Detecting non-relativistic cosmic neutrinos by capture on tritium: phenomenology and physics potential

Abstract

We study the physics potential of the detection of the Cosmic Neutrino Background via neutrino capture on tritium, taking the proposed PTOLEMY experiment as a case study. With the projected energy resolution of ∆ ∼ 0.15 eV, the experiment will be sensitive to neutrino masses with degenerate spectrum, m 1 m 2 m 3 = m ν 0.1 eV. These neutrinos are non-relativistic today; detecting them would be a unique opportunity to probe this unexplored kinematical regime. The signature of neutrino capture is a peak in the electron spectrum that is displaced by 2m ν above the beta decay endpoint. The signal would exceed the background from beta decay if the energy resolution is ∆ 0.7 m ν . Interestingly, the total capture rate depends on the origin of the neutrino mass, being Γ D 4 and Γ M 8 events per year (for a 100 g tritium target) for unclustered Dirac and Majorana neutrinos, respectively. An enhancement of the rate of up to O(1) is expected due to gravitational clustering, with the unique potential to probe the local overdensity of neutrinos. Turning to more exotic neutrino physics, PTOLEMY could be sensitive to a lepton asymmetry, and reveal the eV-scale sterile neutrino that is favored by short baseline oscillation searches. The experiment would also be sensitive to a neutrino lifetime on the order of the age of the universe and break the degeneracy between neutrino mass and lifetime which affects existing bounds.

Nonthermal cosmic neutrino background

“…Recently it has been suggested that measurements of the relic neutrino abundance could discriminate between Dirac and Majorana neutrinos due to their different projected count-rates for PTOLEMY of ∼ 4 yr −1 and ∼ 8 yr −1 , respectively [4]. We see that this proposal may not work in the presence of additional non-thermal Dirac neutrinos which could increase the respective count rate by 64% thereby diminishing the difference between Dirac and Majorana neutrinos.…”

“…Since the thermal neutrinos propagate as mass eigenstates, the different flavors will, to a good approximation, be equilibrated at late times [4]. We further assume that the flavor composition of the non-thermal neutrinos is also roughly 1 : 1 : 1.…”

“…We refer to this process as "left-right equilibration". For the originally thermally coupled LH neutrinos this implies that half of them are converted to the RH component, thus, halving the experimental count rate [4] in neutrino capture experiments. For the originally decoupled RH neutrinos, on the other hand, this implies that half of them are converted to the LH component, thus allowing for their experimental detection through weak currents.…”

Measuring anisotropies in the cosmic neutrino background

“…The generalization to the case of interest, ν j + 3 H → 3 He + e − , is straightforward and will be addressed afterwards (see [7,12,13] for previous calculations of the unpolarized capture rate). The neutron and neutrino are prepared in definite spin states, while the spins of the proton and electron are not observed.…”

“…Their planned target consists of ∼100 g of tritium ( 3 H) atomically bound to graphene. PTOLEMY should observe ∼10 CνB capture events per year, depending on the mass hierarchy and the Dirac versus Majorana nature of the neutrinos; the rate is half as large for non-relativistic Dirac neutrinos [7]. PTOLEMY has a planned energy resolution ∼0.15 eV, though this resolution may be further improved [6].…”