The inverse tritium beta decay (ITBD) reaction, ${\nu }_e+{}^3\mathrm{H}\to e^{-}+{}^3\mathrm{He}$, is a promising experimental tool for observing relic neutrinos created in the early Universe. This reaction has been selected by the PTOLEMY experiment for the search of relic neutrinos. Despite its potential, the ITBD reaction induced by any sources of neutrinos has yet to be observed. We show that an intense ${}^{51}\mathrm{Cr}$ radioactive neutrino source is suitable for observing the ITBD reaction for the first time. As the Sun is another source of intense electron neutrinos, we also examine the ITBD reaction rate from solar neutrinos. Based on our recent studies on the evolution of the helicity of relic neutrinos, we further present the ITBD rate for capturing relic neutrinos as a function of neutrino mass hierarchy, the Dirac versus Majorana nature of neutrino, and the mass of the lightest neutrino.

The sensitivities of ground-based gravitational-wave (GW) detectors are limited by quantum shot noise at a few hundred hertz and above. Nonetheless, one can use a quantum-correlation technique proposed by Martynov et al. [Phys. Rev. A 95, 043831 (2017) to remove the expectation value of the shot noise, thereby exposing underlying classical signals in the cross spectrum formed by cross-correlating the two outputs in a GW interferometer’s antisymmetric port. We explore here the prospects and analyze the sensitivity of using quantum correlation to detect astrophysical GW signals. Conceptually, this technique is similar to the correlation of two different GW detectors as it utilizes the fact that a GW signal will be correlated in the two outputs but the shot noise will be uncorrelated. Quantum correlation also has its unique advantages as it requires only a single interferometer to make a detection. Therefore, quantum correlation could increase the duty cycle, enhance the search efficiency, and enable the detection of highly polarized signals. In particular, we show that quantum correlation could be especially useful for detecting postmerger remnants of binary neutron stars with both short ($<1\mathrm{s}$) and intermediate ($\sim 10–{10}^4\mathrm{s}$) durations and setting upper limits on continuous emissions from unknown pulsars.

Results are presented for a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to allow for spin wandering. This search improves on previous HMM-based searches of Laser Interferometer Gravitational-Wave Observatory data by including the orbital period in the search template grid, and by analyzing data from the latest (third) observing run. In the frequency range searched, from 60 to 500 Hz, we find no evidence of gravitational radiation. This is the most sensitive search for Scorpius X-1 using a HMM to date. For the most sensitive subband, starting at 256.06 Hz, we report an upper limit on gravitational wave strain (at 95% confidence) of $h_0^{95\%}=6.16\times {10}^{-26}$, assuming the orbital inclination angle takes its electromagnetically restricted value $\iota =44°$. The upper limits on gravitational wave strain reported here are on average a factor of $\sim 3$ lower than in the second observing run HMM search. This is the first Scorpius X-1 HMM search with upper limits that reach below the indirect torque-balance limit for certain subbands, assuming $\iota =44°$.

The motion of a scalar field that interacts with a hot plasma, like the inflaton during reheating, is damped, which is a dissipative process. At high temperatures the damping can be described by a local term in the effective equation of motion. The damping coefficient is sensitive to multiple scattering. In the loop expansion its computation would require an all-order resummation. Instead we solve an effective Boltzmann equation, similarly to the computation of transport coefficients. For an interaction with another scalar field we obtain a simple relation between the damping coefficient and the bulk viscosity, so that one can make use of known results for the latter. The numerical prefactor of the damping coefficient turns out to be rather large, of order ${10}^4$.

One of the most promising methods to search for axions is a light-shining-through-walls (LSW) experiment. In this work, we discuss the possibility of performing a LSW experiment at the International Linear Collider facility, where photon beams are generated for positron production. The photon beam is energetic and intense; the energy is of order MeV and the number of photons is about ${10}^{24}$ per year. Because of the high energy and intensity, this LSW experiment can probe a parameter region of the axion unexplored by previous ground-based experiments.

In this paper the general features of the neutrino sector in TeV scale quantum gravity theories, such as Arkani-Hamed-Dimopoulos-Dvali (ADD) model and “many species” theory, are investigated. This class of theories has an inherent way of generating small neutrino masses. After reviewing this mechanism it is generalized to a realistic three-flavor case. Furthermore, a procedure is presented on how to diagonalize a mass matrix which is generated by this class of theories and how one can find the Standard Model flavor eigenstates. The developed general approach is applied to two specific scenarios within ADD and many species theory and possible effects on neutrino oscillations and on unitarity of the lepton mixing matrix are calculated. Finally, a short overview of phenomenology which can be potentially testable by the current neutrino experiments is presented.

