Browsing by Author "Long, Andrew J."
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Item Achieving the highest temperature during reheating with the Higgs condensate(American Physical Society, 2021) Passaglia, Samuel; Hu, Wayne; Long, Andrew J.; Zegeye, DavidWe study the role of the Standard Model Higgs condensate, formed during cosmological inflation, in the epoch of reheating that follows. We focus on the scenario where the inflaton decays slowly and perturbatively, so that there is a long period between the end of inflation and the beginning of radiation domination. The Higgs condensate decays nonperturbatively during this period, and we show that it heats the primordial plasma to much higher temperatures than would result from the slowly decaying inflaton alone. We discuss the effect of this hot plasma on the thermalization of the inflaton’s decay products, and study its phenomenological implications for the formation of cosmological relics like dark matter, with associated isocurvature fluctuations, and the restoration of the electroweak and Peccei-Quinn symmetries.Item An analytic evaluation of gravitational particle production of fermions via Stokes phenomenon(Springer Nature, 2022) Hashiba, Soichiro; Ling, Siyang; Long, Andrew J.The phenomenon of gravitational particle production can take place for quantum fields in curved spacetime. The abundance and energy spectrum of gravitationally produced particles is typically calculated by solving the field’s mode equations on a time-dependent background metric. For purposes of studying dark matter production in an inflationary cosmology, these mode equations are often solved numerically, which is computationally intensive, especially for the rapidly-oscillating high-momentum modes. However, these same modes are amenable to analytic evaluation via the Exact Wentzel-Kramers-Brillouin (EWKB) method, where gravitational particle production is a manifestation of the Stokes phenomenon. These analytic techniques have been used in the past to study gravitational particle production for spin-0 bosons. We extend the earlier work to study gravitational production of spin-1/2 and spin-3/2 fermions. We derive an analytic expression for the connection matrix (valid to all orders in an adiabatic parameter ħ) that relates Bogoliubov coefficients across a Stokes line connecting a merged pair of simple turning points. By comparing the analytic approximation with a direct numerical integration of the mode equations, we demonstrate an excellent agreement and highlight the utility of the Stokes phenomenon formalism applied to fermions. We discuss the implications for an analytic understanding of catastrophic particle production due to vanishing sound speed, which can occur for a spin-3/2 Rarita-Schwinger field.Item Catastrophic production of slow gravitinos(American Physical Society, 2021) Kolb, Edward W.; Long, Andrew J.; McDonough, EvanWe study gravitational particle production of the massive spin-3/2 Rarita-Schwinger field, and its close relative, the gravitino, in Friedmann-Robertson-Walker cosmological spacetimes. For masses lighter than the value of the Hubble expansion rate after inflation, m3/2≲H, we find catastrophic gravitational particle production, wherein the number of gravitationally produced particles is divergent, caused by a transient vanishing of the helicity-1/2 gravitino sound speed. In contrast with the conventional gravitino problem, the spectrum of produced particles is dominated by those with momentum at the UV cutoff. This suggests a breakdown of effective field theory, which might be cured by new degrees of freedom that emerge in the UV. We study the UV completion of the Rarita-Schwinger field, namely N=1, d=4, supergravity. We reproduce known results for models with a single superfield and models with an arbitrary number of chiral superfields, find a simple geometric expression for the sound speed in the latter case, and extend this to include nilpotent constrained superfields and orthogonal constrained superfields. We find supergravity models where the catastrophe is cured and models where it persists. Insofar as quantizing the gravitino is tantamount to quantizing gravity, as is the case in any UV completion of supergravity, the models exhibiting catastrophic production are prime examples of four-dimensional effective field theories that become inconsistent when gravity is quantized, suggesting a possible link to the swampland program. We propose the gravitino swampland conjecture, which is consistent with and indeed follows from the Kachru-Kallosh-Linde-Trivedi and large volume scenarios for moduli stabilization in string theory.Item Completely dark matter from rapid-turn multifield inflation(Springer Nature, 2023) Kolb, Edward W.; Long, Andrew J.; McDonough, Evan; Payeur, GuillaumeWe study cosmological gravitational particle production as applied to “rapid-turn” models of inflation involving two scalar fields. We are interested in the production of massive spin-0 particles that only interact gravitationally and provide a candidate for the dark