LUX-ZEPLIN

Abstract:

We have carried out a series of magnetic field measurements using a portable three-axis SQUID magnetometer at the Laboratoire Souterrain à Bas Bruit (LSBB), Rustrel, France. The magnetometer was originally developed as part of the cryoEDM neutron electric dipole moment experiment [1], where we need to monitor drifts in the magnetic field at a level of ∼0.1 pT. The cryoEDM SQUID system is a 12-channel magnetometer designed to operate in a large cryostat with extensive magnetic shielding [3]. We have tested smaller prototype systems during a series of trips to LSBB [2], primarily to test the SQUIDs, and control and DAQ electronics in a low noise environment. However this investigation also provided an opportunity to characterise the magnetic environment at different locations within the LSBB complex. We monitored the magnetic field at various positions inside the underground laboratory, including the Capsule, the Galerie Anti-Souffe (GAS) and the Galerie Gaz-Brûlés (GGB). We recorded several hours of data at each location to compare with that recorded at the same time by the LSBB [SQUID]2 system permanently installed in the Capsule, and from this we have characterised the relative amplitudes of magnetic field fluctuations in the different locations. SQUID resets are corrected using software, but as this process is not perfect an accurate comparison can only be done during stable periods. Software development and data analysis are still in progress. Preliminary analysis suggests the magnetic field measured in the Capsule is approximately 75% that measured in the GAS and GGB.

Abstract:

There is an abundance of evidence, both astrophysical and cosmological, for the existence of non-luminous, gravitationally interacting matter that cannot be accounted for by the current Standard Model - dark matter. It is estimated that dark matter accounts for ~84% of the matter density of the Universe, but has yet to be directly detected.

The LUX-ZEPLIN experiment (LZ) is searching for a popular dark matter particle candidate, weakly interacting massive particles (WIMPs), in the energy range GeV/c2 – TeV/c2. The LZ detector consists of a dual-phase time projection chamber filled with a 7 t active target of liquid xenon. Recoil interactions of dark matter contained within the galactic halo are expected to produce signals via charge and scintillation channels on the order of a few keV in the xenon target. Owing to passive and active shielding, fiducialisation and background rejection, LZ has reported the world-leading result for spin-independent WIMP–nucleon interactions with data collected during the first science run.

This thesis presents work characterising and constraining the background produced by the cavern environment, namely from naturally occurring radioactive material contained within the cavern rock. This results in a penetrative gamma-ray background from long-lived isotopes of 40K and 238U and 232Th decay chains. Radiogenic neutrons are also produced via spontaneous fission and (α, n) reactions. A new gamma spectroscopy experiment was conducted in the Davis cavern directly outside of the water tank housing LZ in order to gain long-exposure measurements of the gamma-ray background. Simulations are employed to compare our models with real data. The accuracy of our modelling becomes increasingly important as LZ continues to take science data, adding to the live time of the WIMP search and other rare physics searches, such as for low-energy electron recoils and neutrinoless double-beta decay of 136Xe.

Abstract:

The cryoEDM experiment at the Institut Laue-Langevin in Grenoble will measure the electric dipole moment (EDM) of the neutron with unparalleled precision. A neutron EDM arises due to CP violation. The cryoEDM experiment is sensitive to levels of CP violation predicted by many “beyond the standard model” theories and the result will therefore constrain or support these theories. The current limit to the neutron EDM stands at d_n<2.9x 10^-26 e cm as measured with a room temperature experiment. By operating in superfluid helium below 0.9 K and collecting high densities of ultra cold neutrons, the cryoEDM experiment will improve on the existing limit or measure an EDM. High precision magnetometry is essential to reduce the systematic errors in the cryoEDM experiment originating from changes in the magnetic environment. We present the cryoEDM apparatus and technologies.