Siyi Zhou - Academia.edu (original) (raw)

Papers by Siyi Zhou

Research paper thumbnail of Gravitational waves from an inflation triggered first-order phase transition

Journal of High Energy Physics

Large excursion of the inflaton field can trigger interesting dynamics. One important example is ... more Large excursion of the inflaton field can trigger interesting dynamics. One important example is a first-order phase transition in a spectator sector which couples to the inflaton. Gravitational waves (GWs) from such a first-order phase transition during inflation, an example of an instantaneous source, have an oscillatory feature. In this work, we show that this feature is generic for a source in an era of accelerated expansion. We also demonstrate that the shape of the GW signal contains information about the evolution of the early universe following the phase transition. In particular, the slope of the infrared part of the GW spectrum is sensitive to the evolution of the Hubble parameter when the GW modes reenter the horizon after inflation. The slope of the profile of the intermediate oscillatory part and the ultraviolet part of the GW spectrum depend on the evolution of the Hubble parameter when the modes exit horizon during the inflation and when they reenter the horizon durin...

Research paper thumbnail of Strongly coupled quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2018

We study the power spectrum of quasi-single field inflation where strong coupling is considered. ... more We study the power spectrum of quasi-single field inflation where strong coupling is considered. The contribution from the massive propagator can be divided into local and non-local contributions. The local one is the leading contribution and is power-law suppressed as a function of mass, while the non-local contribution is exponentially suppressed in the large mass limit. For the local contribution, it is possible to use the effective field theory approach to study the power spectrum in the strongly coupled region of the parameter space. For the non-local contribution, we developed a partial effective field theory method to simplify the calculation: When there are multiple massive propagators, one can fully compute it after integrating out all but one massive propagator by effective field theory. The result retains the "standard clock" signal, which is interesting for probing the expansion history of the primordial universe and the physics of a "cosmological collider". The error involved compared to the full calculation is power law suppressed by the effective mass of the heavy field.

Research paper thumbnail of Gravitational production of superheavy dark matter

Physical Review D, 2001

The dark matter in the universe can be in the form of a superheavy matter species (wimpzilla). Se... more The dark matter in the universe can be in the form of a superheavy matter species (wimpzilla). Several mechanisms have been proposed for the production of wimpzilla particles during or immediately following the inflationary epoch. Perhaps the most attractive mechanism is through gravitational particle production, where particles are produced simply as a result of the expansion of the universe. In this paper we present a detailed numerical calculation of wimpzilla gravitational production in hybrid-inflation models and natural-inflation models. Generalizing these findings, we also explore the dependence of the gravitational production mechanism on various models of inflation. We show that superheavy dark matter production seems to be robust, with Ω X h 2 ∼ (M X /10 11 GeV) 2 (T RH /10 9 GeV), so long as M X < H I , where M X is the wimpzilla mass, T RH is the reheat temperature, and H I is the expansion rate of the universe during inflation.

Research paper thumbnail of Perturbative unitarity and NEC violation in genesis cosmology

Journal of High Energy Physics

Explorations of the violation of null energy condition (NEC) in cosmology could enrich our unders... more Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the “beyond Horndeski” theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and “beyond Horndeski” genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the “beyond Horndeski” operators.

Research paper thumbnail of Superheavy Dark Matter Production from Symmetry Restoration First-Order Phase Transition During Inflation

We propose a scenario where superheavy dark matter (DM) can be produced via symmetry restoration ... more We propose a scenario where superheavy dark matter (DM) can be produced via symmetry restoration first-order phase transition during inflation triggered by the evolution of the inflaton field. The phase transition happens in a spectator sector coupled to the inflaton field. During the phase transition, the spectator field tunnels from a symmetry-broken vacuum to a symmetry-restored vacuum. The massive particles produced after bubble collisions are protected against decaying by the restored symmetry and may serve as a DM candidate in the later evolution of the Universe. We show that the latent heat released during the phase transition can be sufficient to produce the DM relic abundance observed today. In addition, accompanied with the super heavy DM, this firstorder phase transition also produces gravitational waves detectable via future gravitational wave detectors.