In this work, we derive lower mass bounds on the $Z^{\prime }$ gauge boson based on the dilepton data from the Large Hadron Collider (LHC) with 13 TeV of center-of-mass energy, and forecast the sensitivity of the high-luminosity LHC with $L=3000{\mathrm{fb}}^{-1}$, the high-energy LHC with $\sqrt{s}=27\mathrm{TeV}$, and also at the Future Circular Collider with $\sqrt{s}=100\mathrm{TeV}$. We take into account the presence of exotic and invisible decays of the $Z^{\prime }$ gauge boson to find a more conservative and robust limit, different from previous studies. We investigate the impact of these new decay channels for several benchmark models in the scope of two different 3-3-1 models. We found that in the most constraining cases, LHC with $139{\mathrm{fb}}^{-1}$ can impose $m_{Z^{\prime }}>4\mathrm{TeV}$. Moreover, we forecast high-luminosity LHC, high-energy LHC, and Future Circular Collider reach, and derive the projected bounds $m_{Z^{\prime }}>5.8\mathrm{TeV}$, $m_{Z^{\prime }}>9.9\mathrm{TeV}$, and $m_{Z^{\prime }}>27\mathrm{TeV}$, respectively. Lastly, we put our findings into perspective with dark matter searches to show the region of parameter space where a dark matter candidate with the right relic density is possible.

We consider the prospects for the direct detection of dark matter in pure gravity meditation (PGM) models of supersymmetry breaking. Minimal PGM models require only two parameters, the gravitino mass, $m_{3/2}$, which sets the UV mass for all scalar masses, and $\tan \beta$. Gaugino masses are generated through anomaly mediation. Typically the lightest supersymmetric state (the dark matter candidate) is a wino. Here, we consider a one-parameter extension of the minimal model by allowing the Higgs soft masses to deviate from universality. For simplicity, we take these to be equal and use the $\mu$ term as a surrogate. We also consider nonuniversal stop masses. When $|\mu |\sim 1\mathrm{TeV}$, the Higgsino is a viable dark matter candidate when the gravitino mass is of order $\sim 1\mathrm{PeV}$ and $\tan \beta \simeq 2$. We calculate the spin-dependent and spin-independent cross sections for dark matter scattering on protons. For spin-independent scattering, existing experimental limits place constraints on the PGM parameter space. Much of the currently allowed parameter space lies above the irreducible neutrino background. Thus, future direct detection experiments will be able to probe much of the remaining PGM parameter space.

Confinement of quarks due to the strong interaction and the deconfinement at high temperatures and high densities are a basic paradigm for understanding the nuclear matter. Their simulation, however, is very challenging for classical computers due to the sign problem of solving equilibrium states of finite-temperature quantum chromodynamical systems at finite density. In this paper, we propose a variational approach, using the lattice Schwinger model, to simulate the confinement or deconfinement by investigating the string tension. We adopt an ansatz that the string tension can be evaluated without referring to quantum protocols for measuring the entropy in the free energy. Results of numeral simulation show that the string tension decreases both along the increasing of the temperature and the chemical potential, which can be an analog of the phase diagram of QCD. Through numerical simulations on the classical computer, we demonstrate the potential of exploiting near-term quantum computers for investigating the phase diagram of finite-temperature and finite-density nuclear matters.

One of the most striking evidences of the information loss paradox is that, according to the Hawking’s calculation, the correlation functions of a test scalar field exponentially decay in time. In this paper, I argue that a judicious use of the steepest descent expansion on the classical saddle point (the black hole background) is enough to change this early time decay into a late time growth in agreement with information retrieval. I will explicitly show this in the Jackiw-Teitelboim gravity. There, the so-called “ramp” in the bulk tow-point function is analytically obtained without the need of any other subdominant configurations of the gravity path integral.

We study the conformal dimensions of the would be dual operators to the stabilized moduli for two nonsupersymmetric DeWolfe-Giryavets-Kachru-Taylor (DGKT) vacua. For the first of them, related to the standard supersymmetry DGKT vacuum by $G_4=-G_4^{\mathrm{SUSY}}$, we obtain integer conformal dimensions. For the second of them, which has a nonzero harmonic component in $G_2$, we obtain both integers and real numbers.

