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Anish Ghoshal

Publications and source records attributed to Anish Ghoshal.

At least 91 records · Page 5Linked to original sources
arXiv2024

CMB imprints of high scale non-thermal leptogenesis

We study the imprints of high scale non-thermal leptogenesis on cosmic microwave background (CMB) from the measurements of inflationary spectral index ($n_s$) and tensor-to-scalar ratio ($r$), which otherwise is inaccessible to the conventional laboratory experiments. We argue that non-thermal production of baryon (lepton) asymmetry from subsequent decays of inflaton to heavy right-handed neutrinos (RHN) and RHN to SM leptons is sensitive to the reheating dynamics in the early Universe after the end of inflation. Such dependence provides detectable imprints on the $n_s-r$ plane which is well constrained by the Planck experiment. We investigate two separate cases, (I) inflaton decays to radiation dominantly and (II) inflaton decays to RHN dominantly which further decays to the SM particles to reheat the Universe adequately. Considering a class of $α-$ attractor inflation models, we obtain the allowed mass ranges for RHN for both cases and thereafter furnish the estimates for $n_s$ and $r$. The prescription proposed here is general and can be implemented in various kinds of single-field inflationary models given the conditions for non-thermal leptogenesis are satisfied.

hep-ph↗
arXiv2024

Detectable Gravitational Waves from preheating probes non-thermal Dark Matter

We describe the challenges and pathways when probing inflaton as dark matter with the stochastic gravitational waves (GWs) signal generated during the (p)reheating. Such scenarios are of utmost interest when no other interaction between the visible and dark sectors is present, therefore having no other detectability prospects. We consider the remnant energy in the coherently oscillating inflaton's zeroth mode to contribute to the observed relic dark matter density in the Universe. To fully capture the nonlinear dynamics and the effects of back-reactions during the oscillation, we resort to full nonlinear lattice simulation with pseudo-spectral methods to eliminate the differencing noises. We investigate for models whose behavior during the reheating era is of $m_Φ^2Φ^2$ type and find the typical primordial stochastic GWs backgrounds spectrum from scatterings among highly populated inflaton modes behaving like matter. We comment on the challenges of constructing such viable inflationary models such that the inflaton will account for the total dark matter of the universe while the produced GWs are within the future GWs detectors such as BBO, DECIGO, PTA, AION-MAGIS, and CE. We also describe the necessary modifications to the standard perturbative reheating scenario to prevent the depletion of residual inflaton energy via perturbative decay.

hep-ph↗
arXiv2023

Fate of False Vacuum in Non-perturbative Regimes

We use some exact results in the scalar field theory to revise the analysis by Coleman and Callan about the false vacuum decay and propose a simple non-perturbative formalism. We introduce exact Green's function which incorporates non-perturbative corrections in the strong coupling regimes of the theory. The solution of the scalar field theory involves Jacobi elliptical function and has been used to calculate the effective potential for any arbitrary coupling values. We demonstrate the use of this formalism in a simple $λϕ^4$ theory and show that the effective potential exhibits a false minimum at the origin. We then calculate the false vacuum decay rate and suggest simple analytic formulas which may be useful for the analysis for the first order phase transition beyond the perturbative regime. In our methodology, we show that the standard results obtained in perturbation theory are reproduced by taking the coupling values very small.

hep-th↗
arXiv2023

Axion-like particle (ALP) portal freeze-in dark matter confronting ALP search experiments

The relic density of Dark Matter (DM) in the freeze-in scenario is highly dependent on the evolution history of the universe and changes significantly in a non-standard (NS) cosmological framework prior to Big Bang Nucleosynthesis (BBN). In this scenario, an additional species dominates the energy budget of the universe at early times (before BBN), resulting in a larger cosmological expansion rate at a given temperature compared to the standard radiation-dominated (RD) universe. To investigate the production of DM in the freeze-in scenario, we consider both standard RD and NS cosmological picture before BBN and perform a comparative analysis. We extend the Standard Model (SM) particle content with a SM singlet DM particle $χ$ and an axion-like particle (ALP) $a$. The interactions between ALP, SM particles, and DM are generated by higher dimensional effective operators. This setup allows the production of DM $χ$ from SM bath through the mediation of ALP, via ALP-portal processes. These interactions involve non-renormalizable operators, leading to ultraviolet (UV) freeze-in, which depends on the reheating temperature ($T_{RH}$) of the early universe. In the NS cosmological scenario, the faster expansion rate suppresses the DM production processes, allowing for enhanced effective couplings between the visible and dark sectors to satisfy the observed DM abundance compared to RD scenario. This improved coupling increases the detection prospects for freeze-in DM via the ALP-portal, which is otherwise challenging to detect in RD universe due to small couplings involved. Using an effective field theory set-up, we show that various ALP searches such as in FASER, DUNE, and SHiP, etc. will be able to probe significant parameter space depending on the different model parameters.

