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1. Exciton Bimolecular Annihilation Dynamics in Push–Pull Semiconductor Polymers

   https://doi.org/10.1021/acs.jpclett.3c03094  

   Zheng, Yulong; Venkatesh, Rahul; Rojas-Gatjens, Esteban; Reichmanis, Elsa; Silva-Acuña, Carlos (January 2024, The Journal of Physical Chemistry Letters)

   Exciton−exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push−pull conjugated polymer by fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation0 fluence. Further investigation of the fluence-dependent transients suggests that the exciton−exciton annihilation bimolecular rates are not constant in time, displaying a t−1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push−pull conjugated polymers. 

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2. Dark Matter Annihilation in Small Scales

   Hou, Liqiang (January 2025, North Carolina State University)

   This dissertation explores the impact of dark matter on the early universe and cosmological observables, with a focus on dark matter annihilation effects on thermal history and dark matter annihilation at the small scales, including the formation of the first stars and galaxies. Dark matter annihilation, enhanced by cosmic inhomogeneities, reshapes the gas temperature and ionization history of the early universe. Annihilation injects energy into the IGM, raising the gas temperature and ionization fraction. This process can either suppress or accelerate the star formation. This study examines the effects of dark matter annihilation on the minimum cooling mass of halos at different redshifts. Notably, this work presents the first combined calculation of dark matter annihilation and dark matter baryon velocity offsets, which have previously been treated separately. Our detailed calculations reveal the non-trivial effects of interplay between dark matter annihilation and dark matter baryon velocity offsets affects the evolution of structure formation. To explore these effects further, we extend existing models to include both molecular and atomic cooling halos, allowing star formation to occur in lower-mass halos and offering insights into how dark matter annihilation, streaming velocity, and cooling mechanisms shape early observable signals. Our study calculates the sky-averaged brightness temperature of the high-redshift 21cm absorption signal against the cosmic microwave background, also known as the “global 21cm signal”, including the effects of both dark matter annihilation and velocity offsets. These factors create distinct features in the 21cm signal, providing potential observational signatures of dark matter properties. We also examine energy transfer processes within dark matter halos, including inverse Compton scattering, photoionization, and pair production. By applying a refined Monte Carlo energy-transfer calculation code, we link single-particle simulations to energy deposition fractions. These developments will be crucial for connecting small-scale effects with large- scale galaxy formation models and ultimately interpreting observational data from the early universe. 

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3. Prospects for measuring dark matter microphysics with observations of dwarf spheroidal galaxies

   https://doi.org/10.1088/1475-7516/2021/07/030  

   Baxter, Eric J.; Kumar, Jason; Pace, Andrew B.; Runburg, Jack (July 2021, Journal of Cosmology and Astroparticle Physics)

   Abstract Dark matter annihilation in dwarf spheroidal (dSph) galaxies near the Milky Way has the potential to produce a detectable signature in gamma-rays. The amplitude of this signal depends on the dark matter density in a dSph, the dark matter particle mass, the number of photons produced in an annihilation, and the possibly velocity-dependent dark matter annihilation cross section. We argue that if the amplitude of the annihilation signal from multiple dSphs can be measured, it is possible to determine the velocity-dependence of the annihilation cross section. However, we show that doing so will require improved constraints on the dSph density profiles, including control of possible sources of systematic uncertainty. Making reasonable assumptions about future improvements, we make forecasts for the ability of current and future experiments — including Fermi, CTA and AMEGO — to constrain the dark matter annihilation velocity dependence. 

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4. Searching for MeV-mass neutrinophilic dark matter with large scale dark matter detectors

   https://doi.org/10.1103/PhysRevD.111.063052  

   Suliga, Anna M; Fuller, George M (March 2025, Physical Review D)

   The indirect detection of dark matter (DM) through its annihilation products is one of the primary strategies for DM detection. One of the least constrained classes of models is neutrinophilic DM, because the annihilation products, weakly interacting neutrinos, are challenging to observe. Here, we consider a scenario where MeV-mass DM exclusively annihilates to the third neutrino mass eigenstate, which is predominantly of tau and muon flavor. In such a scenario, the potential detection rate of the neutrinos originating from the DM annihilation in our Galaxy in the conventional detectors would be suppressed by up to approximately two orders of magnitude. This is because the best sensitivity of such detectors for neutrinos with energies below approximately 100 MeV is for electron neutrino flavor. In this work, we highlight the potential of large-scale DM detectors in uncovering such signals in the tens of MeV range of DM masses. In addition, we discuss how coincident signals in direct detection DM experiments and upcoming neutrino detectors such as DUNE, Hyper-Kamiokande, and JUNO could provide new perspectives on the DM problem. Published by the American Physical Society2025 

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5. Bound deuteron-antideuteron system (deuteronium): Leading radiative and internal-structure corrections to bound-state energies

   https://doi.org/10.1103/zrp8-jx3w  

   Adkins, Gregory S; Jentschura, Ulrich D (December 2025, Physical Review Research)

   We evaluate the energy levels of the deuteronium bound system, which consists of a deuteron and an antideuteron, with a special emphasis on states with nonvanishing orbital angular momenta. The excited atomic bound states of deuteronium constitute probes for the understanding of higher-order quantum electrodynamic corrections for spin-1 particles in a bound system where the typical field strength of the binding Coulomb field (at a distance of the generalized Bohr radius) exceeds Schwinger’s critical field strength. For states with nonvanishing angular momenta, effects due to the internal structure of the deuteron and virtual annihilation contributions are highly suppressed. Relevant transitions are found to be in a frequency range accessible by standard laser spectroscopic techniques. We evaluate the leading and next-to-leading energy corrections of orders ${\alpha }^3{m}_d$ and ${\alpha }^4{m}_d$ , where $\alpha$ is the fine-structure constant and $m_d$ is the deuteron mass, and also investigate internal-structure corrections: hadronic vacuum polarization, finite-size effects, and strong-interaction corrections. 

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