LZ team in Sheffield:
|Prof Vitaly Kudryavtsev - Sheffield PI|
|Prof Dan Tovey - Chair of the LZ-UK Institute Board|
|Dr Elena Korolkova - UK Data Centre Production Manager|
|Dr David Woodward (now at Pennsylvania State University)|
|Mr Peter Rossiter|
|Mr Andrew Naylor|
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 that will be operated at Sanford Underground Research Facility (SURF) in Lead (South Dakota, USA). The LZ Collaboration consists of more than 200 scientists and engineers from 38 insitutions in the US, UK, Portugal, Russia and South Korea.
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 first 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, XENON10, XENON100 and finally LUX, XENON1T and PandaX 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 [http://lz.lbl.gov].
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 or background electron recoil within the TPC. Distribution of light between PMTs allows the reconstruciton of the hit 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 significantly more sensitive to WIMPs than any currently running experiment.
The LZ detector is under construction at a depth of about 4850 ft underground (to attenuate cosmic-ray muons by about 7 orders of magnitude) at SURF in Lead, South Dakota. The construction of LZ is supported by the Department of Energy (DoE) and the State of South Dakota in the USA, Science and Technology Facilities Council (STFC) in the UK, as well as by funding agencies in other participating countries. UK scientists from the Imperial College London, University College London, Royal Holloway University of London, the universities of Bristol, Edinburgh, Liverpool, Oxford and Sheffield, and the STFC Rutherford Appleton Laboratory are contributing to the 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 are 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 have been completed with our strong contribution and reported in the Technical Design Report and several papers.
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. We have also simulated neutrons produced by cosmic-ray muons and showed that this background is well under control and does not make a significant contribution to the total background rate.
Figure 3. Left - surface profile above the Davis campus at SURF (located at the centre of the map). The color scheme depicts the altitude above sea level in meters. East direction is to the right. Right - Azimuth angle distribution of 107 muons at SURF as generated by MUSUN. The azimuth angle is counted from East to North (East is pointing to the right on the left figure). Muon intensity is integrated over zenith angle.
In addition we deployed an event biasing method into the LZ detector simulations that is required to propagate particles (gammas and neutrons) through a large thickness of shielding which results in a large attenuation of the flux. This method splits the simulation into multiple stages. The first stage runs as a normal simulation and the events which reach a user-defined boundary are saved. The subsequent stages then re-propagate the saved particles from the previous stage multiple times and then the particles which reach the next user defined boundary are saved. This method helped to significantly speed up the simulations of the cavern rock gamma-rays which were perfomed on the Sheffield HEP HTC cluster. These simulations determined the contribution of the cavern rock gamma-rays to the background in the WIMP search and other physics studies in LZ.
Figure 4. Left - A schematic drawing of the event biaisng method implemented into LZ detector simulations in order to simulate the cavern rock gamma-rays (Dimensions not to scale). Right - The energy spectra of simulated cavern rock gamma-ray events inside the TPC before standard analysis cuts are applied. The insert shows the energy spectra after cuts are applied inside the energy range of interest for WIMP searches.
Our Students have helped with the construction of the LZ detector at SURF in the cleanroom and also with running simulations and analysis on the Lawerence Berkeley National Laboratory supercomputers (NERSC data centre).
Figure 5. Left - Peter Rossiter in the cleanroom at SURF helping construct the top PMT array for the LZ detector. Right - Andrew Naylor on the machine room floor at the LBNL in front of the Cori supercomputer.