hep-ph↗
arXiv2023

Primordial non-Gaussianity as a probe of seesaw and leptogenesis

We present the possibility that the seesaw mechanism and nonthermal leptogenesis can be {investigated} via primordial non-Gaussianities in the context of a majoron curvaton model. Originating as a massless Nambu-Goldstone boson from the spontaneous breaking of the global baryon ($B$) minus lepton ($L$) number symmetry at a scale $v_{B-L}$, majoron becomes massive when it couples to a new confining sector through anomaly. Acting as a curvaton, majoron produces the observed red-tilted curvature power spectrum without relying on any inflaton contribution, and its decay in the post-inflationary era gives rise to a nonthermal population of right-handed neutrinos that participate in leptogenesis. A distinctive feature of the mechanism is the generation of observable non-Gaussianity, {in the parameter space where the red-tilted power spectrum and sufficient baryon asymmetry are produced.} We {find} that the non-Gaussianity parameter $f_{\rm NL} \gtrsim \mathcal{O} (0.1)$ is produced for high-scale seesaw ($v_{B-L}$ at $\mathcal{O}(10^{14-17})$ GeV) and leptogenesis ($M_1 \gtrsim \mathcal{O}(10^6)$ GeV) where the latter represents the lightest right-handed neutrino mass. While the current bounds on local non-Gaussianity excludes some part of parameter space, the rest can be fully probed by future experiments like CMB-S4, LSST, and 21 cm tomography.

hep-ph↗
arXiv2023

Cosmic string gravitational waves from global $U(1)_{B-L}$ symmetry breaking as a probe of the type I seesaw scale

In type I seesaw models, the right-handed neutrinos are typically super-heavy, consistent with the generation of baryon asymmetry via standard leptogenesis. Primordial gravitational waves of cosmological origin provides a new window to probe such high scale physics, which would otherwise be inaccessible. By considering a {\em global} $U(1)_{B-L}$ extension of the type I seesaw model, we explore the connection between the heaviest right-handed neutrino mass and primordial gravitational waves arising from the dynamics of global cosmic string network. As a concrete example, we study a global $U(1)_{B-L}$ extension of the Littlest Seesaw model, and show that the inevitable GW signals, if detectable, probe the parameter space that can accommodate neutrino oscillation data and successful leptogenesis, while respecting theoretical constraints like perturbativity of the theory. Including CMB constraints from polarization and dark radiation leaves a large region of parameter space of the model, including the best fit regions, which can be probed by GW detectors like LISA and ET in the near future. In general, the GW detectors can test high scale type I seesaw models with the heaviest right-handed neutrino mass above $2.5 \times 10^{14}$ GeV, assuming the perturbativity, and $7 \times 10^{13}$ GeV assuming that the coupling between the heaviest right-handed neutrino and the $U(1)_{B-L}$ breaking scalar is less than unity.

hep-ph↗
arXiv2023

Measuring inflaton couplings via dark radiation as $ΔN_{\rm eff}$ in CMB

We study the production of a beyond the Standard Model (BSM) free-streaming relativistic particles which contribute to $N_{eff}$ and investigate how much the predictions for the inflationary analysis change. We consider inflaton decay as the source of this dark radiation (DR) and use the Cosmic Microwave Background (CMB) data from $\textit{Planck}$-2018 to constrain the scenarios and identify the parameter space involving couplings and masses of the inflaton that will be within the reach of next-generation CMB experiments like SPT-3G, CMB-S4, $\text{CMB-Bh$\overline{a}$rat}$, PICO, CMB-HD, etc. We find that if the BSM particle is produced only from the interaction with inflaton along with Standard Model (SM) relativistic particles, then its contribution to $N_{eff}$ is a monotonically increasing function of the branching fraction, $B_X$ of the inflaton to the BSM particle $X$; $\textit{Planck}$ bound on $N_{eff}$ rules out such $B_X \gtrsim 0.09$. Considering two different analyses of $\textit{Planck}$+BICEP data together with other cosmological observations, $N_{eff}$ is treated as a free parameter, which relaxes the constraints on scalar spectral index ($n_s$) and tensor-to-scalar ratio ($r$). The first analysis leads to predictions on the inflationary models like Hilltop inflation being consistent with the data. Second analysis rules out the possibility that BSM particle $X$ producing from the inflaton decay in Coleman-Weinberg Inflation or Starobinsky Inflation scenarios. To this end, we assume that SM Higgs is produced along with the BSM particle. We explore the possibilities that $X$ can be either a scalar or a fermion or a gauge boson and consider possible interactions with inflaton and find out the permissible range on the allowed parameter space Planck and those which will be within the reaches of future CMB observations.

