### Abstract

We propose a simple model in which the baryon asymmetry and dark matter are created via the decays and inverse decays of QCD-triplet scalars, at least one of which must be in the TeV mass range. Singlet fermions produced in these decays constitute the dark matter. The singlets never reach equilibrium, and their coherent production, propagation, and annihilation generates a baryon asymmetry. We find that the out-of-equilibrium condition and the dark matter density constraint typically require the lightest scalar to be long-lived, giving good prospects for detection or exclusion in current and upcoming colliders. In generalizing the leptogenesis mechanism of Akhmedov, Rubakov and Smirnov, our model expands the phenomenological possibilities for low-scale baryogenesis.

### Abstract

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of standard model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the µm scale up to the Big Bang Nucleosynthesis limit of $$\sim 10^7\;\mathrm{m}$$. Neutral LLPs with lifetimes above ~100 m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. We study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC, assuming backgrounds can be rejected as expected. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at ATLAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underllnes the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

### Abstract

We study the potential of the LHCb experiment to discover, for the first time, the true muonium bound state. We propose a search for the vector state, which kinetically mixes with the photon and dominantly decays to electron-positron pairs. We demonstrate that a search for true muonium produced in eta meson decays and decaying at a displaced vertex can exceed a significance of 5 standard deviations assuming statistical uncertainties. We present two possible searches: an inclusive search for the electron-positron vertex, and an exclusive search which requires an additional photon and a reconstruction of the eta mass.

### Abstract

We demonstrate that rare decays of the Standard Model $$Z$$ boson can be used to discover and characterize the nature of new hidden-sector particles. We propose new searches for these particles in soft, high-multiplicity leptonic final states at the Large Hadron Collider. The proposed searches are sensitive to low-mass particles produced in $$Z$$ decays, and we argue that these striking signatures can shed light on the hidden-sector couplings and mechanism for mass generation.

### Abstract

In this paper, we propose a novel powerful strategy to perform searches for new electroweak states. Uncolored electroweak states appear in generic extensions of the Standard Model (SM) and yet are challenging to discover at hadron colliders. This problem is particularly acute when the lightest state in the electroweak multiplet is neutral and all multiplet components are approximately degenerate. In this scenario, production of the charged fields of the multiplet is followed by decay into nearly invisible states; if this decay occurs promptly, the only way to infer the presence of the reaction is through its missing energy signature. Our proposal relies on emission of photon radiation from the new charged states as a means of discriminating the signal from SM backgrounds. We demonstrate its broad applicability by studying two examples: a pure Higgsino doublet and an electroweak quintuplet field.

### Abstract

Dark Matter particles with inelastic interactions are ubiquitous in extensions of the Standard Model, yet remain challenging to fully probe with existing strategies. We propose a series of powerful searches at hadron and lepton colliders that are sensitive to inelastic dark matter dynamics. In representative models featuring either a massive dark photon or a magnetic dipole interaction, we find that the LHC and BABAR could offer strong sensitivity to the thermal relic dark matter parameter space for dark matter masses between ∼100 MeV and 100 GeV and fractional mass-splittings above the percent level; future searches at Belle II with a dedicated monophoton trigger could also offer sensitivity to thermal relic scenarios with masses below a few GeV. Thermal scenarios with either larger masses or splittings are largely ruled out; lower masses remain viable yet may be accessible with other search strategies.

### Abstract

We propose new searches that exploit the unique signatures of decaying sterile neutrinos with masses below MW at the LHC, where they can be produced in rare decays of Standard Model gauge bosons. We show that, for few-GeV-scale sterile neutrinos, the LHC experiments can probe mixing angles at the level of $$10^{-4}$$–$$10^{-3}$$ through powerful searches that look for a prompt lepton in association with a displaced lepton jet. For higher-mass sterile neutrinos, i.e., $$M_N \overset{>}{\sim} 15 \, \mathrm{GeV}$$, run II can explore similarly small mixing angles in prompt multilepton final states. This represents an improvement of up to 2 orders of magnitude in sensitivity to the sterile neutrino production rate.

### Abstract

It has recently been shown that dark-matter annihilation to bottom quarks provides a good fit to the Galactic Center gamma-ray excess identified in the Fermi-LAT data. In the favored dark-matter mass range $$m \sim$$ 30–40 GeV, achieving the best-fit annihilation rate $$\sigma v \sim 5 \times 10^{-26} \, \mathrm{cm^3 s^{-1}}$$ with perturbative couplings requires a sub-TeV mediator particle that interacts with both dark matter and bottom quarks. In this paper, we consider the minimal viable scenarios in which a Standard Model singlet mediates s-channel interactions only between dark matter and bottom quarks, focusing on axial-vector, vector, and pseudoscalar couplings. Using simulations that include on-shell mediator production, we show that existing sbottom searches currently offer the strongest sensitivity over a large region of the favored parameter space explaining the gamma-ray excess, particularly for axial-vector interactions. The 13 TeV LHC will be even more sensitive; however, it may not be sufficient to fully cover the favored parameter space, and the pseudoscalar scenario will remain unconstrained by these searches. We also find that direct- detection constraints, induced through loops of bottom quarks, complement collider bounds to disfavor the vector-current interaction when the mediator is heavier than twice the dark-matter mass. We also present some simple models that generate pseudoscalar-mediated annihilation predominantly to bottom quarks.