We present the ghost-free infinite-derivative extensions of the spherically reduced gravity (SRG) and Callan-Giddings-Harvey-Strominger (CGHS) theories in two space-time dimensions. For the case of SRG, we specify the Schwarzschild-type gauge and diagonalize the quadratic action for field perturbations after taking the background fields to be those of the flat-space solution with a linear dilaton. Using the obtained diagonalization, we construct ghost-free infinite-derivative modifications of the SRG theory. In the context of this modified SRG theory we derive a nonlocal modification of the linearized spherically reduced Schwarzschild solution. For the case of CGHS gravity, we work in the conformal gauge and diagonalize the quadratic action associated with this theory for a general background solution. Using these results, we construct the ghost-free infinite-derivative modifications of the CGHS theory and examine nonlocal modifications to the linearized CGHS black-hole solution.

We study black holes in a modified gravity scenario involving a scalar field quadratically coupled to the Gauss-Bonnet invariant. The scalar is assumed to be in a spontaneously broken phase at spatial infinity due to a bare Higgs-like potential. For a proper choice of sign, the nonminimal coupling to gravity leads to symmetry restoration near the black hole horizon, prompting the development of the scalar wall in its vicinity. The wall thickness depends on the bare mass of the scalar and can be much smaller than the Schwarzschild radius. In a weakly coupled regime, the quadratic coupling to the Gauss-Bonnet invariant effectively becomes linear, and no walls are formed. We find approximate analytical solutions for the scalar field in the test field regime, and obtain numerically static black hole solutions within this setup. We discuss cosmological implications of the model and show that it is fully consistent with the existence of an inflationary stage, unlike the spontaneous scalarization scenario assuming the opposite sign of the nonminimal coupling to gravity. Our model predicts the speed of gravitational waves to be extremely close to unity—in comfortable agreement with the observation of the GW170817 event and its electromagnetic counterpart.

We study the diffuse background free-free emission induced by dark matter halos. Since dark matter halos host ionized thermal plasma, they could be an essential source of cosmological free-free emission. We evaluate the global background intensity and the anisotropy of this free-free emission. We show that the dominant contribution comes from dark matter halos with a mass close to the Jeans mass, $M_{\mathrm{halo}}\sim {10}^{10}{M}_{\odot }$, around the redshift $z\sim 3$. Therefore, the intensity of the free-free emission is sensitive to the small-scale primordial curvature perturbations that form such small-mass dark matter halos. Although our obtained intensity of the global and anisotropic free-free emission is smaller than the 10% level of the free-free emission observed in the high galactic region, we find that the free-free emission signal is modified by $\sim 20\%$ even in the parameter set of the spectral index and the running, which is consistent with the recent Planck result. Therefore, the measurement of the cosmological free-free signals has the potential to provide more stringent constraints on the abundance of small-mass dark matter halos and the curvature perturbations including the spectral index and the running, while carefully removing the Galactic free-free emission is required through the multifrequency radio observation or the cross-correlation study with the galaxy surveys or 21-cm intensity map.

We discuss the possibility to measure particle couplings with stochastic gravitational wave backgrounds (SGWBs). Under certain circumstances a sequence of peaks of different amplitude and frequency—a stairway—emerges in a SGWB spectrum, with each peak probing a different coupling. The detection of such signature opens the possibility to reconstruct couplings (spectroscopy) of particle species involved in high energy phenomena generating SGWBs. Stairwaylike signatures may arise in causally produced backgrounds in the early Universe, e.g., from preheating or first order phase transitions. As a proof of principle we study a preheating scenario with an inflaton $\varphi$ coupled to multiple daughter fields $\{{\chi }_j\}$ with different coupling strengths. As a clear stairway signature is imprinted in the SGWB spectrum, we reconstruct the relevant couplings with various detectors.

