Cosmic Inflation provides an attractive framework for understanding the early universe and the cosmic microwave background. It can readily involve energies close to the scale at which Quantum Gravity effects become important. General considerations of black hole quantum mechanics suggest nontrivial constraints on any effective field theory model of inflation that emerges as a low-energy limit of quantum gravity, in particular the constraint of the Weak Gravity Conjecture. We show that higher-dimensional gauge and gravitational dynamics can elegantly satisfy these constraints and lead to a viable, theoretically-controlled and predictive class of Natural Inflation models.The success of modern cosmology is founded on the simplifying features of homogeneity, isotropy and spatial flatness of the Universe on the largest distances. In this limit, spacetime evolution is given in terms of a single scale-factor, a(t), and its Hubble expansion rate, H ≡ȧ/a. Homogeneity and flatness are themselves puzzling, constituting very special "initial" conditions from the viewpoint of the Hot Big Bang (HBB). But they become more robust if the HBB is preceded by an even earlier era of Cosmic Inflation, exponential expansion of the Universe driven by the dynamics of a scalar field φ (the "inflaton") coupled to General Relativity (see [1] for a review):(We work in fundamental units in which = c = 1. G N is Newton's constant.) If "slow roll" is achieved for a period of time,φ subdominant and V (φ) ≈ constant, we get a ∝ e Ht , H ≈ constant, after which the potential energy is released, "reheating" the Universe to the HBB. Phenomenologically, N e-folds > 40−60 are required to understand the degree of homogeneity/flatness we see today.Remarkably, quantum fluctuations during inflation can seed the inhomogeneities in the distribution of galaxies and in the Cosmic Microwave Background (CMB). In particular, the CMB temperature-fluctuation powerspectrum,is generically predicted by inflation to be approximately scale-invariant, n s ≈ 1, and is measured to be n s ≈ 0.96 [2,3].Slow roll itself requires an unusually flat potential, suggesting that the inflaton φ is a pseudo-Nambu-Goldstone boson of a spontaneously broken global U (1) symmetry, an "axion". If there is a weak coupling that explicitly violates U (1) symmetry by a definite amount of charge, one can generate a potential,where f is a constant determined by the spontaneous breaking dynamics, while V 0 is a constant proportional to the weak coupling. This is the model of "Natural Inflation" [4]. 1 It can be successfully fit to data, and in particular for N e-folds > 50, n s ≈ 0.96, one findsThe Planck scale M pl ≡ 1/ √ 8πG N = 2 × 10 18 GeV is the energy scale above which Quantum Gravity (QG) effects become strong, and effective field theory (EFT) must break down in favor of a more fundamental description such as superstring theory [6].The very high energy scale V 1/4 0 ≈ 0.01M pl is without precedent in observational physics and implies sensitivity to new exotic phenomena. For such large ...

We propose a novel mechanism for dark matter to explain the observed annual modulation signal at DAMA/LIBRA which avoids existing constraints from every other dark matter direct detection experiment including CRESST, CDMS, and XENON10. The dark matter consists of at least two light states with mass $few GeV and splittings $5 keV. It is natural for the heavier states to be cosmologically long-lived and to make up an Oð1Þ fraction of the dark matter. Direct detection rates are dominated by the exothermic reactions in which an excited dark matter state downscatters off of a nucleus, becoming a lower energy state. In contrast to (endothermic) inelastic dark matter, the most sensitive experiments for exothermic dark matter are those with light nuclei and low threshold energies. Interestingly, this model can also naturally account for the observed low-energy events at CoGeNT. The only significant constraint on the model arises from the DAMA/LIBRA unmodulated spectrum but it can be tested in the near future by a low-threshold analysis of CDMS-Si and possibly other experiments including CRESST, COUPP, and XENON100.

The apparent absence of light superpartners at the LHC strongly constrains the viability of the MSSM as a solution to the hierarchy problem. These constraints can be significantly alleviated by R-parity violation (RPV). Bilinear R-parity violation, with the single operator L H_u, does not require any special flavor structure and can be naturally embedded in a GUT while avoiding constraints from proton decay (unlike baryon-number-violating RPV). The LSP in this scenario can be naturally long-lived, giving rise to displaced vertices. Many collider searches, particularly those selecting b-jets or leptons, are insensitive to events with such detector-scale displaced decays owing to cuts on track quality and impact parameter. We demonstrate that for decay lengths in the window ~1-1000 mm, constraints on superpartner masses can be as low as ~450 GeV for squarks and ~40 GeV for LSPs. In some parts of parameter space light LSPs can dominate the Higgs decay width, hiding the Higgs from existing searches. This framework motivates collider searches for detector-scale displaced vertices. LHCb may be ideally suited to trigger on such events, while ATLAS and CMS may need to trigger on missing energy in the event.Comment: 35 pages, 4 figure

The Weak Gravity Conjecture (WGC) is a proposed constraint on theories with gauge fields and gravity, requiring the existence of light charged particles and/or imposing an upper bound on the field theory cutoff Λ. If taken as a consistency requirement for effective field theories (EFTs), it rules out possibilities for model-building including some models of inflation. I demonstrate simple models which satisfy all forms of the WGC, but which through Higgsing of the original gauge fields produce low-energy EFTs with gauge forces that badly violate the WGC. These models illustrate specific loopholes in arguments that motivate the WGC from a bottom-up perspective; for example the arguments based on magnetic monopoles are evaded when the magnetic confinement that occurs in a Higgs phase is accounted for. This indicates that the WGC should not be taken as a veto on EFTs, even if it turns out to be a robust property of UV quantum gravity theories. However, if the latter is true then parametric violation of the WGC at low energy comes at the cost of nonminimal field content in the UV. I propose that only a very weak constraint is applicable to EFTs,M pl where g is the gauge coupling, motivated by entropy bounds. Remarkably, EFTs produced by Higgsing a theory that satisfies the WGC can saturate but not violate this bound.2