arXiv subjects

Canan Karahan

Publications and source records attributed to Canan Karahan.

9 recordsLinked to original sources

arXiv2025

Neutrino Mass, Vacuum Stability and Higgs Inflation with Vector-Like Quarks and a Single Right-Handed Neutrino

We investigate a Standard Model extension containing $n$ degenerate down-type isosinglet vector-like quarks (VLQs) with masses $M_{\mathcal D}$ and Yukawa couplings $y_{\mathcal D}$, supplemented by a single right-handed neutrino (RHN), aiming to simultaneously address neutrino mass generation, electroweak vacuum stability, and Higgs inflation. The VLQs play the dominant role in stabilizing the Higgs potential through their impact on the renormalization-group evolution, while the RHN generates light neutrino masses via a Type-I seesaw mechanism and smooths the high-scale running of the Higgs quartic coupling in the inflationary regime. We perform a two-loop Standard Model renormalization-group equation analysis supplemented by the one-loop contributions of the VLQs and the RHN, with proper matching across their mass thresholds. Using these RG trajectories, we identify the regions in $(n,\, y_{\mathcal D},\, M_{\mathcal D})$ that stabilize the Higgs potential up to the Planck scale while satisfying experimental constraints. Employing the RG-improved Higgs potential in the metric formulation of non-minimal Higgs inflation, we compute the inflationary observables $n_s$ and $r$. The SM+$(n)$VLQ+RHN framework yields predictions consistent with the latest Planck-LB-BK18 and ACT-LB-BK18 data, while simultaneously ensuring electroweak vacuum stability and phenomenologically viable neutrino masses within well-defined regions of parameter space. For comparison, we also investigate the SM+$(n)$VLQ limit and present its vacuum stability and Higgs inflation predictions as a reference to quantify the stabilizing role of the VLQ sector.

hep-ph↗

arXiv2024

Large Field Polynomial Inflation in Palatini $f(R,\phi)$ Gravity

In this paper, we employ the Palatini formalism to investigate the dynamics of large-field inflation using a renormalizable polynomial inflaton potential in the context of $f(R,\phi)$ gravity. Assuming instant reheating, we make a comparative analysis of large-field polynomial inflation (PI). We first consider the minimal and non-minimal coupling of inflaton in $R$ gravity, and then we continue with the minimally and non-minimally coupled inflaton in $f(R,\phi)$ gravity. We scan the parameter space for the inflationary predictions ($n_s$ and $r$) consistent with the Planck and BICEP/Keck 2018 results as well as the sensitivity forecast of the future CMB-S4 and depict the compliant regions in the $\phi_0-\beta$ plane where $\phi_0$ and $\beta$ are two parameters of polynomial inflation model which control the saddle point of the potential and the flatness in the vicinity of this point respectively. We find that a substantial portion of the parameter space aligns with the observational data.

astro-ph.CO↗

arXiv2023

Inflation in Symmergent Metric-Palatini Gravity

In this paper, we study the cosmological inflation phenomenon in symmergent gravity theory. Symmergent gravity is a novel framework which merges gravity and the standard model (SM) so that the gravity emerges from the matter loops and restores the broken gauge symmetries along the way. Symmergent gravity is capable of inducing the gravitational constant $G$ and the quadratic curvature coefficient $c_O$ from the loop corrections of the matter sector in a flat space-time. In the event that all the matter fields, including the beyond the standard model (BSM) sector, are mass degenerate, the vacuum energy can be expressed in terms of $G$ and $c_O$. The parameter which measures the deviation from the mass degeneracy is dubbed $\hat{\alpha}$. The parameters, $c_O$ and $\hat{\alpha}$, of symmergent gravity convey the information about the fermion and boson balance in the matter (SM+BSM) sector in number and in mass, respectively. In our analysis, we have investigated the space of the symmergent parameters $c_O$ and $\hat{\alpha}$ wherein they produce results that comply with the inflationary observables $n_s$, $r$, and $\mathrm{d}n_s/\mathrm{d}\ln k$. We have shown that the vacuum energy together with the quadratic curvature term arising in the symmergent gravity prescription are capable of inflating the universe provided that the quadratic curvature coefficient $c_O$ is negative (which corresponds to fermion dominance in number in the matter sector) and the deviation from the mass degeneracy in the matter sector is minute for both boson mass dominance and fermion mass dominance cases.

