We show that a rotating axion field that makes a transition from a matterlike equation of state to a kinationlike equation of state around the epoch of recombination can significantly ameliorate the Hubble tension, i.e., the discrepancy between the determinations of the present-day expansion rate <math><mrow><msub><mrow><mi>H</mi></mrow><mrow><mn>0</mn></mrow></msub></mrow></math> from observations of the cosmic microwave background on one hand and type Ia supernovae on the other. We consider a specific, UV-complete model of such a rotating axion and find that it can relax the Hubble tension without exacerbating tensions in determinations of other cosmological parameters, in particular the amplitude of matter fluctuations <math><msub><mi>S</mi><mn>8</mn></msub></math>. We subsequently demonstrate how this rotating axion model can also generate the baryon asymmetry of our Universe, by introducing a coupling of the axion field to right-handed neutrinos. This baryogenesis model predicts heavy neutral leptons that are most naturally within reach of future lepton colliders, but in finely tuned regions of parameter space may also be accessible at the high-luminosity LHC and the beam dump experiment SHiP.

United States

Raymond T. Co

Nicolas Fernandez

Akshay Ghalsasi

Japan

Keisuke Harigaya

Jessie Shelton

Axion baryogenesis puts a new spin on the Hubble tension

We assess the capabilities of the CMS and LHCb searches for low-<math><msub><mi>p</mi><mi>T</mi></msub></math> displaced dimuon pairs to discover hidden valley models. To do so, we develop a new benchmark model featuring a light dark photon with dark flavor-violating couplings, which realizes a range of dimuon vertex topologies. We show that the data scouting techniques used in these searches provide unique sensitivity and we make some additional suggestions to further extend the scope of future experimental searches.

Susan Born

Rohith Karur

Simon Knapen

Scouting for dark showers at CMS and LHCb

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

Germany

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

Italy

Giovanni Cortona

CERN

Giuliano Gustavino

Nicoline Hemme

Tova Holmes

South Africa

Deepak Kar

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

Christiane Scherb

Pedro Schwaller

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

Rotations of an axion field in field space provide a natural origin for an era of kination domination, where the energy density is dominated by the kinetic term of the axion field, preceded by an early era of matter domination. Remarkably, no entropy is produced at the end of matter domination and hence these eras of matter and kination domination may occur even after Big Bang Nucleosynthesis. We derive constraints on these eras from both the cosmic microwave background and Big Bang Nucleosynthesis. We investigate how this cosmological scenario affects the spectrum of possible primordial gravitational waves and find that the spectrum features a triangular peak. We discuss how future observations of gravitational waves can probe the viable parameter space, including regions that produce axion dark matter by the kinetic misalignment mechanism or the baryon asymmetry by axiogenesis. For QCD axion dark matter produced by the kinetic misalignment mechanism, a modification to the inflationary gravitational wave spectrum occurs above 0.01 Hz and, for high values of the energy scale of inflation, the prospects for discovery are good. We briefly comment on implications for structure formation of the universe.

Raymond Co

David Dunsky

Lawrence Hall

Gravitational wave and CMB probes of axion kination

Dark matter (DM) freeze-in through a light mediator is an appealing model with excellent detection prospects at current and future experiments. Light mediator freeze-in is UV-insensitive insofar as most DM is produced at late times, and thus the DM abundance does not depend on the unknown early evolution of our universe. However the final DM yield retains a dependence on the initial DM population, which is usually assumed to be exactly zero. We point out that in models with light mediators, the final DM yield will also depend on the initial conditions assumed for the light mediator population. We describe a class of scenarios we call “glaciation” where DM freezing in from the SM encounters a pre-existing thermal bath of mediators, and study the dependence of the final DM yield on the initial temperature of this dark radiation bath. To compute DM scattering rates in this cosmology, we derive for the first time an exact integral expression for the Boltzmann collision term describing interactions between two species at different temperatures. We quantify the dependence of the DM yield on the initial dark temperature and find that it can be sizeable in regions near the traditional (zero initial abundance) freeze-in curve. We generalize the freeze-in curve to a glaciation band, which can extend as much as an order of magnitude below the traditional freeze-in direct detection target, and point out that the DM phase space distribution as well as the yield can be strongly dependent on initial conditions.

Yonatan Kahn

Freeze-in, glaciation, and UV sensitivity from light mediators

We provide five benchmark hidden valley models with perturbative parton showers, which span a wide range of dark shower phenomenology. We consider production through an <math><mi>s</mi></math>-channel, heavy mediator, which can be identified with the SM Higgs. By assuming a set of well-motivated decay portals, one can moreover fix both the branching ratios of the dark mesons and set a lower bound on their lifetime. We provide a public python tool which can be used to generate self-consistent pythia 8 cards for our benchmark models.

