We point out that events with 6 or more top quarks may be observed at the LHC if certain particles exist at the TeV scale. In a model where a vectorlike quark of charge 2/3 decays into a top quark and a pseudoscalar particle, which subsequently decays into a top-antitop pair, the LHC production cross section for events with 6 top quarks may be above 10 fb. If the pseudoscalar is part of a complex scalar field, then longer cascade decays, involving the scalar partner, may lead to events with 8 or even 10 top quarks. We show that for a region of parameter space the dominant LHC signal in this model is 8 top quarks (i.e., four $$ t\overline{t} $$ pairs). The ensuing signals would be spectacular, including many leptons and $\textit{b}$ jets. A discovery in that case would allow several cross section measurements that may determine the masses of all three new particles.

United States

Elias Bernreuther

Bogdan Dobrescu

Multi-top signals of vectorlike quarks at the LHC

Stable dark matter particles may arise as pseudo-Goldstone bosons from the confinement of dark quarks interacting via a non-Abelian gauge force. Their relic abundance is determined not by annihilations into visible particles but by dark pion number-changing processes within the dark sector, such as <math><mn>3</mn><msub><mi>π</mi><mi>D</mi></msub><mo>→</mo><mn>2</mn><msub><mi>π</mi><mi>D</mi></msub></math>. However, if the dark vector mesons <math><msub><mi>ρ</mi><mi>D</mi></msub></math> are light enough for <math><mn>3</mn><msub><mi>π</mi><mi>D</mi></msub><mo>→</mo><msub><mi>π</mi><mi>D</mi></msub><msub><mi>ρ</mi><mi>D</mi></msub></math> annihilations to be kinematically allowed, this process dominates and significantly delays freeze-out. As a result, the preferred dark matter mass scale increases and bounds from the Bullet Cluster can be evaded.

Germany

Nicoline Hemme

Felix Kahlhoefer

Austria

Suchita Kulkarni

Dark matter relic density in strongly interacting dark sectors with light vector mesons

We study a renormalizable theory that includes a heavy weak-singlet vectorlike lepton, τ ′ , that decays into a pseudoscalar boson and a tau lepton. We show that this can be the dominant decay mode of τ ′  provided the pseudoscalar couplings deviate from the case of a Nambu-Goldstone boson. For a range of parameters, the pseudoscalar is long-lived, and mainly decays into a photon pair at one loop, or into a tau pair at tree level. Electroweak production of τ ′+τ ′−  leads to a rich phenomenology at colliders, including signals with many taus or photons. We analyze in detail the LHC signal involving two prompt taus and two displaced photon pairs. Particularly interesting is the case where the pseudoscalar has a decay length of a few meters and thus would typically deposit energy in the muon chambers of the CMS or ATLAS detectors.

Vectorlike leptons and long-lived bosons at the LHC

Michele Lucente

Alessandro Morandini

Erratum to: Searching for dark radiation at the LHC

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

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

Tova Holmes

South Africa

Deepak Kar

Simon Knapen

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

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

Dark showers from strongly interacting dark sectors that confine at the GeV scale can give rise to novel signatures at e + e  −  colliders. In this work, we study the sensitivity of B factory experiments to dark showers produced through an effective interaction arising from a heavy off-shell mediator. We show that a prospective search for displaced vertices from GeV-scale long-lived particles at Belle II can improve the sensitivity to dark showers substantially compared to an existing search at BaBar. We compare the sensitivity of searches for displaced signals to searches for promptly produced resonances at BaBar and KLOE and calculate sensitivity projections for a single-photon search at Belle II to invisible dark showers produced through an effective interaction. The underlying structure of the effective interaction can be resolved at higher-energy experiments, where the mediator can be produced on-shell. To study the resulting constraints, we update electroweak precision bounds on kinetically mixed Z′ bosons and reinterpret a search for low-mass di-muon resonances at LHCb in terms of dark showers. We find that LHCb and Belle II are most sensitive to different particle decay lengths, underscoring the complementarity of LHC and intensity frontier experiments.

Kai Böse

Torben Ferber

Christopher Hearty

Kai Schmidt-Hoberg

Forecasting dark showers at Belle II

In this work we explore the intriguing connections between searches for long-lived particles (LLPs) at the LHC and early universe cosmology. We study the non-thermal production of ultra-relativistic particles (i.e. dark radiation) in the early universe via the decay of weak-scale LLPs and show that the cosmologically interesting range ∆N eff ~ 0.01–0.1 corresponds to LLP decay lengths in the mm to cm range. These decay lengths lie at the boundary between prompt and displaced signatures at the LHC and can be comprehensively explored by combining searches for both. To illustrate this point, we consider a scenario where the LLP decays into a charged lepton and a (nearly) massless invisible particle. By reinterpreting searches for promptly decaying sleptons and for displaced leptons at both ATLAS and CMS we can then directly compare LHC exclusions with cosmological observables. We find that the CMB-S4 target value of ∆N eff = 0.06 is already excluded by current LHC searches and even smaller values can be probed for LLP masses at the electroweak scale.

Searching for dark radiation at the LHC

Many models of dark matter predict long-lived particles (LLPs) that can give rise to striking signatures at the LHC. Existing searches for displaced vertices are however tailored towards heavy LLPs. In this work we show that this bias severely affects their sensitivity to LLPs with masses at the GeV scale. To illustrate this point we consider two dark sector models with light LLPs that decay hadronically: a strongly-interacting dark sector with long-lived exotic mesons, and a Higgsed dark sector with a long-lived dark Higgs boson. We study the sensitivity of an existing ATLAS search for displaced vertices and missing energy in these two models and find that current track and vertex cuts result in very low efficiency for light LLPs. To close this gap in the current search programme we suggest two possible modifications of the vertex reconstruction and the analysis cuts. We calculate projected exclusion limits for these modifications and show that they greatly enhance the sensitivity to LLPs with low mass or short decay lengths.

Juliana Mejia

Michael Krämer

Patrick Tunney

On the challenges of searching for GeV-scale long-lived particles at the LHC

We study the cosmology and LHC phenomenology of a consistent strongly interacting dark sector coupled to Standard Model particles through a generic vector mediator. We lay out the requirements for the model to be cosmologically viable, identify annihilations into dark vector mesons as the dominant dark matter freeze-out process and discuss bounds from direct detection. At the LHC the model predicts dark showers, which can give rise to semi-visible jets or displaced vertices. Existing searches for di-jet resonances and for missing energy mostly probe the parameter regions where prompt decays are expected and constrain our model despite not being optimised for dark showers. We also estimate the sensitivity of dedicated analyses for semi-visible jets and emphasize the complementarity of different search strategies.

Strongly interacting dark sectors in the early Universe and at the LHC through a simplified portal