<p>We consider axions lighter than what their QCD couplings might otherwise suggest. Starting with a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math> axion, we introduce a small explicit <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math> symmetry-breaking coupling between the Standard Model Higgs boson and a reheaton. This small explicit breaking allows us to populate a large portion of the light axion <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><mi>a</mi></msub><mo>−</mo><msub><mi>f</mi><mi>a</mi></msub></math> plane, removes the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1</mn><mo>/</mo><mi>N</mi></math> tuning in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi mathvariant="double-struck">Z</mi><mi>N</mi></msub></math>-axion, and explains why only our sector was reheated. Because of finite temperature effects, axions of this sort undergo either “rigged” misalignment, where the axion misalignment angle is effectively <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>π</mi></math> regardless of its initial value, or “shuffled” misalignment, where the initial angle is effectively randomized.</p>

United States

Abhishek Banerjee

Manuel A. Buen-Abad

Anson Hook

Constructing a light QCD axion without <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1</mn><mo>/</mo><mi>N</mi></math> tuning

We introduce a mechanism by which a misaligned ALP can be dynamically converted into a dark photon in the presence of a background magnetic field. An abundance of non-relativistic ALPs will convert to dark photons with momentum of order the inhomogeneities in the background field; therefore a highly homogeneous field will produce non-relativistic dark photons without relying on any redshifting of their momenta. Taking hidden sector magnetic fields produced by a first order phase transition, the mechanism can reproduce the relic abundance of dark matter for a wide range of dark photon masses down to 10 13 eV.

Edward Broadberry

Saurav Das

Gustavo Marques-Tavares

Adiabatic conversion of ALPs into dark photon dark matter

We study (multi) fermion - monopole bound states, many of which are the states that dyons adiabatically transition into as fermions become light. The properties of these bound states depend critically on the UV symmetries preserved by the fermion mass terms, their relative size, and the value of . Depending on the relative size of the mass terms and the value of , the bound states can undergo phase transitions as well as transition from being stable to unstable. In some simple situations, the bound state solution can be related to the Witten effect of another theory with fewer fermions and larger gauge coupling. These bound states are a result of mass terms and symmetry breaking boundary conditions at the monopole core and, consequently, these bound states do not necessarily have definite quantum numbers under accidental IR symmetries. Additionally, they have binding energies that are $$ \mathcal{O}(1) $$ times the fermion mass and bound state radii of order their inverse mass. As the massless limit is approached, the bound state radii approach infinity, and they become new asymptotic states with odd quantum numbers giving a dynamical understanding to the origin of semitons.

Clayton Ristow

Dyonic bound states

We study the phenomenology of a string bosenova explosion in vector superradiance clouds around spinning black holes, focusing on the observable consequences in gravitational wave detectors and accelerometers. During the superradiance growth of a dark photon cloud — which occurs for dark photon masses   <math> <msub> <mi>m</mi> <msup> <mi>A</mi> <mo>′</mo> </msup> </msub> <mo>~</mo> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>14</mn> </mrow> </msup> <mo>−</mo> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>11</mn> </mrow> </msup> <mi>eV</mi> </math>  $$ {m}_{A^{\prime }}\sim {10}^{-14}-{10}^{-11}\textrm{eV} $$  around stellar-mass black holes (  <math> <msub> <mi>m</mi> <msup> <mi>A</mi> <mo>′</mo> </msup> </msub> <mo>~</mo> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>23</mn> </mrow> </msup> <mo>−</mo> <msup> <mn>10</mn> <mrow> <mo>−</mo> <mn>16</mn> </mrow> </msup> <mi>eV</mi> </math>  $$ {m}_{A^{\prime }}\sim {10}^{-23}-{10}^{-16}\textrm{eV} $$  for supermassive black holes) — the dark electromagnetic field might reach a critical field strength, when a network of dark photon strings is produced via a superheated phase transition. These dark photon strings will then absorb the energy in the background gauge fields and get ejected from the cloud, with total energy of the string network as large as the total rotational energy of the spinning black hole. In this paper, we study the subsequent evolution of this dense string network, and the resulting observational consequences depending on the unknown string tension, or almost equivalently, the ratio between the quartic and the gauge coupling in the Abelian Higgs model. Strings with large tension will dissipate into gravitational waves, detectable over a wide range of frequencies, from ~ nHz near supermassive blackholes, to ≳ 10MHz around stellar mass black holes. This is the first known source of high frequency gravitational waves, unconstrained by cosmological observations. The strain of this gravitational wave can be larger than 10−14 at low frequencies, lasting for longer than typical duration of experiments. Small tension strings, whose string networks can have total lengths as large as 1040 km, can travel to the earth with appreciable rate from any black hole in the Milky Way and interact with earth based accelerometers. If the Standard Model particles are directly charged under the dark photon, e.g. U(1)B−L, this interaction leads to an acceleration of Standard Model particles that is independent of the coupling strength. We work out the spectral density of this acceleration, and project that modern accelerometers and equivalence principle tests can be sensitive to the passing of these strings.

