Precise capture rates of cosmic neutrinos and their implications on cosmology

Abstract

We explore the potential of measurements of cosmological effects, such as neutrino spectral distortions from the neutrino decoupling and neutrino clustering in our Galaxy, via cosmic neutrino capture on tritium. We compute the precise capture rates of each neutrino species including such cosmological effects to probe them. These precise estimates of capture rates are also important in that the would-be deviation of the estimated capture rate could suggest new neutrino physics and/or a non-standard evolution of the universe. In addition, we discuss the precise differences between the capture rates of Dirac and Majorana neutrinos for each species, the required energy resolutions to detect each neutrino species and the method of reconstruction of the spectrum of cosmic neutrinos via the spectrum of emitted electrons, with emphasis on the PTOLEMY experiment.

Unstable cosmic neutrino capture

“…We assume m ν 3 = 0 meV, and (TM T , ∆) = (100 g yr, 40 meV) (top left panel), (100 g yr, 20 meV) (top right panel), (500 g yr, 40 meV) (bottom left panel) and (500 g yr, 20 meV) (top left panel). Note that in an exposure of 100 g yr, the signal of ν 1 with τ ν 1 = 0.5t 0 cannot be observed since the expected number of this event is smaller than unity [55]. Due to the large value of |U e1 |, the constraints on a neutrino lifetime are more severe than those in the NO case.…”

“…Next we forecast the constraints on the neutrino lifetime and the invisible particle mass in the 2-body decays, ν 3 → ν 1 φ for the NO case and ν 1 → ν 3 φ for the IO case. For the IO case, ν 1 → ν 3 φ, as in figure 4, the electron spectrum for ν 3 produced by the decays of ν 1 is extremely suppressed, compared to that for ν 1 produced by the decays JHEP02(2022)132 0.5t 0 cannot be observed since the number of this event is smaller than one [55].…”

“…We set (TM T , ∆) = (100 g yr, 40 meV) (top left panel), (100 g yr, 20 meV) (top right panel), (500 g yr, 40 meV) (bottom left panel) and (500 g yr, 20 meV) (bottom right panel). In an exposure of 100 g yr, the signal of CνB with τ ν10.5t 0 cannot be observed since the number of this event is smaller than one[55].…”

“…The most promising method 2 of direct detections of the CνB is neutrino capture on β-decaying nuclei [49,50], in particular on tritium target [51][52][53][54][55], via the inverse β-decay process, ν i + 3 H → e − + 3 He (see refs. [56][57][58][59][60][61] for studies of physics beyond the SM via cosmic neutrino capture on tritium).…”

“…In the case of non-relativistic Dirac neutrinos, the capture rate is given by Γ i CνB |Uei| 2 σnν i NT[53]. For relativistic Dirac neutrinos, the capture rate is discussed in refs [54,55]…”