The Future of US Particle Physics: Report of the 2021 Snowmass Community Study – Chapter 5: Cosmic Frontier

Abstract

We present the Snowmass 2021 strategy to delve deep, search wide, and aim high for new discoveries at the Cosmic Frontier of particle physics. The Cosmic Frontier is the bedrock of High Energy Physics in the twenty-first century, providing evidence for Beyond the Standard Model physics that motivates much of the Snowmass 2021 program across all frontiers. The scientific scope of the Cosmic Frontier encompasses four of the five science drivers of the field: dark matter, dark energy and cosmic acceleration, neutrinos, and exploring the unknown in search of new particles, new fields, and new principles of Nature. Covering a time frame of 20 years, our plan features a portfolio of small, medium, and large projects which are ready to produce a continuous stream of groundbreaking science results within this decade along with pathfinders for the next generation of projects to come in the following decade. In this report, we articulate the fundamental questions to be addressed for each science driver and identify key measurements required to achieve well-defined scientific thresholds for discovery. We describe the ecosystem of experiments designed to obtain those measurements as well as the associated developments in theory and technology. We plan to delve deep in sensitivity and search wide across many orders of magnitude in mass to discover the particle nature of dark matter. Moreover, we aim high, through billions of light years of cosmic history, to discover the time-evolution of dark energy, make the first experimental observations of the physics of inflation, and search for new physics at the highest energy scales. Search Wide, Aim High, Delve Deep: the Cosmic Frontier Strategy for DiscoveryThe Cosmic Frontier (CF) comprises a broad set of activities aimed at understanding the fundamental physics which governs the evolution of the Universe and its constituents. For Snowmass 2021, the CF is organized into topical groups, each of which produced a report summarizing the open questions and scientific opportunities in its area:• CF1 : particle-like dark matter [1];• CF2 : wave-like dark matter [2];• CF3 : cosmic probes of dark matter [3];• CF4 : dark energy and cosmic acceleration in the modern Universe [4];

String Cosmology: from the Early Universe to Today

“…The current measurements of the present-day Hubble scale, H 0 , tell us the present value of the total energy density ρ T = i ρ i , of the universe. The present value of the Hubble parameter is measured to be 1 H 0 = 100 h 0 kms −1 Mpc −1 , which gives, via (5) with k = 0,…”

“…More generally, an inflationary expansion requires a somewhat unconventional matter content. Indeed, from (5) we see that, for a universe supported by a perfect fluid, the energy density and pressure should satisfy…”

“…At the same time, the difficulties in obtaining runaway directions that sustain accelerated expansion seem to extend to non-supersymmetric setups. The runaway potentials associated with NS-NS tadpoles in non-supersymmetric string theories are also too steep for quintessence, taking the form V = Λ e −γφ with γ = 5 2 , 3 2 , 3 2 in the Einstein frame, for the SO( 16)×SO( 16) theory, the orientifold US p(32) Sugimoto model, and the type 0B' model, respectively (see [76] for an extension of dS no-go theorems to non-supersymmetric string theories).…”

Feebly-interacting particles: FIPs 2020 workshop report

“…FIP − M 2 FIP + iM FIP Γ FIP ) In many models (such as HNLs), all the diagrams contributing to a given process 27 (initial + final state) involve the same small parameters and therefore scale the same, i.e. there is no interference between multiple diagrams which depend on different θ's.…”

“…Several strategic directions have been presented in numerous contributions to the Snowmass 2021 reports (see Figs. 5 and 6) [24,23,26,27,28].…”