Pulsar timing arrays have recently observed a stochastic gravitational wave background at nano-Hertz frequencies. This raises the question whether the signal can be of primordial origin. Supercooled first-order phase transitions are among the few early Universe scenarios that can successfully explain it. To further scrutinise this possibility, a precise theoretical understanding of the dynamics of the phase transition is required. Here we perform such an analysis for a dark sector with an Abelian Higgs model in the conformal limit, which is known to admit large supercooling. We compare simple analytic parametrisations of the bounce action, one-loop finite temperature calculations including Daisy resummation, and results of a dimensionally reduced (3D) effective theory including up to two-loop corrections using the DRalgo framework. Consistent renormalisation group evolution (RGE) of the couplings is essential for a meaningful interpretation of the results. We find that the 3D EFT with consistent expansion in the 4D parameters gives a significantly reduced scale dependence of the phase transition parameters. With a suitable choice of RGE scale, the 4D high temperature expanded effective potential yields results consistent with the 3D calculations, while the analytic parametrisation deviates significantly in the limit of large supercooling.

Germany

Martin Christiansen

Spain

Eric Madge

Cristina Puchades-Ibáñez

Mexico

Maura Ramirez-Quezada

Pedro Schwaller

Beyond the daisy chain: running and the 3D EFT view of supercooled phase transitions

<p>Searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations. In this work, we discuss realistic cosmological scenarios giving rise to neutrino isocurvature. We show that in general both neutrino and matter isocurvature perturbations are generated, whose ratio we parametrize by a newly introduced mixing angle. We obtain the first limits on this new mixing angle from PLANCK data, and discuss novel insights into the early Universe that could be provided by future measurements.</p>

Christopher Gerlach

Israel

Wolfram Ratzinger

Generalized neutrino isocurvature

In the audible axion mechanism, axion-like particles source primordial gravitational waves via their coupling to a dark Abelian gauge field. The original setup, however, relies on a large axion decay constant and coupling to produce sizable signals. In this article, we show that delaying the onset of axion oscillations opens up the testable parameter space and reduces the required coupling to $\textit{α}$ ≳ 1. Furthermore, we investigate the emission of gravitational waves via the axion coupling to the Standard Model photon in the presence of Schwinger pair production, generating a strong signal in the $\textit{μ}$Hz or ultra-high frequency range. Cosmological constraints and gravitational wave projections are provided for both scenarios.

Daniel Schmitt

Supercooled audible axions

We present a minimal composite dark matter model, based on a SU($\textit{N}$ ) dark sector with $\textit{n}$ dark quarks and a heavy t-channel mediator. For $\textit{n}$ ≥ 4, the dark flavor symmetry guarantees the stability of a subset of the dark pions, which serve as our dark matter candidates. Their relic abundance is determined by dark sector annihilation with the remaining dark pions, which are unstable and decay. Due to their degenerate masses, the annihilation cross section is suppressed at low temperatures, thereby avoiding stringent constraints from indirect detection and opening up the GeV mass window. The decaying dark pions are naturally long lived. We obtain limits on the model from semi-visible or emerging jet searches and estimate the reach of future probes.

Adrián Carmona

Fatemeh Elahi

United States

Christiane Scherb

Dark showers from sneaky dark matter

Previous computations of feebly interacting particle production have encountered issues with unphysical (negative) interaction rates at soft momenta. We address this problem by studying the production of Axion-Like Particles (ALPs) coupled to U(1)-gauge fields, employing the full form of 1PI-resummed gauge boson propagators. This approach avoids the need for matching or subtraction procedures. We find that the ALP production rate remains positive across all momentum scales and identify the dominant production mechanisms. At soft ALP momenta ($\textit{p}$ ≲ $\textit{g}$ $\textit{T}$), interactions involving two spacelike gauge bosons dominate the production rate, surpassing other channels by an order of magnitude. In particular, using the full gauge boson propagator suggests that at even softer momenta ($\textit{p}$ ≲ $\textit{g}$ $\textit{T}$), production involving two timelike gauge bosons becomes significant, potentially exceeding other contributions by another order of magnitude. We also find that a leading order accurate result for momenta $\textit{g}$ $\textit{T}$ ≲ $\textit{p}$ ≲ $\textit{g}$ $\textit{T}$ still requires extensions beyond the 1PI resummation. Using these insights, we update the thermal ALP abundance and refine the estimate of the average ALP momentum, providing important input for structure formation constraints on ALP dark matter in the keV mass range.