hep-ph↗
arXiv2023

Non-perturbative Lee-Wick gauge theory: Towards Confinement and RGE with strong couplings

We consider a non-Abelian Lee--Wick gauge theory and discuss Becchi-Rouet-Stora-Tyutin (BRST) invariance. It contains fourth-order derivative as extensions of the kinetic term, leading to massive ghosts in the theory upon quantization. We particularly provide essential clues towards confinement conditions in strongly-coupled regimes, using the Kugo-Ojima approach, and obtain the $β-$functions in the non-perturbative regimes. This is achieved using a set of exact solutions of the corresponding local theory in terms of Jacobi elliptical functions. We obtain a similar $β-$function just as for the ordinary Yang-Mills theory but the main differences are that now, the cut-off arises naturally from the Lee-Wick heavy mass scales (M). We show that the fate of the ghosts is fixed in these regimes: they are no more the propagating degrees of freedom in the infrared (IR)-limit. As it also happens for the ordinary case, confinement is due to the non-Abelian nature of the theory. In the limit $M\rightarrow\infty $, one recovers the standard results for the local non-Abelian Yang-Mills theory.

hep-ph↗
arXiv2023

Confinement in QCD and generic Yang-Mills theories with matter representations

We derive the low-energy limit of quantum chromodynamics (QCD) and provide evidence that in the 't Hooft limit, i.e. for a very large number of colors and increasing 't Hooft coupling, quark confinement is recovered. The low energy limit of the theory turns out to be a non-local Nambu-Jona-Lasinio (NJL) model. The effect of non-locality, arising from a gluon propagator that fits quite well to the profile of an instanton liquid, is to produce a phase transition from a chiral condensate to an instanton liquid, as the coupling increases with lower momentum. This phase transition suffices to move the poles of the quark propagator to the complex plane. As a consequence, free quarks are no longer physical states in the spectrum of the theory.

hep-ph↗
arXiv2023

Imprints of a Supercooled Phase Transition in the Gravitational Wave Spectrum from a Cosmic String network

A network of cosmic strings (CS), if present, would continue emitting gravitational waves (GW) as it evolves throughout the history of the Universe. This results in a characteristic broad spectrum making it a perfect source to infer the expansion history. In particular, a short inflationary period caused by a supercooled phase transition would cause a drop in the spectrum at frequencies corresponding to that event. However, the impact on the spectrum is similar to the ones caused by an early matter-dominated era or from particle production, making it difficult to disentangle these different physical origins. We point out that, in the case of a short inflationary period, the GW spectrum receives an additional contribution from the phase transition itself. This leads to a characteristic imprint of a peak on top of a wide plateau both visible at future GW observatories.

astro-ph.CO↗
arXiv2023

Primordial Black Hole Archaeology with Gravitational Waves from Cosmic Strings

Light primordial black holes (PBHs) with masses smaller than $10^9$ g ($10^{-24} M_\odot$) evaporate before the onset of Big-Bang nucleosynthesis, rendering their detection rather challenging. If efficiently produced, they may have dominated the universe energy density. We study how such an early matter-dominated era can be probed successfully using gravitational waves (GW) emitted by local and global cosmic strings. While previous studies showed that a matter era generates a single-step suppression of the GW spectrum, we instead find a "double-step" suppression for local-string GW whose spectral shape provides information on the duration of the matter era. The presence of the two steps in the GW spectrum originates from GW being produced through two events separated in time: loop formation and loop decay, taking place either before or after the matter era. The second step - called the "knee" - is a novel feature which is universal to any early matter-dominated era and is not only specific to PBHs. Detecting GWs from cosmic strings with LISA, ET, or BBO would set constraints on PBHs with masses between $10^6$ and $10^9$ g for local strings with tension $Gμ= 10^{-11}$, and PBHs masses between $10^4$ and $10^9$ g for global strings with symmetry-breaking scale $η= 10^{15}~\mathrm{GeV}$. Effects from the spin of PBHs are discussed.

