Tree-level Spectrum Research Articles

Curing tachyonic tree-level syndrome in NMSSM light-singlet scenarios

Models with an extended Higgs sector open up the phenomenological possibility of additional scalars, beyond the SM-like boson observed by the LHC, with mass at or below the electroweak scale. Such scenarios are in particular viable in the presence of electroweak-singlet spin-0 fields, as expected for instance in the context of the NMSSM. Given that the size of radiative corrections can substantially affect the Higgs potential, a negative squared mass at the tree level does not necessarily yield a tachyonic spectrum at the physical level, but only indicates a failure of the tree-level description for calculational purposes. We explain how to tackle this technical issue in the example of the NMSSM, in scenarios with light mathcal {CP}-odd or mathcal {CP}-even singlet-dominated states and show how loop corrections to the Higgs masses and decay widths can be derived with the regularized Lagrangian. We further explore how the same flexibility in the definition of tree-level parameters can be exploited to circumvent large deviations of the tree-level spectrum from the kinematical setup in Higgs decays, or to estimate the theoretical uncertainty associated with the discrepancy between tree-level and physical Higgs spectra. The latter is of particular relevance for the properties of the SM-like Higgs boson in supersymmetry-inspired models.

Towards a complete mass spectrum of type-IIB flux vacua at large complex structure

The large number of moduli fields arising in a generic string theory compactification makes a complete computation of the low energy effective theory infeasible. A common strategy to solve this problem is to consider Calabi-Yau manifolds with discrete symmetries, which effectively reduce the number of moduli and make the computation of the truncated Effective Field Theory possible. In this approach, however, the couplings (e.g., the masses) of the truncated fields are left undetermined. In the present paper we discuss the tree-level mass spectrum of type-IIB flux compactifications at Large Complex Structure, focusing on models with a reduced one-dimensional complex structure sector. We compute the tree-level spectrum for the dilaton and complex structure moduli, including the truncated fields, which can be expressed entirely in terms of the known couplings of the reduced theory. We show that the masses of this set of fields are naturally heavy at vacua consistent with the KKLT construction, and we discuss other phenomenologically interesting scenarios where the spectrum involves fields much lighter than the gravitino. We also derive the probability distribution for the masses on the ensemble of flux vacua, and show that it exhibits universal features independent of the details of the compactification. We check our results on a large sample of flux vacua constructed in an orientifold of the Calabi-Yau {mathbbm{W}mathrm{mathbb{P}}}_{left[1,1,1,1,4right]}^4 . Finally, we also discuss the conditions under which the spectrum derived here could arise in more general compactifications.

Effective mass of W boson in a magnetic field

Simple representation for the average value of the W-boson one-loop polarization tensor in a magnetic field B = const, calculated in the ground state of the tree-level spectrum, is derived. It corresponds to Demeur’s formula for electron in QED. The energy of this state, describing effective particle mass, is computed by solving the Schwinger-Dyson equation. As application, we investigate the effective mass squared at the threshold of the tree-level instability, B → Bc = m2/e, and show that it is positive. In this way the stability of the W-boson spectrum is established. Some peculiarities of the results obtained and other applications are discussed.

Primordial power spectrum features and consequences

The present Cosmic Microwave Background (CMB) temperature and polarization anisotropy data is consistent with not only a power law scalar primordial power spectrum (PPS) with a small running but also with the scalar PPS having very sharp features. This has motivated inflationary models with such sharp features. Recently, even the possibility of having nulls in the power spectrum (at certain scales) has been considered. The existence of these nulls has been shown in linear perturbation theory. What shall be the effect of higher order corrections on such nulls? Inspired by this question, we have attempted to calculate quantum radiative corrections to the Fourier transform of the 2-point function in a toy field theory and address the issue of how these corrections to the power spectrum behave in models in which the tree-level power spectrum has a sharp dip (but not a null). In particular, we have considered the possibility of the relative enhancement of radiative corrections in a model in which the tree-level spectrum goes through a dip in power at a certain scale. The mode functions of the field (whose power spectrum is to be evaluated) are chosen such that they undergo the kind of dynamics that leads to a sharp dip in the tree level power spectrum. Next, we have considered the situation in which this field has quartic self interactions, and found one loop correction in a suitably chosen renormalization scheme. Thus, we have attempted to answer the following key question in the context of this toy model (which is as important in the realistic case): In the chosen renormalization scheme, can quantum radiative corrections be enhanced relative to tree-level power spectrum at scales, at which sharp dips appear in the tree-level spectrum?

