Majorana-Raychaudhuri Seminars

Title: Probing cosmological transition redshift through gravitational wave memory

Abstract: Despite numerous attempts over the last two decades to explain the accelerated expansion of our universe, its cause still remains elusive. However, with the advent of gravitational wave (GW) astronomy, there exists promising avenues for exploring unresolved cosmological mysteries. In this presentation we will demonstrate how one such feature of GWs known as GW memory offers a yardstick for understanding the acceleration of the universe. Initially, we will provide a brief overview of the formulation of a master equation describing GW memory within a broad class of spacetimes. Subsequently, we will study its application in FLRW spacetimes and show how GW memory shows observable signatures between accelerated and decelerated universes, potentially enabling the identification of the transition redshift from a matter-dominated to a dark-energy-dominated universe. 

Title: Effect of self-interacting dark matter on supermassive black hole mergers and pulsar timing array signals

Abstract: Mergers of supermassive black holes are a favored process to explain stochastic gravitational waves seen by pulsar timing arrays, but they suffer from a long-standing "final parsec problem," which causes them to stall before reaching the stage of GW emission.  We show that dynamical friction with dark matter spikes can solve this problem, and produce softening of the GW spectrum at low frequencies, if the DM is self-interacting, with cross sections previously identified as ameliorating small scale structure problems of CDM.

Title: Kramers-Kronig relation in gravitational lensing

Abstract: The Kramers-Kronig relation is a well-known relation, especially in the field of optics. The key to this relation is the causality that output comes only after input. We first show that gravitational lensing obeys the causality in the sense that (electromagnetic/gravitational) waves emitted from the source arrive at an observer only after the arrival of the signal in geometrical optics. We then explain the Kramers-Kronig relation in gravitational lensing, as the relation between real and imaginary parts of the amplification factor, which is the amplitude ratio of the lensed wave to the unlensed wave. Finally, we argue that an incorrect separation of the observed gravitational waveform into the amplification factor and the unlensed waveform generically leads to the violation of the Kramers-Kronig relation. This suggests that examining the violation of the Kramers-Kronig relation may be used for correctly extracting the lensing signal in the gravitational wave observations.

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Title: Primordial black holes from slow phase transitions: a model-building perspective

Abstract: We show the relation between the Higgs potential structure and primordial black hole (PBH) formations. Recently, it has been often discussed that PBHs can be formed by first-order phase transition at the early Universe. In this talk, we consider the PBH formation mechanism throught delayed first-order phase transitions at the early Universe. If the phase transition is delayed, the large energy density fluctuation can be realized between symmetry broken and unbroken regions. If the density fluctuation can be larger thant a certain criterion, the overdensity region may collapse to PBHs. We discuss the form of the Higgs potential needed to realize this PBH formation. In addition, we show that the commonly used exponential approximation of the bubble nucleation rate fails to capture such PBH formation.

Title: Probing new physics effects via observations of gravitational waves from first-order phase transition

Abstract: There are some phenomena that the standard model (SM) cannot explain, such as the baryon asymmetry of the universe. Thus the new physics effects beyond the SM are necessary to explain them. The new effects could potentially act as the cause of first-order phase transition in the early universe. The first-order phase transition generates gravitational waves (GWs). Since the GWs travel through the universe virtually unimpeded, it gives us information about the potential of the early universe. In this talk, we discuss the possibility of probing new physics effects via observations of GWs from first-order phase transition.

Title: Lessons for axion dark matter from nonstandard cosmologies

Abstract: Most studies of the early Universe assume a simple standard thermal scenario of radiation domination between the end of cosmic inflation and the start of Big Bang Nucleosynthesis. However, many well-motivated deviations from the standard scenario exist. In such nonstandard cosmologies many processes, including dark matter production, can be significantly affected. I will present a simple model of early matter domination and discuss how the properties of a dark matter axion change relative to the standard thermal model. This will be followed with some more involved, and more realistic, scenarios. Ensuing implications for discovery prospects will also be presented.

