arXiv subjects

Yuhsin Tsai

Publications and source records attributed to Yuhsin Tsai.

At least 37 records · Page 2Linked to original sources

arXiv2024

Challenges in Interpreting the NANOGrav 15-Year Data Set as Early Universe Gravitational Waves Produced by ALP Induced Instability

In this paper, we study a possible early universe source for the recent observation of a stochastic gravitational wave background at the NANOGrav pulsar timing array. The source is a tachyonic instability in a dark gauge field induced by an axion-like particle (ALP), a known source for gravitational waves. We find that relative to the previous analysis with the NANOGrav 12.5-year data set, the current 15-year data set favors parameter space with a relatively larger axion mass and decay constant. This favored parameter space is heavily constrained by $ΔN_{\rm eff}$ and overproduction of ALP dark matter. While there are potential mechanisms for avoiding the second problem, evading the $ΔN_{\rm eff}$ constraint remains highly challenging. In particular, we find that the gravitational wave magnitude is significantly suppressed with respect to the gauge boson dark radiation, which implies that successfully explaining the NANOGrav observation requires a large additional dark radiation, violating the cosmological constraints. Satisfying the $ΔN_{\rm eff}$ constraint will limit the potential contribution from this mechanism to the observed signal to at most a percent level.

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arXiv2023

Finite Bubble Statistics Constrain Late Cosmological Phase Transitions

We consider first order cosmological phase transitions (PT) happening at late times, below Standard Model (SM) temperatures $T_{\rm PT} \lesssim$ GeV. The inherently stochastic nature of bubble nucleation and the finite number of bubbles associated with a late-time PT lead to superhorizon fluctuations in the PT completion time. We compute how such fluctuations eventually source curvature fluctuations with universal properties, independent of the microphysics of the PT dynamics. Using Cosmic Microwave Background (CMB) and Large Scale Structure (LSS) measurements, we constrain the energy released in a dark-sector PT. For 0.1 eV $\lesssim T_{\rm PT} \lesssim$ keV this constraint is stronger than both the current bound from additional neutrino species $ΔN_{\rm eff}$, and in some cases, even CMB-S4 projections. Future measurements of CMB spectral distortions and pulsar timing arrays will also provide competitive sensitivity for keV $\lesssim T_{\rm PT} \lesssim$ GeV.

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arXiv2023

Exploring Mirror Twin Higgs Cosmology with Present and Future Weak Lensing Surveys

We explore the potential of precision cosmological data to study non-minimal dark sectors by updating the cosmological constraint on the mirror twin Higgs model (MTH). The MTH model addresses the Higgs little hierarchy problem by introducing dark sector particles. In this work, we perform a Bayesian global analysis that includes the latest cosmic shear measurement from the DES three-year survey and the Planck CMB and BAO data. In the early Universe, the mirror baryon and mirror radiation behave as dark matter and dark radiation, and their presence modifies the Universe's expansion history. Additionally, the scattering between mirror baryon and photon generates the dark acoustic oscillation process, suppressing the matter power spectrum from the cosmic shear measurement. We demonstrate how current data constrain these corrections to the $Λ$CDM cosmology and find that for a viable solution to the little hierarchy problem, the proportion of MTH dark matter cannot exceed about $30\%$ of the total dark matter density, unless the temperature of twin photon is less than $30\%$ of that of the standard model photon. While the MTH model is presently not a superior solution to the observed $H_0$ tension compared to the $Λ$CDM+$ΔN_{\rm eff}$ model, we demonstrate that it has the potential to alleviate both the $H_0$ and $S_8$ tensions, especially if the $S_8$ tension persists in the future and approaches the result reported by the Planck SZ (2013) analysis. In this case, the MTH model can relax the tensions while satisfying the DES power spectrum constraint up to $k \lesssim 10~h\rm {Mpc}^{-1}$. If the MTH model is indeed accountable for the $S_8$ and $H_0$ tensions, we show that the future China Space Station Telescope (CSST) can determine the twin baryon abundance with a $10\%$ level precision.

