Nima Pourtolami

Repulsive Casimir and Casimir–Polder forces

Casimir and Casimir?Polder repulsions have been known for more than 50 years. The general ?Lifshitz? configuration of parallel semi-infinite dielectric slabs permits repulsion if they are separated by a dielectric fluid that has a value of permittivity that is intermediate between those of the dielectric slabs. This was indirectly confirmed in the 1970s, and more directly by Capasso?s group recently. It has also been known for many years that electrically and magnetically polarizable bodies can experience a repulsive quantum vacuum force. More amenable to practical application are situations where repulsion could be achieved between ordinary conducting and dielectric bodies in vacuum. The status of the field of Casimir repulsion with emphasis on some recent developments will be surveyed. Here, stress will be placed on analytic developments, especially on Casimir?Polder (CP) interactions between anisotropically polarizable atoms, and CP interactions between anisotropic atoms and bodies that also exhibit anisotropy, either because of anisotropic constituents, or because of geometry. Repulsion occurs for wedge-shaped and cylindrical conductors, provided the geometry is sufficiently asymmetric, that is, either the wedge is sufficiently sharp or the atom is sufficiently far from the cylinder.This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical in honour of Stuart Dowker?s 75th birthday devoted to ?Applications of zeta functions and other spectral functions in mathematics and physics?.

Higgs Bosons in Warped Space, from the Bulk to the Brane

In the context of warped extra dimensional models with all fields propagating in the bulk, we address the phenomenology of a bulk scalar Higgs boson, and calculate its production cross section at the LHC as well as its tree-level effects on mediating flavor changing neutral currents. We perform the calculations based on two different approaches. First, we compute our predictions analytically by considering all the degrees of freedom emerging from the dimensional reduction [the infinite tower of Kaluza Klein modes (KK)]. In the second approach, we perform our calculations numerically by considering only the effects caused by the first few KK modes, present in the 4-dimensional effective theory. In the case of a Higgs leaking far from the brane, both approaches give the same predictions as the effects of the heavier KK modes decouple. However, as the Higgs boson is pushed toward the TeV brane, the two approaches seem to be equivalent only when one includes heavier and heavier degrees of freedom (which do not seem to decouple). To reconcile these results it is necessary to introduce higher dimension operators which essentially encode the effects of integrating out the heavy KK modes and dress the brane Higgs so that it looks just like a bulk Higgs. However, in the brane Higgs limit, it is not possible to predict if there will be enhancement or suppression in the Higgs production rate since the corrections depend on the phases of higher dimension operators.

Unified flavor symmetry from warped dimensions

a b s t r a c t In a model of warped extra-dimensions with all matter fields in the bulk, we propose a scenario which explains all the masses and mixings of the SM fermions. In this scenario, the same flavor symmetric structure is imposed on all the fermions of the Standard Model (SM), including neutrinos. Due to the exponential sensitivity on bulk fermion masses, a small breaking of this symmetry can be greatly enhanced and produce seemingly un-symmetric hierarchical masses and small mixing angles among the charged fermion zero-modes (SM quarks and charged leptons), thus washing out visible effects of the symmetry. If the Dirac neutrinos are sufficiently localized towards the UV boundary, and the Higgs field leaking into the bulk, the neutrino mass hierarchy and flavor structure will still be largely dominated and reflect the fundamental flavor structure, whereas localization of the quark sector would reflect the effects of the flavor symmetry breaking sector. We explore these features in an example based on which a family permutation symmetry is imposed in both quark and lepton sectors.

Casimir-Polder repulsion: Polarizable atoms, cylinders, spheres, and ellipsoids

Recently, the topic of Casimir repulsion has received a great deal of attention, largely because of the possibility of technological application. The general subject has a long history, going back to the self-repulsion of a conducting spherical shell and the repulsion between a perfect electric conductor and a perfect magnetic conductor. Recently it has been observed that repulsion can be achieved between ordinary conducting bodies, provided sufficient anisotropy is present. For example, an anisotropic polarizable atom can be repelled near an aperture in a conducting plate. Here we provide new examples of this effect, including the repulsion on such an atom moving on a trajectory nonintersecting a conducting cylinder; in contrast, such repulsion does not occur outside a sphere. Classically, repulsion does occur between a conducting ellipsoid placed in a uniform electric field and an electric dipole. The Casimir-Polder force between an anisotropic atom and an anisotropic dielectric semispace does not exhibit repulsion. The general systematics of repulsion are becoming clear.

REPULSIVE CASIMIR EFFECTS

Like Casimirs original force between conducting plates in vacuum, Casimir forces are usually attractive. But repulsive Casimir forces can be achieved in special circumstances. These might prove useful in nanotechnology. We give examples of when repulsive quantum vacuum forces can arise with conducting materials.

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.

Fermion Masses and Mixing in General Warped Extra Dimensional Models

We analyze fermion masses and mixing in a general warped extra dimensional model, where all the Standard Model (SM) fields, including the Higgs, are allowed to propagate in the bulk. In this context, a slightly broken flavor symmetry imposed universally on all fermion fields, without distinction, can generate the full flavor structure of the SM, including quarks, charged leptons and neutrinos. For quarks and charged leptons, the exponential sensitivity of their wave-functions to small flavor breaking effects yield naturally hierarchical masses and mixing as it is usual in warped models with fermions in the bulk. In the neutrino sector, the exponential wave-function factors can be flavor-blind and thus insensitive to the small flavor symmetry breaking effects, directly linking their masses and mixing angles to the flavor symmetric structure of the 5D neutrino Yukawa couplings. The Higgs must be localized in the bulk and the model is naturally more successful in generalized warped scenarios where the metric background solution is different than AdS5. We study these features in two simple frameworks, flavor complimentarily, and flavor democracy, which provide specific predictions and correlations between quarks and leptons, testable as more precise data in the neutrino sector becomes available.

Higgs production in general 5D warped models

We calculate the production rate of the Higgs boson at the LHC in the context of general five-dimensional (5D) warped scenarios with space-time background modified from the usual

Flavor-changing decays of the top quark in 5D warped models

{\mathrm{AdS}}_{5}

PT-symmetric quantum electrodynamics and unitarity

, and where all the Standard Model fields, including the Higgs, propagate in the bulk. This modification can alleviate considerably the bounds coming from precision electroweak tests and flavor physics. We evaluate the Higgs production rate and show that it is generically consistent with the current experimental results from the LHC for Kaluza-Klein masses as low as 2 TeV, unlike in pure