[Paper Review] Possible detection of relic neutrinos and their mass

Andreas Ringwald|ArXiv.org|Nov 9, 2001

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper proposes a method to indirectly detect relic neutrinos via the Z-burst scenario, where ultrahigh-energy cosmic neutrinos resonantly annihilate with relic neutrinos to produce Z bosons that decay into protons, whose energy spectrum is compared to observed cosmic rays. The analysis yields a best-fit neutrino mass of $ m_\nu = 2.75^{+1.28}_{-0.97} $ eV for halo scenarios and $ 0.26^{+0.20}_{-0.14} $ eV for extragalactic scenarios, with implications for future laboratory and astrophysical neutrino detection.

ABSTRACT

Recently the possibility was widely discussed that a large fraction of the highest energy cosmic rays may be decay products of Z bosons which were produced in the resonant annihilation of ultrahigh energy cosmic neutrinos on cosmological relic neutrinos. If one takes this so-called Z-burst scenario seriously, one may infer the mass of the heaviest relic neutrino as well as the necessary ultrahigh energy cosmic neutrino flux from a comparison of the predicted Z-burst spectrum with the observed cosmic ray spectrum.

Motivation & Objective

• To test the Z-burst scenario as a mechanism for producing ultrahigh-energy cosmic rays (UHECRs) via resonant annihilation of ultrahigh-energy cosmic neutrinos with relic neutrinos.
• To infer the mass of the heaviest relic neutrino by comparing the predicted Z-burst proton spectrum with the observed UHECR spectrum.
• To determine the required flux of ultrahigh-energy cosmic neutrinos consistent with the observed cosmic ray data.
• To assess the viability of the Z-burst model under cosmological and astrophysical constraints, including neutrino overdensity and photon escape from sources.

Proposed method

• Model the Z-burst proton flux using a convolution of the ultrahigh-energy cosmic neutrino flux, relic neutrino number density, Z production cross section, and branching ratio into hadrons.
• Use collider data to determine the momentum distribution of protons from Z decays, parameterized as $ \mathcal{Q}_{p+n}(E_p) $.
• Account for proton propagation from source to Earth, including energy losses from pion and $ e^+e^- $ pair production on the cosmic microwave background and cosmological redshift.
• Apply a maximum likelihood analysis to compare the predicted Z-burst spectrum with the observed UHECR spectrum, extracting confidence intervals on neutrino mass and flux.
• Incorporate both halo and extragalactic scenarios for background protons, and consider the impact of local relic neutrino overdensities.
• Evaluate the required UHE cosmic neutrino flux against theoretical upper limits and detectability in future experiments.

Experimental results

Research questions

• RQ1 What is the mass of the heaviest relic neutrino consistent with the observed ultrahigh-energy cosmic ray spectrum via the Z-burst mechanism?
• RQ2 What flux of ultrahigh-energy cosmic neutrinos is required to reproduce the observed UHECR spectrum in the Z-burst scenario?
• RQ3 How does the presence or absence of local relic neutrino overdensity affect the predicted Z-burst proton spectrum and its compatibility with observations?
• RQ4 Can the required UHE cosmic neutrino flux be reconciled with astrophysical constraints, particularly those from the diffuse gamma-ray background?
• RQ5 What are the implications of the derived neutrino mass for future laboratory experiments such as $ \beta $ decay and neutrinoless double-beta decay?

Key findings

• The best-fit mass for the heaviest relic neutrino is $ m_\nu = 2.75^{+1.28}_{-0.97} $ eV in the halo scenario, with a 95% confidence level lower bound of 0.97 eV.
• For the extragalactic scenario, the best-fit mass is $ 0.26^{+0.20}_{-0.14} $ eV, with a 95% confidence level lower bound of 0.06 eV.
• The derived neutrino mass is consistent with a hierarchical neutrino mass scenario, particularly as suggested by atmospheric neutrino oscillation data.
• The required ultrahigh-energy cosmic neutrino flux is consistent with current observational upper limits and is potentially detectable in upcoming experiments.
• The model requires astrophysical sources of UHE neutrinos that are 'hidden' in gamma rays, possibly due to dense environments or top-down production mechanisms.
• The results are compatible with cosmological constraints, including the upper bound $ \sum m_{\nu_i} \leq 4.4 $ eV from CMBR data, and are within reach of future sensitivity down to $ \sim 0.3 $ eV.