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arXiv2023

Precision Cosmological Constraints on Atomic Dark Matter

Atomic dark matter is a simple but highly theoretically motivated possibility for an interacting dark sector that could constitute some or all of dark matter. We perform a comprehensive study of precision cosmological observables on minimal atomic dark matter, exploring for the first time the full parameter space of dark QED coupling and dark electron and proton masses $(α_{D}, m_{e_{D}}, m_{p_{D}})$ as well as the two cosmological parameters of aDM mass fraction $f_{D}$ and temperature ratio $ξ$ at time of SM recombination. We also show how aDM can accommodate the $(H_0, S_8)$ tension from late-time measurements, leading to a better fit than $Λ$CDM or $Λ$CDM + dark radiation. Furthermore, including late-time measurements leads to closed contours of preferred $ξ$ and dark hydrogen binding energy. The dark proton mass is seemingly unconstrained. Our results serve as an important new jumping-off point for future precision studies of atomic dark matter at non-linear and smaller scales.

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arXiv2023

Unraveling Cosmological Anisotropies within Stochastic Gravitational Wave Backgrounds

Identifying the anisotropies in a cosmologically sourced stochastic gravitational wave background (SGWB) would be of significance in shedding light on the nature of primordial inhomogeneities. For example, if SGWB carries isocurvature fluctuations, it would provide evidence for a multi-field inflationary origin of these inhomogeneities. However, this is challenging in practice due to finite detector sensitivity and also the presence of the astrophysical foregrounds that can compete with the cosmological signal. In this work, we explore the prospects for measuring cosmological SGWB anisotropies in the presence of an astrophysical counterpart and detector noise. To illustrate the main idea, we perform a Fisher analysis using a well-motivated cosmological SGWB template corresponding to a first order phase transition, and an astrophysical SGWB template corresponding to extra-galactic binary mergers, and compute the uncertainty with which various parameters characterizing the isotropic and anisotropic components can be extracted. We also discuss some subtleties and caveats involving shot noise in the astrophysical foreground. Overall, we show that upcoming experiments, e.g., LISA, Taiji, Einstein Telescope, Cosmic Explorer, and BBO, can all be effective in discovering plausible anisotropic cosmological SGWBs.

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arXiv2023

Recent Progress and Next Steps for the MATHUSLA LLP Detector

We report on recent progress and next steps in the design of the proposed MATHUSLA Long Lived Particle (LLP) detector for the HL-LHC as part of the Snowmass 2021 process. Our understanding of backgrounds has greatly improved, aided by detailed simulation studies, and significant R&D has been performed on designing the scintillator detectors and understanding their performance. The collaboration is on track to complete a Technical Design Report, and there are many opportunities for interested new members to contribute towards the goal of designing and constructing MATHUSLA in time for HL-LHC collisions, which would increase the sensitivity to a large variety of highly motivated LLP signals by orders of magnitude.

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arXiv2023

Probing Cosmological Particle Production and Pairwise Hotspots with Deep Neural Networks

Particles with masses much larger than the inflationary Hubble scale, $H_I$, can be pair-produced non-adiabatically during inflation. Due to their large masses, the produced particles modify the curvature perturbation around their locations. These localized perturbations eventually give rise to localized signatures on the Cosmic Microwave Background (CMB), in particular, pairwise hotspots (PHS). In this work, we show that Convolutional Neural Networks (CNN) provide a powerful tool for identifying PHS on the CMB. While for a given hotspot profile a traditional Matched Filter Analysis is known to be optimal, a Neural Network learns to effectively detect the large variety of shapes that can arise in realistic models of particle production. Considering an idealized situation where the dominant background to the PHS signal comes from the standard CMB fluctuations, we show that a CNN can isolate the PHS with $\mathcal{O}(10)\%$ efficiency even if the hotspot temperature is $\mathcal{O}(10)$ times smaller than the average CMB fluctuations. Overall, the CNN search is sensitive to heavy particle masses $M_0/H_I=\mathcal{O}(200)$, and constitutes one of the unique probes of very high energy particle physics.