Research paper thumbnail of Detection of Early-Universe Gravitational Wave Signatures and Fundamental Physics

Detection of a gravitational-wave signal of non-astrophysical origin would be a landmark discover... more Detection of a gravitational-wave signal of non-astrophysical origin would be a landmark discovery, potentially providing a significant clue to some of our most basic, big-picture scientific questions about the Universe. In this white paper, we survey the leading early-Universe mechanisms that may produce a detectable signal-including inflation, phase transitions, topological defects, as well as primordial black holes-and highlight the connections to fundamental physics. We review the complementarity with collider searches for new physics, and multimessenger probes of the large-scale structure of the Universe.

Research paper thumbnail of A unique gravitational wave signal from phase transition during inflation*

Chinese Physics C

We study the properties of gravitational wave (GW) signals produced by first-order phase transiti... more We study the properties of gravitational wave (GW) signals produced by first-order phase transitions during the inflation era. We show that the power spectrum of a GW oscillates with its wave number. This signal can be observed directly by future terrestrial and spatial GW detectors and through the B-mode spectrum in the CMB. This oscillatory feature of the GW is generic for any approximately instantaneous sources occurring during inflation and is distinct from the GW from phase transitions after inflation. The details of the GW spectrum contain information about the scale of the phase transition and the later evolution of the universe.

Research paper thumbnail of CEPC-SPPC Preliminary Conceptual Design Report. 1. Physics and Detector

Research paper thumbnail of with Massive Vector Fields

Research paper thumbnail of Perturbative unitarity in quasi-single field inflation

Journal of High Energy Physics, 2021

We study implications of perturbative unitarity for quasi-single field inflation with the inflato... more We study implications of perturbative unitarity for quasi-single field inflation with the inflaton and one massive scalar. Analyzing high energy scattering, we show that non-Gaussianities with |fNL| ≳ 1 cannot be realized without turning on interactions which violate unitarity at a high energy scale. Then, we provide a relation between fNL and the scale of new physics that is required for UV completion. In particular we find that for the Hubble scale H ≳ × 109 GeV, Planck suppressed operators can easily generate too large non-Gaussanities and so it is hard to realize successful quasi-single field inflation without introducing a mechanism to suppress quantum gravity corrections. Also we generalize the analysis to the regime where the isocurvature mode is heavy and the inflationary dynamics is captured by the inflaton effective theory. Requiring perturbative unitarity of the two-scalar UV models with the inflaton and one heavy scalar, we clarify the parameter space of the P(X, ϕ) mode...

Research paper thumbnail of String Regge trajectories in de Sitter space and implications for inflation

Research paper thumbnail of Spiky strings in de Sitter space

Journal of High Energy Physics, 2021

We study semiclassical spiky strings in de Sitter space and the corresponding Regge trajectories,... more We study semiclassical spiky strings in de Sitter space and the corresponding Regge trajectories, generalizing the analysis in anti-de Sitter space. In particular we demonstrate that each Regge trajectory has a maximum spin due to de Sitter acceleration, similarly to the folded string studied earlier. While this property is useful for the spectrum to satisfy the Higuchi bound, it makes a nontrivial question how to maintain mildness of high-energy string scattering which we are familiar with in flat space and anti-de Sitter space. Our analysis implies that in order to have infinitely many higher spin states, one needs to consider infinitely many Regge trajectories with an increasing folding number.

