[Paper Review] Did the nHZ Gravitational Waves Signatures Observed By NANOGrav Indicate Multiple Sector SUSY Breaking?

Xiao Kang Du, Ming Huang+2 more|arXiv (Cornell University)|Jul 6, 2023

Particle physics theoretical and experimental studies20 citations

TL;DR

The paper proposes that explicitly broken discrete R-symmetry, induced by multiple sector SUSY breaking, can cause domain walls whose collapse emits nHz gravitational waves, potentially matching NANOGrav signals and implying light goldstini.

ABSTRACT

Discrete R symmetries always play an important role in low energy SUSY. The spontaneously broken of such discrete R symmetries, for example, by gaugino condensation, can lead to domain walls, which need to be either inflated away or collapse to avoid cosmic difficulties. We propose that explicitly R symmetry violation needed for collapse of domain walls can be the consequence of multiple sector SUSY breaking. The consistency constraints for the generation of non-problematic domain walls from gaugino condensation are discussed. We also study the emitted gravitational waves related to the collapse of domain walls. We find that, for SUSY breaking scale of order ${\cal O}(1)$ ${ m GeV}$ in one of the sequestered sector (and also a low reheating temperature of order ${ m MeV}$ if the reheating is not completed when the domain walls collapse), the peak frequency of gravitational waves emitted can lie at nHz. Such a low SUSY breaking scale can be consistency and natural in multiple sector SUSY breaking scenario. The GWs signal by NANOGrav could be a signal of such multiple sector SUSY breaking scenario and it may also indicate the existences of light goldstini at ${ m eV}$ mass scale.

Motivation & Objective

• Motivate discrete R-symmetries in low-energy SUSY and the domain-wall problem after spontaneous breaking.
• Show how explicit R-symmetry violation can arise naturally in a multi-sector SUSY breaking framework.
• Derive consistency conditions for domain-wall formation and annihilation to avoid cosmological problems.
• Compute gravitational wave signals from domain-wall collapse and relate them to SUSY-breaking scales.

Proposed method

• Model the explicit discrete R violation via loop-suppressed operators transmitted between sequestered SUSY-breaking sectors and the SSM.
• Describe domain walls from gaugino condensation with a small explicit breaking term w_c in the superpotential.
• Estimate domain-wall tension sigma and bias epsilon, and derive decay conditions to avoid overclosure.
• Compute the gravitational-wave spectrum from domain-wall annihilation, including redshift effects and reheating scenarios.
• Relate peak GW frequency and energy density to am_3/2 (and thus F_i) and to reheating temperature T_R.
• Discuss the possibility of low reheating temperatures and their impact on f_peak^0 and Omega_gw^0.

Experimental results

Research questions

• RQ1 Can multi-sector SUSY breaking generate explicit R-symmetry violation sufficient to annihilate domain walls without cosmological problems?
• RQ2 What are the predicted gravitational-wave signatures from domain-wall collapse, and do they align with NANOGrav observations?
• RQ3 How do the SUSY-breaking scales in sequestered sectors map to GW peak frequency and energy density?
• RQ4 Under what conditions do reheating and gaugino-condensation-induced domain walls produce observable nHz GWs?
• RQ5 Do the results imply the existence of light goldstini at the eV scale?

Key findings

• Domain walls from discrete R-symmetry breaking can be annihilated by loop-suppressed explicit R-violating terms generated in multi-sector SUSY breaking.
• For a sequestered-sector SUSY breaking scale of order 1 GeV and a low reheating temperature around MeV, the GW peak can lie near nHz.
• The present GW density Omega_gw^0 can be around 1.16 x 10^-5 times (A^2)(b/a)^2, with am_3/2 determining the scale.
• Peak GW frequency today scales with am_3/2 and T_R, allowing f_peak^0 ~ 10^-9 Hz for certain parameter choices.
• The scenario naturally suggests light goldstini with masses around eV, due to low F_i in the i-th sector.
• The NANOGrav signal could be interpreted as arising from such multi-sector SUSY breaking GW production.