Halvor Melkild

• Krichevskiy, Daniil; Kolesova, Helena; Melkild, Halvor; Bringmann, Torsten & Pomper, Joachim (2026). Beyond 3→2: Towards Realistic SIMP Dark Matter. Fulltekst i vitenarkiv Vis sammendrag

  Strongly interacting dark sectors with pseudo–Nambu–Goldstone bosons provide a versatile framework for sub-GeV dark matter. While the original SIMP paradigm emphasises number-changing 3 to 2 processes as the origin of the relic abundance, the phenomenology of pionic dark matter is considerably richer. Depending on the spectrum and couplings, the relic density can instead be controlled by 2 to 2 annihilations (WIMP-like regime), semi-annihilation channels involving vector mesons and further processes like \pi\pi\pi to \pi\rho. A consistent treatment therefore requires a framework that simultaneously captures pion dynamics, vector mesons, anomalous interactions, and portal effects. We consider QCD-like dark sectors in which the dark matter candidates are pions stabilized by unbroken dark flavor symmetries. The effective theory is formulated using the Hidden Local Symmetry (HLS) approach. The HLS Lagrangian, supplemented by the gauged Wess–Zumino–Witten term, is constructed in a representation-independent manner and is applicable to complex, real, and pseudo-real fermion theories. As a minimal benchmark, we focus on the pseudo-real symmetry-breaking pattern SU(2N_f)/Sp(2N_f) with Nf=2. We incorporate lattice results for Sp(N_c=4) to obtain realistic relations between m_\pi/f_\pi and m_\rho/m_\pi. A dark photon portal is included to maintain thermal equilibrium with the visible sector and to assess its impact on relic abundance and dark matter stability within the HLS framework.

• Melkild, Halvor; Bringmann, Torsten & Brekke, Anton Andreas (2026). Charged Sterile Neutrinos as Dark Matter. doi: https:/indico2.uis.no/event/64/contributions/2002/. Fulltekst i vitenarkiv
• Melkild, Halvor; Bringmann, Torsten & Brekke, Anton Andreas (2026). Charged Sterile Neutrinos as Dark Matter. Fulltekst i vitenarkiv
• Melkild, Halvor; Krichevskiy, Daniil; Kolesova, Helena & Bringmann, Torsten (2026). Beyond 3→2: Towards Realistic SIMP Dark Matter. Fulltekst i vitenarkiv
• Melkild, Halvor; Krichevskiy, Daniil; Kolesova, Helena & Bringmann, Torsten (2026). Beyond 3→2: Towards Realistic SIMP Dark Matter. Fulltekst i vitenarkiv
• Melkild, Halvor; Bringmann, Torsten; Krichevskiy, Daniil; Kolesova, Helena & Pomper, Joachim (2025). Towards realistic SIMP dark matter. Fulltekst i vitenarkiv Vis sammendrag

  The so-called "strongly interacting massive particles" (SIMPs) refer to a class of dark matter candidates with the relic abundance set by the 3\to2 interactions which allows for dark matter with sub-GeV masses, not yet strongly constrained by direct detection experiments. Interestingly, SIMP dark matter features relatively strong self-interactions that may explain the small-scale structure puzzles. We investigate the realisation of SIMP models where dark pions form dark matter, protected from decay by dark flavour symmetries. We observe that the relic abundance of the dark pions depends sensitively on the masses of heavier states present in dark QCD-like scenarios such as dark vector mesons. We improve on existing results by using recent lattice simulations connecting the masses of dark pions and vector mesons. Furthermore, a portal between dark and visible sectors is usually assumed to maintain thermal equilibrium. To obtain fully realistic results, we explicitly add the dark photon portal and study its effect on dark matter relic abundance and stability.

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