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arXiv2023

Correlating Gravitational Wave and Gamma-ray Signals from Primordial Black Holes

Asteroid-mass primordial black holes (PBH) can explain the observed dark matter abundance while being consistent with the current indirect detection constraints. These PBH can produce gamma-ray signals from Hawking radiation that are within the sensitivity of future measurements by the AMEGO and e-ASTROGAM experiments. PBH which give rise to such observable gamma-ray signals have a cosmic origin from large primordial curvature fluctuations. There must then be a companion, stochastic gravitational wave (GW) background produced by the same curvature fluctuations. We demonstrate that the resulting GW signals will be well within the sensitivity of future detectors such as LISA, DECIGO, BBO, and the Einstein Telescope. The multi-messenger signal from the observed gamma-rays and GW will allow a precise measurement of the primordial curvature perturbation that produces the PBH. Indeed, we argue that the resulting correlation between the two types of observations can provide a smoking-gun signal of PBH.

astro-ph.CO↗

arXiv2022

Early-Universe Model Building

Theoretical investigations into the evolution of the early universe are an essential part of particle physics that allow us to identify viable extensions to the Standard Model as well as motivated parameter space that can be probed by various experiments and observations. In this white paper, we review particle physics models of the early universe. First, we outline various models that explain two essential ingredients of the early universe (dark matter and baryon asymmetry) and those that seek to address current observational anomalies. We then discuss dynamics of the early universe in models of neutrino masses, axions, and several solutions to the electroweak hierarchy problem. Finally, we review solutions to naturalness problems of the Standard Model that employ cosmological dynamics.

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arXiv2022

Improved cosmological constraints on the neutrino mass and lifetime

We present cosmological constraints on the sum of neutrino masses as a function of the neutrino lifetime, in a framework in which neutrinos decay into dark radiation after becoming non-relativistic. We find that in this regime the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO) and (uncalibrated) luminosity distance to supernovae from the Pantheon catalog constrain the sum of neutrino masses $\sum m_ν$ to obey $\sum m_ν< 0.42$ eV at (95$\%$ C.L.). While the the bound has improved significantly as compared to the limits on the same scenario from Planck 2015, it still represents a significant relaxation of the constraints as compared to the stable neutrino case. We show that most of the improvement can be traced to the more precise measurements of low-$\ell$ polarization data in Planck 2018, which leads to tighter constraints on $τ_{\rm reio}$ (and thereby on $A_s$), breaking the degeneracy arising from the effect of (large) neutrino masses on the amplitude of the CMB power spectrum.

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arXiv2022

Constraining decaying dark matter with BOSS data and the effective field theory of large-scale structures

We update cosmological constraints on two decaying dark matter models in light of BOSS-DR12 data analyzed under the Effective Field Theory of Large-Scale Structures (EFTofLSS) formalism, together with Planck, Pantheon and other BOSS measurements of the baryonic acoustic oscillation (BAO). In the first model, a fraction $f_{\rm dcdm}$ of cold dark matter (CDM) decays into dark radiation (DR) with a lifetime $τ$. In the second model (recently suggested as a potential resolution to the $S_8$ tension), all the CDM decays with a lifetime $τ$ into DR and a massive warm dark matter (WDM) particle, with a fraction $\varepsilon$ of the CDM rest mass energy transferred to the DR. Using numerical codes from the recent literature, we perform the first calculation of the mildly non-linear (matter and galaxy) power spectra with the EFTofLSS for these two models. In the case of DR products, we obtain the constraints $f_{\rm dcdm}\lesssim0.022$ (95\% C.L.) for lifetimes shorter than the age of the universe, and $τ/f_{\rm dcdm} \gtrsim 250$ Gyr in the long-lived regime assuming $f_{\rm dcdm}\to1$. We show that Planck data contributes the most to these constraints, with EFTofBOSS providing a marginal improvement over conventional BAO and redshift space distortions ($fσ_8$) data. In the case of DR and WDM decay products, we find that EFTofBOSS data significantly improves the constraints at 68\% C.L. on the CDM lifetime with a $S_8$ prior from KiDS-1000. We show that, in order to fit EFTofBOSS data while lowering $S_8$ to match KiDS-1000, the best-fit model has a longer lifetime $τ= 120$ Gyr, with a larger kick velocity $v_{\rm kick}/c \simeq \varepsilon \simeq 1.2\%$, than that without EFTofBOSS ($τ= 43$ Gyr, $\varepsilon =0.6\%$). We anticipate that future surveys will provide exquisite constraints on such models.

