Extra dimensions are present in many beyond the Standard Model scenarios, most notably in string theory. However, direct signatures of extra dimensions are difficult to observe in many cases. This is the situation, for example, if the energy scales associated with extra dimensions are close to the string or Grand Unification scale. The energetic early universe provides an exciting opportunity to overcome this challenge, since the heavy states associated with high-scale extra dimensions, such as scalar moduli and Kaluza-Klein (KK) gravitons, could have been produced $\textit{on-shell}$ at early epochs. In this work, we illustrate this by focusing on how such states can be produced during inflation and leave signatures in primordial non-Gaussianity (NG). Specifically, we consider a 5D spacetime with a warped extra dimension that remains stabilized as inflation proceeds in the four non-compact dimensions. By discussing an explicit stabilization mechanism, we compute the masses and couplings of the radion modulus and the KK graviton modes. Being gravitational degrees of freedom, these unavoidably couple to the field(s) generating curvature perturbation, and can lead to observable NG with a distinctive oscillatory shape and characteristic angular dependence. We give example benchmarks which can already be probed by the $\textit{Planck}$ data and identify targets for the future. Our study shows that cosmological surveys have the potential to observe on-shell imprints of extra dimensions in the coming years.

United States

Soubhik Kumar

Michael Nee

Warped dimensions at the cosmological collider

We study the physics potential of heavy QCD axions at high-energy muon colliders. Unlike typical axion-like particles, heavy QCD axions solve the strong CP problem with phenomenology driven by the anomalous gluon $$ \left( aG\overset{\sim }{G}\right) $$ couplings. Several ultraviolet scenarios are presented in which QCD axions with TeV-scale masses and decay constants arise consistently with a solution to both the strong CP problem and the axion quality problem. We perform a detailed collider analysis for both a 3 and 10 TeV muon collider, focusing on hadronic axion decays that gives rise to a dijet-resonance signature. Our projections for the axion discovery reach in the multi-TeV mass range demonstrate that a muon collider can significantly extend sensitivity to heavy QCD axions compared to existing experiments.

Ravneet Bedi

Tony Gherghetta

Peiran Li

Zhen Liu

Heavy QCD axions at high-energy muon colliders

We consider first order cosmological phase transitions (PTs) happening at late times below standard model temperatures <math><msub><mi>T</mi><mrow><mi>PT</mi></mrow></msub><mo>≲</mo><mi>GeV</mi></math>. The inherently stochastic nature of bubble nucleation and the finite number of bubbles associated with a late-time PT lead to superhorizon fluctuations in the PT completion time. We compute how such fluctuations eventually source curvature fluctuations with universal properties, independent of the microphysics of the PT dynamics. Using cosmic microwave background (CMB) and large scale structure measurements, we constrain the energy released in a dark-sector PT. For <math><mrow><mn>0.1</mn><mtext> </mtext><mtext> </mtext><mi>eV</mi><mo>≲</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>PT</mi></mrow></msub><mo>≲</mo><mi>keV</mi></mrow></math> this constraint is stronger than both the current bound from additional neutrino species <math><mi>Δ</mi><msub><mi>N</mi><mi>eff</mi></msub></math>, and in some cases, even CMB-S4 projections. Future measurements of CMB spectral distortions and pulsar timing arrays will also provide competitive sensitivity for <math><mrow><mi>keV</mi><mo>≲</mo><msub><mrow><mi>T</mi></mrow><mrow><mi>PT</mi></mrow></msub><mo>≲</mo><mi>GeV</mi></mrow></math>.

Gilly Elor

Japan

Ryusuke Jinno

Robert McGehee

Yuhsin Tsai

Finite Bubble Statistics Constrain Late Cosmological Phase Transitions

The quantum chromodynamics (QCD) axion may solve the strong <math><mi>C</mi><mi>P</mi></math> problem and explain the dark matter (DM) abundance of our Universe. The axion was originally proposed to arise as the pseudo-Nambu-Goldstone boson of global <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>PQ</mi></mrow></msub></math> Peccei-Quinn (PQ) symmetry breaking, but axions also arise generically in string theory as zero modes of higher-dimensional gauge fields. In this work we show that string theory axions behave fundamentally differently from field theory axions in the early Universe. Field theory axions may form axion strings if the PQ phase transition takes place after inflation. In contrast, we show that string theory axions do not generically form axion strings. In special inflationary paradigms, such as D-brane inflation, string theory axion strings may form; however, their tension is parametrically larger than that of field theory axion strings. We then show that such QCD axion strings overproduce the DM abundance for all allowed QCD axion masses and are thus ruled out, except in scenarios with large warping. A loop-hole to this conclusion arises in the axiverse, where an axion string could be composed of multiple different axion mass eigenstates; a heavier eigenstate could collapse the network earlier, allowing for the QCD axion to produce the correct DM abundance and also generating observable gravitational wave signals.

Joshua N. Benabou

Quentin Bonnefoy

Netherlands

Malte Buschmann

Benjamin R. Safdi

Cosmological dynamics of string theory axion strings

Effective field theories (EFTs) of heavy particles coupled to the inflaton are rife with operator redundancies, frequently obscured by sensitivity to both boundary terms and field redefinitions. We initiate a systematic study of these redundancies by establishing a minimal operator basis for an archetypal example, the abelian gauge-Higgs-inflaton EFT. Working up to dimension 9, we show that certain low-dimensional operators are entirely redundant and identify new non-redundant operators with potentially interesting cosmological collider signals. Our methods generalize straightforwardly to other EFTs of heavy particles coupled to the inflaton.