matter. Specifically, we study two models of rapid-turn multifield inflation, motivated in part by the de Sitter swampland conjecture, that are distinguished by the curvature of field space and the presence or absence of field space ‘angular momentum’ conservation. We find that one of these models leads to insufficient particle production and cannot explain the observed dark matter relic abundance. The second model is able to explain the origin of spin-0 dark matter via gravitational production, and we identify the relevant region of parameter space that is consistent with measurements of the dark-matter relic abundance, the dark-matter-photon isocurvature perturbations, and the spectrum of curvature perturbations that is probed by cosmological observations. Our work demonstrates the compatibility of the de Sitter swampland conjecture with completely dark matter.Item Completely dark photons from gravitational particle production during the inflationary era(Springer Nature, 2021) Kolb, Edward W.; Long, Andrew J.Starting with the de Broglie-Proca Lagrangian for a massive vector field, we calculate the number density of particles resulting from gravitational particle production (GPP) during inflation, with detailed consideration to the evolution of the number density through the reheating. We find plausible scenarios for the production of dark-photon dark matter of mass in a wide range, as low as a micro-electron volt to 1014 GeV. Gravitational particle production does not depend on any coupling of the dark photon to standard-model particles.Item Cosmological gravitational particle production of massive spin-2 particles(Springer Nature, 2023) Kolb, Edward W.; Ling, Siyang; Long, Andrew J.; Rosen, Rachel A.The phenomenon of cosmological gravitational particle production (CGPP) is expected to occur during the period of inflation and the transition into a hot big bang cosmology. Particles may be produced even if they only couple directly to gravity, and so CGPP provides a natural explanation for the origin of dark matter. In this work we study the gravitational production of massive spin-2 particles assuming two different couplings to matter. We evaluate the full system of mode equations, including the helicity-0 modes, and by solving them numerically we calculate the spectrum and abundance of massive spin-2 particles that results from inflation on a hilltop potential. We conclude that CGPP might provide a viable mechanism for the generation of massive spin-2 particle dark matter during inflation, and we identify the favorable region of parameter space in terms of the spin-2 particle’s mass and the reheating temperature. As a secondary product of our work, we identify the conditions under which such theories admit ghost or gradient instabilities, and we thereby derive a generalization of the Higuchi bound to Friedmann-Robertson-Walker (FRW) spacetimes.Item Dipole radiation and beyond from axion stars in electromagnetic fields(Springer Nature, 2021) Amin, Mustafa A.; Long, Andrew J.; Mou, Zong-Gang; Saffin, Paul M.We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields (oscillons and axion stars) in the presence of external electromagnetic fields. We delineate different qualitative behaviour of the photon luminosity in terms of an effective dimensionless coupling parameter constructed out of the axion-photon coupling, and field amplitude, oscillation frequency and radius of the axion star. For small values of this dimensionless coupling, we provide a general analytic formula for the dipole radiation field and the photon luminosity per solid angle, including a strong dependence on the radius of the configuration. For moderate to large coupling, we report on a non-monotonic behavior of the luminosity with the coupling strength in the presence of external magnetic fields. After an initial rise in luminosity with the coupling strength, we see a suppression (by an order of magnitude or more compared to the dipole radiation approximation) at moderately large coupling. At sufficiently large coupling, we find a transition to a regime of exponential growth of the luminosity due to parametric resonance. We carry out 3+1 dimensional lattice simulations of axion electrodynamics, at small and large coupling, including non-perturbative effects of parametric resonance as well as backreaction effects when necessary. We also discuss medium (plasma) effects that lead to resonant axion to photon conversion, relevance of the coherence of the soliton, and implications of our results in astrophysical and cosmological settings.Item Filtered Dark Matter at a First Order Phase Transition(American Physical Society, 2020) Baker, Michael J.; Kopp, Joachim; Long, Andrew J.We describe a new mechanism of dark matter production. If dark matter particles acquire mass during a first order phase transition, it is energetically unfavorable for them to enter the expanding bubbles. Instead, most of them are reflected and quickly annihilate away. The bubbles eventually merge as the phase transition completes and only the dark matter particles that have entered