Self-heating of Strongly Interacting Massive Particles

It was recently pointed out that semi-annihilating dark matter (DM) may experience a novel temperature evolution dubbed as self-heating. Exothermic semi-annihilation converts the DM mass to the kinetic energy. This yields a unique DM temperature evolution, $T_{\chi} \propto 1 / a$, in contrast to $T_{\chi} \propto 1 / a^{2}$ for free-streaming non-relativistic particles. Self-heating continues as long as self-scattering sufficiently redistributes the energy of DM particles. In this paper, we study the evolution of cosmological perturbations in self-heating DM. We find that sub-GeV self-heating DM leaves a cutoff on the subgalactic scale of the matter power spectrum when the self-scattering cross section is $\sigma_{\rm self} / m_{\chi} \sim {\cal O} (1) \,{\rm cm}^{2} /{\rm g}$. Then we present a particle physics realization of the self-heating DM scenario. The model is based on recently proposed strongly interacting massive particles with pion-like particles in a QCD-like sector. Pion-like particles semi-annihilate into an axion-like particle, which is thermalized with dark radiation. The dark radiation temperature is smaller than the standard model temperature, evading the constraint from the effective number of neutrino degrees of freedom. It is easily realized when the dark sector is populated from the standard model sector through a small coupling.Comment: 25 pages, 5 figures; minor corrections, version accepted in PR

Late-time magnetogenesis driven by ALP dark matter and dark photon

We propose a mechanism generating primordial magnetic fields after the $e^+e^-$ annihilations. Our mechanism involves an ultra-light axion-like particle (ALP) which constitutes the dark matter, and a dark $U(1)_X$ gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range $10^{-21} {\rm eV}\lesssim m_\phi\lesssim 10^{-17}{\rm eV}$, our mechanism can generate $B\sim 10^{-24} \,{\rm G} \,(m_\phi/10^{-17} {\rm eV})^{5/4}$ with a coherent length $\lambda \sim (m_\phi/10^{-17} {\rm eV})^{-1/2}$ kpc, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark $U(1)_X$ electromagnetic field $E_X \sim B_X\sim 80\,{\rm nG}\, (m_\phi/10^{-17}{\rm eV})^{-1/4}$, which can result in interesting astrophysical/cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.Comment: 6 pages, 2 figures; discussions rearranged, minor numerical errors fixed, conclusion unchanged; discussion improved, accepted for publication in PR

Regulation of BAX/BAK-Dependent Cell Death Program

Mammals have evolved an intricate regulation of a genetically programmed apoptotic cell death that involves mitochondria. Diverse apoptotic signals converge on mitochondria, which causes the release of cytochrome c into the cytosol to activate Apaf-1. This initiates caspase activation, which results in irreversible cellular demise. The BCL-2 family proteins constitute a critical checkpoint in mitochondrion-dependent apoptosis. Multidomain proapoptotic BAX/BAK promotes mitochondrial outer membrane permeabilization, whereas anti-apoptotic BCL-2/BCL-XL/MCL-1 protects mitochondrial integrity and prevents cytochrome c release. The proapoptotic activity of BAX/BAK is triggered by BH3-only molecules: BH3s) which are upregulated by upstream death signals. However, how these subfamilies interact with one another to execute mitochondrial cell death remains unclear. Thus, this thesis aims at elucidating the mechanism regarding how the interplay between BCL-2 subfamilies determines cellular commitment to survival versus death and how BAX/BAK activation is triggered by BH3s. Our laboratory showed that BH3s can be further classified into two subclasses- `activator\u27 tBID/BIM/PUMA that directly activates BAX/BAK to induce cytochrome c release and `inactivator\u27 BH3s that antagonize the function of anti-apoptotic BCL-2 members. Here, a BAX/BAK mutagenesis study indicated that anti-apoptotic BCL-2 members prevent BAX/BAK activation by sequestering activator BH3s rather than by directly binding to BAX/BAK. I further demonstrated that inactivator BH3s are able to displace activator BH3s from anti-apoptotic BCL-2 members with unique specificity, thus preventing their sequestration of activator BH3s. Activator BH3s were shown to act downstream of inactivator BH3s to trigger BAX/BAK activation, establishing the hierarchy of BCL-2 subfamilies in regulating mitochondrial apoptosis. Then, I investigated the molecular mechanism whereby BAX/BAK is activated by activator BH3s. I demonstrated that BAX undergoes stepwise structural reorganization leading to two activation processes-mitochondrial targeting and homo-oligomerization. Activator BH3s initiate BAX activation by attacking and exposing α1 helix of BAX leading to the secondary disengagement of the α9 helix and mitochondrial translocation. Activator BH3s remain associated with BAX to drive homo-oligomerization at the mitochondria. BAK has bypassed the first activation step, but requires activator BH3s for homo-oligomerization. This study further emphasizes the direct engagement of activator BH3s in BAX/BAK-dependent mitochondrial apoptotic pathway. Lastly, our laboratory showed that BH3s trigger caspase-independent mitochondrial dysfunction only in the presence of BAX/BAK. I found that BAK exists as several distinct complexes at the mitochondria, one of which is functionally different from cytochrome c-releasing BAK oligomers but instead includes VDAC/ANT channels that regulate ATP/ADP transport to support ATP production by oxidative phosphorylation. tBID overexpression induces cell death in the absence of Apaf-1 by inhibiting VDAC-mediated ADP import into the mitochondria in a BAK-dependent manner, suggesting that activated BAK antagonizes VDAC activity to initiate mitochondrial dysfunction. This study provides novel insights into how BAK activation couples apoptosis and mitochondrial dysfunction to trigger cell death

