C. Lee

First results from the LUX dark matter experiment at the Sanford Underground Research Facility

The Large Underground Xenon (LUX) experiment is a dual-phase xenon time-projection chamber operating at the Sanford Underground Research Facility (Lead, South Dakota). The LUX cryostat was filled for the first time in the underground laboratory in February 2013. We report results of the first WIMP search data set, taken during the period from April to August 2013, presenting the analysis of 85.3 live days of data with a fiducial volume of 118 kg. A profile-likelihood analysis technique shows our data to be consistent with the background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP-nucleon elastic scattering with a minimum upper limit on the cross section of 7.6 × 10(-46) cm(2) at a WIMP mass of 33 GeV/c(2). We find that the LUX data are in disagreement with low-mass WIMP signal interpretations of the results from several recent direct detection experiments.

Improved Limits on Scattering of Weakly Interacting Massive Particles from Reanalysis of 2013 LUX Data.

We present constraints on weakly interacting massive particles (WIMP)-nucleus scattering from the 2013 data of the Large Underground Xenon dark matter experiment, including 1.4×10^{4}u2009u2009kgu2009day of search exposure. This new analysis incorporates several advances: single-photon calibration at the scintillation wavelength, improved event-reconstruction algorithms, a revised background model including events originating on the detector walls in an enlarged fiducial volume, and new calibrations from decays of an injected tritium β source and from kinematically constrained nuclear recoils down to 1.1xa0keV. Sensitivity, especially to low-mass WIMPs, is enhanced compared to our previous results which modeled the signal only above a 3xa0keV minimum energy. Under standard dark matter halo assumptions and in the mass range above 4u2009u2009GeVu2009c^{-2}, these new results give the most stringent direct limits on the spin-independent WIMP-nucleon cross section. The 90%xa0C.L. upper limit has a minimum of 0.6xa0zb at 33u2009u2009GeVu2009c^{-2} WIMP mass.

Results on the Spin-Dependent Scattering of Weakly Interacting Massive Particles on Nucleons from the Run 3 Data of the LUX Experiment

We present experimental constraints on the spin-dependent WIMP (weakly interacting massive particle)-nucleon elastic cross sections from LUX data acquired in 2013. LUX is a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility (Lead, South Dakota), which is designed to observe the recoil signature of galactic WIMPs scattering from xenon nuclei. A profile likelihood ratio analysis of 1.4×10^{4}u2009u2009kgu2009day of fiducial exposure allows 90%xa0C.L. upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of σ_{n}=9.4×10^{-41}u2009u2009cm^{2} (σ_{p}=2.9×10^{-39}u2009u2009cm^{2}) at 33u2009u2009GeV/c^{2}. The spin-dependent WIMP-neutron limit is the most sensitive constraint to date.

Limits on spin-dependent WIMP-nucleon cross section obtained from the complete LUX exposure

We present experimental constraints on the spin-dependent WIMP-nucleon elastic cross sections from the total 129.5u2009u2009kgu2009yr exposure acquired by the Large Underground Xenon experiment (LUX), operating at the Sanford Underground Research Facility in Lead, South Dakota (USA). A profile likelihood ratio analysis allows 90%xa0C.L. upper limits to be set on the WIMP-neutron (WIMP-proton) cross section of σ_{n}=1.6×10^{-41}u2009u2009cm^{2} (σ_{p}=5×10^{-40}u2009u2009cm^{2}) at 35u2009u2009GeVu2009c^{-2}, almost a sixfold improvement over the previous LUX spin-dependent results. The spin-dependent WIMP-neutron limit is the most sensitive constraint to date.

Tritium calibration of the LUX dark matter experiment

We present measurements of the electron-recoil (ER) response of the LUX dark matter detector based upon 170 000 highly pure and spatially uniform tritium decays. We reconstruct the tritium energy spectrum using the combined energy model and find good agreement with expectations. We report the average charge and light yields of ER events in liquid xenon at 180 and 105 V/cm and compare the results to the NEST model. We also measure the mean charge recombination fraction and its fluctuations, and we investigate the location and width of the LUX ER band. These results provide input to a reanalysis of the LUX run 3 weakly interacting massive particle search.

Interaction of massless fermions with instantons

We develop a systematic method of calculating the massless fermion propagation function and fermion-induced effective action in the background gauge field of an arbitrary number of widely separated instantons and anti-instantons. The results exhibit a strongly non-local nature of instanton interactions with massless fermions. Using these results, we also demonstrate that the ordinary perturbation theory vacuum is not allowed in QCD with massless fermions, since the cluster decomposition for certain correlation functions is violated due to widely separated instanton-anti-instanton bound states. The underlying mechanism is identical to that in the Schwinger model.

General theory of virtual heavy-particle effects in renormalizable field theories (I)

We develop a systematic method of isolating the effects of virtual heavy particles in renormalizable field theories. With a q54-type field-theory model involving two real scalar fields (one with a heavy mass M, and the other light), we show in detail that, up to order 1/M 2 (but to all orders in renormalized couplings), effects of ~irtual heavy particles can be completely incorporated into pure light-particle theory via effective local vertices which involve operators of canonical dimension at most six. All the coupling strengths for such effective local interactions are of order 1/M 2 (the decoupling theorem) and are systematically calculable in renormalized perturbation theory. We also derive a closed set of Callan-Symanzik equations which are satisfied by these coupling strengths. Using these equations, we explicitly sum all the leading logarithms (i.e., g log M ~ O(1)) which appear in the perturbative calculations of the effective coupling strengths. In field-theoretic studies of strong interaction physics, there has been much interest recently in isolating genuine large-momentum processes from complicated longdistance dynamics like confinement - so called factorization. This has been studied in terms of the operator product expansion [1 ] or from the viewpoint of mass singularity cancellation [2]. In the language of the former, genuine large-momentum processes are described by the so-called coefficient functions, and long-distance dynamics by the matrix elements of various local operators. In an asymptotically free field theory like QCD, the coefficient functions may be reliably calculated through perturbation theory and further improved by using the renormalization group [3]. In this paper, we discuss another kind of factorization: heavy particles versus light particles. Specifically, we wish to calculate systematically effects of virtual heavy particles on light-particle Green functions in renormalizable field theories, when all the external momenta are much smaller than the masses of heavy particles. In renormalizable field theories without spontaneous symmetry breaking, Appel

Calibration, event reconstruction, data analysis, and limit calculation for the LUX dark matter experiment

The LUX experiment has performed searches for dark-matter particles scattering elastically on xenon nuclei, leading to stringent upper limits on the nuclear scattering cross sections for dark matter. Here, for results derived from 1.4×104 kg days of target exposure in 2013, details of the calibration, event-reconstruction, modeling, and statistical tests that underlie the results are presented. Detector performance is characterized, including measured efficiencies, stability of response, position resolution, and discrimination between electron- and nuclear-recoil populations. Models are developed for the drift field, optical properties, background populations, the electron- and nuclear-recoil responses, and the absolute rate of low-energy background events. Innovations in the analysis include in situ measurement of the photomultipliers response to xenon scintillation photons, verification of fiducial mass with a low-energy internal calibration source, and new empirical models for low-energy signal yield based on large-sample, in situ calibrations.