Research paper thumbnail of Cosmological signatures of superheavy dark matter

Journal of High Energy Physics, 2020

We discuss two possible scenarios, namely the curvaton mechanism and the dark matter density modu... more We discuss two possible scenarios, namely the curvaton mechanism and the dark matter density modulation, where non-Gaussianity signals of superheavy dark matter produced by gravity can be enhanced and observed. In both scenarios, superheavy dark matter couples to an additional light field as a mediator. In the case of derivative coupling, the resulting non-Gaussianities induced by the light field can be large, which can provide inflationary evidences for these superheavy dark matter scenarios.

Research paper thumbnail of Primordial non-Gaussianity as a probe of high energy physics

Research paper thumbnail of Imprints of Schwinger effect on primordial spectra

Journal of High Energy Physics, 2019

We study the Schwinger effect during inflation and its imprints on the primordial power spectrum ... more We study the Schwinger effect during inflation and its imprints on the primordial power spectrum and bispectrum. The produced charged particles by Schwinger effect during inflation can leave a unique angular dependence on the primodial spectra.

Research paper thumbnail of Operator method and recursion relations for inflationary correlators

Journal of Cosmology and Astroparticle Physics, 2019

We develop the systematics for applying operators on Minkowski correlation functions to get the i... more We develop the systematics for applying operators on Minkowski correlation functions to get the inflationary correlation functions. Simple structures and recursion relations are known for Minkowski correlation functions. Using the operator technique, various novel recursion relations for inflationary correlation functions are obtained.

Research paper thumbnail of Connections between Minkowski and cosmological correlation functions

Journal of Cosmology and Astroparticle Physics, 2018

We show how cosmological correlation functions of massless fields can be rewritten in terms of Mi... more We show how cosmological correlation functions of massless fields can be rewritten in terms of Minkowski correlation functions, by extracting symmetry-breaking operators from the cosmological correlators. This technique simplifies some cosmological calculations. Also, known properties of Minkowski correlation functions can be translated to non-trivial properties of cosmological correlations. To illustrate this idea, inflation to Minkowski and matter bounce to Minkowski relations are presented for the interactions of general single field inflation. And a Minkowski recursion relation is translated to a novel relation for inflation.

Research paper thumbnail of Unsuppressed primordial standard clocks in warm quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2018

We study quasi-single field inflation with a warm inflation background. The thermal effects at sm... more We study quasi-single field inflation with a warm inflation background. The thermal effects at small scales can sufficiently enhance the magnitude of the primordial standard clock signal. This scenario offers us the possibility of probing the UV physics of the very early universe without the exponentially small Boltzmann factor when the mass of the isocurvaton is much heavier than Hubble. The thermal effects at small scales can be studied using the flat space thermal field theory, connected to an effective description using non-Bunch-Davies vacuum at large scales, with large clock signal.

Research paper thumbnail of On the effective field theory for quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2017

We study the effective field theory (EFT) description of the virtual particle effects in quasi-si... more We study the effective field theory (EFT) description of the virtual particle effects in quasi-single field inflation, which unifies the previous results on large mass and large mixing cases. By using a horizon crossing approximation and matching with known limits, approximate expressions for the power spectrum and the spectral index are obtained. The error of the approximate solution is within 10% in dominate parts of the parameter space, which corresponds to less-than-0.1% error in the ns-r diagram. The quasi-single field corrections on the ns-r diagram are plotted for a few inflation models. Especially, the quasi-single field correction drives m 2 φ 2 inflation to the best fit region on the ns-r diagram, with an amount of equilateral non-Gaussianity which can be tested in future experiments.

Research paper thumbnail of KLT-like behaviour of inflationary graviton correlators

Journal of Cosmology and Astroparticle Physics, 2018

We use the spinor helicity formalism to study KLT-like relations for the inflationary graviton fo... more We use the spinor helicity formalism to study KLT-like relations for the inflationary graviton four-point correlation function. New features are observed in this correlation function compared to the graviton scattering amplitude in flat spacetime. After obtaining the general momentum dependence, collinear, squeezed and collapsed limits are considered to further study the features of the correlation function, and the relation to the corresponding flat space scattering amplitude.

