Core Composition Effects on the QCD Axion Mass Limit from Neutron Star Cooling

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Hi, I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Core Composition Effects on the QCD Axion Mass Limit from Neutron Star Cooling".

Jocelyn: Here's the gist of the paper. The gist Neutron stars are very dense media in which axions may be produced, and this work studies how the limit on the mass of QCD axions, namely KSVZ and DFSZ invisible axions, is altered when different equations of state are used to allow for heavier particles to appear in the neutron star core<ref:2606.07742#pg14> Construction of Neutron Star Equations of State The construction of the equations of state was done through the MUSES Calculation Engine, which allows for the construction of NS EOSs with varying particle composition in the core<ref:2606.07742#pg5> The workflow schematically represents a factorization where "Crust-DFT, χEFT, CMF" are combined to build a complete EOS for NS<ref:2606.07742#pg6> This approach is computationally convenient and reflects the fact that the outer crust, the outer core and the inner core are controlled by different effective degrees of freedom and constraints<ref:2606.07742#pg9> The low density regions are described using Crust Density Functional Theory (Crust-DFT), which provides a free-energy density description of matter in thermodynamic space Around saturation density, a Chiral Effective Field theory (χEFT)-based module is employed to bridge the low and high density regions High density regions are performed by the Chiral Mean Field (CMF) module, which is a relativistic mean field realization of a non linear SU(3) sigma model Core Compositions Studied The study considers four scenarios for NS core compositions: 1.

Vera: npeµ This case is the most commonly used, consisting of neutrons, protons, electrons, and muons The relevant neutrino emission processes include Direct URCA, Modified URCA, Bremsstrahlung and PBF in the singlet gaps 2.

Jocelyn: Spin 3/2 resonances This scenario allows for the presence of baryonic resonances with spin 3/2, such as Delta baryons These resonances show up only deep within the NS core, amounting to roughly 1% of the composition at large baryon densities 3.

Vera: Hyperons This allows for the full baryonic octet and decuplet, including all hyperons The inclusion of hyperons implies a large variety of weak processes to be considered, such as DURCA processes and modified URCA processes involving hyperons 4.

Paper summary: Vera: So we've just talked about how different core compositions affect our limits on QCD axion mass using this paper "Core Composition Effects on the QCD Axion Mass Limit from Neutron Star Cooling." Basically, their thesis is that the standard limit we use for these invisible axions depends heavily on whether you include heavier particles like hyperons or delta resonances in the neutron star core >

Jocelyn: And they are testing this by using different equations of state to see how much that limit shifts when you allow those heavier particles to appear >

Subrahmanyan: It matters because neutron stars are perfect laboratories for high energy physics, and this paper shows that the uncertainty in the equation of state doesn't completely destroy our limits, but it does introduce a new layer of complexity >

Vera: They specifically look at two types of axions, KSVZ and DFSZ invisible axions, and they found that the dependence on particle content is generally mild >

Jocelyn: But they also pointed out that for the DFSZ scenario, this change could actually move the limit into a range where an experiment like IAXO could potentially find something >

Subrahmanyan: It’s a subtle point about how we interpret the cooling data; if you don't account for those exotic particles, you might be missing a region of parameter space where an observation could happen >

Vera: Exactly. The paper suggests that allowing these heavier particles to appear doesn't totally invalidate the known limit, which reinforces its reliability, but it does open up new possibilities for what we can probe with future experiments >

Jocelyn: So the big picture is that this study shows us how composition directly impacts our axion search constraints, opening up avenues for more detailed astrophysical measurements >

Conclusion: Vera: So wrapping up this discussion on "Core Composition Effects on the QCD Axion Mass Limit from Neutron Star Cooling," we see that the paper is looking at a very specific problem: how the makeup of a neutron star core affects our ability to constrain QCD axion mass using its cooling properties >

Jocelyn: The authors show that even if you allow for more complex things like hyperons or delta resonances, the change in the limit on those QCD axion masses isn't huge in general >

Subrahmanyan: That’s because the dependence is mild, which means we can still trust our previous limits quite a bit, though we have to be careful about those specific DFSZ axions where it might move into an experimental window >

Vera: So what this means for us observing the cosmos is that if we see an axion signal in the area where IAXO is sensitive, it could actually tell us something about the internal structure of a neutron star core >

Jocelyn: It gives us a way to test those core compositions using axion detection data, which is a pretty exciting avenue for linking particle physics and astrophysics >