Dawid Brzeminski

Canada

Junwu Huang

Searching for string bosenovas with gravitational wave detectors

We show that, if there are conserved flavor symmetries, then some properties of a monopole can depend on <math><mi>θ</mi></math>, even when a fermion is massless. The quantized nature of global symmetries and the fractional nature of the Witten effect can lead to interesting structure. Seen from another point of view, aside from possibly breaking baryon and lepton flavor symmetries (the Callan-Rubakov effect), monopole boundary conditions can also break the axial symmetry that otherwise could have been used to remove <math><mi>θ</mi></math> from the Lagrangian. As an example, in a toy model, we calculate the <math><mi>θ</mi></math> dependence of the mass of the monopole and properties of the nonzero charge density surrounding the monopole.

Theta dependence in the presence of massless fermions

We argue that the striking similarity between the cosmic abundances of baryons and dark matter, despite their very different astrophysical behavior, strongly motivates the scenario in which dark matter resides within a rich dark sector parallel in structure to that of the standard model. The near cosmic coincidence is then explained by an approximate ℤ 2 exchange symmetry between the two sectors, where dark matter consists of stable dark neutrons, with matter and dark matter asymmetries arising via parallel WIMP baryogenesis mechanisms. Taking a top-down perspective, we point out that an adequate ℤ 2 symmetry necessitates solving the electroweak hierarchy problem in each sector, without our committing to a specific implementation. A higher-dimensional realization in the far UV is presented, in which the hierarchical couplings of the two sectors and the requisite ℤ 2-breaking structure arise naturally from extra-dimensional localization and gauge symmetries. We trace the cosmic history, paying attention to potential pitfalls not fully considered in previous literature. Residual ℤ 2-breaking can very plausibly give rise to the asymmetric reheating of the two sectors, needed to keep the cosmological abundance of relativistic dark particles below tight bounds. We show that, despite the need to keep inter-sector couplings highly suppressed after asymmetric reheating, there can naturally be order-one couplings mediated by TeV scale particles which can allow experimental probes of the dark sector at high energy colliders. Massive mediators can also induce dark matter direct detection signals, but likely at or below the neutrino floor.

Arushi Bodas

Mexico

Manuel Buen-Abad

Raman Sundrum

A closer look in the mirror: reflections on the matter/dark matter coincidence

The near equality of the dark matter and baryon energy densities is a remarkable coincidence, especially when one realizes that the baryon mass is exponentially sensitive to UV parameters in the form of dimensional transmutation. We explore a new dynamical mechanism, where in the presence of an arbitrary number density of baryons and dark matter, a scalar adjusts the masses of dark matter and baryons until the two energy densities are comparable. In this manner, the coincidence is explained regardless of the microscopic identity of dark matter and how it was produced. This new scalar causes a variety of experimental effects such as a new force and a (dark) matter density-dependent proton mass.

Dynamical Explanation of the Dark Matter and Baryon Energy Density Coincidence

The presence of a plethora of light spin 0 and spin 1 fields is motivated in a number of BSM scenarios, such as the axiverse. The study of the interactions of such light bosonic fields with the Standard Model has focused mostly on interactions involving only one such field, such as the axion (ϕ) coupling to photons,  $$\phi F\widetilde{F}$$ , or the kinetic mixing between photon and the dark photon, FF  D . In this work, we continue the exploration of interactions involving two light BSM fields and the standard model, focusing on the mixed axion-photon-dark-photon interaction  $$\phi F{\widetilde{F}}_{D}$$ . If either the axion or dark photon are dark matter, we show that this interaction leads to conversion of the CMB photons into a dark sector particle, leading to a distortion in the CMB spectrum. We present the details of these unique distortion signatures and the resulting constraints on the  $$\phi F{\widetilde{F}}_{D}$$  coupling. In particular, we find that for a wide range of masses, the constraints from these effect are stronger than on the more widely studied axion-photon coupling.

CMB spectral distortions from an axion-dark photon-photon interaction

We explore an odd class of quantum field theories where a hierarchy problem is resolved with new dynamics as opposed to new particles. The essential element of our construction is a <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> pseudo-Nambu-Goldstone boson with symmetry breaking interactions all characterized by a large number <math><mi>N</mi></math> of units of the fundamental charge. In the resulting effective theory, quantum corrections, like those to the effective potential and mass, which are normally power divergent and saturated at the UV cutoff, are instead saturated at a much lower scale. This critical scale, which does not involve any new particle, corresponds to the onset of unsuppressed multiparticle production in scattering processes. Remarkably, this all happens within the tractable domain of weak coupling. Terms involving arbitrarily high powers of the Goldstone field must, however, be taken into account. In particular, a truncation to the renormalizable part of the effective Lagrangian would completely miss the physics.

Switzerland

Riccardo Rattazzi

Softening the UV without new particles

We introduce a novel symmetry for quantum 6j-symbols, which we call the tug-the-hook symmetry. Unlike other known symmetries, it is applicable for any representations, including ones with multiplicities. We provide several evidences in favor of the tug-the-hook symmetry. First, this symmetry follows from the eigenvalue conjecture. Second, it is shown by several new examples of explicit coincidence of 6j-symbols with multiplicities. Third, the tug-the-hook symmetry for Wilson loops for knots in the 3d Chern-Simons theory implies the tug-the-hook symmetry for quantum 6j-symbols. An important implication of the analysis is the generalization of the tug-the-hook symmetry for the Chern-Simons Wilson loops to the case of links.