Axion-like particle assisted strongly interacting massive particle

We propose a new realization of strongly interacting massive particles (SIMP) as self-interacting dark matter, where SIMPs couple to the Standard Model sector through an axion-like particle. Our model gets over major obstacles accompanying the original SIMP model, such as a missing mechanism of kinetically equilibrating SIMPs with the SM plasma as well as marginal perturbativity of the chiral Lagrangian density. Remarkably, the parameter region realizing $\sigma_{\rm self}/m_{\rm DM} \simeq 0.1 \textrm{--} 1 \, {\rm cm}^{2}/{\rm g}$ is within the reach of future beam dump experiments such as the Search for Hidden Particles (SHiP) experiment.Comment: 11 pages, 1 figure. v2: figure updated, discussions improve

Self-heating dark matter via semi-annihilation

The freeze-out of dark matter (DM) depends on the evolution of the DM temperature. The DM temperature does not have to follow the standard model one, when the elastic scattering is not sufficient to maintain the kinetic equilibrium. We study the temperature evolution of the semi-annihilating DM, where a pair of the DM particles annihilate into one DM particle and another particle coupled to the standard model sector. We find that the kinetic equilibrium is maintained solely via semi-annihilation until the last stage of the freeze-out. After the freeze-out, semi-annihilation converts the mass deficit to the kinetic energy of DM, which leads to non-trivial evolution of the DM temperature. We argue that the DM temperature redshifts like radiation as long as the DM self-interaction is efficient. We dub this novel temperature evolution as self-heating. Notably, the structure formation is suppressed at subgalactic scales like keV-scale warm DM but with GeV-scale self-heating DM if the self-heating lasts roughly until the matter-radiation equality. The long duration of the self-heating requires the large self-scattering cross section, which in turn flattens the DM density profile in inner halos. Consequently, self-heating DM can be a unified solution to apparent failures of cold DM to reproduce the observed subgalactic scale structure of the Universe.Comment: 6 pages, 4 figures. v2: discussed improved, matches published versio

Dynamics of the cosmological relaxation after reheating

We examine if the cosmological relaxation mechanism, which was proposed recently as a new solution to the hierarchy problem, can be compatible with high reheating temperature well above the weak scale. As the barrier potential disappears at high temperature, the relaxion rolls down further after the reheating, which may ruin the successful implementation of the relaxation mechanism. It is noted that if the relaxion is coupled to a dark gauge boson, the new frictional force arising from dark gauge boson production can efficiently slow down the relaxion motion, which allows the relaxion to be stabilized after the electroweak phase transition for a wide range of model parameters, while satisfying the known observational constraints.Comment: 10 pages, 4 figures; minor revisions, version published in PR

Gravitational Interaction of Ultralight Dark Matter with Interferometers

Ultralight dark matter exhibits an order-one density fluctuation over the spatial scale of its wavelength. These fluctuations gravitationally interact with gravitational wave interferometers, leading to an additional noise floor or signals. We investigate the ultralight dark matter-induced effects in the gravitational wave interferometers. We perform a systematic computation of the power spectrum of ultralight dark matter in interferometers. We show that the ultralight dark matter-induced effect is most relevant for the interferometers with long baseline and that it only constitutes a sub-leading noise floor compared to the estimated noise level in the case of Laser Interferometer Space Antenna or future interferometers with an arm-length comparable to a few astronomical units. Gravitational wave interferometers can then place upper limits on the ultralight dark matter density in the solar system. We find that, under certain assumptions, future interferometers with AU-scale arm-length might probe the dark matter density a few hundred times the local dark matter density, which is measured over a much larger spatial scale.Comment: 20 pages, 5 figure