Dark Quark Nuggets

Abstract: "Dark quark nuggets", a lump of dark quark matter, can be produced in the early universe for a wide range of confining gauge theories and serve as a macroscopic dark matter candidate. The two necessary conditions, a nonzero dark baryon number asymmetry and a first-order phase transition, can be easily satisfied for many asymmetric dark matter models and QCD-like gauge theories with a few massless flavors. For confinement scales from 10 keV to 100 TeV, these dark quark nuggets with a huge dark baryon number have their masses vary from $10^{23}~\mathrm{g}$ to $10^{-7}~\mathrm{g}$ and their radii from $10^{8}~\mathrm{cm}$ to $10^{-15}~\mathrm{cm}$. Such macroscopic dark matter candidates can be searched for by a broad scope of experiments and even new detection strategies. Specifically, we have found that the gravitational microlensing experiments can probe heavier dark quark nuggets or smaller confinement scales around 10 keV; collision of dark quark nuggets can generate detectable and transient electromagnetic radiation signals; the stochastic gravitational wave signals from the first order phase transition can be probed by the pulsar timing array observations and other space-based interferometry experiments; the approximately massless dark mesons can behave as dark radiation to be tested by the next-generation CMB experiments; the free dark baryons, as a subcomponent of dark matter, can have direct detection signals for a sufficiently strong interaction strength with the visible sector.

Analyzing "Dark Quark Nuggets" as a Dark Matter Candidate

The paper titled "Dark Quark Nuggets," authored by Yang Bai, Andrew J. Long, and Sida Lu, explores a novel avenue within the field of dark matter research. It investigates the concept of "dark quark nuggets" (dQNs) as a macroscopic dark matter candidate, which are lumps of dark quark matter that can be produced during a first-order phase transition in the early universe. This intriguing approach situates itself within the broader context of beyond Standard Model (BSM) physics, exploring confining gauge theories that are similar to quantum chromodynamics (QCD).

Theoretical Framework and Formation Dynamics

The authors construct the framework around specific BSM confining gauge theories, referred to collectively as "dark QCD," characterized by varying numbers of colors and flavors, as well as confinement scales. They postulate that dark quark nuggets originate through a cosmological phase transition provided the theory accommodates a dark baryon number asymmetry and undergoes a first-order phase transition.

In their formulation, the dark sector mimics QCD dynamics but with a wider array of possible phase transition behaviors due to varying confinement scales that range from 10 keV to 100 TeV. These transitions result in dark quark nuggets with diverse mass and radius ranges — from (10^{23} ) grams down to (10^{-7} ) grams in mass, and from radii of (10^8) cm down to (10^{-15}) cm.

Detection Prospects

The authors highlight several detection avenues for these macroscopic dark matter candidates. Key among them is gravitational microlensing, which can probe heavier dQNs or those arising from smaller confinement scales. The potential detection of transient electromagnetic signals produced by colliding dark quark nuggets is also discussed, suggesting that these events could prompt observational campaigns looking for cosmic rays emanating from such processes.

Another interesting prediction is the gravitational wave signatures from dark sector phase transitions, which could potentially be detected by pulsar timing arrays and future space-based interferometry experiments. This is especially significant given the renewed interest in gravitational waves as a window into exotic and hidden sector physics.

The differentiation of dark QCD from Standard Model QCD is further supported by the expected behavior of approximately massless dark mesons as dark radiation. The authors speculate that these mesons could inform next-generation cosmic microwave background (CMB) experiments, providing indirect evidence of the peculiarity of dQNs.

Theoretical and Practical Implications

The paper proposes substantial theoretical implications by suggesting a broad class of BSM theories are capable of naturally producing macroscopic dark matter. This not only enriches the dark matter landscape but also suggests a refinement of existing paradigms focused primarily on particle-centric dark matter models.

From a practical perspective, if the outlined signatures of dQNs are observed via microlenses, cosmic rays, or gravitational waves, it would provide a profound insight into early universe dynamics and the properties of dark matter. Such developments could necessitate revisions of cosmological models and implications for structure formation scenarios.

Future Research Directions

The speculative nature of dark quark nuggets opens several avenues for future research. Key areas include exploring the phenomenology of different gauge theory setups, further refining constraints from gravitational wave observatories, and investigating cross-sector interactions that bridge dark matter and visible matter realms.

The research into dark quark nuggets exemplifies the rich interplay between theoretical physics and observational astrophysics. By proposing a robust theoretical framework through which macroscopic dark matter might be explored, this paper contributes significantly to the ongoing inquiry into the true nature of dark matter and its pivotal role in cosmic evolution.