We offer PhD projects to work on the LZ experiment. A PhD student will focus on on data analysis, as well as on modelling background radiations and LZ detector response. The work will also include participation in LZ operation. Involvement in the design of a future dark matter experiment is a possibility.
|2020||Background mitigation in dual phase xenon time projection chambers||Peter Rossiter|
|2017||Simulations of cosmic muons and background radiations for muon tomography and underground experiments||David Woodward|
|April 2019||Background model for the LUX experiment||IoP Joint HEPP and APP Annual Conference 2019, London, UK||Peter Rossiter|
|July 2018||Neutron production in radioactive process relevant to underground experiments||IDM2018, Providence, USA||Vitaly Kudryavtsev|
|July 2018||Simulations for the LZ experiment||IDM2018, Providence, USA||Vitaly Kudryavtsev|
|July 2018||LUX results and LZ sensitivity to dark matter WIMPs||ICNFP2018, Crete, Greece||Vitaly Kudryavtsev|
|March 2018||Simulations of gamma-ray background from rock for dark matter experiments||IoP Joint HEPP and APP Annual Conference 2018, Bristol, UK||Andrew Naylor|
|November 2017||Background radiations in underground experiments||HEPHY, Vienna, Austria||Vitaly Kudryavtsev|
|May 2017||Cosmogenic activation: recent results||LRT2017, Seoul, South Korea||Vitaly Kudryavtsev|
|April 2017||Direct and indirect detection of dark matter||IoP Joint HEPP and APP Annual Conference 2017, Sheffield, UK||Vitaly Kudryavtsev|
|April 2017||Monte Carlo generators for LZ simulations||IoP Joint HEPP and APP Annual Conference 2017, Sheffield, UK||Peter Rossiter|
|January 2017||The LUX-ZEPLIN dark matter experiment||HEP seminar, University of Warwick, UK||Vitaly Kudryavtsev|
|October 2016||The LUX-ZEPLIN dark matter experiment||HEP seminar, University of Birmingham, UK||Vitaly Kudryavtsev|
|October 2016||Dark matter searches with LZ||HEP seminar, University of Sheffield, UK||Vitaly Kudryavtsev|
|September 2016||Prospects for dark matter searches||IPA2016, Orsay, France||Vitaly Kudryavtsev|
|July 2016||The LUX-ZEPLIN dark matter experiment||The 12th International Workshop Dark Side of the Universe, University of Bergen, Norway||David Woodward|
|July 2016||Cosmogenic background in underground laboratories||IDM 2016, Sheffield, UK||Vitaly Kudryavtsev|
|September 2015||Can muon-induced background explain the DAMA data?||TAUP2015, Torino, Italy||Vitaly Kudryavtsev|
|April 2015||Muon-induced background and its impact on rare-event searches||Seminar, LNGS, Italy||Vitaly Kudryavtsev|
|2019||Measurement of the Gamma Ray Background in the Davis Cavern at the Sanford Underground Research Facility||arXiv:1904.02112|
|2018||Projected WIMP sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment||arXiv:1802.06039|
|2017||LUX-ZEPLIN (LZ) Technical Design Report||arXiv:1703.09144|
|2017||Identification of Radiopure Titanium for the LZ Dark Matter Experiment and Future Rare Event Searches||arXiv:1702.02646||D. S. Akerib et al. (LUX-ZEPPLIN), Astropart. Phys. 96, 1 (2017)|
|2015||LUX-ZEPLIN (LZ) Conceptual Design Report||arXiv:1509.02910|
|2019||First direct detection constraint on mirror dark matter kinetic mixing using LUX 2013 data||arXiv:1908.03479|
|2019||Extending light WIMP searches to single scintillation photons in LUX||arXiv:1907.06272|
|2019||Improved Measurements of the β-Decay Response of Liquid Xenon with the LUX Detector||arXiv:1903.12372||D. S. Akerib et al. (LUX), Phys. Rev. D 100, 022002 (2019)|
|2018||Results of a Search for Sub-GeV Dark Matter Using 2013 LUX Data||arXiv:1811.11241||D. S. Akerib et al. (LUX), Phys. Rev. Lett. 122, 131301 (2019)|
|2018||Search for annual and diurnal rate modulations in the LUX experiment||arXiv:1807.07113||D. S. Akerib et al. (LUX), Phys. Rev. D 98, 062005 (2018)|
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