Searching for Light Dark Matter with the SLAC Millicharge Experiment

New sub-GeV gauge forces ("dark photons") that kinetically mix with the photon provide a promising scenario for MeV-GeV dark matter, and are the subject of a program of searches at fixed-target and collider facilities around the world. In such models, dark photons produced in collisions may decay invisibly into dark matter states, thereby evading current searches. We re-examine results of the SLAC mQ electron beam dump experiment designed to search for millicharged particles, and find that it was strongly sensitive to any secondary beam of dark matter produced by electron-nucleus collisions in the target. The constraints are competitive for dark photon masses in the ~1-30 MeV range, covering part of the parameter space that can reconcile the apparent (g-2)_{\mu} anomaly. Simple adjustments to the original SLAC search for millicharges may extend sensitivity to cover a sizable portion of the remaining (g-2)_{\mu} anomaly-motivated region. The mQ sensitivity is therefore complementary to on-going searches for visible decays of dark photons. Compared to existing direct detection searches, mQ sensitivity to electron-dark matter scattering cross sections is more than an order of magnitude better for a significant range of masses and couplings in simple models.Comment: 4 pages, 5 figures, 1 tabl

A Gauge Field Theory of Continuous-Spin Particles

We propose and quantize a local, covariant gauge-field action that unifies the description of all free helicity and continuous-spin degrees of freedom in a simple manner. This is the first field-theory action of any kind for continuous spin particles; it is consistent as a quantum theory and generalizes to any number of dimensions. The fields live on the null cone of an internal four-vector "spin-space"; in D dimensions a linearized gauge invariance reduces their physical content to a single function on a Euclidean (D-2)-plane, on which the little group E(D-2) acts naturally. A projective version of the action further reduces the physical content to S^{D-3}, enabling a new local description of particles with any spin structure, and in particular a tower of all integer-helicity particles for D=4. Gauge-invariant interactions with a background current are added in a straightforward manner.Comment: 35 pages, 1 figure version published in JHEP -- introductory discussion of tensor fields for continuous-spin removed, some typos removed and a few clarifying remarks adde

Labor Market Policy Instruments and the Role of Economic Turbulence

Times of high unemployment always inspire debates on the role of labor market policy and its optimal implementation. This paper uses a dynamic model of search unemployment and bilateral wage bargaining to characterize optimal labor market policy in a possibly turbulent environment. A firing externality, generated by the existence of a partial unemployment insurance system, distorts the pre-policy equilibrium along three margins: job creation, job acceptance, and job destruction. Optimal policy is characterized by a payroll tax, a firing tax, and a hiring subsidy. Endogenous job acceptance demands that a firing tax and a hiring subsidy have to be set equal in any case and cannot be used to correct for the possible failure of the Hosios condition. In that case the optimal policy mix has to be extended by either an output or recruitment tax/subsidy. It is further shown that the derived policy mix is robust to the introduction of economic turbulence in form of state-dependent worker transitions between skill classes. This is crucial as widely discussed intergroup redistribution schemes, like in-work benefits targeted at low-skilled workers, are rendered considerably less effective in that case. Instead of redistribution from high- to low-skilled workers or from firing firms to unemployed workers, the paper identifies a scheme involving redistribution from firing to hiring firms to be optimal.Search and matching, employment subsidies, economic turbulence, policy spill-over

On the Theory of Continuous-Spin Particles: Wavefunctions and Soft-Factor Scattering Amplitudes

The most general massless particles allowed by Poincare-invariance are "continuous-spin" particles (CSPs) characterized by a scale \rho, which at \rho=0 reduce to familiar helicity particles. Though known long-range forces are adequately modeled using helicity particles, it is not known whether CSPs can also mediate long-range forces or what consequences such forces might have. We present sharp evidence for consistent interactions of CSPs with matter: new CSP equations of motion, wavefunctions, and covariant radiation amplitudes. In a companion paper, we use these results to resolve old puzzles concerning CSP thermodynamics and exhibit a striking correspondence limit where CSP amplitudes approach helicity-0, 1 or 2 amplitudes.Comment: 37 pages, 5 figure. version published in JHEP. References added, redundant covariant basis of states added. Fixed some typos and inconsistent sign conventions in earlier versio

High Energy Electron Signals from Dark Matter Annihilation in the Sun

In this paper we discuss two mechanisms by which high energy electrons resulting from dark matter annihilations in or near the Sun can arrive at the Earth. Specifically, electrons can escape the sun if DM annihilates into long-lived states, or if dark matter scatters inelastically, which would leave a halo of dark matter outside of the sun. Such a localized source of electrons may affect the spectra observed by experiments with narrower fields of view oriented towards the sun, such as ATIC, differently from those with larger fields of view such as Fermi. We suggest a simple test of these possibilities with existing Fermi data that is more sensitive than limits from final state radiation. If observed, such a signal will constitute an unequivocal signature of dark matter.Comment: 6 pages, 5 figures, 1 table, clarifications added, published versio

Simplified Models for a First Characterization of New Physics at the LHC

Low-energy SUSY and several other theories that address the hierarchy problem predict pair-production at the LHC of particles with Standard Model quantum numbers that decay to jets, missing energy, and possibly leptons. If an excess of such events is seen in LHC data, a theoretical framework in which to describe it will be essential to constraining the structure of the new physics. We propose a basis of four deliberately simplified models, each specified by only 2-3 masses and 4-5 branching ratios, for use in a first characterization of data. Fits of these simplified models to the data furnish a quantitative presentation of the jet structure, electroweak decays, and heavy-flavor content of the data, independent of detector effects. These fits, together with plots comparing their predictions to distributions in data, can be used as targets for describing the data within any full theoretical model.Comment: 76 pages, 24 figures, 9 table