[Paper Review] Particle Probes with Superradiant Pulsars

David E. Kaplan, Surjeet Rajendran+1 more|arXiv (Cornell University)|Aug 27, 2019

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper proposes using rotational superradiance in millisecond pulsars as a probe for ultra-light bosons, leveraging their precisely measured spin frequencies to constrain scalar particles with masses below 10^{-11} eV. It demonstrates that the absence of pulsars above ~700 Hz may signal a new particle with mass ~10^{-11} eV, improving fifth-force constraints by up to three orders of magnitude and ruling out QCD axions with Planck-scale decay constants in specific mass ranges.

ABSTRACT

We demonstrate that rotational superradiance can be efficient in millisecond pulsars. Measurements from the two fastest known pulsars PSR J1748-2446ad and PSR B1937+21 can place bounds on bosons with masses below 10^{-11} eV. The bounds are maximally good at masses corresponding to the rotation rate of the star, where scalar interactions that mediate forces ~ 10^6 times weaker than gravity are ruled out, exceeding existing fifth force constraints by 3 orders of magnitude. For certain neutron star equations of state, these measurements would also constrain the QCD axion with masses between 5 10^{-13} and 3 10^{-12} eV. Despite the ability of most neutron star equations of state to support frequencies as high as ~ 1500 Hz, the observed absence of pulsars above ~ 700 Hz could be due to the existence of a new particle of mass ~ 10^{-11} eV with a Yukawa coupling to nucleons.

Motivation & Objective

• To explore whether millisecond pulsars can serve as sensitive probes for ultra-light bosons through rotational superradiance.
• To improve existing constraints on weakly coupled scalar particles, particularly those mediating forces weaker than gravity.
• To investigate whether the observed cutoff in pulsar spin frequencies above ~700 Hz could be due to superradiative losses from a new light particle.
• To assess the viability of neutron star equations of state in supporting superradiant instabilities and constraining the QCD axion.

Proposed method

• Modeling superradiance in neutron stars using the rotating frame formalism and scalar field equations with Yukawa couplings to nucleons.
• Calculating the superradiant growth rate and absorption rate for scalar modes in the presence of a neutron star's equatorial bulge and ellipticity.
• Using observed spin frequencies of PSR J1748-2446ad (716 Hz) and PSR B1937+21 (642 Hz) to set bounds on scalar couplings.
• Assessing mode stability by analyzing mixing with absorptive modes via phonon coupling and disruptive companions.
• Comparing theoretical constraints with existing torsion balance experiments and the QCD axion mass-coupling relation.
• Evaluating the impact of neutron star equation of state on effective coupling and superradiant threshold.

Experimental results

Research questions

• RQ1 Can superradiance in millisecond pulsars constrain ultra-light bosons more effectively than current laboratory experiments?
• RQ2 What is the maximum coupling strength of scalar particles to neutrons that can be ruled out using known pulsar spin frequencies?
• RQ3 Why do no pulsars with spin frequencies above ~700 Hz exist, and could this be due to a new particle's superradiative damping?
• RQ4 To what extent do neutron star equations of state affect the effective coupling of the QCD axion to nucleons in this context?
• RQ5 How do dissipative mechanisms like phonon mixing or companion disruption affect the stability of superradiant modes?

Key findings

• Superradiance in millisecond pulsars constrains scalar particles with masses below 10^{-11} eV, improving existing fifth-force bounds by up to three orders of magnitude.
• For scalar interactions weaker than gravity by a factor of 10^6, such couplings are ruled out at the mass corresponding to the pulsar's rotation frequency.
• The observed absence of pulsars above ~700 Hz may be explained by superradiative damping from a new particle with mass ~10^{-11} eV and frequency ~1500–3000 Hz.
• For certain neutron star equations of state, the QCD axion with masses between 5×10^{-13} eV and 3×10^{-12} eV is ruled out if θeff ~ 1.
• Constraints from PSR J1748-2446ad and PSR B1937+21 exceed torsion balance limits by 2–4 orders of magnitude in the mass range 2×10^{-12} eV to 6×10^{-12} eV.
• The method is robust against mode mixing with absorptive channels due to the known axisymmetry and composition of neutron stars, unlike black hole-based approaches.