the bubbles survive to constitute the observed dark matter today. This mechanism can produce dark matter with masses from the TeV scale to above the PeV scale, surpassing the Griest-Kamionkowski bound.Item Gravitino Swampland Conjecture(American Physical Society, 2021) Kolb, Edward W.; Long, Andrew J.; McDonough, EvanWe extend the swampland from effective field theories (EFTs) inconsistent with quantum gravity to EFTs inconsistent with quantum supergravity. This enlarges the swampland to include EFTs that become inconsistent when the gravitino is quantized. We propose the “gravitino swampland conjecture”: the gravitino sound speed must be nonvanishing in all EFTs that are low-energy limits of quantum supergravity. This seemingly simple statement has important consequences for both theories and observations. The conjecture is consistent with and supported by the Kachru-Kallosh-Linde-Trivedi and large volume scenarios for moduli stabilization in string theory.Item Neutron star cooling with lepton-flavor-violating axions(American Physical Society, 2024) Zhang, Hong-Yi; Hagimoto, Ray; Long, Andrew J.The cores of dense stars are a powerful laboratory for studying feebly coupled particles such as axions. Some of the strongest constraints on axionlike particles and their couplings to ordinary matter derive from considerations of stellar axion emission. In this work we study the radiation of axionlike particles from degenerate neutron star matter via a lepton-flavor-violating coupling that leads to muon-electron conversion when an axion is emitted. We calculate the axion emission rate per unit volume (emissivity) and by comparing with the rate of neutrino emission, we infer upper limits on the lepton-flavor-violating coupling that are at the level of |gaeμ|≲10−6. For the hotter environment of a supernova, such as SN 1987A, the axion emission rate is enhanced and the limit is stronger, at the level of |gaeμ|≲10−11, competitive with laboratory limits. Interestingly, our derivation of the axion emissivity reveals that axion emission via the lepton-flavor-violating coupling is suppressed relative to the familiar lepton-flavor-preserving channels by the square of the plasma temperature to muon mass ratio, which is responsible for the relatively weaker limits.Item Phase of confined electroweak force in the early Universe(American Physical Society, 2019) Berger, Joshua; Long, Andrew J.; Turner, JessicaWe consider a modified cosmological history in which the presence of beyond-the-Standard-Model physics causes the weak gauge sector, SU(2)L, to confine before it is Higgsed. Under the assumption of chiral symmetry breaking, quark and lepton weak doublets form condensates that break the global symmetries of the Standard Model, including baryon and lepton number, down to a U(1) subgroup under which only the weak singlet fermions and Higgs boson transform. The weakly coupled gauge group SU(3)c×U(1)Y is also broken to an SU(2)c×U(1)Q gauge group. The light states include (pseudo-)Goldstone bosons of the global symmetry breaking, mostly elementary fermions primarily composed of the weak singlet quarks and leptons, and the gauge bosons of the weakly coupled gauge group. We discuss possible signatures from early Universe cosmology including gravitational wave radiation, topological defects, and baryogenesis.Item Quantum interference in gravitational particle production(Springer Nature, 2022) Basso, Edward; Chung, Daniel J.H.; Kolb, Edward W.; Long, Andrew J.Previous numerical investigations of gravitational particle production during the coherent oscillation period of inflation displayed unexplained fluctuations in the spectral density of the produced particles. We argue that these features are due to the quantum interference of the coherent scattering reactions that produce the particles. We provide accurate analytic formulae to compute the particle production amplitude for a conformally- coupled scalar field, including the interference effect in the kinematic region where the production can be interpreted as inflaton scattering into scalar final states via graviton exchange.Item Resonant conversion of axion dark radiation into terahertz electromagnetic radiation in a neutron star magnetosphere(American Physical Society, 2024) Long, Andrew J.; Schiappacasse, Enrico D.In the strong magnetic field of a neutron star’s magnetosphere, axions coupled to electromagnetism develop a nonzero probability to convert into photons. Past studies have revealed that the axion-photon conversion can be resonantly enhanced. We recognize that the axion-photon resonance admits two parametrically distinct resonant solutions, which we call the mass-matched resonance and the Euler-Heisenberg assisted resonance. The mass-matched resonance occurs at a point in the magnetosphere where the radially-varying plasma frequency crosses the axion mass 𝜔pl ≈𝑚𝑎. The Euler-Heisenberg assisted resonance occurs where the axion energy satisfies 𝜔 ≈(2𝜔2pl/7𝑔𝛾𝛾𝛾𝛾¯𝐵2)1/2. This second resonance is made possible though the strong background magnetic field ¯𝐵, as well as the nonzero