Research paper thumbnail of Gravitational waves from an inflation triggered first-order phase transition

Journal of High Energy Physics

Large excursion of the inflaton field can trigger interesting dynamics. One important example is ... more Large excursion of the inflaton field can trigger interesting dynamics. One important example is a first-order phase transition in a spectator sector which couples to the inflaton. Gravitational waves (GWs) from such a first-order phase transition during inflation, an example of an instantaneous source, have an oscillatory feature. In this work, we show that this feature is generic for a source in an era of accelerated expansion. We also demonstrate that the shape of the GW signal contains information about the evolution of the early universe following the phase transition. In particular, the slope of the infrared part of the GW spectrum is sensitive to the evolution of the Hubble parameter when the GW modes reenter the horizon after inflation. The slope of the profile of the intermediate oscillatory part and the ultraviolet part of the GW spectrum depend on the evolution of the Hubble parameter when the modes exit horizon during the inflation and when they reenter the horizon durin...

Research paper thumbnail of Strongly coupled quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2018

We study the power spectrum of quasi-single field inflation where strong coupling is considered. ... more We study the power spectrum of quasi-single field inflation where strong coupling is considered. The contribution from the massive propagator can be divided into local and non-local contributions. The local one is the leading contribution and is power-law suppressed as a function of mass, while the non-local contribution is exponentially suppressed in the large mass limit. For the local contribution, it is possible to use the effective field theory approach to study the power spectrum in the strongly coupled region of the parameter space. For the non-local contribution, we developed a partial effective field theory method to simplify the calculation: When there are multiple massive propagators, one can fully compute it after integrating out all but one massive propagator by effective field theory. The result retains the "standard clock" signal, which is interesting for probing the expansion history of the primordial universe and the physics of a "cosmological collider". The error involved compared to the full calculation is power law suppressed by the effective mass of the heavy field.

Research paper thumbnail of Gravitational production of superheavy dark matter

Physical Review D, 2001

The dark matter in the universe can be in the form of a superheavy matter species (wimpzilla). Se... more The dark matter in the universe can be in the form of a superheavy matter species (wimpzilla). Several mechanisms have been proposed for the production of wimpzilla particles during or immediately following the inflationary epoch. Perhaps the most attractive mechanism is through gravitational particle production, where particles are produced simply as a result of the expansion of the universe. In this paper we present a detailed numerical calculation of wimpzilla gravitational production in hybrid-inflation models and natural-inflation models. Generalizing these findings, we also explore the dependence of the gravitational production mechanism on various models of inflation. We show that superheavy dark matter production seems to be robust, with Ω X h 2 ∼ (M X /10 11 GeV) 2 (T RH /10 9 GeV), so long as M X < H I , where M X is the wimpzilla mass, T RH is the reheat temperature, and H I is the expansion rate of the universe during inflation.

Research paper thumbnail of Perturbative unitarity and NEC violation in genesis cosmology

Journal of High Energy Physics

Explorations of the violation of null energy condition (NEC) in cosmology could enrich our unders... more Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the “beyond Horndeski” theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and “beyond Horndeski” genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the “beyond Horndeski” operators.

Research paper thumbnail of Superheavy Dark Matter Production from Symmetry Restoration First-Order Phase Transition During Inflation

We propose a scenario where superheavy dark matter (DM) can be produced via symmetry restoration ... more We propose a scenario where superheavy dark matter (DM) can be produced via symmetry restoration first-order phase transition during inflation triggered by the evolution of the inflaton field. The phase transition happens in a spectator sector coupled to the inflaton field. During the phase transition, the spectator field tunnels from a symmetry-broken vacuum to a symmetry-restored vacuum. The massive particles produced after bubble collisions are protected against decaying by the restored symmetry and may serve as a DM candidate in the later evolution of the Universe. We show that the latent heat released during the phase transition can be sufficient to produce the DM relic abundance observed today. In addition, accompanied with the super heavy DM, this firstorder phase transition also produces gravitational waves detectable via future gravitational wave detectors.