Effective Mass of W Boson in a Magnetic Field

Simple representation for the average value of the W-boson one-loop polarization tensor in a magnetic field B=const, calculated in the ground state of the tree-level spectrum, is derived. It corresponds to Demeur's formula for electron in QED. The energy of this state, describing effective particle mass, is computed by solving the Schwinger-Dyson equation. As application, we investigate the effective mass squared at the threshold of the tree-level instability, $B \to B_c = m^2/e$, and show that it is positive. In this way the stability of the W-boson spectrum is established. Some peculiarities of the results obtained and other applications are discussed.

Inflationary infrared divergences: geometry of the reheating surface vs. δN formalism

We describe a simple way of incorporating fluctuations of the Hubble scale during the horizon exit of scalar perturbations into the δN formalism. The dominant effect comes from the dependence of the Hubble scale on low-frequency modes of the inflaton. This modifies the coefficient of the log-enhanced term appearing in the curvature spectrum at second order in field fluctuations. With this modification, the relevant coefficient turns out to be proportional to the second derivative of the tree-level spectrum with respect to the inflaton ϕ at horizonexit. A logarithm with precisely the same coefficient appears in acalculation of the log-enhancement of the curvature spectrum basedpurely on the geometry of the reheating surface. We take thisagreement as strong support for the proposed implementation of theδN formalism. Moreover, our analysis makes it apparent that the log-enhancement of the inflationary power-spectrum is indeed physical if this quantity isdefined using a global coordinate system on the reheating surface (or anyother post-inflationary surface of constant energy density). However, it can be avoided by defining the spectrum using invariant distances on this surface.

Model for light scalar mesons in QCD

We propose a systematic procedure to study a generalized linear sigma model which can give a physical picture of possible mixing between qq and qqq q low lying spin zero states. In the limit of zero quark masses, we derive the model independent results for the properties of the Nambu-Goldstone pseudoscalar particles. For getting information on the scalars it is necessary to make a specific choice of terms. We impose two plausible physical criteria-the modeling of the axial anomaly and the suppression of effective vertices representing too many fermion lines-for limiting the large number of terms which are allowed on general grounds. We calculate the tree-level spectrum based on the leading terms in our approach and find that it prominently exhibits a very low mass isosinglet scalar state. Finally we point out that the low energy result for scattering of pions continues to hold in the general version of the model.

Brane-antibrane constructions

In type II string theories, we examine intersecting brane constructions containing brane-antibrane pairs suspended between 5-branes, and more general non-BPS constructions. The tree-level spectra are obtained in each case. We identify various models with distinct physics: parallel brane-antibrane pairs, adjacent pairs, non-adjacent pairs, and configurations which break all supersymmetry even though any pair of branes preserves some supersymmetry. In each case we examine the possible decay modes. Some of these configurations turn out to be tachyon-free, stable non-BPS states. We use T-duality to map some of our brane constructions to brane-antibrane pairs at ALE singularities. This enables us to explicitly derive the spectra by the analogue of the quiver construction, and to compute the sign of the brane-antibrane force in each case.

Monopole backgrounds on the world sheet

We consider a generalization of a coset construction involving monopole gauge field configurations. Among other results this generalization leads to a new tree level spectrum and has interesting modular properties at one loop.

A supergravity model without elementary gauge singlets

We present some further calculations regarding a previously derived model which exhibits spontaneous supergravity breaking without using elementary gauge singlet scalars. The model in its simplest form does not possess a hidden sector since all particles have gauge interactions. The tree level mass matrices are shown to be completely diagonalisable. The resulting tree level spectrum is not phenomenologically useful. Some extensions of the model, which have better behaviour, are then discussed.