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 – former PhD student, now at Pennsylvania
State University, and Sanford Underground Research Facility, USA
Dr Peter Rossiter – former PhD student, now back in
Australia
Dr Andrew Naylor – former PhD student, now at NERSC, LBNL,
USA
Mr Tom Rushton – PhD student
Miss Jemima Tranter – PhD student (also on G3 DM search at Boulby)
Mr Jo Orpwood – PhD student
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.
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 37 institutions in the US, UK, Portugal 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 uses 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 are 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), has a strong electric field allowing position reconstruction of events. PMTs, viewing this central part, detect two signals from each particle interaction 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 along the drift field of the nuclear recoil or background electron recoil within the TPC. Distribution of light between PMTs allows the reconstruction of the hit position in the x-y-plane perpendicular to the drift field.
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 now operating 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 and operation of LZ has been 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 have been and are contributing to the LZ construction and operation.
The team in the University of Sheffield has been focusing on modelling background radiations for the LZ experiment and is now working of characterisation of backgrounds in the experiment contributing to the construction of the background model included in the interpretation of the results in terms of WIMP limits or potential signal. One of the main background in the 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 and are now assessing these background using available data from the first science run. An example from simulations is shown below.
Neutron spectra from stainless steel
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.
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.
Surface profile at SURF Azimuthal angle distribution of the muon flux
Figure 3. Left: surface profile above the
Davis campus at SURF (located at the centre of the map). The colour 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 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 performed 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.
We are now working with the LZ data focusing on discriminating single against multiple neutron scattering events, activation studies and measurements of the muon flux and muon-induced background.
Diagram of the rock gamma simulations Spectra of energy depositions from rock gammas
Figure 4. Left: a schematic drawing of the
event biasing 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 PhD students have helped with the construction and operation of the LZ detector at SURF.
Peter Rossiter with PMT array Andrew Naylor next to the Cori supercomputer an NERSC
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 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 also possible.
|
Year |
Title |
Author |
|
2022 2020 |
Background studies for LUX
and LZ: activation, gammas from rock and krypton removal Background
mitigation in dual phase xenon time projection chambers |
Andrew
Naylor Peter
Rossiter |
|
2017 |
Simulations
of cosmic muons and background radiations for muon tomography and underground
experiments
|
David
Woodward |
|
Date |
Title |
Venue |
Speaker |
|
July
2022 |
Identification
of dark matter 2022, Vienna, Austria |
Vitaly
Kudryavtsev |
|
|
June
2022 |
Neutron
yield calculation from (a,n) reactions with SOURCES4 |
Low
radioactivity techniques 2022, Rapid City, SD, USA |
Vitaly
Kudryavtsev |
|
May
2019 |
Neutron
production in (alpha,n) reactions: where we were
and where we are now |
Low
radioactivity techniques 2019, LSC, Jaca, Spain |
Vitaly
Kudryavtsev |
|
April
2019 |
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 |
IDM2018,
Providence, USA |
Vitaly
Kudryavtsev |
|
|
July
2018 |
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 |
LRT2017,
Seoul, South Korea |
Vitaly
Kudryavtsev |
|
|
April
2017 |
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 |
IPA2016,
Orsay, France |
Vitaly
Kudryavtsev |
|
|
July
2016 |
The
12th International Workshop Dark Side of the Universe, University of Bergen,
Norway |
David
Woodward |
|
|
July
2016 |
IDM
2016, Sheffield, UK |
Vitaly
Kudryavtsev |
|
|
September
2015 |
TAUP2015,
Torino, Italy |
Vitaly
Kudryavtsev |
|
|
April
2015 |
Muon-induced
background and its impact on rare-event searches |
Seminar,
LNGS, Italy |
Vitaly
Kudryavtsev |
LZ
publications from inspirehep.net
More information about the LZ experiment can be found at the official LZ web-page: http://lz.lbl.gov and also at the official LZUK web-page: http://lz.ac.uk/.
To find out what's currently going on with experiment check our twitter
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