We present the science case, reference design, and project plan for the Stage-4 ground-based cosmic microwave background experiment CMB-S4.

CMB-S4 Science Case, Reference Design, and Project Plan

The landscape of QCD axion models

We review the landscape of QCD axion models. Theoretical constructions that extend the window for the axion mass and couplings beyond conventional regions are highlighted and classified. Bounds from cosmology, astrophysics and experimental searches are reexamined and updated.

We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO) . Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as gaγ~ 10−12 GeV−1, or to electrons gae~10−13, IAXO has the potential to find the QCD axion in the 1 meV~1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums.

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Physics potential of the International Axion Observatory (IAXO)

We present a new bound on the ultralight axion (ULA) dark matter mass ${m}_{\mathrm{a}}$, using the Lyman-alpha forest to look for suppressed cosmic structure growth: a 95% lower limit ${m}_{\mathrm{a}}g2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}\text{ }\text{ }\mathrm{eV}$. This strongly disfavors ($g99.7%$ credibility) the canonical ULA with ${10}^{\ensuremath{-}22}\text{ }\text{ }\mathrm{eV}l{m}_{\mathrm{a}}l{10}^{\ensuremath{-}21}\text{ }\text{ }\mathrm{eV}$, motivated by the string axiverse and solutions to possible tensions in the cold dark matter model. We strengthen previous equivalent bounds by about an order of magnitude. We demonstrate the robustness of our results using an optimized emulator of improved hydrodynamical simulations.

Strong Bound on Canonical Ultralight Axion Dark Matter from the Lyman-Alpha Forest

Author(s): Drlica-Wagner, Alex; Mao, Yao-Yuan; Adhikari, Susmita; Armstrong, Robert; Banerjee, Arka; Banik, Nilanjan; Bechtol, Keith; Bird, Simeon; Boddy, Kimberly K; Bonaca, Ana; Bovy, Jo; Buckley, Matthew R; Bulbul, Esra; Chang, Chihway; Chapline, George; Cohen-Tanugi, Johann; Cuoco, Alessandro; Cyr-Racine, Francis-Yan; Dawson, William A; Rivero, Ana Diaz; Dvorkin, Cora; Erkal, Denis; Fassnacht, Christopher D; Garcia-Bellido, Juan; Giannotti, Maurizio; Gluscevic, Vera; Golovich, Nathan; Hendel, David; Hezaveh, Yashar D; Horiuchi, Shunsaku; Jee, M James; Kaplinghat, Manoj; Keeton, Charles R; Koposov, Sergey E; Lam, Casey Y; Li, Ting S; Lu, Jessica R; Mandelbaum, Rachel; McDermott, Samuel D; McNanna, Mitch; Medford, Michael; Meyer, Manuel; Marc, Moniez; Murgia, Simona; Nadler, Ethan O; Necib, Lina; Nuss, Eric; Pace, Andrew B; Peter, Annika HG; Polin, Daniel A; Prescod-Weinstein, Chanda; Read, Justin I; Rosenfeld, Rogerio; Shipp, Nora; Simon, Joshua D; Slatyer, Tracy R; Straniero, Oscar; Strigari, Louis E; Tollerud, Erik; Tyson, J Anthony; Wang, Mei-Yu; Wechsler, Risa H; Wittman, David; Yu, Hai-Bo; Zaharijas, Gabrijela; Ali-Haimoud, Yacine; Annis, James; Birrer, Simon; Biswas, Rahul; Blazek, Jonathan; Brooks, Alyson M; Buckley-Geer, Elizabeth; Caputo, Regina; Charles, Eric; Digel, Seth; Dodelson, Scott; Flaugher, Brenna; Frieman, Joshua; Gawiser, Eric; Hearin, Andrew P; Hložek, Renee; Jain, Bhuvnesh; Jeltema, Tesla E; Koushiappas, Savvas M; Lisanti, Mariangela | Abstract: Astrophysical and cosmological observations currently provide the only robust, empirical measurements of dark matter. Future observations with Large Synoptic Survey Telescope (LSST) will provide necessary guidance for the experimental dark matter program. This white paper represents a community effort to summarize the science case for studying the fundamental physics of dark matter with LSST. We discuss how LSST will inform our understanding of the fundamental properties of dark matter, such as particle mass, self-interaction strength, non-gravitational couplings to the Standard Model, and compact object abundances. Additionally, we discuss the ways that LSST will complement other experiments to strengthen our understanding of the fundamental characteristics of dark matter. More information on the LSST dark matter effort can be found at https://lsstdarkmatter.github.io/ .

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Denis Erkal

Papers

Probing the Fundamental Nature of Dark Matter with the Large Synoptic Survey Telescope