Title: Darkest Before The Dawn: Exploring the Very Early Universe

Abstract: The visible universe is 60 orders of magnitude larger than the Planck length, offering a rough lower bound on its overall expansion since the Big Bang. In typical theories, inflation accounts for 30 of these 60 “decades" and during the next 15 decades typical interaction energies are above the TeV scale.  In contrast to the final 15 decades of growth, which take roughly 13.8 billion years, the middle phase lasts just a trillionth of a second and occurs at energy scales which are experimentally untested. Despite its brevity, this epoch can host a complex range of nonlinear phenomena including gravitationally-driven “structure formation” from the collapse of self-gravitating quantum matter and gravitational wave production and I will discuss possible fingerprints from this era that might be visible today.

Title: Dark matter minihalos from primordial magnetic fields

Abstract: Primordial magnetic fields (PMFs) offer a simple explanation for the origin of galactic magnetic fields as well as of the purportedly detected magnetic fields in cosmic voids. A unique signature of the presence of PMFs is how they enhance baryon perturbations on small scales. However, on scales below the baryon thermal jeans scale, PMFs cannot enhance baryon density perturbations. In this talk, I show that on such small scales, PMFs instead enhance dark matter power perturbation purely by gravitational influence. I conclude by arguing how the search for 10^−11 to 10^3 solar mass dark matter minihalos can potentially provide the most sensitive probe for PMFs. 

Title: Neutrinos as an indirect new physics probe: oscillations, scattering, and thermal decoupling in the context of the SMEFT

Abstract: Neutrinos have been playing a significant role in our understanding of nature. Examples include the debate on momentum and energy conservation originated from nuclear beta decay in the 1930s, and the discovery of the weak neutral current following the pioneering Gargamelle experiment at CERN in the 1970s. While the modern particle physics can be well described by the standard model (SM), with a recent milestone in the discovery of the Higgs particle in 2012 at the LHC, other well-established phenomena such as neutrino oscillations, dark matter and the matter-antimatter asymmetry, require an extension of the SM. Tremendous efforts have been put in this direction in the past decades. In this talk, I will utilize neutrinos as an indirect new physics probe in the SM Effective Field Theory (SMEFT) framework, with a special focus on neutrino oscillations, neutrino scattering experiments like CEvNS, and neutrino decoupling in the early Universe.

Title: Detecting Rare Species of Dark Matter with Terrestrial Detectors

Abstract: A sub-component of dark matter with a short collision length compared to a planetary size leads to efficient accumulation of dark matter in astrophysical bodies. Such particles represent an interesting physics target since they can evade existing bounds from direct detection due to their rapid thermalization in high-density environments. In this talk, I will demonstrate that terrestrial probes, such as, large-volume neutrino telescopes as well as commercial/research nuclear reactors, can provide novel ways to constrain or discover such particles.

Title: Regularization and renormalization on cosmological backgrounds, primordial gravitational waves, and N_{eff} bounds revisited

Abstract: In this talk I will present on some recent and ongoing work that follows through on the renormalization process on cosmological backgrounds to completion. Doing so by adopting established techniques is clarifying to the point of novelty. Among the results we'll arrive at include -- Clarifying the roll played by the (UV and IR) scales corresponding to the beginning and end of inflation relative to the UV and IR scales corresponding to the unknown completion of the theory and its observables; Showing how certain IR divergences are an artifact of assuming a past infinite de Sitter phase as opposed to finite duration inflation; Finally, deriving a stress tensor for gravitational wave that does not presume a prior scale separation (as with the standard Isaacson form), and is therefore fit for the purposes of renormalization, highlighting how any attempts to extract N_{eff} bounds is inextricable from the process of renormalization.