Italy

Mathias Becker

Julia Harz

Enrico Morgante

ALP production from abelian gauge bosons: beyond hard thermal loops

In an external electric or magnetic field, a gravitational wave (GW) may be converted into electromagnetic radiation. We present a coordinate-invariant framework to describe the GW signal in a detector that is based on this effect, such as cavities for axion searches. In this framework, we pay special attention to the definition of manifestly coordinate-independent expressions for the electromagnetic fields that an external observer would detect. A careful assessment of the detector’s perceived motion allows us to treat both its mechanical and its electromagnetic response to the GW consistently. We further introduce well-defined approximations for which this motion may be neglected, and hence provide suggestions on which coordinate frame is suitable to characterise the GW signal in practice. We illustrate our findings in two examples, an infinitesimally thin rod and a spherical electromagnetic cavity.

Sebastian Schenk

A coordinate-independent formalism for detecting high-frequency gravitational waves

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.

Nicklas Ramberg

Primordial gravitational waves in the nano-Hertz regime and PTA data — towards solving the GW inverse problem

We analyze the phase transition in improved holographic QCD to obtain an estimate of the gravitational wave signal emitted in the confinement transition of a pure SU(3) Yang-Mills dark sector. We derive the effective action from holography and show that the energy budget and duration of the phase transition can be calculated with minor errors. These are used as input to obtain a prediction of the gravitational wave signal. To our knowledge, this is the first computation of the gravitational wave signal in a holographic model designated to match lattice data on the thermal properties of pure Yang-Mills.

Gravitational waves from dark SU(3) Yang-Mills theory

In this work, we consider the case of a strongly coupled dark/hidden sector, which extends the Standard Model (SM) by adding an additional non-Abelian gauge group. These extensions generally contain matter fields, much like the SM quarks, and gauge fields similar to the SM gluons. We focus on the exploration of such sectors where the dark particles are produced at the LHC through a portal and undergo rapid hadronization within the dark sector before decaying back, at least in part and potentially with sizeable lifetimes, to SM particles, giving a range of possibly spectacular signatures such as emerging or semi-visible jets. Other, non-QCD-like scenarios leading to soft unclustered energy patterns or glueballs are also discussed. After a review of the theory, existing benchmarks and constraints, this work addresses how to build consistent benchmarks from the underlying physical parameters and present new developments for the pythia Hidden Valley module, along with jet substructure studies. Finally, a series of improved search strategies is presented in order to pave the way for a better exploration of the dark showers at the LHC.

France

Guillaume Albouy

Canada

Jared Barron

Taiwan

Hugues Beauchesne

Elias Bernreuther

United Kingdom

Marcella Bona

Switzerland

Cesare Cazzaniga

Cari Cesarotti

Timothy Cohen

Annapaola Cosa

David Curtin

Zeynep Demiragli

Sweden

Caterina Doglioni

Alison Elliot

Karri DiPetrillo

Florian Eble

Carlos Erice

Chad Freer

Aran Garcia-Bellido

Caleb Gemmell

Marie-Hélène Genest

Giovanni Cortona

CERN

Giuliano Gustavino

Nicoline Hemme

Tova Holmes

South Africa

Deepak Kar

Simon Knapen

Austria

Suchita Kulkarni

Luca Lavezzo

Belgium

Steven Lowette

Benedikt Maier

Seán Mee

Stephen Mrenna

India

Harikrishnan Nair

Jeremi Niedziela

Christos Papageorgakis

Nukulsinh Parmar

Christoph Paus

Kevin Pedro

Ana Peixoto

Alexx Perloff

Tilman Plehn

Jessie Shelton

Akanksha Singh

Sukanya Sinha

Torbjörn Sjöstrand

Aris Spourdalakis

Daniel Stolarski

Matthew Strassler

Netherlands

Andrii Usachov

Carlos Sierra

Christopher Verhaaren

Long Wang

Theory, phenomenology, and experimental avenues for dark showers: a Snowmass 2021 report

We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in   <math> <mi>t</mi> <mover> <mi>t</mi> <mo>¯</mo> </mover> </math>  $$ t\overline{t} $$  events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter, and show that the ratio of energy deposits in the electromagnetic and hadronic calorimeters as well as track vetoes can efficiently suppress Standard Model backgrounds. Our proposed search can probe exotic top branching ratios smaller than 10 −4 with a conservative strategy at the upcoming LHC run, and potentially below the 10 −7 level with more advanced methods. Finally we also show that measurements of single top production probe these branching ratios in the very short and very long lifetime limit at the 10 −3 level.

Adrian Carmona

The ALPs from the top: searching for long lived axion-like particles from exotic top decays