Light scalars in neutron star mergers

Due to their unique set of multimessenger signals, neutron star mergers have emerged as a novel environment for studies of new physics beyond the Standard Model (SM). As a case study, we consider the simplest extension of the SM scalar sector involving a light CP-even scalar singlet S mixing with the SM Higgs boson. These S particles can be produced abundantly in neutron star mergers via the nucleon bremsstrahlung process. We show that the S particles may either be trapped in or stream freely out of the merger remnant, depending on the S mass, its mixing with the SM Higgs boson, and the temperature and baryon density in the merger. In the free-streaming region, the scalar S will provide an extra channel to cool down the merger remnant, with cooling timescales as small as 𝒪(ms). On the other hand, in the trapped region, the Bose gas of S particles could contribute a larger thermal conductivity than the trapped neutrinos in some parts of the parameter space, thus leading to faster thermal equilibration than expected. Therefore, future observations of the early postmerger phase of a neutron star merger could effectively probe a unique range of the S parameter space, largely complementary to the existing and future laboratory and supernova limits. In view of these results, we hope the merger simulation community will be motivated to implement the effects of light CP-even scalars into their simulations in both the free-streaming and trapped regimes.

Light(ly)-coupled dark matter in the keV range: freeze-in and constraints

Dark matter produced from thermal freeze-out is typically restricted to have masses above roughly 1 MeV. However, if the couplings are small, the freeze-in mechanism allows for production of dark matter down to keV masses. We consider dark matter coupled to a dark photon that mixes with the photon and dark matter coupled to photons through an electric or magnetic dipole moment. We discuss contributions to the freeze-in production of such dark matter particles from standard model fermion-antifermion annihilation and plasmon decay. We also derive constraints on such dark matter from the cooling of red giant stars and horizontal branch stars, carefully evaluating the thermal processes as well as the bremsstrahlung process that dominates for masses above the plasma frequency. We find that the parameters needed to obtain the observed relic abundance from freeze-in are excluded below a few tens of keV, depending on the value of the dark gauge coupling constant for the dark photon portal model, and below a few keV, depending on the reheating temperature for dark matter with an electric or magnetic dipole moment. While laboratory probes are unlikely to probe these freeze-in scenarios in general, we show that for dark matter with an electric or magnetic dipole moment and for dark matter masses above the reheating temperature, the couplings needed for freeze-in to produce the observed relic abundance can be probed partially by upcoming direct-detection experiments.

Study the Effect the Laser Intensity on the Behavior of the Kinetic Energy, Drift Energy and the Total Energy of the Silver Plasma

The particle-in-cell plasma simulation program in two dimensions was developed to display the properties of silver plasma under the effect of Ruby laser 694.3 nm with different intensities; 1012Wcm-2, 1015 Wcm-2, 1018 Wcm-2, and 1020 Wcm-2. The time evolution and the properties of total energy, kinetic energy, and drift energy of the system were examined in the region near the critical density (ne=0.2ncr). The charged particles respond to the laser pulse after a specified period of interaction time in the form of an increase in the energy of the system. This response depends on the intensity of the laser pulse used in this work. A significant increase was observed in plasma energy due to the efficient transfer of laser energy to plasma particles by the Inverse Bremsstrahlung process. The effectiveness of this process is reduced when the laser intensity is increased. This result is shown especially when using 1020 Wcm-2 laser intensity. The results indicated that the plotting of the electron velocity distributions during different time steps of interaction is Maxwellian and it was observed that the curves have a strong energy tail that indicates energy transfers and heating to the plasma.

Effect of Coulomb Focusing on the Electron–Atom Bremsstrahlung Cross Section for Tungsten and Iron in Nonthermal Lorentzian Plasmas

The Coulomb focusing effect on the electron–atom bremsstrahlung spectrum is investigated in nonthermal Lorentzian plasmas. The universal expression of the cross section of nonrelativistic electron–atom bremsstrahlung process is obtained by the solution of the Thomas-Fermi equation with the effective atomic charge. The effective Coulomb focusing for the electron–atom bremsstrahlung cross section near the threshold domain is also investigated by adopting the modified Elwert-Sommerfeld factor with the mean effective charge for the bremsstrahlung process. In addition, the bremsstrahlung emission rates are obtained by considering encounters between nonthermal electrons and atoms such as Fe and W atoms. We found that the bremsstrahlung emission rates for nonthermal electron–atoms are lower than those for thermal plasmas. Various nonthermal effects on the bremsstrahlung emission rates in Lorentzian plasmas are also discussed.

Backward peaking radiation pattern from a relativistic particle accelerated by lightning leader tip electric field

<div> <p>Charged particles being accelerated by the lightning leader tip electric field emit electromagnetic radiation due to the Bremsstrahlung process (Celestin et al., JGR, 2012). Bremsstrahlung has a continuous spectrum of radiation which includes radio waves and ionising radiation such as gamma rays which can be recorded by detectors on board the ASIM payload on the International Space Station, the forthcoming TARANIS satellite, or on the ground (Abbasi et al., JGR, 2018).  </p> </div><div> <p>The radiation pattern of this Bremsstrahlung is not well known. Displays of radiation patterns of accelerated particles are normally limited either to a low frequency approximation for radio waves, or to linear acceleration in a high frequency approximation for gamma rays. Here we report the radiation patterns from accelerated relativistic particles at low and high frequencies of the Bremsstrahlung process. It is found that the radiation patterns have four relative maxima with two backward peaking and two forward peaking.  </p> </div><div> <p>The shape of the radiation pattern is only determined by the velocity of the particle whilst the intensity of the radiation pattern is determined by the velocity and the acceleration of the particle. For example, relativistic particles with a large velocity exhibit a radiation pattern which is more forward peaking when compared to a non-relativistic particle with a smaller velocity. Similarly, relativistic particles with a large acceleration exhibit a radiation pattern with a larger intensity when compared to relativistic particles with a smaller acceleration. All these radiation patterns exhibit backward peaking radiation. The asymmetry of the radiation pattern, I.e., the different intensities of forward and backward peaking lobes, is controlled by the asymmetric frequencies of the Bremsstrahlung radiation caused by the Doppler effect.  </p> </div><div> <p>These results are important because they enable a determination of particle properties which can be inferred from observations with networks of radio receivers and arrays of gamma ray detectors. </p> </div>

Effect of quantized conductivity on the anomalous photon emission radiated from atomic-size point contacts

We observe anomalous visible to near-infrared electromagnetic emission from electrically driven atomic-size point contacts. We show that the number of photons released strongly depends on the quantized conductance steps of the contact. Counterintuitively, the light intensity features an exponential decay dependence with the injected electrical power. We propose an analytical model for the light emission considering an out-of-equilibrium electron distribution. We treat photon emission as a Bremsstrahlung process resulting from hot electrons colliding with the metal boundary, and find qualitative accord with the experimental data.