astro-ph.CO↗

arXiv2023

Dimensional Regularization in Quantum Field Theory with Ultraviolet Cutoff

In view of various field-theoretic reasons, in the present work, we study the question of if the usual dimensional regularization can be extended to quantum field theories with an ultraviolet cutoff (Poincare-breaking scale) in a way preserving all the properties of the dimensional regularization. And we find that it can indeed be. The resulting extension gives a framework in which the power-law and logarithmic divergences get detached to involve different scales. This new regularization scheme, the detached regularization as we call it, enables one to treat the power-law and logarithmic divergences differently and independently. We apply the detached regularization to the computation of the vacuum energy and to two well-known QFTs namely the scalar and spinor electrodynamics. As a case study, we consider Fujikawa's subtractive renormalization in the framework of the detached regularization, and show its effectiveness up to two loops by specializing to scalar self energy. We discuss various application areas of the detached regularization.

hep-th↗

arXiv2021

Ricci-Determinant Gravity: Dynamical Aspects and Astrophysical Implications

The Palatini gravitational action is enlarged by an arbitrary function $f(X)$ of the determinants of the Ricci tensor and the metric, $X=|\textbf{det}.R|/|\textbf{det}.g|$. The resulting Ricci-determinant theory exhibits novel deviations from general relativity. We study a particular realization where the extension is characterized by the square-root of the Ricci-determinant, $f(X)=\lambda_\text{Edd}\sqrt{X}$, which corresponds to the famous Eddington action. We analyze the obtained equations for a perfect fluid source and show that the affine connection can be solved in terms of the energy density and pressure of the fluid through an obtained disformal metric. As an application, we derive the hydrostatic equilibrium equations for relativistic stars and inspect the significant effects induced by the square-root of the Ricci tensor. We find that an upper bound on $\lambda_{\rm Edd}$, at which deviations from the predictions of general relativity on neutron stars become prominent, corresponds to the hierarchy between the Planck and the vacuum mass scales. The Ricci-determinant gravity that we propose here is expected to have interesting implications in other cosmological domains.

gr-qc↗

arXiv2021

Type-3/2 Seesaw Mechanism

Type-I seesaw provides a natural explanation for the tiny neutrino masses. The right-handed neutrino masses it requires are, however, too large to keep the Higgs boson mass at its measured value. Here we show that vector-spinors, singlet leptons like the right-handed neutrinos, generate the tiny neutrino masses naturally by the exchange of its spin-1/2 and spin-3/2 components. This one-step seesaw mechanism, which we call Type-3/2 seesaw, keeps the Higgs boson mass unchanged at one loop, and gives cause therefore to no fine-tuning problem. If the on-shell vector-spinor is a pure spin-3/2 particle then it becomes a potential candidate for hidden dark matter diluting due only to the expansion of the Universe. The Type-3/2 seesaw provides a natural framework for the neutrino, Higgs boson and dark matter sectors, with overall agreement with current experiments and observations.

hep-ph↗

arXiv2020

Affine gravitational scenario for dark matter decay

Since the current evidence of its existence is revealed only through its gravitational influence, the way dark matter couples to gravity must be then of primary importance. Here, unlike the standard model sector which is typically coupled to metric, dark matter is supposed to couple only to spacetime affine connection through a $Z_2$-symmetry breaking term. We show that this structure leads to a coupling between dark matter, which is considered scalar, and the standard model Higgs potential. This induces dark matter decays into standard model particles through the Higgs which acts as a portal between the visible and the dark sectors. We study thoroughly the resulting decay modes for various mass ranges, and provide relevant bounds on the nonminimal coupling to affine gravity in line with observational data. Moreover, we find that the coupling to Higgs can be sufficiently large to facilitate production of dark matter lighter than 10 GeV at current and future high energy colliders.

hep-ph↗

arXiv2020

Electroweak Stability and Discovery Luminosities for New Physics

What is the luminosity needed for discovering new physics if the electroweak scale is to remain stable? In this work we study this question, with the example of a real singlet scalar which couples to the Higgs field already at the renormalizable level. Observing that the electroweak scale remains stable if the two scalars couple in a seesawic fashion, we show that the HL-LHC, expected to deliver an integrated luminosity around 3/ab, can discover scalars weighing up to 800 GeV. The FCC-hh, on the other hand, can discover scalars as heavy as 2.3 TeV at 100/ab luminosity. It thus follows that the new physics that does not destabilize the electroweak scale can be accessed only at high luminosities, and is not possible exclude by the current LHC results.

hep-ph↗

arXiv2017

Hidden Spin-3/2 Field in the Standard Model

Here we show that a massive spin-3/2 field can hide in the SM spectrum in a way revealing itself only virtually. We study collider signatures and loop effects of this field, and determine its role in Higgs inflation and its potential as Dark Matter. We show that this spin-3/2 field has a rich linear collider phenomenology and motivates consideration of a neutrino-Higgs collider. We also show that study of Higgs inflation, dark matter and dark energy can reveal more about the neutrino and dark sector.

hep-ph↗