LZ team in Sheffield:
|Dr. Vitaly Kudryavtsev|
|Dr. Elena Korolkova|
|Mr. David Woodward|
Brief introduction to dark matter search
Non-baryonic dark matter is believed to be responsible for about 85% of the total matter content and for about 23% of the total mass-energy content of the Universe. The most likely dark matter candidate - Weakly Interacting Massive Particle (WIMP), is a natural product of Supersymmetric theories of particle physics. A search for these particles with accelerators (particle physics) and using direct or indirect methods of WIMP detection (particle astrophysics) complement each other and in case of discovery will allow physicists to determine or to severely constrain parameters of supersymmetric models.
LUX-ZEPLIN (LZ) experiment
LZ is a next-generation direct WIMP search experiment to be designed, constructed and operated at Sanford Underground Research Facility (SURF) in Lead (South Dakota, USA). The LZ Collaboration consists of 128 scientists and engineers from 29 insitutions in the US, UK, Portugal and Russia.
The LZ experiment will utilise the liquid noble gas technology using two-phase xenon detector with liquid xenon as the target for WIMP interactions. The technology was developed in the UK with single-phase liquid xenon experiment ZEPLIN-I and the world-first two-phase xenon dark matter experiment ZEPLIN-II (both with Sheffield involvement). Both experiment have set world-competitive limits on WIMP interactions at that time. The technology was later advanced in ZEPLIN-III, XENON-10, XENON-100 and finally LUX detectors. A number of liquid and two-phase argon experiments have also been running or are at the design or construction stages.
The key principle of selecting WIMP-induced signals from a much bigger rate of background events is the discrimination between nuclear recoils expected from WIMP interactions and the majority of background events due to electrons caused by gamma-rays or beta-decays.
LZ will have about 7 tons of active liquid xenon in a cryostat surrounded by an additional thin region of xenon ('skin'), liquid organic scintillator and water, all being viewed by photomultiplier tubes (PMTs). Xenon skin, organic scintillator and water will be used as an anticoincidence system to identify and reject events caused by various particles but WIMPs. Below is the schematic of the LZ detector.
Figure 1. Cross-sectional schematic view of the LZ detector (from lzdarkmatter.org).
The central part of the detector, liquid xenon time projection chamber (TPC), will have a strong electric field allowing position reconstruction of events. PMTs, viewing this central part, will detect two signal from each particle track within the TPC: the first signal is due to the prompt scintillation in liquid xenon, the second one is due to ionisation electrons drifting in electric field upwards into the gas phase producing electroluminescence signal in gaseous xenon. The delay between the two signals is proportional to the drift time of electrons and hence, to the z-position of the nuclear recoil track or background electron recoil track within the TPC. Distribution of light between PMTs allows the reconstruciton of the track position in the x-y-plane.
A powerful discrimination between nuclear and electron recoil events is achieved by measuring the ratio of the two signals: ionisation to scintillation which is measured to be significantly smaller for nuclear recoils than for electron recoils for the same magnitude of the scintillation pulse. The LZ experiment will be many times more sensitive to WIMPs than any currently running experiment.
The LZ detector will be built at a depth of about 4850 ft underground (to attenuate cosmisc-ray muons by about 7 orders of magnitude) at SURF in Lead, South Dakota. In July 2014, the US Department of Energy's Office of Science and the National Science Foundation announced support for the LZ experiment at SURF. UK scientists from the Imperial College London, University College London, the Universities of Edinburgh, Liverpool, Oxford and Sheffield, and the STFC Rutherford Appleton and Daresbury Laboratories are also seeking funding for LZ construction.
The team in the University of Sheffield is currently focusing on modelling background radiations for the LZ experiment. One of the main background in the future experiment, which should be sufficiently attenuated is neutrons from radioactivity in major detector components and cosmic-ray muons. Based on our previous experience with simulations, we have calculated neutron yields and spectra from various materials that will be used in the LZ construction. An example is shown below.
Figure 2. Neutron spectra from uranium and thorium contamination in stainless steel. Concentrations of 1 ppb of U and 1 ppb of Th were assumed. Calculations have been done using the modified SOURCES4 code.
Full Monte Carlo simulations of the background from various components are in progress.
We have also developed a model for cosmic-ray muons at SURF. The model takes into account the surface profile above the underground laboratory and muon transport through rock using accurate interaction cross-sections. The model predicts muon fluxes, energy spectra and angular distributions that can be used in simulating background neutron events and their suppression by the outer detector anticoincidence systems: water Cherenkov and liquid scintillator detectors viewed by PMTs. Surface profile above SURF and azimuthal angular distribution of muons at SURF are shown below in Figure 3.
Figure 3. Left - syrface profile above the Davis campus at SURF (located at the centre of the map). The lines drawn from the centre divide each quadrant into 4 angles similar in size: 20o-25o to guide the eye. Right - muon azimuth angle distribution. The vertical lines show approximately the division of quadrants on the left figure where the azimuth angle is counted from East to North (East is pointing to the right on the left figure). Moving from East to North and further on counterclockwise on the map on the left figure, you can see how dips and peaks on the surface profile correspond to peaks and dips in the number of muons on the right graph.
We offer PhD projects to work on the LZ experiment. A PhD student will focus on modelling background radiations and LZ detector response in preparation for future data analysis. The work may also include material screening at Boulby Underground Laboratory and participation in LZ construction. It can be combined with the data analysis from the currently running EDELWEISS dark matter experiment.
More information about the LZ experiment can be found at the official LZ web-page: http://lz.lbl.gov.