<p>We apply the criterion of finite naturalness to the limiting case of a generic heavy sector decoupled from the Standard Model. The sole and unavoidable exception to this decoupling arises from gravitational interactions. We demonstrate that gravity can couple the Higgs to the heavy scale significantly earlier than the well-known three-loop top-quark-mediated diagrams discussed in previous literature. As an application, we show that finite naturalness disfavors large-field inflationary models involving super-Planckian field excursions. In contrast, in the small-field regime, achieving successful inflation requires substantial fine-tuning of the initial conditions, in agreement with previous results. Recent data from the Atacama Cosmology Telescope further amplify the tension between naturalness and fine-tuning, challenging the theoretical robustness of single-field inflation as a compelling explanation for the origin of the Universe.</p>

Italy

Pier Giuseppe Catinari

United States

Loris Del Grosso

Luca Di Giovanni

Alfredo Urbano

Insights into the highest natural scale: Finite naturalness challenges inflationary dynamics

<p>In this work, we critically reanalyze the explicit breaking of the Peccei-Quinn global symmetry—and the corresponding corrections to the QCD axion potential—induced by gravity. Specifically, we examine the role of gravitational instantons, which are nonperturbative, finite-action solutions to the Euclidean Einstein equations. These instantons represent topologically nontrivial configurations of spacetime and are analogous to instantons in gauge theory. The amount of symmetry breaking induced by gravitational instantons can be computed in a controlled way within the framework of semiclassical gravity, using ’t Hooft operators, in full analogy to the computation of the axion potential arising from QCD small instanton effects. Contrary to previous results in the literature, we find that the effects of gravitational instantons are extremely small and therefore do not give rise to a significant quality problem for the axion solution to the strong <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>C</mi><mi>P</mi></mrow></math> problem, both within the Standard Model and in beyond-the-Standard-Model scenarios that involve multiple copies of the Standard Model. In conclusion, we argue that, assuming the ultraviolet completion of gravity is weakly coupled, the axion solution to the strong <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>C</mi><mi>P</mi></mrow></math> problem remains free from any quality issues due to gravity. Along the way, we derive the effective Lagrangian of the QCD axion, including its gravitational coupling.</p>

Gravitational instantons and the quality problem of the QCD axion: Facts, speculations, and statements in between

We consider a theory in which a real scalar field is Yukawa coupled to a fermion and has a potential with two nondegenerate vacua. If the coupling is sufficiently strong, a collection of <math><mi>N</mi></math> fermions deforms the true vacuum state, creating energetically favored false-vacuum pockets in which fermions are trapped. We embed this model within general relativity and prove that it admits self-gravitating compact objects where the scalar field acquires a nontrivial profile due to nonperturbative effects. We discuss some applications of this general mechanism: (i) “neutron soliton stars” in low-energy effective QCD, which naturally happen to have masses around <math><mn>2</mn><msub><mi>M</mi><mo>⊙</mo></msub></math> and radii around 10 km even without neutron interactions; (ii) “Higgs false-vacuum pockets” in and beyond the standard model; (iii) “dark soliton stars” in models with a dark sector. In the latter two examples, we find compelling solutions naturally describing centimeter-size compact objects with masses around <math><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup><msub><mi>M</mi><mo>⊙</mo></msub></math>, intriguingly in a range compatible with the Optical Gravitational Lensing <math><mrow><mi>Experiment</mi><mtext> </mtext><mo>(</mo><mi>OGLE</mi><mo>)</mo><mo>+</mo><mtext>Hyper</mtext></mrow></math> Suprime-Cam (HSC) microlensing anomaly. In addition to these interesting examples, the mechanism of nonperturbative vacuum scalarization may play a role in various contexts in and beyond the standard model, providing a support mechanism for new compact objects that can form in the early Universe, can collapse into primordial black holes through accretion past their maximum mass, and serve as dark matter candidates.

Paolo Pani

Compact objects in and beyond the standard model from nonperturbative vacuum scalarization

The cancellation of infrared (IR) divergences is an old topic in quantum field theory whose main results are condensed into the celebrated Kinoshita-Lee-Nauenberg (KLN) theorem. In this paper we consider mass-suppressed corrections to the leading (i.e. double-logarithmic) IR divergences in the context of spontaneously broken gauge theories. We work in a simplified theoretical set-up based on the spontaneously broken U′(1) ⨂ U(1) gauge group. We analyze, at the one-loop level and including mass-suppressed terms, the double-logarithmic corrections to the decay channels of an hypothetical heavy Z′ gauge boson coupled to light chiral fermions and mixed with a light massive Z gauge boson. Limited to this theoretical framework, only final state IR corrections are relevant. We find that full exploitation of the KLN theorem requires non-trivial combinations of various decay channels in order to get rid of the mass-suppressed IR corrections. Based on this observation we show that, starting from any two-body decay of the heavy Z′ gauge boson, the cancellation of the mass-suppressed double-logarithmic corrections requires the sum over the full decay width (thus enforcing the inclusion of final states which are naïvely unrelated to the starting one). En route, we prove a number of technical results that are relevant for the computation of mass-suppressed double-logarithms of IR origin. Our results are relevant for models that enlarge the Standard Model by adding a heavy Z′.

Paolo Ciafaloni

Denis Comelli

Power suppressed corrections show new features of infrared cancellations

This paper constructs, making use of the on-shell spinor-helicity formalism, a possible ultraviolet completion of gravity following a “bottom-up” approach. The assumptions of locality, unitarity, and causality i) require an infinite tower of resonances with increasing spin and quantized mass, ii) introduce a duality relation among crossed scattering channels, and iii) dress all gravitational amplitudes in the Standard Model with a form factor that closely resembles either the Veneziano or the Virasoro-Shapiro amplitude in string theory. As a consequence of unitarity, the theory predicts leading order deviations from General Relativity in the coupling of gravity to fermions that could be explained if space-time has torsion in addition to curvature.