astro-ph.CO↗

arXiv2022

Free-streaming and Coupled Dark Radiation Isocurvature Perturbations: Constraints and Application to the Hubble Tension

Dark radiation (DR) appears as a new physics candidate in various scenarios beyond the Standard Model. While it is often assumed that perturbations in DR are adiabatic, they can easily have an isocurvature component if more than one field was present during inflation, and whose decay products did not all thermalize with each other. By implementing the appropriate isocurvature initial conditions (IC), we derive the constraints on both uncorrelated and correlated DR density isocurvature perturbations from the full Planck 2018 data alone, and also in combination with other cosmological data sets. Our study on free-streaming DR (FDR) updates and generalizes the existing bound on neutrino density isocurvature perturbations by including a varying number of relativistic degrees of freedom, and for coupled DR (CDR) isocurvature, we derive the first bound. We also show that for CDR qualitatively new physical effects arise compared to FDR. One such effect is that for isocurvature IC, FDR gives rise to larger CMB anisotropies compared to CDR -- contrary to the adiabatic case. More generally, we find that a blue-tilt of DR isocurvature spectrum is preferred. This gives rise to a larger value of the Hubble constant $H_0$ compared to the standard $Λ$CDM+$ΔN_{\rm eff}$ cosmology with adiabatic spectra and relaxes the $H_0$ tension.

astro-ph.CO↗

arXiv2022

Mirror Twin Higgs Cosmology: Constraints and a Possible Resolution to the $H_0$ and $S_8$ Tensions

The mirror twin Higgs model (MTH) is a solution to the Higgs hierarchy problem that provides well-predicted cosmological signatures with only three extra parameters: the temperature of the twin sector, the abundance of twin baryons, and the vacuum expectation value (VEV) of twin electroweak symmetry breaking. These parameters specify the behavior of twin radiation and the acoustic oscillations of twin baryons, which lead to testable effects on the cosmic microwave background (CMB) and large-scale structure (LSS). While collider searches can only probe the twin VEV, through a fit to cosmological data we show that the existing CMB (Planck18 TTTEEE+lowE+lowT+lensing) and LSS (KV450) data already provide useful constraints on the remaining MTH parameters. Additionally, we show that the presence of twin radiation in this model can raise the Hubble constant $H_0$ while the scattering twin baryons can reduce the matter fluctuations $S_8$, which helps to relax the observed $H_0$ and $S_8$ tensions simultaneously. This scenario is different from the typical $Λ$CDM + $ΔN_{\rm eff}$ model, in which extra radiation helps with the Hubble tension but worsens the $S_8$ tension. For instance, when including the SH0ES and Planck SZ data in the fit, we find that a universe with $\gtrsim 20\%$ of the dark matter comprised of twin baryons is preferred over $Λ$CDM by $\sim4σ$. If the twin sector is indeed responsible for resolving the $H_0$ and $S_8$ tensions, future measurements from the Euclid satellite and CMB Stage 4 experiment will further measure the twin parameters to $O(1-10\%)$-level precision. Our study demonstrates how models with hidden naturalness can potentially be probed using precision cosmological data.