hep-ph↗
arXiv2023

Bremsstrahlung High-frequency Gravitational Wave Signatures of High-scale Non-thermal Leptogenesis

Inflaton seeds non-thermal leptogenesis by pair producing right-handed neutrinos in the seesaw model. We show that the inevitable graviton bremsstrahlung associated with inflaton decay can be a unique probe of non-thermal leptogenesis. The emitted gravitons contribute to a high-frequency stochastic gravitational waves background with a characteristic fall-off below the peak frequency. Besides leading to a lower bound on the frequency ($f\gtrsim 10^{11}$ Hz), the seesaw-perturbativity condition makes the mechanism sensitive to the lightest neutrino mass. For an inflaton mass close to the Planck scale, the gravitational waves contribute to sizeable dark radiation, which is within the projected sensitivity limits of future experiments such as CMB-S4 and CMB-HD.

hep-ph↗
arXiv2023

Probing Freeze-in Dark Matter via Heavy Neutrino Portal

We explore the possibility of probing freeze-in dark matter (DM) produced via the right-handed neutrino (RHN) portal using the RHN search experiments. We focus on a simplified framework of minimally-extended type-I seesaw model consisting of only four free parameters, namely the RHN mass, the fermionic DM mass, the Yukawa coupling between the DM and the RHN, and a real singlet scalar mass. We consider two cases for the DM production either via decay of the thermal RHN or via scattering of the bath particles mediated by the RHN. In both cases, we show that for sub-TeV scale DM masses, the allowed model parameter space satisfying the observed DM relic density for freeze-in scenario falls within the reach of current and future collider, beam dump and forward physics facilities looking for feebly-coupled heavy neutrinos.

hep-ph↗
arXiv2023

Growth of curvature perturbations for PBH formation \& detectable GWs in non-minimal curvaton scenario revisited

We revisit the growth of curvature perturbations in non-minimal curvaton scenario with a non-trivial field metric $λ(ϕ)$ where $ϕ$ is an inflaton field, and incorporate the effect from the non-uniform onset of curvaton's oscillation in terms of an axion-like potential. The field metric $λ(ϕ)$ plays a central role in the enhancement of curvaton field perturbation $δχ$, serving as an effective friction term which can be either positive or negative, depending on the first derivative $λ_{,ϕ}$.Our analysis reveals that $δχ$ undergoes the superhorizon growth when the condition $η_\text{eff} \equiv - 2 \sqrt{2ε} M_\text{Pl} { λ_{,ϕ} \over λ} < -3$ is satisfied. This is analogous to the mechanism responsible for the amplification of curvature perturbations in the context of ultra-slow-roll inflation, namely the growing modes dominate curvature perturbations. As a case study, we examine the impact of a Gaussian dip in $λ(ϕ)$ and conduct a thorough investigation of both the analytical and numerical aspects of the inflationary dynamics.Our findings indicate that the enhancement of curvaton perturbations during inflation is not solely determined by the depth of the dip in $λ(ϕ)$. Rather, the first derivative $λ_{,ϕ}$ also plays a significant role, a feature that has not been previously highlighted in the literature. Utilizing the $δ\mathcal{N}$ formalism, we derive analytical expressions for both the final curvature power spectrum and the non-linear parameter $f_\text{NL}$ in terms of an axion-like curvaton's potential leading to the non-uniform curvaton's oscillation. Additionally, the resulting primordial black hole abundance and scalar-induced gravitational waves are calculated, which provide observational windows for PBHs.

astro-ph.CO↗
arXiv2023

Measuring Inflaton Couplings via Primordial Gravitational Waves

We investigate the reach of future gravitational wave (GW) detectors in probing inflaton couplings with visible sector particles that can either be bosonic or fermionic in nature. Assuming reheating takes place through perturbative quantum production from vacuum in presence of classical inflaton background field, we find that the spectral energy density of the primordial GW generated during inflation becomes sensitive to inflaton-matter coupling. We conclude, obeying bounds from Big Bang Nucleosysthesis and Cosmic Microwave Background, that, e.g., inflaton-scalar couplings of the order of $\sim\mathcal{O}(10^{-20})$ GeV fall within the sensitivity range of several proposed GW detector facilities. However, this prediction is sensitive to the size of the inflationary scale, nature of the inflaton-matter interaction and shape of the potential during reheating. Having found the time-dependent effective inflaton decay width, we also discuss its implications for dark matter (DM) production from the thermal plasma via UV freeze-in during reheating. It is shown, that one can reproduce the observed DM abundance for its mass up to several PeVs, depending on the dimension of the operator connecting DM with the thermal bath and the associated scale of the UV physics. Thus we promote primordial GW to observables sensitive to feebly coupled inflaton, which is very challenging if not impossible to test in conventional particle physics laboratories or astrophysical measurements.