Japan

Rodrigo Alonso

Amplitudes, resonances, and the ultraviolet completion of gravity

There exist non-trivial stationary points of the Euclidean action for an axion particle minimally coupled to Einstein gravity, dubbed wormholes. They explicitly break the continuos global shift symmetry of the axion in a non-perturbative way, and generate an effective potential that may compete with QCD depending on the value of the axion decay constant. In this paper, we explore both theoretical and phenomenological aspects of this issue. On the theory side, we address the problem of stability of the wormhole solutions, and we show that the spectrum of the quadratic action features only positive eigenvalues. On the phenomenological side, we discuss, beside the obvious application to the QCD axion, relevant consequences for models with ultralight dark matter and black hole superradiance. We conclude discussing wormhole solutions for a generic coset and the potential they generate.

CERN

Wormholes and masses for Goldstone bosons

We critically reexamine two possible dark matter candidates within the Standard Model. First, we consider the <math><mi>u</mi><mi>u</mi><mi>d</mi><mi>d</mi><mi>s</mi><mi>s</mi></math> hexaquark. Its QCD binding energy could be large enough to make it (quasi)stable. We show that the cosmological dark matter abundance is reproduced thermally if its mass is 1.2 GeV. However, we also find that such a mass is excluded by the stability of oxygen nuclei. Second, we consider the possibility that the instability in the Higgs potential leads to the formation of primordial black holes while avoiding vacuum decay during inflation. We show that the nonminimal Higgs coupling to gravity must be as small as allowed by quantum corrections, <math><mo>|</mo><msub><mi>ξ</mi><mi>H</mi></msub><mo>|</mo><mo>&lt;</mo><mn>0.01</mn></math>. Even so, one must assume that the Universe survived in <math><msup><mi>e</mi><mn>120</mn></msup></math> independent regions to fluctuations that lead to vacuum decay with probability <math><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></math> each.

Christian Gross

Antonello Polosa

Alessandro Strumia

Wei Xue

Dark matter in the standard model?

We show that, despite stringent constraints on the shape of the main part of the cosmic microwave background (CMB) spectrum, there is considerable room for its modification within its Rayleigh-Jeans (RJ) end, <math><mi>ω</mi><mo>≪</mo><msub><mi>T</mi><mrow><mi>CMB</mi></mrow></msub></math>. We construct explicit new physics models that give an order one (or larger) increase of photon count in the RJ tail, which can be tested by existing and upcoming experiments aiming to detect the cosmological 21 cm emission or absorption signal. This class of models stipulates the decay of unstable particles to dark photons <math><msup><mi>A</mi><mo>′</mo></msup></math> that have a small mass, <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><msup><mrow><mi>A</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>∼</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>14</mn></mrow></msup><mi>–</mi><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>9</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>eV</mi></mrow></math>, nonvanishing mixing angle <math><mi>ε</mi></math> with electromagnetism, and energies much smaller than <math><msub><mi>T</mi><mrow><mi>CMB</mi></mrow></msub></math>. The nonthermal number density of dark photons can be many orders of magnitude above the number density of CMB photons, and even a small probability of <math><msup><mi>A</mi><mo>′</mo></msup><mo>→</mo><mi>A</mi></math> oscillations, for values as small as <math><mi>ε</mi><mo>∼</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>9</mn></mrow></msup></math>, can significantly increase the number of RJ photons. In particular, we show that resonant oscillations of dark photons into regular photons in the interval of redshifts <math><mn>20</mn><mo>&lt;</mo><mi>z</mi><mo>&lt;</mo><mn>1700</mn></math> can be invoked as an explanation of the recent tentative observation of a stronger-than-expected absorption signal of 21 cm photons. We present a model that realizes this possibility, where meV mass dark matter decays to dark photons, with a lifetime longer than the age of the Universe.

Canada

Maxim Pospelov

Austria

Josef Pradler

Joshua T. Ruderman

Room for New Physics in the Rayleigh-Jeans Tail of the Cosmic Microwave Background

The linear dilaton geometry in five dimensions, rediscovered recently in the continuum limit of the clockwork model, may offer a solution to the hierarchy problem which is qualitatively different from other extra-dimensional scenarios and leads to distinctive signatures at the LHC. We discuss the structure of the theory, in particular aspects of naturalness and UV completion, and then explore its phenomenology, suggesting novel strategies for experimental searches. In particular, we propose to analyze the diphoton and dilepton invariant mass spectra in Fourier space in order to identify an approximately periodic structure of resonant peaks. Among other signals, we highlight displaced decays from resonantly-produced long-lived states and high-multiplicity final states from cascade decays of excited gravitons.

Gian Giudice

Israel

Yevgeny Kats

Matthew McCullough

Riccardo Torre

Clockwork/linear dilaton: structure and phenomenology

The LHCb measurement of the μ/e ratio R K ∗ indicates a deficit with respect to the Standard Model prediction, supporting earlier hints of lepton universality violation observed in the R K ratio. We show that the R K and R K ∗ ratios alone constrain the chiralities of the states contributing to these anomalies, and we find deviations from the Standard Model at the 4 σ level. This conclusion is further corroborated by hints from the theoretically challenging b → sμ + μ − distributions. Theoretical interpretations in terms of Z ′, lepto-quarks, loop mediators, and composite dynamics are discussed. We highlight their distinctive features in terms of the chirality and flavour structures relevant to the observed anomalies.