Title: Second leptogenesis: a source of large lepton asymmetry

Abstract: Right-handed neutrinos play an important role in explaining various BSM phenomena, for example, tiny neutrino masses and baryon asymmetry of the Universe. Recently, a scenario was proposed where they have time-(and temperature-)dependent masses due to their coupling with wave dark matter, a coherently oscillating scalar field, and their effect has been studied variously. In this talk, we discuss the effect of the temperature-dependent mass in the early Universe, in particular, that in the lepton number production via their decay (leptogenesis). We suggested that such masses can cause the decoupling of the heavy neutrinos twice; in other words, it leads to two-fold leptogenesis. We consider a scenario where the lepton number produced at the first leptogenesis is partly converted to the baryon number via the sphaleron process as in the normal leptogenesis scenario, while the second leptogenesis occurs after the decoupling of the sphaleron process, and the produced lepton number remains until the current Universe. This extra production of the lepton number may cause fairly large lepton asymmetry compared with baryon asymmetry, which is suggested by the latest 4-Helium observation. We will show that the second leptogenesis significantly amplifies the lepton asymmetry and potentially explains this discrepancy. We will also discuss distinctive phenomenological predictions of the scenario. This talk is based on JHEP 03 (2024) 003.

Title:  Gravitational waves from quasi-stable cosmic strings and PTA data

Abstract: We discuss the stochastic gravitational wave background emitted from a network of 'quasi-stable' strings (QSS) and its realization in grand unified theories. A symmetry breaking in the early universe produces monopoles that suffer partial inflation. A subsequent symmetry breaking at a lower energy scale creates cosmic strings that are effectively stable against the breaking via Schwinger monopole-pair creation. As the monopoles reenter the horizon, we will have monopole-antimonopoles connected by strings, and further loop formation essentially ceases. Consequently, the lower frequency part of the gravitational wave spectrum will be suppressed compared to that of topologically stable cosmic strings. The gravitational radiation emitted in the early universe by QSS with a dimensionless string tension $G\mu\sim 10^{-6}$, is compatible with the exciting evidence of low-frequency gravitational background in PTA data, as well as the recent LIGO-VIRGO constraints, provided the superheavy strings and monopoles experience a certain amount of inflation.

Title: Generic Predictions for Primordial Perturbations and their implications

Abstract: We introduce a novel framework for studying small-scale primordial perturbations and

their cosmological implications. The framework uses a deep reinforcement learning to

generate scalar power spectrum profiles that are consistent with current observational

constraints. The framework is shown to predict the abundance of primordial black

holes and the production of secondary induced gravitational waves. We demonstrate

that the set up under consideration is capable of generating predictions that are

beyond the traditional model-based approaches.

Title: Analytical Results on Dark Matter Velocity Distributions

Abstract: We review recent work on determining dark matter velocity distributions from analytical methods  of classical mechanics, and the potential impact of these results on dark matter indirect detection  strategies.  In particular, we discuss velocity-dependent dark matter annihilation in subhalos and in the Galactic Center.  We compare the results of these analytic methods to those obtained in large N  numerical simulations.

Title:  Parity Solution to the Strong CP Problem and its Experimental Tests

Abstract:  In theories where Parity (P) is a spontaneously broken symmetry, the strong CP problem can be solved by P symmetry alone, without the need for an axion.  In this talk I shall elaborate on this proposal and present left-right symmetric theories which offer such a solution to the strong CP problem.  These theories have a natural embedding in SU(5) x SU(5) grand unification, which will be discussed. Phenomenological implications of this class of models will be discussed, focusing on neutrino oscillations and certain flavor anomalies.  Neutrinos are predicted to be naturally light Dirac fermions in these models.

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Title: Neutrinos masses, ultra-light dark matter and cosmology

Abstract:  Can the neutrino mass be a function of time? In this talk, I 

will present a consistent scenario of time-varying neutrino masses, and 

discuss its impact on cosmology, beta decay, and neutrino oscillation 

experiments. Such time-varying masses are assumed to be generated by the 

coupling between a neutrino and an ultralight scalar dark matter 

candidate. I will discuss how such a model fares in the light of various 

cosmological bounds, such as those coming from Big Bang Nucleosynthesis, 

the cosmic microwave background, as well as large-scale structures. This 

scenario can be further constrained using neutrino oscillation 

experiments, beta-decay experiments like KATRIN, as well as reactor and 

gallium experiments. Finally, I will also touch upon some recent ideas 

of whether the neutrino mass can be entirely generated by neutrino-dark matter interactions.