Dong Xu

Perturbative benchmark models for a dark shower search program

We consider the scattering of dark matter particles from superfluid liquid <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math>, which has been proposed as a target for their direct detection. Focusing on dark matter masses below <math><mo>∼</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>, we demonstrate from sum-rule arguments the importance of the production of single phonons with energies <math><mi>ω</mi><mo>≲</mo><mn>1</mn><mtext> </mtext><mtext> </mtext><mi>meV</mi></math>. We show further that the anomalous dispersion of phonons in liquid <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math> at low pressures [i.e., <math><msup><mi>d</mi><mn>2</mn></msup><mi>ω</mi><mo>(</mo><mi>q</mi><mo>)</mo><mo>/</mo><mi>d</mi><msup><mi>q</mi><mn>2</mn></msup><mo>&gt;</mo><mn>0</mn></math>, where <math><mi>q</mi></math> and <math><mi>ω</mi><mo>(</mo><mi>q</mi><mo>)</mo></math> are the phonon momentum and energy] has the important consequence that a single phonon will decay over a relatively short distance into a shower of lower-energy phonons centered on the direction of the original phonon. Thus, the experimental challenge in this regime is to detect a shower of low-energy phonons, not just a single phonon. Additional information from the distribution of phonons in such a shower could enhance the determination of the dark matter mass.

Denmark

Gordon Baym

D. H. Beck

Jeffrey P. Filippini

C. J. Pethick

Searching for low mass dark matter via phonon creation in superfluid <math><mrow><mmultiscripts><mrow><mi>He</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>4</mn></mrow></mmultiscripts></mrow></math>

A wide range of mechanisms predict present-day <math><mi>s</mi></math>-wave dark matter (DM) annihilation cross sections that are orders of magnitude below current experimental sensitivity. We explore the capability of DM density spikes around the Milky Way’s supermassive black hole to probe such faint signals of DM annihilations, considering a range of possible spike and halo distributions. As an exemplar of a theory with a suppressed <math><mi>s</mi></math>-wave annihilation cross section, we consider a hidden sector axion portal model of DM. In this model, the leading contribution to the annihilation cross section in the early Universe is <math><mi>p</mi></math>-wave, while <math><mi>s</mi></math>-wave annihilations occur at higher order in the coupling constant. We provide a unified treatment of DM freeze-out in this model including both <math><mi>s</mi></math>- and <math><mi>p</mi></math>-wave annihilations and analytically determine the photon spectrum for the dominant DM annihilation process in the Universe today. We find that Fermi and H.E.S.S. observations of the Galactic center offer excellent sensitivity to this model over a wide range of parameter space, with prospects depending sensitively on the properties of the DM spike as well as the central halo.

Barry T. Chiang

Stuart L. Shapiro

Faint dark matter annihilation signals and the Milky Way’s supermassive black hole

During inflation, scalar fields with masses less than the Hubble scale acquire vacuum expectation values (VEVs) via stochastic processes driven by quantum fluctuations. For nearly massless spectator scalars transforming nontrivially under a continuous symmetry group, we demonstrate that the evolution of the VEV depends on the dimensionality of the scalar field space. Fields in larger representations both attain larger vacuum expectation values and converge more rapidly to equilibrium. We present an argument demonstrating how this higher-dimensional evolution can be obtained in unitary gauge for fields transforming under local symmetries with a mass gap that is small compared to the Hubble scale. Finally, we show that accounting for the full number of degrees of freedom in the Standard Model Higgs multiplet tightens Higgs stability constraints on the inflationary scale at the percent level and has more dramatic consequences for both the VEV and the energy stored in the Higgs field after inflation.

Peter Adshead

Lauren Pearce

Zachary J. Weiner

Stochastic evolution of scalar fields with continuous symmetries during inflation

Light new physics weakly coupled to the Higgs can induce a strong first-order electroweak phase transition (EWPT). Here, we argue that scenarios in which the EWPT is driven first-order by a light scalar with mass between <math><mrow><mo>∼</mo><mn>10</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi><mo>−</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>h</mi></mrow></msub><mo>/</mo><mn>2</mn></mrow></math> and small mixing with the Higgs will be conclusively probed by the high-luminosity LHC and future Higgs factories. Our arguments are based on analytic and numerical studies of the finite-temperature effective potential and provide a well-motivated target for exotic Higgs decay searches at the LHC and future lepton colliders.

Jonathan Kozaczuk

China

Michael J. Ramsey-Musolf

Exotic Higgs boson decays and the electroweak phase transition