The nature of dark matter, observed to date only through its gravitational interactions on galactic and cosmological scales, remains a profound mystery. The indirect evidence for dark matter includes galactic matter orbital rates inconsistent with visible and invisible baryonic matter, gravitational lensing of galaxies, lensing of the cosmic microwave background (CMB), and cosmological scale observations including CMB temperature fluctuation angular power spectra and CMB polarization distributions[1]. All observations are consistent with the now-standard LCDM cosmological model of so-called cold (nonrelativistic) relic dark matter, relic standard-model matter, and dark energy characterized by a cosmological constant. The WMAP data constrains the matter density, baryonic matter density, and neutrino density to 0.136, 0.023, and 0.003 of the critical density [2] implying the dark component dominates the matter in the universe.

Relic weakly interacting massive particles (WIMPs) may be a significant dark matter component according to models motivated by both the galactic and cosmological observations and beyond-the-standard-model elementary particle ideas such as supersymmetry (SUSY). The simplest SUSY WIMP model assumes a single relic lightest supersymmetric particle protected from decay to known matter particles by R-parity symmetry. If such particles interact with each other with weak-scale cross sections, implying their interactions are mediated by particles with weak scale (~TeV) or heavier masses, the density of the relics can be similar to that inferred from indirect measurements. In a naïve model, WIMP interactions are spin-independent and coherent amongst nucleons in a nucleus. Their detection therefore favors a high Z target. In some models, WIMP interactions may be suppressed because the interactions are spin-dependent and largely cancel amongst nucleons [3]. Other suppression mechanisms may be imagined [4].

Direct searches for dark matter attempt to observe signals of WIMP elastic interactions with normal matter (nuclei). The velocity distribution of cold dark matter particles in the Milky Way relative to a target nucleus on Earth and the (essentially unconstrained) mass of the relic particle governs the mean energy transfer. Sensitive direct search measurements based on nuclear recoil have probed masses as low as a few GeV. At low WIMP mass, the energy transfer is difficult to observe. At high masses, the expected flux, constrained by the measured dark matter density, limits sensitivity. Additional searches for axion-like light weight dark matter particles require other techniques.

Direct searches complement searches for WIMP production in the form of anomalous missing energy events in collider experiments like CMS and ATLAS, and complement indirect astrophysical annihilation signatures investigated by the space-based and terrestrial cosmic ray/gamma ray detection experiments. A concordance of observations of missing energy events ascribable to WIMPS of a certain mass in colliders, of new mediators or standard model mediated interactions, along with direct observations of WIMP elastic collisions with recoil energies consistent with that mass and with a cross section consistent with predictions, and finally of galactic scale annihilation or other dark matter interactions could provide a compelling picture of dark matter.

The search for direct signals of dark matter was identified by 2014 Particle Physics Project Prioritization Panel [P5] as a priority for research [5]. The direct dark matter search field has been active for some years and a variety of technologies have been deployed or tested including cryogenic crystal devices with low thresholds but also low target size, gaseous detectors with directional sensitivity but also low mass, and liquid noble gas scintillation and ionization detectors. The latter, while lacking directionality, may be pushed to the multi-ton scale as recognized by P5. Presently in operation, the LUX experiment [6] uses a 370-kg liquid-xenon target as a time projection chamber to search for high mass WIMP dark matter. Limits from the initial LUX run have led the field. A factor of ten in collection time during the run due to end in 2016 will improve these limits or possibly provide first evidence.

The LUX-Zeplin (LZ) liquid-xenon WIMP dark matter [7] direct search project concept was selected by the DOE Office of High Energy Physics for support as one 2nd generation direct dark matter search for the Cosmic Frontier Program. The LZ experiment will scale up proven two-phase liquid xenon detection technology [8][9] to significantly extend previous searches and discover or provide the best limits on WIMP dark matter for WIMP mass above a few GeV. Our proposal is focused upon the LZ project.

A signal for WIMP dark matter would be the first direct evidence for physics beyond the standard model. The next step would be a further scale up and experiments dedicated to providing directional information to validate the WIMP wind model[10]. The indication of the mass scale and interaction cross section would focus searches for signatures at colliders. The LZ experiment can search for WIMPS without background approaching a “floor” represented by interactions of solar and galactic neutrinos. These signals will be of interest as well as searches for axionic dark matter and other exotic phenomena.

LZSensitivityThe figure at left from the LZ Conceptual Design Report (CDR) [11] compares the projected sensitivity of the LZ to existing and expected LUX limits along with some SUSY motivated models. The figure shows the projected 90% confidence level (CL) sensitivity for the SI WIMP-nucleon cross sections for LZ (solid blue) along with the current world’s-best limits from LUX (dashed blue), the LUX 300-day projection (dotted blue), and the final ZEPLIN result (dot-dashed blue). Regions above the curves are excluded. The green and yellow bands display the 68% (1s) and 95% (2s) ranges of the expected LZ 90% CL limit. The grey small-dashed line is an estimate of the 90% CL for the S2-only technique. The grey long-dashed line indicates the potential improved low-mass reach if the lower energy threshold is lowered. The regions where background NRs from cosmic neutrinos emerge, and an ultimate neutrino floor, are shown. The grey-colored regions are favored by recent scans of the five-parameter CMSSM, which include the most current constraints from LHC results. The purple and blue points are pMSSM models, where 15 parameters are scanned. The number of standard deviations (s) that quantify consistency are higher for models that are more inconsistent with very recent LHC data. Further explanation and references are provided in the CDR.

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