Title: Signatures of Primordial Black Holes: Correlation of various measurements

Abstract: Primordial black holes (PBH) can explain the observed dark matter abundance while being consistent with various constraints. In this talk, I will explore the correlation of various types of signals that can arise from the PBH and its formation history. For example, PBHs which give rise to observable gamma-ray signals from Hawking radiation at AMEGO, e-ASTROGAM can have a companion, stochastic gravitational wave (GW) background. Additionally, the PBH superradiance emerging from axion-like particles can produce complementary signals. In this talk, I will investigate the PBH as a dark matter candidate utilizing various complementary searches.

Title: Flavor Matters, but Matter Flavors: Matter Effects on Flavor Composition of Astrophysical Neutrinos

Abstract: We show that high-energy astrophysical neutrinos produced in the  

cores of heavily obscured active

galactic nuclei (AGNs) can undergo strong matter effects, thus  

significantly influencing their source

flavor ratios. In particular, matter effects can completely modify the  

standard interpretation of the

flavor ratio measurements in terms of the physical processes occurring  

in the sources (e.g., pp versus

pγ, full pion-decay chain versus muon-damped pion decay). We contrast  

our results with the existing

flavor ratio measurements at IceCube, as well as with projections for  

next-generation neutrino

telescopes like IceCube-Gen2. Signatures of these matter effects in  

neutrino flavor composition

would not only bring more evidence for neutrino production in central  

AGN regions, but would

also be a powerful probe of heavily Compton-thick AGNs, which escape  

conventional observation in X-rays and other electromagnetic wavelengths

Title: Dark Matter and Matter-Antimatter Asymmetry from Long-Lived Particles 

Abstract: In this, I will first discuss how sub-TeV long-lived particles (LLPs) in the visible sector may drive an epoch of early matter domination (EMD). I will then present a minimal extension of the standard model that includes a weak-scale LLP and a light dark matter (DM) candidate with O(GeV) mass. The LLP decay at the end of an EMD period yields the correct DM abundance and generates the observed baryon asymmetry of the universe. The parameter space of this model can be probed by proposed LLP searches as well as next-generation neutron-antineutron oscillation experiments.

Title: Back-reaction in the early universe

Abstract: Models of inflation might lead to enhanced scalar fluctuations on scales much smaller than those seeding the large-scale structure formation. In these scenarios, it is possible that the spike of power at high wavenumber might induce large corrections to the scalar power spectrum, e.g. in the form of loop corrections, potentially endangering the perturbativity of the underlying models. In this talk we discuss recent developments in the calculation of the 1-loop correction.

Title: Nordic Walking: A Dynamical Route to Hierarchies, and its Application to 3D Wess-Zumino-Witten theory

Abstract: A hierarchy in QFT refers to the existence of two scales with a large separation between them. If the coupling is dimensionful, the running between these scales is power-law and leads to a fine-tuning problem; if it is dimensionless, the running is logarithmic and only needs mild tuning. The fine-tuning problem for dimensionful couplings can be alleviated entirely if the coupling's beta function has a pair of complex conjugate fixed points close to the real line; this is called Walking, which I shall review briefly. I shall then discuss in detail our main result: a dimensionful coupling can be rendered (nearly) marginal dynamically, softening the fine-tuning problem to a mild one. It is an intermediate between walking and running, and christened hence 'Nordic Walking'. As an example for its emergence, I shall present our efforts to study 3D WZW theory on 4-sphere target manifold, which is an effective theory for deconfined quantum criticality, a proposed mechanism for certain (putative) order-to-order transitions beyond the Ginzburg-Landau paradigm in statistical/condensed matter physics. Time permitting, I shall also highlight some details of the calculational procedure, which entails a very instructive (and rather entertaining) application of higher-order regulators in functional RG. This may be useful beyond the present context in constructing non-perturbative gauge/diffeomorphism invariant RG flows.

(Based on 2312.11614.)