hep-ph↗
arXiv2023

Probing the Dark Matter density with gravitational waves from super-massive binary black holes

Supermassive black hole binaries source gravitational waves measured by Pulsar Timing Arrays. The frequency spectrum of this stochastic background is predicted more precisely than its amplitude. We argue that Dark Matter friction can suppress the spectrum around nHz frequencies, where it is measured, allowing to derive robust and significant bounds on the Dark Matter density, which, in turn, controls indirect detection signals from galactic centers. A precise spectrum of gravitational waves would translate in a tomography of the DM density profile, potentially probing DM particle-physics effects that induce a characteristic DM density profile, such as DM annihilations or de Broglie wavelength.

astro-ph.CO↗
arXiv2023

Cosmological probes of Grand Unification: Primordial Blackholes & scalar-induced Gravitational Waves

We investigate the inflationary cosmology involving an SU(5) GUT (grand unified theory) singlet scalar with non-minimal coupling to the Ricci scalar. In this scenario the scale of grand unification is set by the inflaton vev when the inflaton rolls down its potential towards its minimum $v$, thereby relating inflationary dynamics to GUT symmetry breaking with a prediction $r \simeq 0.025$ for the tensor-to-scalar ratio to be tested by the next generation CMB experiments. We show in this inflationary framework involving inflection-point how a suitable choice of parameters in $SU(5)$ leads to a bump in the scalar power spectrum with production of Primordial Blackholes (PBH) of masses $10^{17}-10^{18}$g ($ 10 - 100 M_\odot$). We derive the constraints on the self quartic and mixed quartic couplings of the inflaton in SU(5) that are consistent with the inflationary analysis. Moreover, we also show that this scenario leads to large amplitude induced second-order tensor perturbations propagating as Gravitational Waves (GW) with amplitude $Ω_{\rm GW}h^2 \sim 10^{-17}$ and peak frequency $f_{\rm peak} \sim$ (0.1 - 1) Hz, which can be detected in the next generation GW observatories like LISA, BBO, ET, etc. Thus, we unify the $SU(5)$ framework with PBH via inflection-point inflation showing how the upcoming measurements of PBH and GW will enable us to probe the scale of $SU(5)$ symmetry breaking, and thereby complementing the laboratory based experiments. We also discuss scenarios involving the Pati-Salam and Trinification gauge groups and its impact on quartic and mixed-quartic couplings that may lead to PBH and detectable GW signals.

hep-ph↗
arXiv2023

Impact of high-scale Seesaw and Leptogenesis on inflationary tensor perturbations as detectable gravitational waves

We discuss the damping of inflationary gravitational waves (GW) that re-enter the horizon before or during an epoch, where the energy budget of the universe is dominated by an unstable right handed neutrino (RHN), whose out of equilibrium decay releases entropy. Starting from the minimal Standard Model extension, motivated by the observed neutrino mass scale, with nothing more than 3 RHN for the Seesaw mechanism, we discuss the conditions for high scale leptogenesis assuming a thermal initial population of RHN. We further address the associated production of potentially light non-thermal dark matter and a potential component of dark radiation from the same RHN decay. One of our main findings is that the frequency, above which the damping of the tensor modes is potentially observable, is completely determined by successful leptogenesis and a Davidson-Ibarra type bound to be at around $0.1\;\text{Hz}$. To quantify the detection prospects of this GW background for various proposed interferometers such as AEDGE, BBO, DECIGO, Einstein Telescope or LISA we compute the signal-to-noise ratio (SNR). This allows us to investigate the viable parameter space of our model, spanned by the mass of the decaying RHN $M_1 \gtrsim 2.4\times 10^8\;\text{GeV} \cdot \sqrt{2\times 10^{-7}\;\text{eV}/\tilde{m}_1}$ (for leptogenesis) and the effective neutrino mass parameterizing its decay width $\tilde{m}_1< 2.9\times 10^{-7}\;\text{eV}$ (for RHN matter domination). Thus gravitational wave astronomy is a novel way to probe both the Seesaw and the leptogenesis scale, which are completely inaccessible to laboratory experiments in high scale scenarios.

hep-ph↗