PandaX-4T reports the first direct spectral search for scalar-mediated neutrino self-interactions in 136Xe double beta decay, setting new upper limits for mediator masses below 2 MeV/c².
A tale of two modes: Neutrino free-streaming in the early universe
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abstract
We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant $ G_{\rm eff}$, and resulting in a neutrino opacity $\dot{\tau}_\nu\propto G_{\rm eff}^2 T_\nu^5$. Using a conservative prior on the parameter $\log_{10}\left(G_{\rm eff} {\rm MeV}^2\right)$, we find a bimodal posterior distribution. The first of these modes is consistent with the standard $\Lambda$CDM cosmology and corresponds to neutrinos decoupling at redshift $z_{\nu,{\rm dec}} > 1.3\times10^5$. The other mode of the posterior, dubbed the "interacting neutrino mode", corresponds to neutrino decoupling occurring within a narrow redshift window centered around $z_{\nu,{\rm dec}}\sim8300$. This mode is characterized by a high value of the effective neutrino coupling constant, together with a lower value of the scalar spectral index and amplitude of fluctuations, and a higher value of the Hubble parameter. Using both a maximum likelihood analysis and the ratio of the two mode's Bayesian evidence, we find the interacting neutrino mode to be statistically disfavored compared to the standard $\Lambda$CDM cosmology. Interestingly, the addition of CMB polarization and direct Hubble constant measurements significantly raises the statistical significance of this secondary mode, indicating that new physics in the neutrino sector could help explain the difference between local measurements of $H_0$, and those inferred from CMB data. A robust consequence of our results is that neutrinos must be free streaming long before the epoch of matter-radiation equality.
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A new scheme for the neutrino collision term valid from light to heavy mediator regimes, with smooth transition as the universe cools, for scalar-mediated NSI in Dirac and Majorana cases.
Derives suppression of adiabatic perturbations and scale-dependent growth of isocurvature power in warm wave dark matter, verifies with Schrödinger-Poisson simulations, and proposes an analytic halo mass function.
Final Planck CMB data confirms the flat 6-parameter ΛCDM model with Ω_c h² = 0.120 ± 0.001, Ω_b h² = 0.0224 ± 0.0001, n_s = 0.965 ± 0.004, τ = 0.054 ± 0.007, H_0 = 67.4 ± 0.5 km/s/Mpc, and no strong evidence for extensions.
Presents the science case, reference design, and project plan for the CMB-S4 ground-based CMB experiment.
citing papers explorer
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Probing scalar-neutrino and scalar-dark-matter interactions with PandaX-4T
PandaX-4T reports the first direct spectral search for scalar-mediated neutrino self-interactions in 136Xe double beta decay, setting new upper limits for mediator masses below 2 MeV/c².
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Towards a complete scheme of cosmological neutrino self-interactions: Collision term for a wide range of mediator masses
A new scheme for the neutrino collision term valid from light to heavy mediator regimes, with smooth transition as the universe cools, for scalar-mediated NSI in Dirac and Majorana cases.
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Early Growth of Structure in Warm Wave Dark Matter
Derives suppression of adiabatic perturbations and scale-dependent growth of isocurvature power in warm wave dark matter, verifies with Schrödinger-Poisson simulations, and proposes an analytic halo mass function.
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Planck 2018 results. VI. Cosmological parameters
Final Planck CMB data confirms the flat 6-parameter ΛCDM model with Ω_c h² = 0.120 ± 0.001, Ω_b h² = 0.0224 ± 0.0001, n_s = 0.965 ± 0.004, τ = 0.054 ± 0.007, H_0 = 67.4 ± 0.5 km/s/Mpc, and no strong evidence for extensions.
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CMB-S4 Science Case, Reference Design, and Project Plan
Presents the science case, reference design, and project plan for the CMB-S4 ground-based CMB experiment.