for Kaluza–Klein graviton modes at the LHC, when confronted with the discovery of the Higgs, have been construed to have severely limited the efficacy of the Randall–Sundrum model as an explanation of the hierarchy problem. We show, though, that the presence of multiple warping offers a natural resolution of this conundrum through modifications in both the graviton spectrum and their couplings to the Standard Model fields.

Antisymmetric tensor fields in a generalized Randall–Sundrum scenario

Abstract Bulk antisymmetric tensor fields of different ranks have been studied in the context of a generalized Randall–Sundrum model with a non-vanishing induced cosmological constant on the visible brane. It is shown that instead of the usual exponential suppression of the couplings of the zero modes of these bulk fields with the brane fermions in the original Randall–Sundrum model, here the couplings are proportional to the brane cosmological constant. Thus in an era of large cosmological constant these fields have significant role in physical phenomena because of their enhanced couplings with the visible brane fermions.

Why has spacetime torsion such negligible effect on the Universe

We attempt an answer to the question as to why the evolution of a four-dimensional universe is governed by spacetime curvature but not torsion. An answer is found if there is an additional compact spacelike dimension with a warped geometry, with torsion caused by a Kalb-Ramond (KR) antisymmetric tensor field in the bulk. Starting from a Randall-Sundrum type of warped extra dimension, and including the inevitable backreaction ensuing from the radius stabilization mechanism, we show that there is always an extra exponential suppression of the KR field on the four-dimensional projection that constitutes our visible Universe. The backreaction is found to facilitate the process of such suppression.

Gauge and moduli hierarchy in a multiply warped braneworld scenario

Abstract Discovery of Higgs-like boson near the mass scale ∼ 126 Gev generates renewed interest to the gauge hierarchy problem in the standard model related to the stabilisation of the Higgs mass within Tev scale without any unnatural fine tuning. One of the successful attempts to resolve this problem has been the Randall–Sundrum warped geometry model. Subsequently this 5-dimensional model was extended to a doubly warped 6-dimensional (or higher) model which can offer a geometric explanation of the fermion mass hierarchy in the standard model of elementary particles (D. Choudhury and S. SenGupta, 2007 [1] ). In an attempt to address the dark energy issue, we in this work extend such 6-dimensional warped braneworld model to include non-flat 3-branes at the orbifold fixed points such that a small but non-vanishing brane cosmological constant is induced in our observable brane. We show that the requirements of a Planck to Tev scale warping along with a vanishingly small but non-zero cosmological constant on the visible brane with non-hierarchical moduli, each with scale close to Planck length, lead to a scenario where the 3-branes can have energy scales either close to Tev or close to Planck scale. Such a scenario can address both the gauge hierarchy as well as fermion mass hierarchy problem in standard model without introducing hierarchical scales between the two moduli. Thus simultaneous resolutions to the gauge hierarchy problem, fermion mass hierarchy problem and non-hierarchical moduli problem are closely linked with the near flatness condition of our universe.

Higher rank antisymmetric tensor fields in Klebanov-Strassler geometry

In string theory, higher rank antisymmetric tensor fields appear as massless excitations of closed strings. Till date there is no experimental support in favour of their existence. In a stringy framework, starting from a warped throat-like Klebanov-Strassler geometry, we show that all the massless higher rank antisymmetric tensor fields are heavily suppressed due to the background fluxes leading to their invisibility in our Universe.