Cosmological Lithium Solution from Discrete Gauged B-L.

The cosmological lithium problem-that theory predicts a primordial abundance far higher than the observed value-has resisted decades of attempts by cosmologists, nuclear physicists, and astronomers alike to root out systematics. We reconsider this problem in the setting of the standard model extended by gauged baryon minus lepton number, which we spontaneously break by a scalar with charge six. Cosmic strings from this breaking can support interactions converting three protons into three positrons, and we argue that an "electric"-"magnetic" interplay can give this process an amplified, strong-scale cross section in an analog of the Callan-Rubakov effect. We suggest such cosmic strings have disintegrated O(1) of the primordial lithium nuclei, and lay out what is necessary for this scheme to succeed. To our knowledge this is the first new physics mechanism with microphysical justification for the abundance of lithium uniquely to be modified after big bang nucleosynthesis.

The muon Smasher's guide.

We lay out a comprehensive physics case for a future high-energy muon collider, exploring a range of collision energies (from 1 to 100 TeV) and luminosities. We highlight the advantages of such a collider over proposed alternatives. We show how one can leverage both the point-like nature of the muons themselves as well as the cloud of electroweak radiation that surrounds the beam to blur the dichotomy between energy and precision in the search for new physics. The physics case is buttressed by a range of studies with applications to electroweak symmetry breaking, dark matter, and the naturalness of the weak scale. Furthermore, we make sharp connections with complementary experiments that are probing new physics effects using electric dipole moments, flavor violation, and gravitational waves. An extensive appendix provides cross section predictions as a function of the center-of-mass energy for many canonical simplified models.

Supersoft Top Squarks.

In a supersymmetric theory, the IR contributions to the Higgs mass are calculable below the mediation scale Λ_{UV} in terms of the IR field content and parameters. However, logarithmic sensitivity to physics at Λ_{UV} remains. In this Letter, we present a first example of a framework, dictated by symmetries, to supersoften these logarithms from the matter sector. The result is a model with finite, IR-calculable corrections to the Higgs mass. This requires the introduction of new fields-the "lumberjacks"-whose role is to screen the UV-sensitive logs. These models have considerably reduced fine-tuning, by more than an order of magnitude for high-scale supersymmetry. This impacts interpretations of the natural parameter space, suggesting it may be premature to declare a naturalness crisis for high-scale supersymmetry.