Euler-Heisenberg four-photon self-interaction, which has the coupling 𝑔𝛾𝛾𝛾𝛾 =8𝛼2/45𝑚4𝑒. We study the resonant conversion of relativistic axion dark radiation into photons via the Euler-Heisenberg assisted resonance, and we calculate the expected electromagnetic radiation assuming different values for the axion-photon coupling 𝑔𝑎𝛾𝛾 and different amplitudes for the axion flux onto the neutron star Φ𝑎. We briefly discuss several possible sources of axion dark radiation. Achieving a sufficiently strong axion flux to induce a detectable electromagnetic signal seems unlikely.Item Snowmass2021 cosmic frontier white paper: Ultraheavy particle dark matter(SciPost Foundation, 2023) Carney, Daniel; Raj, Nirmal; Bai, Yang; Berger, Joshua; Blanco, Carlos; Bramante, Joseph; Cappiello, Christopher; Dutra, Maíra; Ebadi, Reza; Engel, Kristi; Kolb, Edward; Harding, J. Patrick; Kumar, Jason; Krnjaic, Gordan; Lang, Rafael F.; Leane, Rebecca K.; Lehmann, Benjamin V.; Li, Shengchao; Long, Andrew J.; Mohlabeng, Gopolang; Olcina, Ibles; Pueschel, Elisa; Rodd, Nicholas L.; Rott, Carsten; Sengupta, Dipan; Shakya, Bibhushan; Walsworth, Ronald L.; Westerdale, ShawnWe outline the unique opportunities and challenges in the search for "ultraheavy" dark matter candidates with masses between roughly 10 TeV and the Planck scale mpl≈1016 TeV. This mass range presents a wide and relatively unexplored dark matter parameter space, with a rich space of possible models and cosmic histories. We emphasize that both current detectors and new, targeted search techniques, via both direct and indirect detection, are poised to contribute to searches for ultraheavy particle dark matter in the coming decade. We highlight the need for new developments in this space, including new analyses of current and imminent direct and indirect experiments targeting ultraheavy dark matter and development of new, ultra-sensitive detector technologies like next-generation liquid noble detectors, neutrino experiments, and specialized quantum sensing techniques.Item Thermal pressure on ultrarelativistic bubbles from a semiclassical formalism(IOP Publishing, 2024) Long, Andrew J.; Turner, JessicaWe study a planar bubble wall that is traveling at an ultrarelativistic speed through a thermal plasma. This situation may arise during a first-order electroweak phase transition in the early universe. As particles cross the wall, it is assumed that their mass grows from m a to m b , and they are decelerated causing them to emit massless radiation (m c = 0). We are interested in the momentum transfer to the wall, the thermal pressure felt by the wall, and the resultant terminal velocity of the wall. We employ the semiclassical current radiation (SCR) formalism to perform these calculations. An incident-charged particle is treated as a point-like classical electromagnetic current, and the spectrum of quantum electromagnetic radiation (photons) is derived by calculating appropriate matrix elements. To understand how the spectrum depends on the thickness of the wall, we explore simplified models for the current corresponding to an abrupt and a gradual deceleration. For the model of abrupt deceleration, we find that the SCR formalism can reproduce the P therm ∝ γ 0 w scaling found in earlier work by assuming that the emission is soft, but if the emission is not soft the SCR formalism can be used to obtain P therm ∝ γ 2 w instead. For the model of gradual deceleration, we find that the wall thickness L w enters to cutoff the otherwise log-flat radiation spectrum above a momentum of ∼ γ 2 w / L w , and we discuss the connections with classical electromagnetic bremsstrahlung.Item Upper Limit on the QCD Axion Mass from Isolated Neutron Star Cooling(American Physical Society, 2022) Buschmann, Malte; Dessert, Christopher; Foster, Joshua W.; Long, Andrew J.; Safdi, Benjamin R.The quantum chromodynamics (QCD) axion may modify the cooling rates of neutron stars (NSs). The axions are produced within the NS cores from nucleon bremsstrahlung and, when the nucleons are in superfluid states, Cooper pair breaking and formation processes. We show that four of the nearby isolated magnificent seven NSs along with PSR J0659 are prime candidates for axion cooling studies because they are coeval, with ages of a few hundred thousand years known from kinematic considerations, and they have well-measured surface luminosities. We compare these data to dedicated NS cooling simulations incorporating axions, profiling over uncertainties related to the equation of state, NS masses, surface compositions, and superfluidity. Our calculations of the axion and neutrino emissivities include high-density suppression factors that also affect SN 1987A and previous NS cooling limits on axions. We find no evidence for axions in the isolated NS data, and within the context of the Kim-Shifman-Vainshtein-Zakharov QCD axion model, we constrain ma≲16 meV at 95% confidence level. An improved understanding of NS cooling and nucleon superfluidity could further improve these limits or lead to the discovery of the axion at weaker couplings.