Nathaniel Craig

Amara McCune

An effective cosmological collider

Axion strings are horizon-size topological defects that may be produced in the early Universe. Ultralight axion-like particles may form strings that persist to temperatures below that of big bang nucleosynthesis. Such strings have been considered previously as sources of gravitational waves and cosmic microwave background (CMB) polarization rotation. In this work we show, through analytic arguments and dedicated adaptive mesh refinement cosmological simulations, that axion strings deposit a subdominant fraction of their energy into high-energy Standard Model (SM) final states, for example, by the direct production of heavy radial modes that subsequently decay to SM particles. This high-energy SM radiation is absorbed by the primordial plasma, leading to novel signatures in precision big bang nucleosynthesis, the CMB power spectrum, and gamma-ray surveys. In particular, we show that CMB power spectrum data constrains axion strings with decay constants <math><mrow><msub><mrow><mi>f</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>≲</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>12</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, up to model dependence on the ultraviolet completion, for axion masses <math><msub><mi>m</mi><mi>a</mi></msub><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>29</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></math>; future CMB surveys could find striking evidence of axion strings with lower decay constants.

Yujin Park

Signatures of primordial energy injection from axion strings

Positivity bounds represent nontrivial limitations on effective field theories (EFTs) if those EFTs are to be completed into a Lorentz-invariant, causal, local, and unitary framework. While such positivity bounds have been applied in a wide array of physical contexts to obtain useful constraints, their application to inflationary EFTs is subtle since Lorentz invariance is spontaneously broken during cosmic inflation. One path forward is to employ a Breit parameterization to ensure a crossing-symmetric and analytic S-matrix in theories with broken boosts. We extend this approach to a theory with multiple fields, and uncover a fundamental obstruction that arises unless all fields obey a dispersion relation that is approximately lightlike. We then apply the formalism to various classes of inflationary EFTs, with and without isocurvature perturbations, and employ this parameterization to derive new positivity bounds on such EFTs. For multifield inflation, we also consider bounds originating from the generalized optical theorem and demonstrate how these can give rise to stronger constraints on EFTs compared to constraints from traditional elastic positivity bounds alone. We compute various shapes of non-Gaussianity (NG), involving both adiabatic and isocurvature perturbations, and show how the observational parameter space controlling the strength of NG can be constrained by our bounds.

Marat Freytsis

Grant Remmen

CERN

Nicholas Rodd

Multifield positivity bounds for inflation

We study the prospect of measuring the electroweak quantum numbers of beyond the Standard Model color sextet particles that decay into same-sign top-quark pairs. Among these particles, the color sextet scalars give rise to top quarks with the same chirality, while the top quarks coming from the color sextet vector would have opposite chirality. This difference gets encoded in the angular distributions of the bottom quarks and leptons originating from the decays of the top quarks. We utilize this feature and the energy distributions of the final-state jets and leptons to distinguish among the three possible color sextet resonances, taking into account various SM background processes at the 13 TeV LHC.

India

Rafiqul Rahaman

Ritesh K. Singh

Measuring electroweak quantum numbers of color sextet resonances at the LHC

Particles with masses much larger than the inflationary Hubble scale, <math><msub><mi>H</mi><mi>I</mi></msub></math>, can be pair-produced nonadiabatically during inflation. Due to their large masses, the produced particles modify the curvature perturbation around their locations. These localized perturbations eventually give rise to localized signatures on the cosmic microwave background (CMB), in particular, pairwise hotspots (PHSs). In this work, we show that convolutional neural networks (CNNs) provide a powerful tool for identifying PHSs on the CMB. While for a given hotspot profile a traditional matched filter analysis is known to be optimal, a neural network learns to effectively detect the large variety of shapes that can arise in realistic models of particle production. Considering an idealized situation where the dominant background to the PHS signal comes from the standard CMB fluctuations, we show that a CNN can isolate the PHSs with <math><mi>O</mi><mo>(</mo><mn>10</mn><mo>)</mo><mo>%</mo></math> efficiency even if the hotspot temperature is <math><mi>O</mi><mo>(</mo><mn>10</mn><mo>)</mo></math> times smaller than the average CMB fluctuations. Overall, the CNN search is sensitive to heavy particle masses <math><msub><mi>M</mi><mn>0</mn></msub><mo>/</mo><msub><mi>H</mi><mi>I</mi></msub><mo>=</mo><mi>O</mi><mo>(</mo><mn>200</mn><mo>)</mo></math>, and constitutes one of the unique probes of very high energy particle physics.

Taegyun Kim

South Korea

Jeong Han Kim

Adam Martin

Moritz Münchmeyer

Probing cosmological particle production and pairwise hotspots with deep neural networks

Heavy QCD axions are well-motivated extensions of the QCD axion that address the quality problem while still solving the strong CP problem. Owing to the gluon coupling, critical for solving the strong CP problem, these axions can be produced in significant numbers in beam dump and collider environments for axion decay constants as large as PeV, relevant for addressing the axion quality problem. In addition, if these axions have leptonic couplings, they can give rise to long-lived decay into lepton pairs, in particular, dominantly into muons above the dimuon threshold and below the GeV scale in a broad class of axion models. Considering existing constraints, primarily from rare meson decays, we demonstrate that current and future neutrino facilities and long-lived particle searches have the potential to probe significant parts of the heavy QCD axion parameter space via dimuon final states.

Raymond Co

Searches for heavy QCD axions via dimuon final states