### Abstract

We show that the existence of new, light gauge interactions coupled to Standard Model (SM) neutrinos gives rise to an abundance of sterile neutrinos through the sterile neutrinos’ mixing with the SM. Specifically, in the mass range of MeV–GeV and coupling of $$g' \sim 10^{-6}$$–$$10^{-3}$$, the decay of this new vector boson in the early Universe produces a sufficient quantity of sterile neutrinos to account for the observed dark matter abundance. Interestingly, this can be achieved within a natural extension of the SM gauge group, such as a gauged $$L_{\mu}$$ − $$L_{\tau}$$ number, without any tree-level coupling between the new vector boson and the sterile neutrino states. Such new leptonic interactions might also be at the origin of the well-known discrepancy associated with the anomalous magnetic moment of the muon.

### Brian Shuve

Student authorFaculty author

1. Brian Shuve and David Tucker-Smith Baryogenesis and Dark Matter From Freeze-In Physical Review D 101 (2020) 115023. David Curtin, Marco Drewes, John C. McCullough, Patrick Meade, Rabindra N Mohapatra, Jessie Shelton, Brian Shuve, Elena Accomando, Cristiano Alpigiani, Stefan Antusch, Juan Carlos Arteaga-Velázquez, Brian Batell, Martin Bauer, Nikita Blinov, Karen Salomé Caballero-Mora, Jae Hyeok Chang, Eung Jin Chun, Raymond T Co, Timothy Cohen, Peter Cox, Nathaniel Craig, Csaba Csáki, Yanou Cui, Francesco D'Eramo, Luigi Delle Rose, P S Bhupal Dev, Keith R Dienes, Jeff A Dror, Rouven Essig, Jared A Evans, Jason L Evans, Arturo Fernández Tellez, Oliver Fischer, Thomas Flacke, Anthony Fradette, Claudia Frugiuele, Elina Fuchs, Tony Gherghetta, Gian F Giudice, Dmitry Gorbunov, Rick S Gupta, Claudia Hagedorn, Lawrence J Hall, Philip Harris, Juan Carlos Helo, Martin Hirsch, Yonit Hochberg, Anson Hook, Alejandro Ibarra, Seyda Ipek, Sunghoon Jung, Simon Knapen, Eric Kuflik, Zhen Liu, Salvator Lombardo, H J Lubatti, David McKeen, Emiliano Molinaro, Stefano Moretti, Natsumi Nagata, Matthias Neubert, Jose Miguel No, Emmanuel Olaiya, Gilad Perez, Michael E Peskin, David Pinner, Maxim Pospelov, Matthew Reece, Dean J Robinson, Mario Rodríguez Cahuantzi, Rinaldo Santonico, Matthias Schlaffer, Clair Shepherd-Themistocleous, Andrew Spray, Daniel Stolarski, Martin A Subieta Vasquez, Raman Sundrum, Andrea Thamm, Brooks Thomas, Yuhsin Tsai, Brock Tweedie, Stephen M West, Charles Young, Felix Yu, Bryan Zaldivar, Yongchao Zhang, Kathryn Zurek, and José Zurita Long-Lived Particles At the Energy Frontier: the Mathusla Physics Case Reports on Progress in Physics 82 (2019) 116201. Xabier Cid Vidal, Philip Ilten, Jonathan Plews, Brian Shuve, and Yotam Soreq Discovering True Muonium At LHCb Physical Review D 100 (2019) 053003. Nikita Blinov, Eder Izaguirre, and Brian Shuve Rare Z Boson Decays to a Hidden Sector Physical Review D 97 (2018) 015009. Brian Shuve and Michael E. Peskin Revision of the LHCb Limit on Majorana Neutrinos Physical Review D 94 (2016) 113007. Ahmed Ismael, Eder Izaguirre, and Brian Shuve Illuminating New Electroweak States at Hadron Colliders Physical Review D 94 (2016) 015001. Eder Izaguirre, Gordan Krnjaic, and Brian Shuve Discovering Inelastic Thermal Relic Dark Matter at Colliders Physical Review D 93 (2016) 063523. Eder Izaguirre and Brian Shuve Multilepton and Lepton Jet Probes of Sub-Weak-Scale Right-Handed Neutrinos Physical Review D 91 (2015) 093010. Eder Izaguirre, Gordan Krnjaic, and Brian Shuve Bottom-up Approach to the Galactic Center Excess Physical Review D 90 (2014) 18. Brian Shuve and Itay Yavin Dark Matter Progenitor: Light Vector Boson Decay into Sterile Neutrinos Physical Review D 89 (2014) 113004.