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arXiv2022

Unleashing the full power of LHCb to probe Stealth New Physics

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.

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arXiv2021

B-modes from Post-inflationary Gravitational Waves Sourced by Axionic Instabilities at Cosmic Reionization

We show that axion-like particles that only couple to invisible dark photons can generate visible B-mode signals around the reionization epoch. The axion field starts rolling shortly before reionization, resulting in a tachyonic instability for the dark photons. This generates an exponential growth of the dark photon quanta sourcing both scalar metric modes and gravitational waves that leave an imprint on the reionized baryons. The tensor modes modify the cosmic microwave background (CMB) polarization at reionization, generating visible B-mode signatures for the next generation of CMB experiments for parameter ranges that satisfy the current experimental constraints.

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arXiv2021

Cosmologically Degenerate Fermions

Dark matter (DM) with a mass below a few keV must have a phase space distribution that differs substantially from the Standard Model particle thermal phase space: otherwise, it will free stream out of cosmic structures as they form. We observe that fermionic DM psi in this mass range will have a non-negligible momentum in the early Universe, even in the total absence of thermal kinetic energy. This is because the fermions were inevitably more dense at higher redshifts, and thus experienced Pauli degeneracy pressure. They fill up the lowest-momentum states, such that a typical fermion gains a momentum ~ O(p_F) that can exceed its mass m_psi. We find a simple relation between m_psi, the current fraction f_psi of the cold DM energy density in light fermions, and the redshift at which they were relativistic. Considering the impacts of the transition between nonrelativistic and relativistic behavior as revealed by measurements of DNeff and the matter power spectrum, we derive qualitatively new bounds in the f_psi-m_psi plane. We also improve existing bounds for f_psi = 1 by an order of magnitude to m_psi=2 keV. We remark on implications for direct detection and suggest models of dark sectors that may give rise to cosmologically degenerate fermions.

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arXiv2021

Cosmological Particle Production and Pairwise Hotspots on the CMB

Heavy particles with masses much bigger than the inflationary Hubble scale $H_*$, can get non-adiabatically pair produced during inflation through their couplings to the inflaton. If such couplings give rise to time-dependent masses for the heavy particles, then following their production, the heavy particles modify the curvature perturbation around their locations in a time-dependent and scale non-invariant manner. This results into a non-trivial spatial profile of the curvature perturbation that is preserved on superhorizon scales and eventually generates localized hot or cold spots on the CMB. We explore this phenomenon by studying the inflationary production of heavy scalars and derive the final temperature profile of the spots on the CMB by taking into account the subhorizon evolution, focusing in particular on the parameter space where pairwise hot spots (PHS) arise. When the heavy scalar has an $\mathcal{O}(1)$ coupling to the inflaton, we show that for an idealized situation where the dominant background to the PHS signal comes from the standard CMB fluctuations themselves, a simple position space search based on applying a temperature cut, can be sensitive to heavy particle masses $M_0/H_*\sim\mathcal{O}(100)$. The corresponding PHS signal also modifies the CMB power spectra and bispectra, although the corrections are below (outside) the sensitivity of current measurements (searches).

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arXiv2021

Bounds on Gauge Bosons Coupled to Non-conserved Currents

We discuss new bounds on vectors coupled to currents whose non-conservation is due to mass terms, such as $U(1)_{L_μ- L_τ}$. Due to the emission of many final state longitudinally polarized gauge bosons, inclusive rates grow exponentially fast in energy, leading to constraints that are only logarithmically dependent on the symmetry breaking mass term. This exponential growth is unique to Stueckelberg theories and reverts back to polynomial growth at energies above the mass of the radial mode. We present bounds coming from the high transverse mass tail of mono-lepton+missing transverse energy events at the LHC, which beat out cosmological bounds to place the strongest limit on Stueckelberg $U(1)_{L_μ- L_τ}$ models for most masses below a keV. We also discuss a stronger, but much more uncertain, bound coming from the validity of perturbation theory at the LHC.

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