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Register a new userCosmological Constraint on Dark Photon from $N_{rm eff}$
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A new U(1) gauge symmetry is the simplest extension of the Standard Model and has various theoretical and phenomenological motivations. In this paper, we study the cosmological constraint on the MeV scale dark photon. After the neutrino decoupling era at $T = mathcal{O}(1),$MeV, the decay and annihilation of the dark photon heats up the electron and photon plasma and accordingly decreases the effective number of neutrino $N_{mathrm{eff}}$ in the recombination era. We derive a conservative lower-limit of the dark photon mass around 8.5 MeV from the current Planck data if the mixing between the dark photon and ordinary photon is larger than $mathcal{O}(10^{-9})$. We also find that the future CMB stage-$rm I! V$ experiments can probe up to 17 MeV dark photon.
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If neutrinos are Dirac particles the existence of light right-handed neutrinos $ u_{R}$ is implied. Those would contribute to the effective number of relativistic neutrino species $N_{{rm eff}}$ in the early Universe. With pure standard model interactions, the contribution is negligibly small. In the presence of new interactions, however, the contribution could be significantly enhanced. We consider the most general effective four-fermion interactions for neutrinos (scalar, pseudo-scalar, vector, axial-vector and tensor), and compute the contribution of right-handed neutrinos to $N_{{rm eff}}$. Taking the Planck 2018 measurement of $N_{{rm eff}}$, strong constraints on the effective four-fermion coupling are obtained, corresponding to interaction strengths of $10^{-5}sim10^{-3}$ in units of the Fermi constant. This translates in new physics scales of up to 43 TeV and higher. Future experiments such as CMB-S4 can probe or exclude the existence of effective 4-neutrino operators for Dirac neutrinos. Ways to avoid this conclusion are discussed.
We discuss Dirac neutrinos whose right-handed component $ u_R$ has new interactions that may lead to a measurable contribution to the effective number of relativistic neutrino species $N_{rm eff}$. We aim at a model-independent and comprehensive study on a variety of possibilities. Processes for $ u_R$-genesis from decay or scattering of thermal species, with spin-0, spin-1/2, or spin-1 initial or final states are all covered. We calculate numerically and analytically the contribution of $ u_R$ to $N_{rm eff}$ primarily in the freeze-in regime, since the freeze-out regime has been studied before. While our approximate analytical results apply only to freeze-in, our numerical calculations work for freeze-out as well, including the transition between the two regimes. Using current and future constraints on $N_{rm eff}$, we obtain limits and sensitivities of CMB experiments on masses and couplings of the new interactions. As a by-product, we obtain the contribution of Higgs-neutrino interactions, $Delta N_{rm eff}^{rm SM} approx 7.5times10^{-12}$, assuming the neutrino mass is 0.1 eV and generated by the standard Higgs mechanism.
We evaluate the contribution to $N_{rm eff}$ of the extra sterile states in low-scale Type I seesaw models (with three extra sterile states). We explore the full parameter space and find that at least two of the heavy states always reach thermalisation in the Early Universe, while the third one might not thermalise provided the lightest neutrino mass is below ${mathcal O}(10^{-3}$eV). Constraints from cosmology therefore severely restrict the spectra of heavy states in the range 1eV- 100 MeV. The implications for neutrinoless double beta decay are also discussed.
We investigate whether the $4.4sigma$ tension on $H_0$ between SH$_{0}$ES 2019 and Planck 2018 can be alleviated by a variation of Newtons constant $G_N$ between the early and the late Universe. This changes the Hubble rate before recombination, similarly to adding $Delta N_{rm eff}$ extra relativistic degrees of freedom. We implement a varying $G_N$ in a scalar-tensor theory of gravity, with a non-minimal coupling $(M^2+beta phi^2)R$. If the scalar $phi$ starts in the radiation era at an initial value $phi_I sim 0.5~M_p$ and with $beta<0$, a dynamical transition occurs naturally around the epoch of matter-radiation equality and the field evolves towards zero at late times. As a consequence, the $H_0$ tension between SH$_{0}$ES (2019) and Planck 2018+BAO slightly decreases, as in $Delta N_{rm eff}$ models, to the 3.8$sigma$ level. We then perform a fit to a combined Planck, BAO and supernovae (SH$_0$ES and Pantheon) dataset. When including local constraints on Post-Newtonian (PN) parameters, we find $H_0=69.08_{-0.71}^{+0.6}~text{km/s/Mpc}$ and a marginal improvement of $Deltachi^2simeq-3.2$ compared to $Lambda$CDM, at the cost of 2 extra parameters. In order to take into account scenarios where local constraints could be evaded, we also perform a fit without PN constraints and find $H_0=69.65_{-0.78}^{+0.8}~text{km/s/Mpc}$ and a more significant improvement $Deltachi^2=-5.4$ with 2 extra parameters. For comparison, we find that the $Delta N_{rm eff}$ model gives $H_0=70.08_{-0.95}^{+0.91}~text{km/s/Mpc}$ and $Deltachi^2=-3.4$ at the cost of one extra parameter, which disfavors the $Lambda$CDM limit just above 2$sigma$, since $Delta N_{rm eff}=0.34_{-0.16}^{+0.15}$. Overall, our varying $G_N$ model performs similarly to the $Delta N_{rm eff}$ model in respect to the $H_0$ tension, if a physical mechanism to remove PN constraints can be implemented.
Gravitinos are a fundamental prediction of supergravity, their mass ($m_{G}$) is informative of the value of the SUSY breaking scale, and, if produced during reheating, their number density is a function of the reheating temperature ($T_{text{rh}}$). As a result, constraining their parameter space provides in turn significant constraints on particles physics and cosmology. We have previously shown that for gravitinos decaying into photons or charged particles during the ($mu$ and $y$) distortion eras, upcoming CMB spectral distortions bounds are highly effective in constraining the $T_{text{rh}}-m_{G}$ space. For heavier gravitinos (with lifetimes shorter than a few $times10^6$ sec), distortions are quickly thermalized and energy injections cause a temperature rise for the CMB bath. If the decay occurs after neutrino decoupling, its overall effect is a suppression of the effective number of relativistic degrees of freedom ($N_{text{eff}}$). In this paper, we utilize the observational bounds on $N_{text{eff}}$ to constrain gravitino decays, and hence provide new constaints on gravitinos and reheating. For gravitino masses less than $approx 10^5$ GeV, current observations give an upper limit on the reheating scale in the range of $approx 5 times 10^{10}- 5 times 10^{11}$GeV. For masses greater than $approx 4 times 10^3$ GeV they are more stringent than previous bounds from BBN constraints, coming from photodissociation of deuterium, by almost 2 orders of magnitude.
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