γ-Ray Flashes from Dark Photons in Neutron Star Mergers.

In this Letter we begin the study of visible dark sector signals coming from binary neutron star mergers. We focus on dark photons emitted in the 10 ms-1 s after the merger, and show how they can lead to bright transient γ-ray signals. The signal will be approximately isotropic, and for much of the interesting parameter space will be close to thermal, with an apparent temperature of ∼100 keV. These features can distinguish the dark photon signal from the expected short γ-ray bursts produced in neutron star mergers, which are beamed in a small angle and nonthermal. We calculate the expected signal strength and show that for dark photon masses in the 1-100 MeV range it can easily lead to total luminosities larger than 10^{46} ergs for much of the unconstrained parameter space. This signal can be used to probe a large fraction of the unconstrained parameter space motivated by freeze-in dark matter scenarios with interactions mediated by a dark photon in that mass range. We also comment on future improvements when proposed telescopes and midband gravitational detectors become operational.

Scalars Gliding through an Expanding Universe.

In this Letter, we investigate the effects of single derivative mixing in massive bosonic fields. In the regime of large mixing, we show that this leads to striking changes of the field dynamics, delaying the onset of classical oscillations and decreasing, or even eliminating, the friction due to Hubble expansion. We highlight this phenomenon with a few examples. In the first example, we show how an axionlike particle can have its number abundance parametrically enhanced. In the second example, we demonstrate that the QCD axion can have its number abundance enhanced allowing for misalignment driven axion dark matter all the way down to f_{a} of order astrophysical bounds. In the third example, we show that the delayed oscillation of the scalar field can also sustain a period of inflation. In the last example, we present a situation where an oscillating scalar field is completely frictionless and does not dilute away in time.

Probing Muonphilic Force Carriers and Dark Matter at Kaon Factories.

Rare kaon decays are excellent probes of light, new weakly coupled particles. If such particles X couple preferentially to muons, they can be produced in K→µνX decays. We evaluate the future sensitivity for this process at NA62 assuming X decays either invisibly or to dimuons. Our main physics target is the parameter space that resolves the (g-2)_{µ} anomaly, where X is a gauged L_{µ}-L_{τ} vector or a muonphilic scalar. The same parameter space can also accommodate dark matter freeze-out or reduce the tension between cosmological and local measurements of H_{0} if the new force decays to dark matter or neutrinos, respectively. We show that for invisible X decays, a dedicated single muon trigger analysis at NA62 could probe much of the remaining (g-2)_{µ} favored parameter space. Alternatively, if X decays to muons, NA62 can perform a dimuon resonance search in K→3µν events and greatly improve existing coverage for this process. Independently of its sensitivity to new particles, we find that NA62 is also sensitive to the standard model predicted rate for K→3µν, which has never been measured.