Question

I am interested in invisibly decaying dark photon to the dark sector. Let's assume the particles in dark sector are dark photon (γ_D), dark matter (DM) , and long lived particle scalar (ϕ_D).

Now, from my understanding, invisible dark photons are when dark photon decays to the particle in the dark sector. However, I will set that dark photon can decay to ϕ_D but not to DM. And finally, ϕ_D will decay to SM particle. In this case, do we say that dark photon is invisible since it decays to a long-lived particle in the dark sector (The life time of this particle is suppressed by the kinetic mixing parameter)? Or do you say this is visible decaying dark photon since it eventually decays to SM particle?

May be I am messing up with the definition of dark sector.

Answer 1

In particle physics, a dark photon decay to a dark sector particle, which only later decays into SM particles, is considered invisible due to delayed detection. The term 'invisible' refers to the lack of immediate experimental detectability, not to the ultimate fate of the decay products. The study of such decays is part of broader efforts to understand dark matter and its weakly interacting constituents like WIMPs, neutralinos, and axions.

Step-by-step explanation:

The question you've asked pertains to whether a dark photon that decays into a long-lived scalar particle, which in turn decays to Standard Model (SM) particles, is considered invisibly or visibly decaying. In the context of particle physics and dark matter investigations, an 'invisible decay' typically refers to decays that do not produce detectable SM particles directly, thus escaping detection in experiments designed to observe such decay products. Given that the dark photon first decays into a dark sector particle, φ_D, which only later decays into SM particles and assuming the lifetime of φ_D is significantly long due to suppressed interactions by the kinetic mixing parameter, the initial decay of the dark photon would be considered 'invisible.'

This is because the detection in experiments would not be immediate; the visibility lies in the final decay of the scalar φ_D into SM particles. Thus, while the entire process eventually results in visible products, the specific decay of the dark photon to φ_D would be considered invisible. This reflects the complex nature of detecting and studying components of the dark sector, including particles like WIMPs, neutralinos, and axions.

By analogy, in the study of hadrons, certain decay lifetimes and patterns led to the concept of 'strangeness' as a quantum number that is conserved by the strong force but not by the weak force. Similarly, dark sector particles and their decays present challenges to contemporary physics, especially when trying to detect and describe them within the cosmological context of dark matter that is believed to interact only weakly with ordinary matter and exists in a halo around galaxies.