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Register a new userNeutrino physics with the PTOLEMY project: active neutrino properties and the light sterile case
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The PTOLEMY project aims to develop a scalable design for a Cosmic Neutrino Background (CNB) detector, the first of its kind and the only one conceived that can look directly at the image of the Universe encoded in neutrino background produced in the first second after the Big Bang. The scope of the work for the next three years is to complete the conceptual design of this detector and to validate with direct measurements that the non-neutrino backgrounds are below the expected cosmological signal. In this paper we discuss in details the theoretical aspects of the experiment and its physics goals. In particular, we mainly address three issues. First we discuss the sensitivity of PTOLEMY to the standard neutrino mass scale. We then study the perspectives of the experiment to detect the CNB via neutrino capture on tritium as a function of the neutrino mass scale and the energy resolution of the apparatus. Finally, we consider an extra sterile neutrino with mass in the eV range, coupled to the active states via oscillations, which has been advocated in view of neutrino oscillation anomalies. This extra state would contribute to the tritium decay spectrum, and its properties, mass and mixing angle, could be studied by analyzing the features in the beta decay electron spectrum.
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We investigate the implications of one light eV scale sterile neutrino on the physics potential of the proposed long-baseline experiment DUNE. If the future short-baseline experiments confirm the existence of sterile neutrinos, then it can affect the mass hierarchy (MH) and CP-violation (CPV) searches at DUNE. The MH sensitivity still remains above 5$sigma$ if the three new mixing angles ($theta_{14}, theta_{24}, theta_{34}$) are all close to $theta_{13}$. In contrast, it can decrease to 4$sigma$ if the least constrained mixing angle $theta_{34}$ is close to its upper limit $sim 30^0$. We also assess the sensitivity to the CPV induced both by the standard CP-phase $delta_{13} equiv delta$, and the new CP-phases $delta_{14}$ and $delta_{34}$. In the 3+1 scheme, the discovery potential of CPV induced by $delta_{13}$ gets deteriorated compared to the 3$ u$ case. In particular, the maximal sensitivity (reached around $delta_{13}$ $sim$ $pm$ $90^0$) decreases from $5sigma$ to $4sigma$ if all the three new mixing angles are close to $theta_{13}$. It can further diminish to almost $3sigma$ if $theta_{34}$ is large ($sim 30^0$). The sensitivity to the CPV due to $delta_{14}$ can reach 3$sigma$ for an appreciable fraction of its true values. Interestingly, $theta_{34}$ and its associated phase $delta_{34}$ can influence both the $ u_e$ appearance and $ u_mu$ disappearance channels via matter effects, which in DUNE are pronounced. Hence, DUNE can also probe CPV induced by $delta_{34}$ provided $theta_{34}$ is large. We also reconstruct the two phases $delta_{13}$ and $delta_{14}$. The typical 1$sigma$ uncertainty on $delta_{13}$ ($delta_{14}$) is $sim20^0$ ($30^0$) if $theta_{34} =0$. The reconstruction of $delta_{14}$ (but not that of $delta_{13}$) degrades if $theta_{34}$ is large.
We provide a consistent framework to set limits on properties of light sterile neutrinos coupled to all three active neutrinos using a combination of the latest cosmological data and terrestrial measurements from oscillations, $beta$-decay and neutrinoless double-$beta$ decay ($0 ubetabeta$) experiments. We directly constrain the full $3+1$ active-sterile mixing matrix elements $|U_{alpha4}|^2$, with $alpha in ( e,mu ,tau )$, and the mass-squared splitting $Delta m^2_{41} equiv m_4^2-m_1^2$. We find that results for a $3+1$ case differ from previously studied $1+1$ scenarios where the sterile is only coupled to one of the neutrinos, which is largely explained by parameter space volume effects. Limits on the mass splitting and the mixing matrix elements are currently dominated by the cosmological data sets. The exact results are slightly prior dependent, but we reliably find all matrix elements to be constrained below $|U_{alpha4}|^2 lesssim 10^{-3}$. Short-baseline neutrino oscillation hints in favor of eV-scale sterile neutrinos are in serious tension with these bounds, irrespective of prior assumptions. We also translate the bounds from the cosmological analysis into constraints on the parameters probed by laboratory searches, such as $m_beta$ or $m_{beta beta}$, the effective mass parameters probed by $beta$-decay and $0 ubetabeta$ searches, respectively. When allowing for mixing with a light sterile neutrino, cosmology leads to upper bounds of $m_beta < 0.09$ eV and $m_{beta beta} < 0.07$ eV at 95% C.L, more stringent than the limits from current laboratory experiments.
Dark matter detectors will soon be sensitive to Solar neutrinos via two distinct channels: coherent neutrino-nucleus scattering and neutrino electron elastic scattering. We establish an analysis method for extracting Solar model properties and neutrino properties from these measurements, including the possible effects of sterile neutrinos which have been hinted at by some reactor experiments and cosmological measurements. Even including sterile neutrinos, through the coherent scattering channel a 1 ton-year exposure with a low-threshold Germanium detector could improve on the current measurement of the normalization of the $^8$B Solar neutrino flux down to 3% or less. Combining with the elastic scattering data will provide constraints on both the high and low energy survival probability, and will improve on the uncertainty on the active-to-sterile mixing angle by a factor of two. This sensitivity to active-to-sterile transitions is competitive and complementary to forthcoming dedicated short baseline sterile neutrino searches with nuclear decays.
Light sterile neutrinos have been introduced as an explanation for a number of oscillation signals at $Delta m^2 sim 1$ eV$^2$. Neutrino oscillations at relatively short baselines provide a probe of these possible new states. This paper describes an accelerator-based experiment using neutral current coherent neutrino-nucleus scattering to strictly search for active-to-sterile neutrino oscillations. This experiment could, thus, definitively establish the existence of sterile neutrinos and provide constraints on their mixing parameters. A cyclotron-based proton beam can be directed to multiple targets, producing a low energy pion and muon decay-at-rest neutrino source with variable distance to a single detector. Two types of detectors are considered: a germanium-based detector inspired by the CDMS design and a liquid argon detector inspired by the proposed CLEAR experiment.
ESS$ u$SB is a proposed neutrino super-beam project at the ESS facility. We study the performance of this setup in the presence of a light eV-scale sterile neutrino, considering 540 km baseline with 2 years (8 years) of $ u$ ($bar u$) run-plan. This baseline offers the possibility to work around the second oscillation maximum, providing high sensitivity towards CP-violation (CPV). We explore in detail its capability in resolving CPV generated by the standard CP phase $delta_{13}$, the new CP phase $delta_{14}$, and the octant of $theta_{23}$. We find that the sensitivity to CPV induced by $delta_{13}$ deteriorates noticeably when going from $3 u$ to 4$ u$ case. The two phases $delta_{13}$ and $delta_{14}$ can be reconstructed with a 1$sigma$ uncertainty of $sim15^0$ and $ sim35^0$ respectively. Concerning the octant of $theta_{23}$, we find poor sensitivity in both $3 u$ and $4 u$ schemes. Our results show that a setup like ESS$ u$SB working around the second oscillation maximum with a baseline of 540 km, performs quite well to explore CPV in 3$ u$ scheme, but it is not optimal for studying CP properties in 3+1 scheme.
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