Research paper thumbnail of Detection of Early-Universe Gravitational Wave Signatures and Fundamental Physics

Detection of a gravitational-wave signal of non-astrophysical origin would be a landmark discover... more Detection of a gravitational-wave signal of non-astrophysical origin would be a landmark discovery, potentially providing a significant clue to some of our most basic, big-picture scientific questions about the Universe. In this white paper, we survey the leading early-Universe mechanisms that may produce a detectable signal-including inflation, phase transitions, topological defects, as well as primordial black holes-and highlight the connections to fundamental physics. We review the complementarity with collider searches for new physics, and multimessenger probes of the large-scale structure of the Universe.

Research paper thumbnail of A unique gravitational wave signal from phase transition during inflation*

Chinese Physics C

We study the properties of gravitational wave (GW) signals produced by first-order phase transiti... more We study the properties of gravitational wave (GW) signals produced by first-order phase transitions during the inflation era. We show that the power spectrum of a GW oscillates with its wave number. This signal can be observed directly by future terrestrial and spatial GW detectors and through the B-mode spectrum in the CMB. This oscillatory feature of the GW is generic for any approximately instantaneous sources occurring during inflation and is distinct from the GW from phase transitions after inflation. The details of the GW spectrum contain information about the scale of the phase transition and the later evolution of the universe.

Research paper thumbnail of CEPC-SPPC Preliminary Conceptual Design Report. 1. Physics and Detector

Research paper thumbnail of with Massive Vector Fields

Research paper thumbnail of Perturbative unitarity in quasi-single field inflation

Journal of High Energy Physics, 2021

We study implications of perturbative unitarity for quasi-single field inflation with the inflato... more We study implications of perturbative unitarity for quasi-single field inflation with the inflaton and one massive scalar. Analyzing high energy scattering, we show that non-Gaussianities with |fNL| ≳ 1 cannot be realized without turning on interactions which violate unitarity at a high energy scale. Then, we provide a relation between fNL and the scale of new physics that is required for UV completion. In particular we find that for the Hubble scale H ≳ × 109 GeV, Planck suppressed operators can easily generate too large non-Gaussanities and so it is hard to realize successful quasi-single field inflation without introducing a mechanism to suppress quantum gravity corrections. Also we generalize the analysis to the regime where the isocurvature mode is heavy and the inflationary dynamics is captured by the inflaton effective theory. Requiring perturbative unitarity of the two-scalar UV models with the inflaton and one heavy scalar, we clarify the parameter space of the P(X, ϕ) mode...

Research paper thumbnail of String Regge trajectories in de Sitter space and implications for inflation

Research paper thumbnail of Spiky strings in de Sitter space

Journal of High Energy Physics, 2021

We study semiclassical spiky strings in de Sitter space and the corresponding Regge trajectories,... more We study semiclassical spiky strings in de Sitter space and the corresponding Regge trajectories, generalizing the analysis in anti-de Sitter space. In particular we demonstrate that each Regge trajectory has a maximum spin due to de Sitter acceleration, similarly to the folded string studied earlier. While this property is useful for the spectrum to satisfy the Higuchi bound, it makes a nontrivial question how to maintain mildness of high-energy string scattering which we are familiar with in flat space and anti-de Sitter space. Our analysis implies that in order to have infinitely many higher spin states, one needs to consider infinitely many Regge trajectories with an increasing folding number.

Research paper thumbnail of Cosmological signatures of superheavy dark matter

Journal of High Energy Physics, 2020

We discuss two possible scenarios, namely the curvaton mechanism and the dark matter density modu... more We discuss two possible scenarios, namely the curvaton mechanism and the dark matter density modulation, where non-Gaussianity signals of superheavy dark matter produced by gravity can be enhanced and observed. In both scenarios, superheavy dark matter couples to an additional light field as a mediator. In the case of derivative coupling, the resulting non-Gaussianities induced by the light field can be large, which can provide inflationary evidences for these superheavy dark matter scenarios.