Second-order tensor modes induced by nonlinear gravity are a key component of the cosmological background of gravitational waves. A detection of this background would allow us to probe the primordial power spectrum at otherwise inaccessible scales. Usually, the energy density of these gravitational waves is studied within perturbation theory in a particular gauge—a connection between our physical spacetime and a fictitious background. It is a widely recognized issue that the second-order, scalar-induced gravitational waves are gauge dependent. This issue arises because they are not well-defined as tensors in the physical spacetime at second-order and are thus unphysical. In this paper, we propose the covariant transverse-traceless projection of the extrinsic curvature to study cosmological gravitational waves on a spatial hypersurface. We define a new energy density, which is based purely on spacetime tensors, independent of perturbation theory, and thus, is gauge invariant by definition. We show that, in the context of second-order perturbation theory, this new energy density contains only propagating modes in the constant-time hypersurface in the Newtonian gauge. We further show that we can recover the same gravitational waves after a transformation to the synchronous gauge, so long as we correctly identify the Newtonian hypersurface.

We present new bounds on the cosmic abundance of magnetic monopoles based on the survival of primordial magnetic fields during the reheating and radiation-dominated epochs. The new bounds can be stronger than the conventional Parker bound from Galactic magnetic fields, as well as bounds from direct searches. We also apply our bounds to monopoles produced by the primordial magnetic fields themselves through the Schwinger effect, and derive additional conditions for the survival of the primordial fields.

We consider a general quantum field relativistic scattering involving two half-spin fermions, $A$ and $B$, which are initially entangled with another fermion $C$ that does not participate in the scattering dynamics. We construct general expressions for the reduced spin matrices for the out-state considering a general tripartite spin-entangled state. In particular we study an inelastic QED process at tree-level, namely $e^{-}{e}^{+}\to {\mu }^{-}{\mu }^{+}$ and a half-spin fermion $C$ as a spectator particle which can be entangled to the $AB$ system in the following ways: W state, GHZ state, $|{\mathrm{A}}^{\alpha }⟩\otimes |{\mathrm{\Psi }}^{\pm }{⟩}_{\mathrm{BC}}$, and $|{\mathrm{A}}^{\alpha }⟩\otimes |{\mathrm{\Phi }}^{\pm }{⟩}_{\mathrm{BC}}$, where $\{|{\mathrm{\Psi }}^{\pm }⟩,|{\mathrm{\Phi }}^{\pm }⟩\}$ are the Bell basis states and $|{\mathrm{A}}^{\alpha }⟩$ is a spin superposition state of system $A$. We calculate the von Neumann entropy variation before and after the scattering for the particle $C$ and show that spin measurements in $C$ contain numerical information about the total cross section of the process. We compare the initial states W and GHZ as well as study the role played by the parameter $\alpha$ in the evaluation of the entropy variations and the cross section encoded in the spectator particle.

In the presence of an electromagnetic background plane wave field, electron, positron, and photon states are not stable because electrons and positrons emit photons and photons decay into electron-positron pairs. This decay of the particle states leads to an exponential damping term in the probabilities of single nonlinear Compton scattering and nonlinear Breit-Wheeler pair production. In this paper, we investigate analytically and numerically the probabilities of nonlinear Compton scattering and nonlinear Breit-Wheeler pair production including the particle states’ decay. For this, we first compute spin- and polarization-resolved expressions of the probabilities, provide some of their asymptotic behaviors, and show that the results of the total probabilities are independent of the spin and polarization bases. Then, we present several plots of the total and differential probabilities for different pulse lengths and for different spin and polarization quantum numbers. We observe that it is crucial to take into account the damping of the states in order for the probabilities to stay always below unity, and we show that the damping factors also scale with the intensity and pulse duration of the background field. In the case of nonlinear Compton scattering, we show numerically that the total probability behaves like a Poissonian distribution in the regime where the photon recoil is negligible. In all considered cases, the kinematic conditions are such that the final particles momenta transverse to the propagation direction of the plane wave are always much smaller than the particles longitudinal momenta and the main spread of the momentum distribution on the transverse plane is along the direction of the plane wave electric field.

Over the last several years, our understanding of neutrino oscillations has developed significantly due to the long-baseline measurements of muon-neutrino disappearance and muon-to-electron-neutrino appearance at the T2K and NOvA experiments. However, when interpreted under the standard-three-massive-neutrinos paradigm, a tension has emerged between the two experiments’ data. Here, we examine whether this tension can be alleviated when a fourth, very light neutrino is added to the picture. Specifically, we focus on the scenario in which this new neutrino has a mass similar to, or even lighter than, the three mostly active neutrinos that have been identified to date. We find that, for some regions of parameter space, the four-neutrino framework is favored over the three-neutrino one with moderate ($\lesssim 2\sigma$) significance. Interpreting these results, we provide a future outlook for near-term and long-term experiments if this four-neutrino framework is indeed true.