[Paper Review] Gravitational wave complementarity and impact of NANOGrav data on gravitational leptogenesis: cosmic strings

Rome Samanta, Satyabrata Datta|arXiv (Cornell University)|Sep 28, 2020

Neutrino Physics ResearchPhysics and Astronomy175 references96 citations

TL;DR

This paper proposes a complementary probe of gravitational leptogenesis via cosmic string-induced gravitational waves (GWs) and neutrinoless double beta decay (0νββ) experiments. It shows that the right-handed (RH) neutrino-induced gravitational leptogenesis mechanism generates lepton asymmetry through quantum gravity effects, with NANOGrav's 12.5-year pulsar timing data disfavoring m1 ≳25 meV at 2σ, linking GW constraints to 0νββ sensitivity.

ABSTRACT

In seesaw mechanism, if right handed (RH) neutrino masses are generated dynamically by a gauged $U(1)$ symmetry breaking, a stochastic gravitational wave background (SGWB) sourced by a cosmic string network could be a potential probe of leptogenesis. We show that the leptogenesis mechanism that facilitates the dominant production of lepton asymmetry via the quantum effects of right-handed neutrinos in gravitational background, can be probed by GW detectors as well as next-generation neutrinoless double beta decay ($0 u\beta\beta$) experiments in a complementary way. We infer that for a successful leptogenesis, an exclusion limit on $f-\Omega_{ m GW}h^2$ plane would correspond to an exclusion on the $|m_{\beta\beta}|-m_1$ plane as well. We consider a normal light neutrino mass ordering and discuss how recent NANOGrav pulsar timing data (if interpreted as GW signal) e.g., at 95$\%$ CL, would correlate with the potential discovery or null signal in $0 u\beta\beta$ decay experiments.

Motivation & Objective

• To establish a connection between gravitational wave signals from cosmic strings and the viability of right-handed neutrino-induced gravitational leptogenesis (RIGL).
• To explore how NANOGrav's 12.5-year pulsar timing data can constrain the RIGL mechanism and the effective electron neutrino mass in 0νββ decay.
• To derive a complementary exclusion region in the |mββ| − m1 plane based on GW detection limits from cosmic strings.
• To investigate the interplay between the string tension parameter Gµ and the lightest neutrino mass m1 in the context of successful leptogenesis.
• To assess the consistency of the RIGL mechanism with recent NANOGrav data, particularly under normal neutrino mass ordering.

Proposed method

• Formulates the RIGL mechanism where lepton asymmetry arises from two-loop quantum gravity effects involving right-handed neutrinos, generating a chemical potential via ∂µRjµ/M² operators.
• Derives the effective coupling 'b' in the Lagrangian using two-loop self-energy diagrams in a conformally flat gravitational background, leading to a CPT-violating chemical potential.
• Relates the string tension µ ∼ Λ²_CS to the GW energy density spectrum, using the Nambu–Goto action to model cosmic string loop decay and GW emission.
• Computes the stochastic gravitational wave background (SGWB) power spectrum from cosmic strings and compares it with NANOGrav's observed common-spectrum process at f ∼1/year.
• Uses the observed GW amplitude to constrain the string tension parameter Gµ, translating this into bounds on the lightest neutrino mass m1 via the RIGL mechanism.
• Applies the 0νββ decay parameter |mββ| as a complementary observable, deriving a corresponding exclusion region in the |mββ| − m1 plane based on GW constraints.

Experimental results

Research questions

• RQ1 Can the RIGL mechanism be probed by both gravitational wave detectors and future 0νββ experiments in a complementary manner?
• RQ2 How does the NANOGrav 12.5-year pulsar timing data constrain the RIGL mechanism, particularly in terms of the lightest neutrino mass m1?
• RQ3 What is the relationship between the string tension Gµ and the upper bound on m1 for successful leptogenesis?
• RQ4 To what extent does the NANOGrav signal at f ∼1/year favor cosmic string models over other GW sources like supermassive black hole binaries?
• RQ5 What is the predicted exclusion limit on |mββ| corresponding to a given GW detection threshold in the f − ΩGWh² plane?

Key findings

• A successful RIGL mechanism requires Gµ > 4.4 × 10⁻¹¹, which sets an upper bound of m1 ≲ 12 meV for the lightest neutrino mass.
• NANOGrav data at 95% CL disfavor m1 ≳ 25 meV, indicating that the RIGL mechanism is disfavored if the NANOGrav signal is interpreted as a stochastic GW background from cosmic strings.
• The exclusion limit on the string tension parameter Gµ translates into a corresponding exclusion in the |mββ| − m1 plane, enabling complementary testing via 0νββ experiments.
• The mechanism allows for a lower lightest RH neutrino mass scale M1 ∼ 10⁷ GeV when flavor effects are included, compared to the standard thermal leptogenesis lower bound of M1 > 10⁹ GeV.
• The model predicts a strong correlation between the SGWB amplitude and the effective electron neutrino mass |mββ|, with higher Gµ allowing larger m1 and thus less stringent constraints from 0νββ.
• The cosmic string model provides a better fit to the NANOGrav data than the single-power-law model, supporting the viability of this GW source for probing BSM physics.