Research paper thumbnail of Primordial non-Gaussianity as a probe of high energy physics

Research paper thumbnail of Imprints of Schwinger effect on primordial spectra

Journal of High Energy Physics, 2019

We study the Schwinger effect during inflation and its imprints on the primordial power spectrum ... more We study the Schwinger effect during inflation and its imprints on the primordial power spectrum and bispectrum. The produced charged particles by Schwinger effect during inflation can leave a unique angular dependence on the primodial spectra.

Research paper thumbnail of Operator method and recursion relations for inflationary correlators

Journal of Cosmology and Astroparticle Physics, 2019

We develop the systematics for applying operators on Minkowski correlation functions to get the i... more We develop the systematics for applying operators on Minkowski correlation functions to get the inflationary correlation functions. Simple structures and recursion relations are known for Minkowski correlation functions. Using the operator technique, various novel recursion relations for inflationary correlation functions are obtained.

Research paper thumbnail of Connections between Minkowski and cosmological correlation functions

Journal of Cosmology and Astroparticle Physics, 2018

We show how cosmological correlation functions of massless fields can be rewritten in terms of Mi... more We show how cosmological correlation functions of massless fields can be rewritten in terms of Minkowski correlation functions, by extracting symmetry-breaking operators from the cosmological correlators. This technique simplifies some cosmological calculations. Also, known properties of Minkowski correlation functions can be translated to non-trivial properties of cosmological correlations. To illustrate this idea, inflation to Minkowski and matter bounce to Minkowski relations are presented for the interactions of general single field inflation. And a Minkowski recursion relation is translated to a novel relation for inflation.

Research paper thumbnail of Unsuppressed primordial standard clocks in warm quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2018

We study quasi-single field inflation with a warm inflation background. The thermal effects at sm... more We study quasi-single field inflation with a warm inflation background. The thermal effects at small scales can sufficiently enhance the magnitude of the primordial standard clock signal. This scenario offers us the possibility of probing the UV physics of the very early universe without the exponentially small Boltzmann factor when the mass of the isocurvaton is much heavier than Hubble. The thermal effects at small scales can be studied using the flat space thermal field theory, connected to an effective description using non-Bunch-Davies vacuum at large scales, with large clock signal.

Research paper thumbnail of On the effective field theory for quasi-single field inflation

Journal of Cosmology and Astroparticle Physics, 2017

We study the effective field theory (EFT) description of the virtual particle effects in quasi-si... more We study the effective field theory (EFT) description of the virtual particle effects in quasi-single field inflation, which unifies the previous results on large mass and large mixing cases. By using a horizon crossing approximation and matching with known limits, approximate expressions for the power spectrum and the spectral index are obtained. The error of the approximate solution is within 10% in dominate parts of the parameter space, which corresponds to less-than-0.1% error in the ns-r diagram. The quasi-single field corrections on the ns-r diagram are plotted for a few inflation models. Especially, the quasi-single field correction drives m 2 φ 2 inflation to the best fit region on the ns-r diagram, with an amount of equilateral non-Gaussianity which can be tested in future experiments.

Research paper thumbnail of KLT-like behaviour of inflationary graviton correlators

Journal of Cosmology and Astroparticle Physics, 2018

We use the spinor helicity formalism to study KLT-like relations for the inflationary graviton fo... more We use the spinor helicity formalism to study KLT-like relations for the inflationary graviton four-point correlation function. New features are observed in this correlation function compared to the graviton scattering amplitude in flat spacetime. After obtaining the general momentum dependence, collinear, squeezed and collapsed limits are considered to further study the features of the correlation function, and the relation to the corresponding flat space scattering amplitude.